Differential binding of two monoclonal antibodies directed against the chicken CD8 alpha molecule

Two murine monoclonal antibodies (mAb), CT8 and AV14, have been shown to recognise the avian homologue of the mammalian CD8 alpha molecule. In previous flow cytometry studies we could discriminate two subpopulations of CD8(+) T cells, expressing either a high level (CD8(Bright+)) or a low level (CD8(Dim+)) 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 Leghom chickens as well as ((BB14)-B-14)-MHC inbred Wellcome chickens. The results show that mAb AV14 and mAb CT8 recognise different epitopes on the chicken CD8 alpha molecule, The CD8(Bright+) cells appeared to express the CD8 alpha beta heterodimer and the CD8(Dim+) cells the CD8 alpha alpha homodimer. Conformational differences between the alpha beta heterodimer and the alpha alpha homodimer could account for the differences in binding characteristics found for the two mAb, The existence of a polymorphism of the CD8 alpha 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.