High avidity CTLs for two self-antigens demonstrate superior in vitro and in vivo antitumor efficacy

A majority of the human tumor-associated Ags characterized to date are deri ved from nonmutated "self"-proteins. Little is currently understood about t he nature of the self-reactive lymphocytes that recognize these Ags. We rec ently characterized two nonmutated tumor-associated Ags for the B16 murine melanoma: tyrosinase-related protein-2 (TRP-2) and the endogenous retrovira l envelope protein, p15E, We previously reported that both TRP-2 and p15E r eactive CTL could be detected in the spleens of naive animals after a singl e in vitro stimulation using 10(-5)-10(-6) M of the appropriate K-b-binding 9-amino acid epitope. In this report we show that the CTL found in naive a nimals are low avidity lymphocytes, that respond only to high concentration s of peptide in vitro. We demonstrate that titration of in vitro-stimulatin g peptide to limiting concentrations distinguishes qualitative differences in the lymphocyte reactivity to these two Ags between vaccinated and unvacc inated animals. We further demonstrate that in vitro expansion of CTL in ei ther high or low concentrations of stimulating peptide generated CTL cultur es with different avidities for the relevant epitopes, CTL expanded in low concentrations demonstrated higher avidity for peptide-pulsed targets and b etter tumor recognition, when compared to CTL generated in the presence of high concentrations of Ag, More importantly, high avidity CTL demonstrated superior in vivo antitumor activity. These results demonstrate that qualita tive differences in the CTL that recognize these two self-Ags are criticall y important to their in vitro and in vivo anti-tumor efficacy.