Differential binding of two monoclonal antibodies directed against the chicken CD8α molecule

Two murine monoclonal antibodies (mAb), CT8 and AV14, have been shown to recognise the avian homologue of the mammalian CD8α molecule. In previous flow cytometry studies we could discriminate two subpopulations of CD8+ T cells, expressing either a high level (CD8Bright+) or a low level (CD8Dim+) of CD8 molecules. The staining patterns of mAb AV14 and mAb CT8 were not always identical for individual chickens. In this study the discrepancy in the reactivity of these mAb was examined, using outbred White Leghorn chickens as well as (B14B14)-MHC inbred Wellcome chickens. The results show that mAb AV14 and mAb CT8 recognise different epitopes on the chicken CD8α molecule. The CD8Bright+ cells appeared to express the CD8αβ heterodimer and the CD8Dim+ cells the CD8αα homodimer. Conformational differences between the αβ heterodimer and the αα homodimer could account for the differences in binding characteristics found for the two mAb. The existence of a polymorphism of the CD8α molecule in outbred White Leghorn chickens was suggested by the failure of peripheral blood leucocytes from some chickens to react with mAb AV14. This heterogeneity was not observed in the Wellcome line.