Rh, Kell, Duffy, Kidd Antigens & Antibodies Chapter

Chapter 7
Rh, Kell, Duffy, and Kidd Antigens
and Antibodies
Connie M. Westhoff
●
Marion E. Reid
This chapter summarizes four clinically significant blood
group systems that are defined by protein polymorphisms.
The proteins that carry these blood group antigens were
difficult to isolate because they are integral membrane proteins present as minor components of the total red blood
cell (RBC) protein. Biochemical techniques developed in
the 1970s and 1980s enabled their purification and partial
amino acid sequencing. In the 1990s, the protein sequence
data were used to construct nucleic acid probes for amplification and screening of bone marrow cDNA libraries to
isolate the genes. In the past decade, there has been a considerable increase in the amount of information about these
blood group antigens; principally, the development and use
of the polymerase chain reaction (PCR) has resulted in the
rapid elucidation of the molecular basis for the antigens and
phenotypes. In addition, the structure and function of these
membrane proteins is an active area of investigation.
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80
Rh BLOOD GROUP SYSTEM
History
The Rh system is second only to the ABO system in importance in transfusion medicine. Rh antigens, especially D, are
highly immunogenic and can cause hemolytic disease of the
newborn (HDN) and severe transfusion reactions. HDN was
first described by a French midwife in 1609 in a set of twins,
of whom one was hydropic and stillborn, and the other was
jaundiced and died of kernicterus.1,2 In 1939, Levine and
Stetson3 described a woman who delivered a stillborn fetus
and suffered a severe hemolytic reaction when transfused
with blood from her husband. Her serum agglutinated the
RBCs of her husband and 80 of 104 ABO-compatible donors.
Subsequently, in 1941, Levine and colleagues4 correctly concluded that the mother had been immunized by the fetus,
which carried an antigen inherited from the father, and suggested that the cause of the erythroblastosis fetalis was maternal antibody in the fetal circulation. Attempts to immunize
rabbits against this new antigen were not successful.1 Levine
and colleagues did not name the antigen.5
Meanwhile, Landsteiner and Wiener, in an effort to discover additional blood groups, injected rabbits and guinea
pigs with rhesus monkey RBCs. The antiserum agglutinated
not only rhesus cells but also the RBCs of 85% of a group of
white subjects from New York, whom the researchers called
Rh positive; the remaining 15% were Rh negative.1 Because
the anti-Rhesus appeared to have reactivity indistinguishable
Ch07-F039816.indd 80
from the maternal antibody reported by Levine and Stetson,
the antigen responsible for HDN was named Rh. Later it was
realized that the rabbit antiserum was not recognizing the
same antigen but was detecting an antigen found in greater
amounts on Rh-positive than on Rh-negative RBCs.6 This
antigen was named LW for Landsteiner and Wiener,1 and the
original human specificity became known as anti-D.
As early as 1941, it was obvious that Rh was not a simple
single antigen system. Fisher named the C and c antigens
(A and B had been used for ABO) on the basis of the reactivity of two antibodies that recognized antithetical antigens,
and used the next letters of the alphabet, D and E, to define
antigens recognized by two additional antibodies.1 Anti-e,
which recognized the e antigen, was identified in 1945.7
Nomenclature
The Rh system has long been acknowledged as one of the
most complex blood group systems because of its large
number of antigens (45) and the heterogeneity of its antibodies. The introduction of two different Rh nomenclatures
reflected the differences in opinion concerning the number of genes that encoded these antigens. The Fisher-Race
nomenclature was based on the premise that three closely
linked genes — C/c, E/e, and D — were responsible, whereas
the Wiener nomenclature (Rh-Hr) was based on the belief
that a single gene encoded one agglutinogen that carried
several blood group factors.
Even though neither theory was correct (there are two
genes, RHD and RHCE, correctly proposed by Tippett8), the
Fisher-Race designation (CDE) for haplotypes is often preferred for written communication, and a modified version of
Wiener’s nomenclature (the original form is nearly obsolete)
is preferred for spoken communication (Table 7–1). A capital
“R” indicates that D is present, and a lowercase “r” (or “little
r”) indicates that it is not. The C or c and E or e Rh antigens carried with D are represented by subscripts: 1 for Ce
(R1), 2 for cE (R2), 0 for ce (R0), and Z for CE (Rz). The CcEe
antigens present without D (r) are represented by superscript
symbols: “prime” for Ce (r’), “double-prime” for cE (r”) and
“y” for CE (ry) (see Table 7–1). The “R” versus “r” terminology allows one to convey the common Rh antigens present
on one chromosome in a single term (a phenotype). Dashes
are used to represent missing antigens of the rare deletion
(or CE-depleted) phenotypes; for example, D– – (referred to
as D dash, dash) lacks C/c and E/e antigens.
In 1962, Rosenfield and associates9 introduced numerical
designations for the Rh antigens to more accurately represent
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Nomenclature and Prevalence for Rh Haplotypes
Prevalence (%)
Haplotype Based on
Antigens Present
Shorthand
for Haplotype
White
DCe
DcE
Dce
DCE
ce
Ce
cE
CE
R1
R2
R0
RZ
r
r’
r”
ry
42
14
4
<0.01
37
2
1
<0.01
African American
17
11
44
<0.01
26
2
<0.01
<0.01
Asian
70
21
3
1
3
2
<0.01
<0.01
the serologic data, to be free of genetic interpretation, and to
be more compatible for computer use (Table 7–2). However,
this numerical nomenclature, with a few exceptions (Rh17,
Rh32, Rh33), is not widely used in the clinical laboratory.
or lack of expression of, RhAG results in a lack of Rh antigen
expression (Rh-null) or a marked reduction of Rh antigen
expression (Rh-mod).17 RhAG has one N-glycan chain that
also carries ABO and Ii specificities (Fig. 7–1).
Terminology
Rh-null and the Rh-Core Complex
Current Rh terminology distinguishes the genes and the
proteins from the antigens, which are referred to by the letter designations, D, C, c, E, e, and so on. To indicate the RH
genes, capital letters, with or without italics, are used (i.e.,
RHD, RHCE, and RHAG). The different alleles of the RHCE
gene are designated RHce, RHCe, RHcE, according to which
antigens they encode. In contrast, the proteins are designated
RhD, RhCE (or according to the specific antigens they carry,
Rhce, RhCe, or RhcE), and RhAG. RH haplotypes are designated Dce, DCe, DcE, etc., or ce, Ce, cE when referring to a
specific CE haplotype.
Rh-null erythrocytes, which lack expression of Rh antigens,
are stomatocytic and spherocytic, and affected individuals have variable degrees of anemia.18,19 The phenotype is
rare, occurring on two different genetic backgrounds; the
regulator type, caused by mutations in the RHAG gene, and
the amorph type, which maps to the RH locus. The amorph
Rh-null results from mutations in RHCE on a deleted RHD
background.20 Regulator RBCs express no Rh and RhAG
proteins, whereas amorph type RBCs have no Rh proteins
but have reduced (20% of normal) RhAG.
Rh and RhAG proteins are associated in the membrane,
possibly as a tetramer consisting of two molecules of each
as a core complex,21 although the precise configuration is
not yet known. Evidence that other proteins interact with
this Rh-core complex comes from observations that Rh-null
RBCs have reduced expression of CD47 (an integrin-associated protein) and of glycophorin B (a sialoglycoprotein that
carries S or s and U antigens). Rh-null RBCs also lack LW
(ICAM-4), a glycoprotein of approximate Mr 42,000 that
belongs to the family of intercellular adhesion molecules.
Band 3 (the red cell anion exchanger AE1) may also be associated with the Rh complex.22 The Rh complex is linked to
the membrane skeleton via Rh/RhAG–ankyrin interaction23
and CD47–protein 4.2 association.24 Some mutations underlying Rh-null disease involve potential contact sites between
RhAG and RhCE/RhD and cytoskeleton ankyrin, which may
explain the morphologic defect in these erythrocytes.23
Proteins
Despite the clinical importance of the Rh blood groups,
the RBC membrane proteins that carry them were identified only in the late 1980s.10,11 The extremely hydrophobic
nature of these multipass transmembrane proteins made the
biochemical isolation difficult and limited progress in their
characterization until the genes were cloned.
The Rh proteins, designated RhD and RhCE, are 417amino acid, nonglycosylated proteins; one carries the D antigen, and the other carries various combinations of the CE
antigens (ce, cE, Ce, or CE).12–14 RhD differs from RhCE by
32 to 35 amino acids (depending on which form of RhCE is
present), and both are predicted to span the membrane 12
times. They migrate in SDS-PAGE (sodium dodecyl sulfate–
polyacrylamide gel electrophoresis) gels with an approximate molecular weight ratio (Mr) of 30,000 to 32,000, and
hence are sometimes referred to as the Rh30 proteins. They
are covalently linked to fatty acids (palmitate) in the lipid
bilayer.15
A 409-amino acid glycosylated protein that coprecipitates
with RhD and RhCE proteins and migrates with an approximate Mr of 40,000 to 100,000 is called RhAG (Rh-associated
glycoprotein) or Rh50 glycoprotein to reflect its apparent
molecular weight.16 RhAG (Rh50) shares 37% amino acid
identity with the RhD and RhCE proteins and has the same
predicted membrane topology. RhAG is not polymorphic
and does not carry Rh antigens. It is important for targeting
the RhD and RhCE to the membrane, because mutations in,
Ch07-F039816.indd 81
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
Table 7–1
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81
Genes
Two genes, designated RHD and RHCE, encode the Rh
proteins. Rh-positive individuals have both genes, whereas
most Rh-negative white people have only the RHCE gene.25
The genes are 97% identical. Each gene has 10 exons and
is the result of a gene duplication on chromosome 1p34–
p36.26 A comprehensive diagram of the intron–exon structure of RHD and RHCE can be found in the review by Avent
and Reid.27
The single gene, RHAG, located at chromosome 6p11–
p21.1 encodes RhAG. RHAG is 47% identical to the RH genes
and also has 10 exons.16,28
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BLOOD BANKING
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Table 7–2 Rosenfield Numerical Terminology
for Rh Antigens
Numerical Term
ISBT Symbol
Rh1
Rh2
Rh3
Rh4
Rh5
Rh6
Rh7
Rh8
Rh9
Rh10
Rh11
Rh12*
Rh17
Rh18
Rh19
Rh20
Rh21
Rh22
Rh23*
Rh26
Rh27
Rh28
Rh29
Rh30
Rh31
Rh32
Rh33
Rh34
Rh35
Rh36
Rh37
Rh39
Rh40
Rh41
Rh42
Rh43
Rh44
Rh45
Rh46
Rh47
Rh48
Rh49
Rh50
Rh51
Rh52
Rh53
Rh54
Rh55
Rh56
D
C
E
c
e
ce or f
Ce
CW
CX
V
EW
G
Hr0†
Hr
hrs
VS
CG
CE
DW
c-like
cE
hrH
Rh29
Goa
hrB
Rh32‡
Rh33§
HrB
Rh35||
Bea
Evans
Rh39
Tar
Rh41
Rh42
Crawford
Nou
Riv
Sec
Dav
JAL
STEM
FPTT
MAR
BARC
JAHK
DAK
LOCR
CENR
*
Rh13 through Rh16, Rh24, and Rh25 are obsolete.
High-incidence antigen; the antibody is made by D– –/D– and
similar phenotypes.
=N
‡
Low-incidence antigen expressed by R and DBT phenotypes.
Har
§
=N Originally described on R0 phenotype, but also found on
R and DVIa (C)−, R0JOH, and R1Lisa.
||
Low-incidence antigen on D(C)(e) cells.
ISBT, International Society for Blood Transfusion.
†
Antigens
As previously described, the major Rh antigens are D, C, c,
E, and e. The many other Rh antigens (see Table 7–2) define
compound antigens in cis (e.g., f (ce), Ce, and CE), low-incidence antigens arising from partial D hybrid proteins (e.g.,
Dw, Goa, BARC), high-incidence antigens, and other variant
antigens. The molecular bases of most Rh antigens have been
determined.
Ch07-F039816.indd 82
RhD
Extracellular
Lipid
Bilayer
Intracellular
NH2
RhCE
417
C/c (Ser/Pro)
103
E/e (Pro/Ala)
226
NH2
417
RhAG
Rh-associated glycoprotein (Rh50)
NH2
409
Figure 7–1 Predicted 12-transmembrane domain model of the RhD,
RhCE, and RhAG proteins in the red blood cell membrane. The amino
acid differences between RhD and RhCE are shown as symbols. The
eight extracellular differences between RhD and RhCE are indicated as
open circles. The zigzag lines represent the location of possible palmitoylation sites. Positions 103 and 226 in RhCE that are critical for C/c
and E/e expression, respectively, are indicated as black circles. The Nglycan on the first extracellular loop of the Rh-associated glycoprotein
is indicated by the branched structure.
Studies to estimate the number of D, C/c, and E/e antigen sites on RBCs found differences between Rh phenotypes.
The number of D antigens ranges from 10,000 on Dce/ce
RBCs to 33,000 on DcE/DcE. The number of C, c, and e antigens per RBC varies from 8500 to 85,000.6 Because C or c
and E or e are carried on the same protein, their numbers
should be equivalent. The equivalency was not always demonstrated in these studies, reflecting the inherent difficulty
in the use of radiolabeled polyclonal antiserum to estimate
antigen numbers. Results of tests with monoclonal antibodies to high-incidence Rh antigens suggest that the total
number of Rh proteins (RhD and RhCE) per RBC is 100,000
to 200,000. The number of RhAG is also estimated to be
100,000 to 200,000,29 consistent with predictions that Rh and
RhAG may be present in the membrane as a tetramer of two
molecules of each.21
D Antigen
Rh positive and Rh negative refer to the presence and absence,
respectively, of the D antigen, which is the most immunogenic
of the Rh antigens. The Rh-negative (D-negative) phenotype
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WEAK D (FORMERLY Du)
An estimated 0.2% to 1% of white persons (and a greater
number of African Americans) have reduced expression of
the D antigen, which is characterized serologically as failure of the RBCs to agglutinate directly with anti-D typing
reagents and requires the use of the indirect antiglobulin
test (IAT) for detection. The number of samples classified as
weak D depends on the characteristics of the typing reagent.
These weak D antigens were previously referred to as Du, a
term that has been abolished. The molecular basis of weak
D expression is heterogeneous. In a large study of samples
from Germany,36 expression of weak D was found to be associated with the presence of point mutations in RHD. More
than 70% of the samples had a Val270Gly amino acid change,
but a total of 16 different mutations were found in the initial study. Since then, a large number of different mutations have been associated with the weak D phenotype (42
to date).37 Collectively, the weak D phenotypes have amino
acid changes predicted to be intracellular or in the transmembrane regions of the RhD protein and not on the outer
surface of the RBC,36 suggesting that the mutations affect the
efficiency of insertion and, therefore, the quantity of RhD
protein in the membrane but do not affect D epitopes.
The majority of individuals with a weak D phenotype
can safely receive D-positive blood and do not make anti-D.
However, two weak D types (type 4.2.2 and type 15) have been
reported to make anti-D.38 These weak D individuals would
more accurately be classified as partial-D. The serologic differentiation of weak D from partial D is not unequivocal.
Importantly, donor center typing procedures must detect
and label weak D RBC components as D-positive.
A very weak form of D, designated Del, detected by
absorption and elution of anti-D, has a high incidence in
Hong Kong Chinese and Japanese persons. The Del phenotype most often results from a splice site mutation or deletion
that results in the absence of amino acids encoded by exon
9 of RHD39 in Asians or a M295I mutation in whites. These
Ch07-F039816.indd 83
RBCs type as D negative (even when tested using the IAT),
and they are usually only recognized if they stimulate production of anti-D in a D-negative recipient.40,41
PARTIAL D ANTIGENS (D CATEGORIES OR D MOSAICS)
The D antigen has long been described as a mosaic on the
basis of the observation that some Rh-positive individuals make alloanti-D when exposed to normal D antigen.
It was hypothesized that the RBCs of these individuals
lack some part of RhD and that they can produce antibodies to the missing portion. Molecular analysis has
shown that this hypothesis is correct, but what was not
predicted is that the missing portions of RHD are replaced
by corresponding portions of RHCE (Fig. 7–2).27,42 Some
replacements involve entire exons, and the novel sequence
of amino acids generates new antigens (e.g., Dw, BARC,
Rh32) (Table 7–3). In addition, several exon rearrangements can give rise to the same partial D category (e.g., DVI
can result from type I, II, or III rearrangements; see Fig.
7–2). Other partial D antigens result from multiple amino
acid conversions between RHCE and RHD, and some are
the result of single point mutations in RHD (DVI, DMH,
DFW; see Table 7–3). These point mutations are predicted
to be located on an extracellular loop or portion of the
RhD protein, in contrast to the weak D antigens already
described, which are predicted to have mutations located
in cytoplasmic or transmembrane regions. Individuals
with partial D antigens can make anti-D and therefore
ideally should receive D-negative donor blood. In practice,
however, most are typed as D positive and are recognized
only after they have made anti-D.
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
occurs in 15% to 17% of white persons but is not common
in other ethnic populations. The absence of D in people of
European descent is primarily the result of deletion of the entire RHD gene and occurred on a Dce (R0) haplotype because
the allele most often carried with the deletion is ce. However,
rare D-negative white persons carry an RHD gene that is not
expressed because of a premature stop codon,30 a 4-base pair
(bp) insertion at the intron 3/exon 4 junction,31 point mutations, or RHD/CE hybrids.32,33 Most of these are associated
with the uncommon Ce (r⬘) or cE (r⬘⬘) haplotypes.
D-negative phenotypes in Asian or African persons, however, are most often caused by inactive or silent RHD rather
than a complete gene deletion. Asian D-negative individuals occur with a frequency of less than 1%, and most carry
mutant RHD genes associated with Ce,34 indicating that the
gene mutations probably originated on a DCe (R1) haplotype. Only 3% to 7% of South African black persons are D
negative, but 66% of this group have RHD genes that contain
a 37-bp internal duplication,35 which results in a premature
stop codon. The 37-bp insert RHD-pseudogene was also
found in 24% of D-negative African Americans. In addition,
15% of the D-negative phenotypes in Africans result from a
hybrid RHD-CE-Ds gene characterized by expression of VS,
weak C and e, and no D antigen.35 Only 18% of D-negative
Africans and 54% of D-negative African Americans completely lack RHD.
ELEVATED D
Several phenotypes, including D– –, Dc−, and DCW−, have
an elevated expression of D antigen and no, weak, and variant
CE antigens, respectively.6 They are caused by replacement
of portions of RHCE by RHD,27,42 analogous to the partial
D rearrangements already described. The additional RHD
sequences in RHCE along with a normal RHD may explain
the enhanced D and accounts for the reduced or missing CE
antigens. Immunized people with these CE-depleted phenotypes make anti-Rh17.
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D EPITOPES EXPRESSED ON RHCE PROTEINS
(R0HAR, ceCF, ceRT)
Some Rhce proteins carry D-specific amino acids that react
with some monoclonal anti-D reagents, adding further complexity to D antigen typing. Two examples, R0Har (DHAR),
found in individuals of German ancestry,43 and Crawford
(ceCF, Rh43), found in individuals of African ancestry,
deserve attention because of their strong reactivity (3+4+) with some FDA-licensed monoclonal reagents and lack
of reactivity with others (including the weak D test) (Table
7–4). Individuals with RoHar do not have a RHD gene, but
exon 5 of the RHCE gene is from RHD (see Fig. 7–2). Those
that carry a Crawford allele, ceCF, also do not have a RHD
gene, but have a Rhce protein with three amino acid changes,
Trp16Cys, Leu245Val, and Gln233Glu. The latter two are Dspecific residues responsible for reactivity with a monoclonal anti-D (see Table 7–4 and Fig. 7–2).44,45 Individuals with
R0Har or Crawford phenotypes make anti-D when stimulated46 (our unpublished observations), and should be considered Rh negative for transfusion and Rh immune globulin
prophylaxis.
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BLOOD BANKING
Origin of the common RHCE genes
RHD
RHCE (ce)
226A
103P
G
antigen
16W
16C
Exon 2 from RHD
Point mutation
226P
103S
Ce
16C G
antigen
cE
Examples of some
RHCE rearrangements
Examples of some
RHD rearrangements
RHCE
rG
No RHD
DIIIc
R0HAR
No RHD
DVa Type II
RHCE
DW
ceCF
No RHD
N
R
RHCE
DVI Type I
16C
Q233E L245V
RHD
RHCE
DVI Type II
BARC
EII
RHD
RHCE
DBT Type I
Rh32
Figure 7–2 Top panel, Diagram of the RHD and RHCE genes, indicating the changes that resulted in the common RhCE polymorphisms. The
shared exon 2 of RHD and RHCE (shown as a black box) explains the expression of G antigen on RhCe and RhD proteins. Bottom panel, Examples
of some RHCE and RHD rearrangements.
II
Table 7–3
84
Molecular Basis of Some Rh Antigens, Partial D, and Unusual Phenotypes
Molecular Basis
Gene
Phenotype/Antigen/Genotype
Single point mutations
RHD
Partial D: DMH, DVII, D+G−, DFW, DHR, DVa, DHMi, DNU, DII, DNB, DHO
Weak D (previously called Du)
CX, CW, Rh−26, E type I, IV, V+,VS+
Partial D: DIIIa, DIVa, DVa, DFR type I
E type III, IV, V+VS+
Partial D: DIIIb, DIIIc, DIVb, DVa, DVI, DFR type II, DBT r”G, (Ce)Ce, (C)ces VS+V−
=N
DHar, rG, R
E type II
DCW−
D− −, D••, Dc−
D••
D••
Multiple mutations
(gene conversions)
Rearranged gene(s) RHD
RHCE
RHD: RHCE
RHCE
RHD
RHCE
RHD-CE-D
RHCE-D-CE
RHD-CE
RHD-CE: RHCE-D
RHD: RHCE-D-CE
RHD: RHD-CE
RHCE-D: RHD-CE
Table 7–4 Composition (IgM and IgG Clones) and Reactivity of FDA-Licensed Anti-D Reagents with
Some Rh Variant RBCs That Can Result in D Typing Discrepancies
Reagent
IgM Monoclonal
IgG
DVI
DBT
DHAR (Whites)
Crawford (Blacks)
Gammaclone
Immucor Series 4
Immucor Series 5
Ortho BioClone
Ortho Gel (ID-MTS)
Polyclonal
GAMA401
MS201
Th28
MAD2
MS201
F8D8 monoclonal
MS26 monoclonal
MS26 monoclonal
Polyclonal
Neg/Pos*
Neg/Pos
Neg/Pos
Neg/Pos
Neg
Neg/Pos
Pos
Pos
Pos
Neg/Pos
Pos
Neg/Pos
Pos
Pos
Vary/Pos
Neg/Neg
Pos
Neg/Neg
Pos
Neg
Neg
Neg
Neg
Neg/Neg
*
Result following slash denotes anti-D test result by the IAT, as permitted by the manufacturer.
Ch07-F039816.indd 84
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C/c and E/e Antigens
There are four major allelic forms of RHCE: ce, Ce, cE, and
CE.14 C and c differ by four amino acids: Cys16Trp (cysteine
at residue 16 replaced by tryptophan) encoded by exon 1,
and Ile60Leu, Ser68Asn, and Ser103Pro encoded by exon 2.
Only residue 103 is predicted to be extracellular; it is located
on the second loop of RhCE (see Fig. 7–1). The amino acids
encoded by exon 2 of RHC are identical to those encoded
by exon 2 of RHD. At the genomic level, RHCe appears to
have arisen from transfer of exon 2 from RHD into RHce (see
Fig. 7–2). The shared exon 2 explains the expression of the G
antigen on both RhD and RhC proteins.
E and e differ by one amino acid, Pro226Ala. This polymorphism, predicted to reside on the fourth extracellular loop
of the protein (see Fig. 7–1), is encoded by exon 5. A single
point mutation in RHce resulted in RHcE (see Fig. 7–2).14,48
CW and CX antigens result from single amino acid changes,
encoded by exon 1, which are predicted to be located on the
first extracellular loop of the RhCE protein.49
The antigens V and VS are expressed on RBCs of more than
30% of black persons. They are the result of a Leu245Val substitution located in the predicted eighth transmembrane segment of Rhce.50 The close location of the e antigen, Ala226 on
the fourth extracellular loop, suggests that Leu245Val causes
a local conformation change responsible for the weakened
expression of e antigen in many black persons who are V and
VS positive. The V−VS+ phenotype results from a Gly336Cys
change on the 245Val background,51 and these alleles are
referred to as ceS (Fig. 7–3). Loss of VS expression (the V+VS−
phenotype) is associated with additional amino acid changes
and is characteristic of the ceAR haplotype (see Fig. 7–3).
Other modifications of RHCE, which are uncommon, are
the hybrids rG, RN, and several E/e variants (see Fig. 7–2). RN
RBCs are found in people of African origin and type as e-weak
(or negative) with polyclonal reagents, but are indistinguishable from “normal” e-positive RBCs with some monoclonal
anti-e. The E variants—EI, EII, and EIII—result either from
a point mutation (EI) or from gene conversion events that
lead to replacement of several extracellular RhcE amino acids
with RhD residues (EII and EIII) and loss of some E epitope
expression.52 Category EIV RBCs, which have an amino acid
substitution in an intracellular domain, do not lack E epitopes but have reduced E expression.53 The very rare RH:-26
results from a Gly96Ser transmembrane amino acid change
that abolishes Rh26 and weakens c expression.54
VARIATION IN
e EXPRESSION AND SICKLE CELL PATIENTS
Variation in expression of the e antigen is not uncommon
and can result from several different mutations. Deletion of
the codon for Arg229, which is close to the Ala226 residue
found in normal e expression,55 and substitution of a Cys
residue for Trp at position 16 in the Rhce protein56 weaken
expression of the antigen. Individuals of African ancestry
often have RHce genes that encode variant e antigens. The
RBCs type as e positive, but they often make alloantibodies
with e-like specificities. The antibodies, designated anti-hrS,
-hrB, -RH18, and -RH34, are difficult to identify serologically,
are clinically significant, and have caused transfusion fatalities.57 The prevalence of e variants in this population, together
with the incidence of sickle cell disease requiring transfusion
support often provided by white donors with conventional
RHce, make the occurrence of alloanti-e in these patients not
uncommon. Some of the RH genetic backgrounds have now
been defined58 and include the RHCE haplotypes shown in
Figure 7–3. All encode the Trp16Cys difference in exon 1, and
have additional changes, primarily localized to exon 5. The ceS
allele is associated with RBCs that are hrB−, whereas individuals homozygous for ceAR, ceMO, ceEK, and ceBI alleles lack
RHD
RHCE
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
Lastly, an Rhce protein with an R154T mutation, designated
ceRT, demonstrates weak reactivity with some anti-D monoclonal
reagents, and the reactivity is enhanced at lower temperatures.
Interestingly, this variant does not carry any D-specific amino
acid but mimics a D-epitope (epD6) structure.47
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85
Figure 7–3 Diagram of the RHD
and RHCE genes indicating changes
often found in African backgrounds
that complicate transfusion in sickle
cell patients.
ceS
D/Ce/D (D-negative, C-positive)
W16C
L245V
G336C
ceMO
DIIIa
(hrS–)
W16C
N152T T201R F223V
DIII
Type 5
V223F
ceEK
(hrS–)
W16C M238V
M267K
R263G
L62F
T201R F223V
A137V
N152T
ceBI
(hrS–)
DIVa
L62F N152T
D350H
W16C M238V A273V L378V
I306V
DAR
T201R F223V
ceAR
I342T
W16C M238V
M267K
L245V R263G
DOL
M170T
Ch07-F039816.indd 85
(hrB–)
(hrS–)
F223V
9/1/06 8:10:58 PM
BLOOD BANKING
II
86
the high-incidence hrS antigen (see Fig. 7–3). Importantly,
because of the multiple molecular backgrounds responsible
for the hrB− and hrS− phenotypes, some of which are not yet
elucidated, the antibodies produced are not all serologically
compatible. This explains why it is difficult to find compatible blood for patients with these antibodies, and often only
rare deleted D– – RBCs appear compatible. As an additional
complication, these variant RHce can often be inherited with
an altered RHD (e.g., DAR, DAU, or DIIIA), so they can also
make anti-D (see Fig. 7–3).
Anti-c, clinically the most important Rh antibody after
anti-D, may cause severe HDN. Anti-C, anti-E, and anti-e do
not often cause HDN, and when they do, it is usually mild.6,59
Autoantibodies to high-incidence Rh antigens often occur
in the sera of patients with warm autoimmune hemolytic
anemia and in some cases of drug-induced autoimmune
hemolytic anemia. These autoantibodies are nonreactive
with Rh-null cells.60
Rh Genotyping
A large number of IgM, direct-agglutinating, anti-D monoclonals have been generated by immortalizing human B lymphocytes in vitro with Epstein-Barr virus. D-typing reagents
licensed for use in the United States are a blend of monoclonal IgM reactive at room temperature along with monoclonal
or polyclonal IgG reactive by the IAT for the determination
of weak D. Four different FDA-licensed reagents are available for tube testing and one for gel (see Table 7–4). All but
two contain different IgM clones. The reactivity of each with
variant D antigens may result in D typing discrepancies.
Importantly, the IgM anti-D component of these reagents
has been selected to not react with DVI RBCs (see Table 7–4).
DVI is the most common partial D found in white populations, and these individuals often make anti-D when exposed
to conventional D. Most agree that they should be classified
as Rh-negative for transfusion or Rh immune globulin. The
IgG component in these reagents reacts with DVI RBCs in the
IAT phase (see Table 7–4). DVI RBCs can stimulate production of anti-D in an Rh-negative recipient and must be typed
as Rh-positive as donors. The composition of current FDAlicensed reagents has prompted the movement away from
weak D testing in the hospital and prenatal setting to classify
individuals with DVI RBCs, or some of the other partial D
phenotypes (see Table 7–4), as Rh negative.
Monoclonal anti-D has not yet been tested clinically for
its ability to prevent immunization after pregnancy but has
been shown to suppress D immunization in D-negative male
volunteers.61
RH genotyping is a useful means to determine the Rh phenotype of patients who have been recently transfused or whose
RBCs are coated with IgG. RH genotyping in the prenatal
setting can be used to determine paternal RHD zygosity and
to predict fetal D status to prevent invasive and expensive
monitoring for the possibility of HDN. The ethnic background of the parents is important to the design of the assay,
because the different molecular events responsible for Dnegative phenotypes must be considered. Testing of samples
from the parents limits the possibility of misinterpretation.
RH genotyping can aid resolution of D typing discrepancies. These often are the result of differences in manufacturers’ reagents, but in the donor setting they can be FDA
reportable. Genotyping can determine a specific weak D
type, partial D category, or the presence of Del.
Genotyping to detect the inheritance of altered or variant
e alleles, which are often linked to variant RHD, aids resolution of antibody specificities and is helpful to determine
alloantibody versus autoantibody specificity. This can be
of significance, especially in sensitized sickle cell patients.
Molecular genotyping can aid in the selection of compatible blood for transfusion and ultimately improve long-term
transfusion support. Currently, these investigations can
sometimes be cumbersome, often requiring complete gene
sequencing. The development of automated, high-throughput platforms that sample many regions of both RHD and
RHCE, along with detailed algorithms for accurate interpretation, are needed.
Antibodies
Most Rh antibodies are IgG, subclasses IgG1 and IgG3 (IgG2
and IgG4 have also been detected), and some sera have an
IgM component.59 Rh antibodies do not activate complement, although two rare exceptions have been reported. The
lack of complement activation by Rh antibodies is thought
to be due to the distance between antigens, but is probably
due to a lack of mobility.6 Reactivity of Rh antibodies is
enhanced by enzyme treatment of the test RBCs, and most
react optimally at 37°C.
Anti-D can cause severe transfusion reactions and severe
HDN. Approximately 80% to 85% of D-negative persons
make anti-D after exposure to D-positive RBCs. The lack of
response in 15% may be due to antigen dose, recipient HLADR alleles, and other as yet unknown genetic factors. AntiD was the most common Rh antibody, but its incidence has
greatly diminished with the prophylactic use of Rh immune
globulin for prevention of HDN. ABO incompatibility
between the mother and the fetus has a partial protective
effect against immunization to D; this finding suggested the
rationale for development of Rh immune globulin.59
Ch07-F039816.indd 86
Anti-D reagents
Expression
Northern blot analysis indicated that Rh and RhAG messenger RNA (mRNA) are restricted to cells of erythroid and
myeloid lineage, but reverse transcriptase-PCR (RT-PCR)
found Rh mRNA splicing isoforms in B and T lymphocytes
and monocytes.62 The significance of this observation is not
yet known. During erythropoiesis, RhAG appears early (on
CD34+ progenitors), but the Rh proteins appear later—RhCE
first, followed by RhD.63
Evolution
The RHD and RHCE genes arose from an early duplication
of the erythrocyte RHAG gene. RH and RHAG have been
investigated in nonhuman primates64,65 and rodents,66–68 and
most, with the exception of gorillas and chimpanzees, have
a RHAG and only one RH gene. The RH gene duplicated
in some common ancestor of gorillas, chimpanzees, and
humans, leading to RHCE and RHD. Chimpanzees and some
gorillas have three RH genes, indicating that a third duplication has taken place in these species.
Function
The predicted membrane structures of Rh and RhAG suggest that they are transport proteins, and the analysis of their
9/1/06 8:10:59 PM
Summary
The molecular basis of many of the Rh antigens has now
been elucidated. The revelation that RhD and RhCE proteins differ by 35 amino acids explains why D antigen is so
immunogenic. In addition, exchanges between RHD and
RHCE, mainly by gene conversion, have generated many
Rh polymorphisms. The proximity of the two genes on the
same chromosome probably affords greater opportunity for
exchange. This finding finally explains the myriad of antigens
observed in the Rh blood group system and gives interesting
insight into the evolutionary history of duplicated genes and
the interactions that can take place between them. The complexity hampers molecular genotyping, and additional Rh
variants are still being discovered. The challenge is to develop
automated platforms that sample several regions of the genes
for unequivocal interpretation.
The discovery that members of the Rh family of proteins,
RhAG, RhBG, and RhCG, are involved in ammonia/ammonium transport and are ideally positioned in key tissues
essential for ammonium elimination is a significant finding
because it was long assumed that the high membrane permeability of ammonia would obviate the need for specific
transport pathways in mammalian cells.
RBC membrane protein–cytoskeleton and protein–
protein interactions are an active area of investigation.
Although the major attachment sites between the erythro-
Ch07-F039816.indd 87
cyte cytoskeleton and the lipid bilayer are understood to be
through glycophorin C and band 3, an additional attachment site mediated by the Rh complex explains the Rh-null
defect. Additional studies are needed to determine the protein–protein associations and the dynamics of the assembly
of the Rh-membrane complex.
THE KELL AND Kx SYSTEM
History
The Kell blood group system was discovered in 1946, just a
few weeks after the introduction of the antiglobulin test. The
RBCs from a newborn baby who was thought to be suffering
from HDN gave a positive reaction in the direct antiglobulin
test.84 The serum of the mother reacted with RBCs from her
husband, her older child, and 9% of random donors. The
system was named from Kelleher, the mother’s surname, and
the antigen is referred to as K (synonyms: Kell, K1). Three
years later, the more common antigen, k (synonyms: Cellano,
K2), which has a high incidence in all populations, was identified through the typing of large numbers of RBC samples
with an antibody that had also caused a mild case of HDN.85
The Kell system remained a simple two-antigen system
until 1957, when the antithetical Kpa and Kpb antigens were
reported, as was the K0 (Kell-null) phenotype.6 Subsequently,
the number of Kell antigens has grown to 25, making Kell
one of the most polymorphic blood group systems known.
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
amino acid sequence reveals distant similarity to ammonium transporters in bacteria, fungi, and plants.69 Evidence
for ammonia transport by RhAG comes from yeast complementation experiments,70,71 expression studies in Xenopus
oocytes,72 and direct evidence in RBCs.73 Rh and RhAG
homologues have been found in many organisms, including the sponge (Geodia), the slime mold (Dictyostelium), the
fruit fly (Drosophila), and the frog (Xenopus),74 indicating
that they are conserved throughout evolution.
Nonerythroid homologues, designated RhCG and RhBG,
are found in the kidney,75 liver,76,77 testis, brain, gastrointestinal tract,78 and skin79,80 and are localized to regions where
ammonium production and elimination are critical in mammalian tissues, strongly suggesting a role for these proteins
in ammonia/ammonium homeostasis. When expressed in
Xenopus oocytes, RhBG and RhCG also mediate transport of
ammonia.81 In the kidney ammonium ions act as expendable
cations that facilitate excretion of acids, and renal ammonium metabolism and transport are critical for acid-base
balance. In the collecting segment and collecting duct, where
large amounts of ammonia are excreted, RhBG and RhCG
are found on the basolateral and apical membranes, respectively, of the intercalated cells.76 These localization studies
suggest that RhBG and RhCG are ideally situated to mediate
transepithelial movement of ammonium from the interstitium to the lumen of the collecting duct. In support, mouse
collecting duct (mIMCD-3) cells, which show polarized
expression of these proteins, demonstrate transportermediated movement of ammonia.82
The function of RhCE and RhD has not been determined.
Co-expression of RhCE/RhAG did not influence the rate or
total substrate accumulated in oocytes compared to that seen
with expression of RhAG alone.83 Although further studies
are necessary to determine if RhCE/D are involved in membrane transport, RhCE/D may have lost transport function
and may have a structural role in the RBC membrane.
Proteins
The Kell protein is a type II glycoprotein with an approximate Mr of 93,000. It has a 665-amino acid carboxyl terminal
extracellular domain, a single 20-amino acid transmembrane domain, and a 47-amino acid N-terminal cytoplasmic
domain.86 The protein has five N-glycosylation sites and 15
extracellular cysteine residues that cause folding through the
formation of multiple intrachain disulfide bonds (Fig. 7–4).
This explains why Kell blood group antigens are inactivated
when RBCs are treated with reducing agents, such as dithiothreitol and aminoethylisothiouronium bromide, which
disrupt disulfide bonds.87 All Kell system antigens are carried on this glycoprotein, which is present at 3500 to 17,000
copies per RBC.88,89 All but two (Jsa and Jsb) of the Kell
antigens are localized in the N-terminal half of the protein
before residue 550, strongly suggesting that the C-terminal
domain does not tolerate change and is functionally important. Indeed, the Kell glycoprotein is a zinc endopeptidase,
and the C terminus contains a zinc-binding domain that is
the catalytic site.90,91
Kx is a 444-amino acid, 37-kD protein that is linked by a
disulfide bond to the Kell protein.17 Kx is predicted to span
the membrane 10 times (see Fig. 7–4), is not glycosylated,
and may be a membrane transport protein. RBCs lacking
Kx have the McLeod phenotype, which is characterized by
a marked reduction of Kell antigens, acanthocytosis, and
reduced in vivo RBC survival.
7
87
Genes
The KEL gene has been localized on chromosome 7q33. It
consists of 19 exons spanning approximately 21.5 kb.92,93 The
Kell antigens result from nucleotide mutations that cause
9/1/06 8:11:00 PM
BLOOD BANKING
Catalytic domain
597
Jsb/Jsa
Leu597Pro
Kpb/Kpa
Arg281Trp
281
k/K
Thr193Met
193
S
Extracellular
COOH
Lipid
Bilayer
Intracellular
NH2
Kx
Kell
NH2
COOH
Figure 7–4 Kell and Kx proteins. Kell is a single-pass protein, but
Kx is predicted to span the red blood cell membrane 10 times. Kell
and Kx are linked by a disulfide bond, shown as -S—. The amino acids
that are responsible for the more common Kell antigens are shown.
The N-glycosylation sites are shown as Y. The hollow Y represents the
N-glycosylation site that is not present on the K (K1) protein.
very weak expression of Kell antigens. It is now evident that
he lacked Kx, which is important for expression of Kell, and
that lack of Kx is the basis for the McLeod syndrome. Males
with the McLeod syndrome have muscular and neurologic
defects, including skeletal muscle wasting, elevated serum
creatine phosphokinase, psychopathology, seizures with basal
ganglia degeneration, and cardiomyopathy.90,100 Most symptoms develop after the fourth decade of life. The syndrome is
very rare. Approximately 60 males have been identified, and
all but 2 have been white; however, because of the plethora of
symptoms, the syndrome is probably underdiagnosed.
Fifteen different mutations in the XK gene were found
in a study of 17 families; these mutations involve major and
minor deletions, point mutations, and splice site or frameshift mutations that result in the absence or truncation of
Kx protein.17,100 At one time, chronic granulomatous disease
(CGD) was thought to be related to the McLeod syndrome,
but the gene controlling CGD is near the XK gene on the
X chromosome, and the small minority of patients with
CGD who have the McLeod phenotype have X-chromosome
deletions encompassing both genes.6,101
Antigens
The Kell system consists of five sets of high-incidence and
low-incidence antigens, as follows (the names of the highincidence antigens appear in boldface):
●
II
88
single amino acid substitutions in the protein (Table 7–5).
The lack of Kell antigens, K0, is caused by several different
molecular defects, including nucleotide deletion, defective
splicing, premature stop codons, and amino acid substitutions.91,94,95
The XK gene is on the short arm of the X chromosome at
Xp21.96 XK has three exons, and mutations cause the McLeod
syndrome, which, because the gene is X linked, affects males.
Carrier females, because of X-chromosome inactivation,
have two populations of RBCs (one of the McLeod phenotype and one normal), and the proportion varies from 5%
McLeod:95% normal to 85% McLeod:15% normal.97,98
McLeod Syndrome
When testing medical students in 1961, Allen and coworkers99
found that one of the students, Mr. McLeod, had RBCs with
Table 7–5 Molecular Basis of Antigens in the
Kell Blood Group System
Antigen
Amino Acid
Position
k(K2)
K(K1)
Kpa(K3)
Kpb(K4)
Kpc(K21)
Jsa(K6)
Jsb(K7)
K11
K17
K14
K24
Ula
Threonine
Methionine
Tryptophan
Arginine
Glutamine
Proline
Leucine
Valine
Alanine
Arginine
Proline
Glutamic acid→Valine
193
Ch07-F039816.indd 88
281
597
302
180
494
●
●
●
●
K and k
Kpa, Kpb, and Kpc
Jsa and Jsb
K11 and K17
K14 and K24
In addition, 14 independently expressed antigens, 3 lowincidence (Ula, K23, VLAN), and 11 high-incidence (Ku,
Km, K12, K13, K16, K18, K19, K22, Tou, KALT, KTIM),
have been identified. The null phenotype, K0, lacks Kell
antigens, and Kell-mod phenotypes have a weak expression
of Kell antigens.
Kell antigens show population variations (Table 7–6). K
has an incidence of 9% in white persons but is much less
common in people of other ethnic backgrounds.1,58 Kpa and
K17 are also mainly found in white persons. Jsa is almost
exclusively found in African Americans, with an incidence of
20%. Ula is found in Finnish and Japanese persons.1,58
The molecular basis of most of the Kell antigens has been
determined (see Table 7–5).91,102 The K methionine substitution disrupts a glycosylation consensus sequence so that K has
one less N-glycan than k.103 Jsa and Jsb are located within a
cluster of cysteine residues,104 a finding that explains why they
are more susceptible than other Kell antigens to treatment
with reducing agents.105 No Kell haplotype has been found to
express more than one low-incidence Kell antigen, not because
of structural constraints, but because multiple expression
would require more than one mutation encoding a recognized
Kell system antigen to occur in the same gene.106
Weaker expression of Kell antigens is found when RBCs
carry Kpa in cis.107–109 Weak expression of Kell antigens can
be inherited or can be acquired and transient. Inherited
weak expression occurs when the Kell-associated Kx protein
is absent (McLeod phenotype), when glycophorins C and D
are absent (Leach phenotype), or when a portion of the extracellular domain of glycophorin C and D, specifically exon 3, is
9/1/06 8:11:00 PM
Kell Phenotypes and Prevalence
Prevalence (%)
Phenotype
White
African American
K−k+
K+k+
K+k−
Kp(a+b−)
Kp(a−b+)
Kp(a+b+)
Kp(a−b−c+)
Js(a+b−)
Js(a−b+)
Js(a+b+)
K11
K17
K14
K24
Ula
91
8.8
0.2
Rare
97.7
2.3
0.32 Japanese
0
100
Rare
High incidence
Low incidence
High incidence
Low incidence
Low incidence
(2.6% in Finns,
0.46% in Japanese)
98
2
Rare
0
100
Rare
0
1
80
19
deleted (some Gerbich-negative phenotypes).109,110 Transient
depression of Kell system antigens has been associated with the
presence of autoantibodies mimicking alloantibodies in autoimmune hemolytic anemia and with microbial infections. Kell
expression was reduced in two cases of idiopathic thrombocytopenic purpura but returned to normal after remission.6
Antibodies
Kell antigens are highly immunogenic, and anti K is common.
However, because more than 90% of donors are K negative, it
is not difficult to find compatible blood for patients with antiK. The other Kell system antibodies are less common but are
also usually IgG, and they have caused transfusion reactions
and HDN or neonatal anemia. In HDN due to Kell antibodies,
neither maternal antibody titers nor amniotic bilirubin levels are good predictors of the severity of the disease. Reports
demonstrate that Kell antigens are expressed very early during
erythropoiesis63 and that Kell antibodies can cause suppression
of erythropoiesis in vitro.111 This finding suggests that the low
level of bilirubin observed, in the presence of neonatal anemia,
is due to Kell antibodies that bind to erythroid progenitors and
exert effects before hemoglobinization.
Anti-Ku is the antibody made by immunized K0 individuals, and Ku represents the high-incidence or “total-Kell”
antigen. McLeod males with CGD make anti-Kx+Km; this
antibody reacts strongly with K0 cells, weaker with RBCs of
common Kell phenotype, and not at all with McLeod phenotype RBCs. Anti-Km, made by McLeod persons who do not
have CGD, reacts with RBCs of common Kell phenotypes
but not with K0 or McLeod RBCs, suggesting that it detects
one or more epitopes requiring both the Kell and Kx proteins. One case has been reported of a McLeod male without
CGD who made an apparent anti-Kx without the presence
of anti-Km.101
Expression
Kell system antigens appear to be erythroid specific and are
expressed very early during erythropoiesis.63 Kx mRNA is
found in muscle, heart, brain, and hematopoietic tissue.17
Ch07-F039816.indd 89
Evolution
Primate RBCs express the k antigen but not the K antigen,
indicating that the K mutation appeared in human lineage.
Chimpanzees are Js(a+b−),112 and Jsa is also present on
RBCs from Old World monkeys,113 suggesting that Jsb also
arose after human speciation. Kell protein is not found on
immunoblots of RBCs from sheep, goat, cattle, rabbit, horse,
mouse, donkey, cat, dog, or rat.114
Function
The Kell glycoprotein is a member of the M13 or neprilysin
family of zinc endopeptidases that cleave a variety of physiologically active peptides. One member of this family, endothelin converting enzyme 1 (ECE-1), is a membrane-bound
metalloprotease that catalyzes the proteolytic activation of
big endothelin-1 (big ET-1).115 Like ECE-1, Kell protein can
proteolytically cleave endothelins, specifically big ET-3 to generate ET-3, which is a potent vasoconstrictor.116 ET-3 is also
involved in the development of the enteric nervous system
and in migration of neural crest-derived cells. The biologic
role of the endothelins is not yet completely elucidated, but
they act on two G protein–coupled receptors, ETA and ETB,
which are found on many cells. Kell-null individuals, who lack
Kell protein, do not have any obvious defect, so they do not
immediately give insight into the biologic function of the Kell
protein. This lack of defect in Kell-null people may be because
other enzymes probably also cleave ET-3,116 and determining
whether Kell-null individuals have abnormal levels of plasma
endothelins is an active area of investigation.
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
Table 7–6
7
DUFFY (Fy) BLOOD GROUP SYSTEM
89
History
The Duffy (Fya) blood group antigen was first reported in
1950 by Cutbush and associates,117 who described the reactivity of an antibody found in a hemophiliac male who
had received multiple transfusions. This blood group system bears the patient’s surname, Duffy, the last two letters
of which provide the abbreviated nomenclature (Fy). Fyb
was found 1 year later.118 In 1975, Fy was identified as the
receptor for the malarial parasite Plasmodium vivax.119 This
discovery explained the predominance of the Fy(a−b−)
(Fy-null) phenotype, which confers resistance to malarial
invasion, in persons originating from West Africa.
Proteins
The Fy protein is a transmembrane glycoprotein of 35 to
43 kD consisting of a glycosylated amino terminal region,
which protrudes from the membrane and has seven transmembrane-spanning domains (Fig. 7–5).120,121 In 1993, it was
realized that Fy was the erythrocyte chemokine receptor that
could bind interleukin-8 and monocyte chemotactic peptide1 (MCP-1).122 The cloning of the FY gene123 confirmed that it
belongs to the conserved family of chemokine receptors.
Gene
The FY gene is located on the long arm of chromosome
1q22–q23124 and spans 1.5 kb. The gene, which has only two
9/1/06 8:11:01 PM
BLOOD BANKING
NH2
31
29
18
Fy6
Table 7–7
Prevalence (%)
Fya/Fyb
Gly42Asp
40 42
Duffy Phenotypes and Prevalence
Phenotype
Caucasians
Blacks
Chinese Japanese Thai
Fy(a+b−)
Fy(a−b+)
Fy(a+b+)
Fy(a−b−)*
FyX
17
34
49
Rare
1.4
9
22
1
68
0
90.8
0.3
8.9
0
0
Fy3
Extracellular
Lipid
Bilayer
81.5
0.9
17.6
0
0
69
3
28
0
0
*
Fy(a−b−), incidence in Israeli Arabs 25%; Israeli Jews, 4%
Intracellular
Fyx
Arg89Cys
COOH
Figure 7–5 The predicted seven-transmembrane domain structure of
the Duffy protein. The amino acid change responsible for the Fya/Fyb
polymorphism, the mutation responsible for Fyx glycosylation sites, and
the regions where Fy3 and Fy6 map are indicated.
exons, contains two ATG codons. The upstream exon contains the major start site.125,126 The same two-exon organization is also found in genes for other chemokine receptors.127
Antigens
II
90
The Fya and Fyb antigens are encoded by two allelic forms
of the gene, designated FYA and FYB, and are responsible
for the Fy(a+b−), Fy(a−b+), and Fy(a+b+) phenotypes.126
They differ by a single amino acid located on the extracellular domain (see Fig. 7–5).
Fyx, which is a weak expression of Fyb, is found in white
persons and is due to a single mutation in the FYB gene.128,129
The amino acid change, which is located in the first intracellular cytoplasmic loop, is associated with a decrease in the
amount of protein in the membrane and results in diminished expression of Fyb, Fy3, and Fy6 antigens as well as
reduced binding of chemokine.127,130,131
Fy3, as determined with one monoclonal anti-Fy3, is
located on the third extracellular loop,132 whereas Fy6 maps
to the amino terminal loop of the protein. The aspartic acid
at amino acid 25 and glutamic acid at 26 are critical for antiFy6 binding.133
The Fy(a−b−) phenotype found in African Americans is
caused by a mutation in the promoter region of FYB (T>C
at position −46), which disrupts a binding site for the erythroid transcription factor GATA-1 and results in the loss of
Fy expression on RBCs.134,135 The Fy protein has also been
found on endothelial cells. Because the erythroid promoter
controls expression only in erythroid cells, expression of Fy
proteins on endothelium is normal in these Fy(a−b−). All
African persons with a mutated GATA sequence to date have
been shown to carry FYB; therefore, Fyb is expressed on their
nonerythroid tissues. This finding explains why Fy(a−b−)
individuals make anti-Fya but not anti-Fyb.136 It also is relevant in the selection of antigen-matched units for Fy(a−b−)
patients with sickle cell disease, because they would not be
expected to make anti-Fyb. Rare Fy(a−b−) who do make
anti-Fyb should be investigated at the genetic level, because
the molecular basis for this observation has not yet been
explained. Lastly, a FYA allele with a mutated GATA sequence
has been found in Papua New Guinea.137
Ch07-F039816.indd 90
The Fy(a−b−) phenotype in white persons is very rare
(Table 7–7).1 One propositus, an Australian (AZ) woman,
appears to be homozygous for a 14-bp deletion in FYA, which
introduces a stop codon in the protein138; a Cree Indian
female (Ye), a white female (NE), and a Lebanese Jewish
male (AA) carry different Trp to stop codon mutations.139
Because these mutations would result in a truncated protein,
these people would not be expected to express endothelial or
erythroid Fy protein. All four people made anti-Fy3.
There are 13,000 to 14,000 Fy antigen sites per RBC,140
and Fya, Fyb, and Fy6 antigens are sensitive to proteolytic
enzyme treatment of antigen-positive RBCs, although Fy3 is
resistant.
Antibodies
Fy antigens are estimated to be 40 times less immunogenic
than K antigens, and most Fy antibodies arise from stimulation by blood transfusion. They are mostly IgG, subclass
IgG1, and only rarely are IgM. Anti-Fyb is less common than
anti-Fya, and Fy antibodies are often found in sera with
other antibodies. Anti-Fy3 is made by rare white Fy(a−b−)
individuals.6 Anti-Fy6 is a mouse monoclonal antibody.127
Expression
Duffy mRNA is present in kidney, spleen, heart, lung, muscle,
duodenum, pancreas, placenta, and brain.123,127 Cells responsible for Fy expression in these tissues are the endothelial cells
lining postcapillary venules,141–143 except in the brain, where
expression is localized to the Purkinje cell neurons.144,145 The
same polypeptide is expressed in endothelial cells and RBCs,
but in brain, a larger, 8.5-kb mRNA is present. The function
of Fy on neurons is an area of active investigation.
In the fetus, Fy antigens can be detected at 6 to 7 weeks’
gestation and are well developed at birth.1 The expression of
these antigens was found to occur late during erythropoiesis
and RBC maturation.63
Evolution
RBCs of monkeys and apes react with anti-Fyb, and the conserved GT repeat sequences in the 3 flanking region of the
gene both suggest that FYB was the ancestral gene.146,147 The
first human divergence occurred when a mutation in the
erythroid promoter region of FYB resulted in the loss of
Fyb expression on RBCs. This mutation conferred resistance
to malaria infection in regions where P. vivax was endemic
and selected for the high proportion of Fy(a−b−) in populations of African ancestry. Later, a single nucleotide change
9/1/06 8:11:01 PM
211
Jka/Jkb
Asp280Asn
Extracellular
Lipid
Bilayer
Function
The importance of Fy as a receptor for the malarial parasite
P. vivax is well established, but its biologic role as a chemokine receptor on RBCs, endothelial cells, and brain is not yet
clear. The chemokine receptors are a family of proteins that
are receptors on target cells for the binding of chemokines.
Chemokines are so named because they are cytokines that are
chemotactic (cause cell migration), and their receptors are
an active area of investigation.148 Chemokine receptors have
been found principally on lymphocytes, where they are coupled to G proteins and activate intracellular signaling pathways that regulate cell migration into tissues. Unlike other
chemokine receptors, Fy does not have a conserved amino
acid DRY-motif in the second extracellular loop, and cells
transfected with Fy and stimulation with chemokines do
not mobilize free calcium.120 Also, Fy can bind chemokines
from both the CXC (IL-8, MGSA) and the CC (RANTES,
MCP-1, MIP-1) classes of chemokines.127 These features have
led investigators to hypothesize that it may act as a scavenger or sink for excess chemokine release into the circulation.149,150 If the function of Fy on RBCs is to scavenge excess
chemokine, it may be that Fy(a−b−) individuals would be
more susceptible to septic shock143 or to cardiac damage
after infarction. Renal allografts have been reported to have
shorter survival in African American Fy(a−b−) recipients,151
and Fy has been shown to be upregulated in the kidney
during renal injury.152
Abundant expression of Fy on high endothelial venules
and sinusoids in the spleen,143 which is a site central to chemokine-induced leukocyte trafficking, suggests a role in
leukocyte migration into the tissues. Fy is also similar to
the receptor for endothelins (ETB), vasoactive proteins that
strongly influence vascular biology and that may also be
mitogenic.127
THE KIDD BLOOD GROUP SYSTEM
History
The Kidd blood group system was discovered in 1951, when
a “new” antibody in the serum of Mrs. Kidd (who also had
anti-K) caused HDN in her sixth child. Jk came from the
initials of the baby (John Kidd), because K had been used
previously for Kell.153 Anti-Jkb was found 2 years later in
the serum of a woman who had a transfusion reaction (she
also had anti-Fya).154 Kidd system antibodies are characteristically found in sera with other blood group antibodies,
suggesting that Kidd antigens are not particularly immunogenic. However, Kidd antibodies induce a rapid and robust
anamnestic response that is responsible for their reputation
for causing severe delayed hemolytic transfusion reactions.
Proteins
The Kidd glycoprotein has an approximate Mr of 46,000 to
60,000 and is predicted to span the membrane 10 times. The
Ch07-F039816.indd 91
Intracellular
COOH
NH2
Figure 7–6 Predicted 10-transmembrane domain structure of the
Kidd/urea transporter. The polymorphism responsible for the Kidd antigens and the site for the N-glycan are indicated.
RH, KELL, DUFFY, AND KIDD ANTIGENS AND ANTIBODIES
in the FYB gene caused the FYA polymorphism in people of
European and Asian ancestry. Fy has been cloned from several nonhuman primates, including chimpanzees, squirrel
monkeys, and rhesus monkeys, and from cows, pigs, rabbits,
and mice.127
third extracellular loop is large and carries an N-glycan at
Asn211 (Fig. 7–6). The protein has 10 cysteine residues, but
only 1 is predicted to be extracellular, a fact that explains why
the antigens are not sensitive to disulfide reagents. There is
internal homology between the N-terminal and C-terminal halves of the protein, with each containing an LP box
(LPXXTXPF) characteristic of urea transporters.155 Isolation
of the Kidd protein from RBCs was elusive, and cloning of the
Kidd gene was accomplished with primers complementary
to the rabbit kidney urea transporter. A clone isolated from
a human bone marrow library with the PCR product was
confirmed by in vitro transcription–translation to encode a
protein that carried Kidd blood group antigens.156
7
Gene
The Kidd blood group gene (HUT11 or SLC14A1) is located
at chromosome 18q11–q12.157,158 The HUT11 gene has 11
exons and spans approximately 30 kb. Two alternative polyadenylation sites generate transcripts of 4.4 and 2.0 kb, which
appear to be used equally. Exons 4 to 11 encode the mature
protein.17,159
91
Antigens
Two antigens, Jka and Jkb, are responsible for the three common phenotypes—Jk(a+b−), Jk(a−b+), and Jk(a+b+). The
two antigens are found with similar frequency in white persons but show large differences in other ethnic groups (Table
7–8).1 The Jk(a−b−) phenotype is rare but occurs with
greater incidence in Asian and Polynesian people.
Table 7–8
Kidd Phenotypes and Prevalence
Prevalence(%)
Phenotype
White
African
Asian
Jk(a+b−)
Jk(a−b+)
Jk(a+b+)
Jk(a−b−)
26.3
23.4
50.3
Rare
51.5
8.1
40.8
Rare
23.2
26.8
49.1
0.9 (Polynesian)
9/1/06 8:11:02 PM
BLOOD BANKING
The Jka and Jkb polymorphism is located on the fourth
extracellular loop and is caused by a single amino acid substitution (see Fig. 7–6).160 The null phenotype has been
reported to arise on the following two genetic backgrounds:
homozygous inheritance of a silent allele and inheritance
of a dominant inhibitor gene, In(Jk), which is unlinked to
JK (HUT11).1 The silent alleles were found to have acceptor
or donor splice site mutations that cause skipping of exon 6
or exon 7.159,161 In Finnish persons, a single point mutation
(Ser291Pro) was the only change associated with silencing
of the expression of Jkb.161 The unlinked dominant inhibitor
gene has not yet been identified.
The Jk3 antigen (“total Jk”) is found on RBCs that are
positive for Jka, Jkb, or both, but the specific amino acid residues responsible for Jk3 are unknown. There are approximately 14,000 Jka antigen sites on Jk(a+b−) RBCs.140
Antibodies
II
92
Neither anti-Jka nor anti-Jkb is common, and anti-Jkb is
found less often than anti-Jka. Once a Kidd antibody is
identified, compatible blood is not difficult to find, because
25% of donors are negative for each antigen. Unfortunately,
Kidd antibodies are often found in sera that contain other
alloantibodies, so the situation is often more complicated. In
addition, the antibodies are known to cause delayed hemolytic transfusion reactions and are responsible for at least one
third of all cases of delayed hemolytic transfusion reactions.59
The antibodies often decrease in titer, react only with cells
that are from persons homozygous for the antigen (the dosage phenomenon), or may escape detection altogether in the
sensitized patient’s serum before transfusion. If the patient
is transfused with antigen-positive RBCs, an anamnestic
response results, with an increase in the antibody titer and
hemolysis of the transfused RBCs.59
Because Kidd antibodies are mainly IgG, it was assumed
that they must activate complement to cause such prompt
RBC destruction. Kidd antibodies can be partially IgM, however, and the minor IgM component may be responsible for
the pattern of destruction of incompatible RBCs in delayed
hemolytic transfusion reactions. This theory is based on evidence that serum fractions containing only IgG anti-Jka do
not bind complement.162 Kidd antibodies only rarely cause
HDN; if they do, it is typically not severe. Anti-Jka has occasionally been found as an autoantibody, and the concurrent,
temporary suppression of antigen expression has also been
reported.59 Anti-Jk3, sometimes referred to as anti-Jkab, is
produced by Jk(a−b−) individuals and reacts with RBCs that
are positive for Jka, Jkb, or both.163
Expression
In the fetus, Kidd antigens can be detected at 11 weeks’ gestation and are well developed at birth.6 Immunohistochemical
and in situ hybridization have shown that Kidd/HUT11 is
also expressed on endothelial cells of vasa recta in the medulla
of the human kidney.17
Evolution
Expression of RBC Kidd antigens has not been investigated
in nonhuman primates, but the human RBC Kidd protein
is clearly a member of a family of urea transporters, which
Ch07-F039816.indd 92
have homologues in the human kidney and in nonhuman
primates.
Function
A first clue to the function of Kidd came from the observation in 1982 that the RBCs of a male of Samoan descent
were resistant to lysis in 2M urea, which was being used to
lyse RBCs for platelet counting.164 His RBCs were Jk(a−b−),
and subsequent investigations revealed that movement of
urea into Jk(a−b−) RBCs was equivalent to passive diffusion, in contrast to RBCs with normal Kidd antigens, with
rapid urea influx. Fast urea transport in human RBCs is
thought to be advantageous for RBC osmotic stability in the
kidney, especially during transit through the vasa recta from
the papilla to the isoosmotic renal cortex.165 Urea transport in the kidney contributes to urine concentration and
water preservation. Two types of urea transporters have now
been characterized, vasopressin-sensitive transporters and
constitutive transporters. The Kidd/HUT11 in RBCs and
kidney medulla is a constitutive transporter. A vasopressinsensitive transporter, HUT2, which is expressed in the collecting ducts of the kidney, has 62% sequence identity with
Kidd/HUT11. HUT2 is also located on chromosome 18q12,
suggesting that both transporters evolved from an ancestral
gene duplication.166 Jk-null individuals do not suffer a clinical syndrome except for a reduced capacity to concentrate
urine,167 suggesting that other mechanisms or other gene
family members, such as HUT2, can compensate for the
missing Kidd/HUT11 protein.
Summary
The last decade has witnessed the rapid elucidation of the
molecular basis for the various blood group antigens and
phenotypes. In the next decade, research focus is turned to
determination of the structure and function of the proteins
carrying these blood group antigens. Null individuals (natural knockouts), persons who lack the antigens and the proteins that carry them, exist for all four of these blood group
systems. Reminiscent of what is being found in many genetically engineered knockout mice, most of the individuals with
null phenotypes do not have serious or any obvious defects.
This observation is consistent with growing evidence that
many genes have evolved as members of larger gene families,
which appear to have overlapping abilities to substitute for
the disrupted gene family member. Evidence is accumulating
that genes encoding blood group proteins are also members
of larger gene families. The mouse homologues of some of
these blood group genes have been cloned, and knockout
mice are being generated to study them. This process may
aid in the further elucidation of the structure and function
of the molecules carrying blood group antigens.
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