Relative Immunogenicity of Fy^a and K Antigens Based on HLA Class II Restriction Analysis.

Background: The relative immunogenicity of protein red blood cell antigens is a longstanding observation. The immunogenicity of a specific antigen has been evaluated first by comparing the observed frequency of alloantibodies with the calculated frequency of the opportunity for immunization. For example in the Caucasian population, it can be deduced that K antigen is 9 times more immunogenic than Fy^a antigen. The aim of this work was to elucidate the underlying mechanisms of the protein red blood cell relative immunogenicity.

Study Design: We chose the model of K and Fy^a antigens since both result from of a single nucleotide polymorphism and present different levels of immunogenicity. 29 individuals producing anti-Fy^a and 30 producing anti-K alloantibodies were analyzed. Each individual was selected according to the presence of only one antibody: Fy^a or K specificity. FY* and KEL* genotyping was performed using the real time fluorescence PCR method. All individuals were Caucasian. DRB1* polymorphisms were determined at the generic level by reverse dot-blot hybridization. DRB1*04 was determined at the allelic level by PCR-SSP. The observed DRB1* frequencies were compared to those of a previously published control population of 350 Caucasian individuals. The TEPITOPE algorithm was applied to predict DRB1*-binding peptide sequences containing the specific K and Fy^a polymorphisms.

Results: Within the anti-Fy^a group, the DRB1*04 was the unique restriction molecule as the DRB1*04 phenotypic frequency was 100%. In the control population, the DRB1*04 phenotypic frequency was 19.01%. The frequency of DRB1*04 was significantly higher as compared to the control population (P<0.0001). The distribution of DRB1*04 alleles was scattered. Conversely, within the anti-K group, a high degree of histocompatibility promiscuity was observed as numerous DRB1* molecules corresponding to the predominant molecules in the Caucasian population were identified.

The DRB1* peptide binding motif prediction algorithm TEPITOPE confirmed these findings and evidenced a higher DRB1* affinity for K-derived peptides as compared to Fy^a-derived peptides.

Conclusion: These results show that DRB1*04 is the major restriction molecule for Fy^a-derived peptides and suggest that when stimulated, only 19.01% of Caucasians are capable of binding the Fy^a-derived peptides and producing anti-Fy^a antibodies. Conversely, all or nearly all individuals in the general population are capable of binding the K-derived peptides and therefore could produce, when stimulated anti-K antibodies.

These results suggest that within a particular ethnic group, the intrinsic immunogenicity of a given red blood cell antigen is related both to the nature of the antigen (number and affinity of potential derived peptides) and to the distribution within the population of DRB1* molecules capable of presenting the derived peptide to the T lymphocyte. Although this first immunological signal is crucial for initiating the immunologic response, future studies regarding T lymphocyte recognition will be necessary in order to fully understand red blood cell immunogenicity.