MUTATIONS IN MHC CLASS-II DIMER-OF-DIMERS CONTACT RESIDUES - EFFECTS ON ANTIGEN PRESENTATION

The recent solutions of the MHC class II crystal structure reveal dime rization of the ap heterodimers. These dimer of dimers structures may also exist either on resting cells or after engagement by TCR, and may be involved in B cell signaling and up-regulation of co-stimulatory m olecules such as B7 which facilitate T cell activation. By combining c rystallographic data on HLA-DR1 with the sequence of murine I-A(k) and refining the resulting structure through energy minimization calculat ions, we have predicted the contact amino acids expected to stabilize the I-A(k) dimer of dimers structure. As in HLA-DR1, three salt bridge s in I-A(k) (D alpha 162-H beta 112, H alpha 181-E beta 163, E alpha 1 83-H beta 113) appear to provide the main interaction, Guided by this structural data, we prepared 45 B cell transfectants representing 20 d ifferent class II mutation phenotypes in the contact region containing these salt bridges. We examined their abilities to activate three T c ell hybrids. Antigen-specific h4Ly50.5 cells were not greatly affected by changes in the dimer of dimer contact residues. In contrast, autor eactive C8.A3 T cells were very sensitive to changes in this region bu t presentation of class II of many mutation phenotypes could be rescue d by treatments that up-regulate B7-1, The alloreactive hybridoma 2H40 .2.5 was less sensitive to changes in the contact residues. A simple m odel was developed that summarizes the effects of the mutations for th e T cells tested. Mutations at D alpha 162, E alpha 183, H alpha 181 a nd R beta 106 had the largest negative impact, while D alpha 166, E al pha 185, H beta 112, H beta 113 and E beta 163 were less disruptive. R esults are consistent with mutations interfering with class II interac tion with another molecule which might or might not be another class I I heterodimer, However, the larger negative impact of a chain mutation s in salt bridge pairs suggests that these sites also help maintain so me essential conformation of the alpha chain apart from any possible i mpact on dimer of dimers stability.