Tumor allograft rejection is mainly mediated by CD8⁺ cytotoxic T lymphocytes stimulated with class I alloantigens in cooperation with CD4⁺ helper T cells recognizing class II alloantigens

Presence of the three major pathways (self-Ia restricted, all-K/D restricted, and allo-Ia restricted pathways) in generating class I-restricted CTL has been reported. The present study was conducted in order to clarify which of the three is the main pathway in mediating tumor allograft rejection. One million EL-4 tumor cells derived from C57BL/6 (B6;H-2^b) were inoculated into the various strains of mice that were genetically different from B6. Class I (K/D) Ag-disparate but IA Ag-matched B6.C-H-2^bm1 (bm1;K^bm1, IA^b, IE^-, D^b) mice or B10.A (5R) (5R; b, b, k, d) mice could not reject 1 x 10⁶ EL-4 tumor cells in spite of the strong generation of CTL against the B6 Ag, suggesting the inability of the self-Ia restricted pathway and the allo-K/D restricted pathway in rejecting tumor allografts. The strains of mice being capable of rejecting EL-4 tumor were disparate from B6 mice in both class I and class II (IA) Ag, suggesting the importance of the allo-Ia restricted pathway in rejecting tumor allografts. To generate CTL against K^b Ag via the allo-Ia restricted pathway in the bm1 mice, 2 x 10⁷ B6.H-2^bm12 (bm12; b, bm12, -, b) spleen cells were injected into the bm1 mice as a supplementary source of allogeneic APC that possibly raise CTL through CD4⁺ Th cells of bm1 origin. These bm1 mice became capable of rejecting 1 x 10⁶ EL-4 tumor cells. The same was observed in the combination of bm12→B10.A (5R) (b, b, k, d) mice. To further elucidate the role of the class II restricted CD4⁺ Th cells, anti-CD4 antibody was repeatedly i.v. administered into the C3H/He (C3H; H-2(k)) or the DBA/2 (DBA; H-2(d)) mice on days 0, 1, and 4. Injection of anti-CD4 antibody led 1 x 10⁶ EL-4 tumor cells to grow and kill the C3H and DBA mice. These results suggest that the main effector CTL pathway involved in tumor allograft rejection is allo-Ia restricted pathway where CD8⁺ precursor CTL were stimulated by the class II-restricted CD4⁺ Th cells.