CYTOTOXIC T-CELL REPERTOIRE SELECTION - A SINGLE AMINO-ACID DETERMINES ALTERNATIVE CLASS-I RESTRICTION

CTL responses are governed by intracellular Ag processing, affinity of peptides for MHC class I molecules, and the T cell repertoire. In thi s report we demonstrate that a class I D-d-restricted 10-mer CTL epito pe within the gp160 envelope glycoprotein of HIV-1 strain IIIB (residu es 318-327) contains a 9-amino acid peptide (residues 319-327), which efficiently binds to both the D-d and L(d) class I molecules in vitro. The potential for broadening the naturally limited CTL response to in clude presentation on the L(d) class I molecules in vivo was examined using a minigene-based vaccine strategy to insure cytosolic expression of ''preprocessed'' forms of the gp160 epitope. Immunization with rec ombinant vaccinia viruses (vac) expressing either the gp160 10 mer or 9 mer, both including an initiation methionine (M318-327 and M319-327, respectively), induced predominantly D-d-restricted CTL specific for native gp160. By contrast, recombinant vac expressing eight gp160 amin o acids (M320-327) generated predominantly L(d)-restricted CTL which a re specific for synthetic gp160 peptides but not native gp160. The abi lity to induce L(d)-restricted CTL suggests that the absence of an L(d )-restricted response to native gp160 cannot be attributed to a limite d T cell repertoire, but to inefficient processing of gp160 for presen tation on L(d). The switch in class I restriction, controlled by a sin gle amino acid within one epitope, demonstrates that nonanchor residue s have a profound effect on differential MHC restriction and CTL induc tion. Thus, minigene-based vaccines expressing minimal epitopes may be useful in inducing a more heterogeneous CTL response than previously appreciated.