The mouse Kell blood group gene (Kel): cDNA sequence, genomic organization, expression, and enzymatic function

Alloantigen Copy Number as a Critical Factor in RBC Alloimmunization

RBC alloimmunization remains a significant barrier to ongoing transfusion therapy leading to morbidity, and in extreme cases mortality, due to delayed or insufficient units of compatible RBCs. In addition, the monitoring and characterization of alloantibodies, often with multiple specificities in a single patient, consumes substantial health care resources. Extended phenotypic matching has mitigated, but not eliminated, RBC alloimmunization and is only logistically available for specialized populations. Thus, RBC alloimmunization remains a substantial problem. In recent decades it has become clear that mechanisms of RBC alloimmunization are distinct from other antigens and lack of mechanistic understanding likely contributes to the fact that there are no approved interventions to prevent RBC alloimmunization from transfusion. The combination of human studies and murine modeling have identified several key factors in RBC alloimmunization. In both humans and mice, immunogenicity is a function of alloantigen copy number on RBCs. Murine studies have further shown that copy number not only changes rates of immunization but the mechanisms of antibody formation. This review summarizes the current understanding of quantitative and qualitative effects of alloantigen copy number on RBC alloimmunization.

IgG Subclass Determines Suppression Versus Enhancement of Humoral Alloimmunity to Kell RBC Antigens in Mice

It has long been appreciated that immunoglobulins are not just the effector endpoint of humoral immunity, but rather have a complex role in regulating antibody responses themselves. Donor derived anti-RhD IgG has been used for over 50 years as an immunoprophylactic to prevent maternal alloimmunization to RhD. Although anti-RhD has dramatically decreased rates of hemolytic disease of the fetus and newborn (for the RhD alloantigen), anti-RhD also fails in some cases, and can even paradoxically enhance immune responses in some circumstances. Attempts to generate a monoclonal anti-RhD have largely failed, with some monoclonals suppressing less than donor derived anti-RhD and others enhancing immunity. These difficulties likely result, in part, because the mechanism of anti-RhD remains unclear. However, substantial evidence exists to reject the common explanations of simple clearance of RhD + RBCs or masking of antigen. Donor derived anti-RhD is a mixture of 4 different IgG subtypes. To the best of our knowledge an analysis of the role different IgG subtypes play in immunoregulation has not been carried out; and, only IgG1 and IgG3 have been tested as monoclonals. Multiple attempts to elicit alloimmune responses to human RhD epitopes in mice have failed. To circumvent this limitation, we utilize a tractable animal model of RBC alloimmunization using the human Kell glycoprotein as an antigen to test the effect of IgG subtype on immunoregulation by antibodies to RBC alloantigens. We report that the ability of an anti-RBC IgG to enhance, suppress (at the level of IgM responses), or have no effect is a function of the IgG subclass in this model system.

Giant axon formation in mice lacking Kell, XK, or Kell and XK: animal models of McLeod neuroacanthocytosis syndrome

McLeod neuroacanthocytosis syndrome (MLS) is a rare X-linked multisystem disease caused by XK gene mutations and characterized by hematological and neurological abnormalities. XK, a putative membrane transporter, is expressed ubiquitously and is covalently linked to Kell, an endothelin-3-converting enzyme (ECE-3). Absence of XK results in reduction of Kell at sites where both proteins are coexpressed. To elucidate the functional roles of XK, Kell, and the XK-Kell complex associated with pathogenesis in MLS, we studied the pathology of the spinal cord, anterior roots, sciatic nerve, and skeletal muscle from knockout mouse models, using Kel(-/-), Xk(-/-), Kel(-/-)Xk(-/-), and wild-type mice aged 6 to 18 months. A striking finding was that giant axons were frequently associated with paranodal demyelination. The pathology suggests probable anterograde progression from the spinal cord to the sciatic nerve. The neuropathological abnormalities were found in all three genotypes, but were more marked in the double-knockout Kel(-/-)Xk(-/-) mice than in either Kel(-/-) or Xk(-/-) mice. Skeletal muscles from Xk(-/-) and Kel(-/-)Xk(-/-) mice showed mild abnormalities, but those from Kel(-/-) mice were similar to the wild type. The more marked neuropathological abnormalities in Kel(-/-)Xk(-/-) mice suggest a possible functional association between XK and Kell in nonerythroid tissues.

Resolution of translation start site for the human Kell glycoprotein

BACKGROUND

The human Kell blood group system currently contains 35 antigens determined by allelic polymorphisms in the Kell glycoprotein, a single-pass Type II transmembrane protein. The Kell glycoprotein was initially cloned through screening of a cDNA library; however, direct amino acid sequencing of most of the Kell glycoprotein has not been reported. The N-terminus of the Kell glycoprotein contains two potential translational start sites, which result in differences in the cytoplasmic tail.

STUDY DESIGN AND METHODS

Protein extracts were isolated from human red blood cell membranes and were digested with trypsin. The resulting peptides were subjected to liquid chromatography-tandem mass spectrometry, allowing resolution of peptides from the N-terminus of the Kell glycoprotein.

RESULTS

Peptides were isolated and sequenced that correspond to the upstream methionine start site predicted by the full cDNA sequence. No evidence of internal translation initiation at Methionine 20 was detected.

CONCLUSIONS

These findings identify the translational start site and define the full cytoplasmic tail of the human Kell glycoprotein.

Transfusion of murine red blood cells expressing the human KEL glycoprotein induces clinically significant alloantibodies

BACKGROUND

Red blood cell (RBC) alloantibodies to nonself antigens may develop after transfusion or pregnancy, leading to morbidity and mortality in the form of hemolytic transfusion reactions or hemolytic disease of the newborn. A better understanding of the mechanisms of RBC alloantibody induction, or strategies to mitigate the consequences of such antibodies, may ultimately improve transfusion safety. However, such studies are inherently difficult in humans.

STUDY DESIGN AND METHODS

We recently generated transgenic mice with RBC-specific expression of the human KEL glycoprotein, specifically the KEL2 or KEL1 antigens. Herein, we investigate recipient alloimmune responses to transfused RBCs in this system.

RESULTS

Transfusion of RBCs from KEL2 donors into wild-type recipients (lacking the human KEL protein but expressing the murine KEL ortholog) resulted in dose-dependent anti-KEL glycoprotein immunoglobulin (Ig)M and IgG antibody responses, enhanced by recipient inflammation with poly(I:C). Boostable responses were evident upon repeat transfusion, with morbid-appearing alloimmunized recipients experiencing rapid clearance of transfused KEL2 but not control RBCs. Although KEL1 RBCs were also immunogenic after transfusion into wild-type recipients, transfusion of KEL1 RBCs into KEL2 recipients or vice versa failed to lead to detectable anti-KEL1 or anti-KEL2 responses.

CONCLUSIONS

This murine model, with reproducible and clinically significant KEL glycoprotein alloantibody responses, provides a platform for future mechanistic studies of RBC alloantibody induction and consequences. Long-term translational goals of these studies include improving transfusion safety for at-risk patients.

Ablation of the Kell/Xk complex alters erythrocyte divalent cation homeostasis

XK is a putative transporter of unknown function that is ubiquitously expressed and linked through disulfide bonds to Kell protein, an endothelin-3 (ET-3)-converting enzyme. We generated three knockout (KO) mice that lacked either Xk, Kell or both proteins and characterized erythrocyte cation levels, transport and hematological parameters. Absence of Xk or Kell was accompanied by changes in erythrocyte K(+), Mg(2+), Na(+) and Ca(2+) transport that were associated with changes in mean cellular volume and corpuscular hemoglobin concentration mean. Baseline Ca(2+)-ATPase activity was undetected in erythrocytes from all three mouse types but was restored upon pre-incubation with ET-3. Consistent with these alterations in Ca(2+) handling, we observed increased Gardos channel activity in Kel and Xk KO mice. In addition Kel deletion was associated with increased Mg(2+) permeability while Xk deletion blocked Na/Mg exchanger activity. Our results provide evidence that cellular divalent cation regulation is functionally coupled to the Kell/XK system in erythrocytes and loss of this complex may contribute to acanthocytosis formation in McLeod syndrome.

Spontaneously arising red cells with a McLeod-like phenotype in normal donors

Very few human genes can be used to identify spontaneous inactivating somatic mutations. We hypothesized that because the XK gene is X-linked, it would be easy to identify spontaneously arising red cells with a phenotype resembling the McLeod syndrome, which results from inherited XK mutations. Here, by flow cytometry, we detect such phenotypic variants at a median frequency of 9 x 10(-6) in neonatal cord blood samples and 39 x 10(-6) in healthy adults (p=0.004). It may be possible to further investigate the relationship between aging, mutations, and cancer using this approach.

Changes in red cell ion transport, reduced intratumoral neovascularization, and some mild motor function abnormalities accompany targeted disruption of the Mouse Kell gene (Kel)

Kell (ECE-3), a highly polymorphic blood group glycoprotein, displays more than 30 antigens that produce allo-antibodies and, on red blood cells (RBCs), is complexed through a single disulfide bond with the integral membrane protein, XK. XK is a putative membrane transporter whose absence results in a late onset form of neuromuscular abnormalities known as the McLeod syndrome. Although Kell glycoprotein is known to be an endothelin-3-converting enzyme, the full extent of its physiological function is unknown. To study the functions of Kell glycoprotein, we undertook targeted disruption of the murine Kel gene by homologous recombination. RBCs from Kel(-/-) mice lacked Kell glycoprotein, Kell/XK complex, and endothelin-3-converting enzyme activity and had reduced levels of XK. XK mRNA levels in spleen, brain, and testis were unchanged. In Kel(-/-) mice RBC Gardos channel activity was increased and the normal enhancement by endothelin-3 was blunted. Analysis of the microvessels of tumors produced from LL2 cells indicated that the central portion of tumors from wild-type mice were populated with many mature blood vessels, but that vessels in tumors from Kel(-/-) mice were fewer and smaller. The absence of Kell glycoprotein mildly affected some motor activities identified by foot splay on the drop tests. The targeted disruption of Kel in mouse enabled us to identify phenotypes that would not be easily detected in humans lacking Kell glycoprotein. In this regard, the Kell knockout mouse provides a good animal model for the study of normal and/or pathophysiological functions of Kell glycoprotein.

Molecular characterization of a family of metalloendopeptidases from the intestinal brush border of Haemonchus contortus

Substantial protection against the economically important parasitic nematode Haemonchus contortus has been achieved by immunizing sheep with a glycoprotein fraction isolated from the intestinal membranes of the worm (H-gal-GP). Previous studies showed that one of the major components of H-gal-GP is a family of at least 4 zinc metalloendopeptidases, designated MEPs 1–4. This paper describes aspects of the molecular architecture of this protease family, including the proteomic analysis of the MEP fraction of the H-gal-GP complex. These enzymes belong to the M13 zinc metalloendopeptidase family (EC 3.4.24.11), also known as neutral endopeptidases or neprilysins. The sequences of MEPs 1 and 3 suggested a typical Type II integral membrane protein structure, whilst MEPs 2 and 4 had putative cleavable signal peptides, typical of secreted proteins. Proteomic analysis of H-gal-GP indicated that the extracellular domain of all 4 MEPs had been cleaved close to the transmembrane region/signal peptide with additional cleavage sites mid-way along the polypeptide. MEP3 was present as a homo-dimer in H-gal-GP, whereas MEP1 or MEP2 formed hetero-dimers with MEP4. It was found that expression of MEP3 was confined to developing 4th-stage larvae and to adult worms, the stages of Haemonchus which feed on blood. MEP-like activity was detected in the H-gal-GP complex over a broad pH range (5–9). Since all 4 MEPs must share a similar microenvironment in the complex, this suggests that each might have a different substrate specificity.

Multiple loci are linked with anti-red blood cell antibody production in NZB mice -- comparison with other phenotypes implies complex modes of action

The New Zealand Black (NZB) mouse strain is a model of autoimmune haemolytic anaemia (AHA) and systemic lupus erythematosus (SLE), characterized by the production of anti-red blood cell (RBC) antibodies and anti-nuclear antibodies (ANA), respectively. A linkage analysis was carried out in an (NZB x BALB/c) F(2) cross in order to identify loci involved in the production of both anti-RBC IgM and IgG antibodies. These regions of linkage were compared with linkage data to ANA from the same cohort and other linkage analyses involving New Zealand mice. Four previously described NZB loci linked to anti-RBC antibodies were confirmed, and eight novel loci linked to this trait were also mapped: five of which were of NZB origin, and three derived from the non-autoimmune BALB/c background. A comparison between loci linked with anti-RBC antibodies and ANA demonstrated many that co-localize, suggesting the presence of genes that result in the general breaking of tolerance to self-antigen. Furthermore, the observation that some loci were associated only with the anti-RBC response suggests an antigen specific mechanism in addition to a general breaking of tolerance. A locus linked with anti-RBC antibodies and ANA on distal chromosome 7 in this cohort is orthologous to one on the q arm of human chromosome 11, a region linked to AHA and ANA in human SLE.

Kell and XK immunohistochemistry in McLeod myopathy

The McLeod syndrome is an X-linked neuroacanthocytosis manifesting with myopathy and progressive chorea. It is caused by mutations of the XK gene encoding the XK protein, a putative membrane transport protein of yet unknown function. In erythroid tissues, XK forms a functional complex with the Kell glycoprotein. Here, we present an immunohistochemical study in skeletal muscle of normal controls and a McLeod patient with a XK gene point mutation (C977T) using affinity-purified antibodies against XK and Kell proteins. Histological examination of the affected muscle revealed the typical pattern of McLeod myopathy including type 2 fiber atrophy. In control muscles, Kell immunohistochemistry stained sarcoplasmic membranes. XK immunohistochemistry resulted in a type 2 fiber-specific intracellular staining that was most probably confined to the sarcoplasmic reticulum. In contrast, there was only a weak background signal without a specific staining pattern for XK and Kell in the McLeod muscle. Our results demonstrate that the lack of physiological XK expression correlates to the type 2 fiber atrophy in McLeod myopathy, and suggest that the XK protein represents a crucial factor for the maintenance of normal muscle structure and function.