Human CD4 residue Phe 43 is critical for repertoire development and maturation of MHC class II restricted CD4 single-positive T lineage cells in vivo

Abstract

To determine the functional significance of structural alteration of CD4-MHC class II interaction in vivo, two human (h)CD4-transgenic (tg) mice were established on a murine (m)CD4– / – H-2b background. The MHC class II binding-competent hCD4 (R240AhCD4) rescues the number and helper activity of hCD4+CD8– single-positive (SP) mature T cells in mCD4– / – mice. In contrast, the MHC class II binding-deficient F43I hCD4 mutant cannot facilitate normal differentiation of double-positive thymocytes to CD4+CD8– SP thymocytes. Hence, only 20 – 25 % of CD4+CD8– SP T cells found in wild-type or R240A hCD4tg mice are generated, with resultant diminished helper responses. Differentiation of F43I hCD4 SP T cells is MHC class II but not class I dependent as demonstrated by crossing F43I hCD4tg mice onto MHC-deficient mice. These cells show a different pattern of TCR Vα and Vβ gene usage relative to comparable R240A hCD4 SP T cells from R240 AhCD4tg animals. Expression of activation markers including CD25 and CD69 on F43I hCD4 SP T cells suggests that autoreactive specificites may not have been eliminated intrathymically. Collectively, the results show that CD4-MHC class II interaction significantly influences intrathymic repertoire selection.