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

To determine the functional significance of structural alteration of CD4-MH C class II interaction in vivo, two human (h)CD4-transgenic (tg) mice were established on a murine (m)CD4(-/-) H-2(b) background. The MHC class II bin ding-competent hCD4 (R240AhCD4) rescues the number and helper activity of h CD4(+)CD8(-) single-positive (SP) mature T cells in mCD4(-/-) mice. In cont rast, the MHC class II binding-deficient F431 hCD4 mutant cannot facilitate normal differentiation of double-positive thymocytes to CD4(+)CD8(-) SP th ymocytes. Hence, only 20-25 % of CD4(+)CD8(-) SP T cells found in wild-type or R240A hCD4tg mice are generated, with resultant diminished helper respo nses. Differentiation of F431 hCD4 SP T cells is MHC class II but not class I dependent as demonstrated by crossing F431 hCD4tg mice onto MHC-deficien t mice. These cells show a different pattern of TCR Vu and VP gene usage re lative to comparable R240A hCD4 SP T cells from R240 AhCD4tg animals. Expre ssion of activation markers including CD25 and CD69 on F431 hCD4 SP T cells suggests that autoreactive specificities may not have been eliminated intr athymically. Collectively, the results show that CD4-MHC class II interacti on significantly influences intrathymic repertoire selection.