MAJOR HISTOCOMPATIBILITY COMPLEX REGULATION OF T-HELPER FUNCTIONS MAPPED TO A PEPTIDE C-TERMINUS THAT CONTROLS LIGAND DENSITY

The functional status (Th1- versus Th2-like) of CD4 T cells primed aga inst human collagen type IV (hCol IV) or a single 30mer peptide from t he alpha 2 chain of this molecule is predicted by the major histocompa tibility complex (MHC) class II (I-A) genotype of the responding mice. H-2(s) mice elicit Th1-like cell-mediated responses to these antigens , whereas Th2-like humoral responses are primed in H-2(b,d,k) mice. We now report that the ability of MHC to dictate T helper function in th is system depends upon a single amino acid of the minimal alpha 2(IV) peptide. The C terminus of this minimal (12mer) peptide is -G-G-P-K, w hich is predicted to form a p-turn. The present data demonstrate that the terminal lysine (K) stabilizes the immunogens full biological effe cts necessary for exclusive cell-mediated responses in H-2(s) mice. Th e lysine-truncated (11mer) peptide with otherwise identical sequence e ffectively primes T helper function in both H-2(b) and H-2(s) genotype s. Most importantly, our direct analysis of these peptides' presentati on by live antigen-presenting cells (APC) reveals that the 12mer is bo und at a log higher density on H-2(s) APC than on H-2(b) APC, and that the 11mer is presented at an equally low relative density on APC from both genotypes. In vitro analyses of 12mer/11mer cross reactive Th cl ones demonstrate that I-AS restricted clones require about 1-2 log low er doses of 12mer peptide than 11mer peptide to stimulate equivalent t hymidine incorporation and cytokine release. By contrast, I-Ab-restric ted (12mer/11mer cross-reactive) Th clones show no preference for the 12mer and require relatively high peptide doses similar to those requi red to stimulate the I-AS clones with the 11mer peptide. Thus, the pep tide dose requirements of Th clones reflect the high density of presen tation associated with the 12mer: I-AS ligand. Taken together, the res ults directly support the role of ligand density as an important contr ol point in the functional decision of CD4 T cells.