At least one YMHCI molecule in the chicken is alloimmunogenic and dynamically expressed on spleen cells during development

Transcriptionally active, MHC class I (MHCI) loci are located in two separate polymorphic genomic regions in the chicken called B and Y. The YMHCI gene sequences encode molecules with uncommon substitutions in the antigen-binding region indicating that YMHCI molecules are likely unique and may bind a specialized form of antigen distinct from that of other antigen-binding MHCI molecules. To learn whether YMHCI gene expression results in the production of alloantigens at the cell surface, we immunized 15I5 × 7 2 chickens using syngeneic RP9 cells expressing transduced YF1w*7.1, a potentially alloimmunogenic YMHCI allele from the Y7 haplotype present in line C. The resulting antisera show that YF1w*7.1 MHCI molecules are immunogenic and expressed on the surfaces of cells in blood and spleen of line C chickens. Virtually all CD3+, CD4+, and CD8+ cells circulating in line C blood are positive, as are BU1+ cells. The YF1w*7.1 MHCI allele is dynamically expressed at levels comparable to but transcriptionally independent of classical BMHCI on erythrocytes, lymphocytes, granulocytes, monocytes, and thrombocytes within the spleen pre- and post-hatching. The antisera react with cells from two among four haplotypes segregating in closed populations of lines N and P. YMHCI shares features associated with both classical and non-classical MHCI. It is becoming increasingly likely that YMHCI has a fundamental role in avian immunity and thereby needs to be included in the growing spectrum of functionally active, diverse MHCI molecules no longer adequately described by the classical/non-classical dichotomy.