Regulation of primary alloantibody response through antecedent exposure to a microbial T-cell epitope

Abstract

Humoral alloimmunization to red blood cell (RBC) antigens is a clinically significant problem that can lead to transfusion reactions and difficulty in locating future compatible blood for transfusion. However, factors regulating responder/ nonresponder status are only partially understood. Herein, we identify a series of microbes with 100% identity in 8-to 9-amino acid peptides containing the variant amino acids in Kell, Kidd, and Duffy antigens. To test the hypothesis that infection with such a microbe could predispose to RBC alloimmunization, a mouse model was developed using murine polyoma virus expressing a defined CD4 ؉ Tcell epitope ovalbumin [323][324][325][326][327][328][329][330][331][332][333][334][335][336][337][338][339] IntroductionHumoral immunization to red blood cell (RBC) alloantigens can occur as a result of transfusion or pregnancy. Antibodies against clinically significant blood group alloantigens (ie, RhD, Kell, Kidd, etc) can lead to both hemolysis of transfused RBCs and/or to hemolytic disease of the newborn. 1,2 However, unlike humoral immunization to microbial infection, which approaches 100% in immunocompetent persons, exposure to RBC alloantigens induces a measurable antibody response in only a subset of recipients. Alloimmunization to the RhD antigen on RBCs ranges from 20% to 80%, with only 3% to 10% of recipients becoming immunized to the remaining RBC antigens (eg, Kell, Duffy, Kidd), despite chronic transfusion. [3][4][5] The reason why some transfused patients but not others become alloimmunized is unclear, and factors influencing alloimmunization have been only partially defined. Immunogenetics plays some role in variability of alloimmunization to blood products, as antibody responses to certain alloantigens are confined to distinct recipient human leukocyte antigen (HLA) types. [6][7][8] In addition, genetic variants outside of HLA may regulate RBC alloimmunization. 9 Environmental differences between recipients also likely affect alloimmunization, as genetically identical animals still have variable alloantibody responses to transfused RBCs. 10,11 One such environmental variable may be the inflammatory status of the recipient, which has been shown to have a substantial effect upon alloimmunization to transfused RBCs in mice, 10,12,13 and potentially in humans. 14 In the current report, we hypothesize that an additional potential factor is small peptide homology between microbial-derived peptides and blood group antigens.It has long been appreciated that alloimmunity can be induced through exposure to microbial antigens that mimic the 3-dimensional structure and epitopes of alloantigens (molecular mimicry); thus, antibodies generated against the microbial antigen can cross-react with alloantigens. This is a well-documented event with anti-ABO antibodies, where humans make antibodies against "non-self-" ABO antigens without any prior exposure through transfusion, because of immunization with gut microbes that express A and/or B antigens. 15 In the context of immunoglobulin G (...

Antibody-Mediated Immune Suppression of Erythrocyte Alloimmunization Can Occur Independently from Red Cell Clearance or Epitope Masking in a Murine Model

“…Although an Ab response to the HEL portion of the molecule was clearly detectable at different RBC doses ( Fig. 1), consistent with a previous report, no immune response was detected to OVA or the Duffy portion of the Ag (40). The lowest dose of red cells giving rise to a significant response was 10 5 cells per mouse, which induced both IgM ( Fig.…”

Identification, immunomodulatory activity, and immunogenicity of the major helper T-cell epitope on the K blood group antigen

“…It has long been recognized that certain bacteria-such as Escherichia coli or Shigella sonnei-are capable of expressing K-like Ags on their cell surface, 29 and there are more recent suggestions of potential molecular mimicry between K Th epitopes and microbial peptides from Haemophilus influenzae, Bacteroides fragilis, and Vibrio species. 28 Our own homology searches also illustrate that the potential for mimicry is extensive, and while it will be a major challenge to identify the particular environmental Ag(s) responsible for priming, animal models confirm that preexisting crossreactive Th memory is a powerful factor in increasing the immunogenicity of both auto- 34 and allo- 28 RBC Ags. Such effects may be common in the induction of blood group responses.…”

“…Their potency is illustrated by the finding that murine Ab responses to an RBC-bound complex of foreign Th-and B-cell determinants were increased 100-to 1000-fold after prior infection with virus expressing the helper epitope. 28 The second characteristic of the K polymorphism that may enhance its ability to induce Th responses is the loss of the glycosylation motif encoded by the antithetical k sequence. 9,31 The Kell protein displays carbohydrate residues at amino acid 191 in the k, but not K, type because substitution of T 193 for M 193 ablates the N-glycosylation site.…”

“…19,22,23 The confirmation of Th memory for particular Kell peptides in unimmunized individuals raises the possibility that the cells are primed in vivo by another Ag. Searches for homology between the sequence of Kell and microbial Ags have already reported several key matches 28,29 and, to further illustrate the potential for such cross-reactivity, we used the basic local alignment search tool (BLAST) 30 to interrogate the SwissProt database. The search revealed extensive homologies between the Kell protein sequences spanned by the peptide panels and many bacterial, parasite, viral, and fungal sequences (examples illustrated in supplemental Table 1, available on the Blood Web site; see the Supplemental Materials link at the top of the online article).…”

Absence of the spleen and the occurrence of primary red cell alloimmunization in humans

“…Second, the induction of anti-M (if from the IgM class) might implicate a T cell-independent humoral immune response, for which the spleen is known to be essential. 13 Although an accessory spleen, present in over 10% of humans, was not identified via post-splenectomy CT scanning of the abdomen, some functional splenic tissue might have remained after splenectomy which mediated alloimmunization. Third, the specific combination of a donor liver transplant with splenectomy could have caused red cell alloimmunization via pre-primed lymphocytes derived from the donor's liver transplant (i.e., passenger lymphocyte syndrome).…”