Regulation of Ia+ reticulum cell sarcoma (RCS) growth in syngeneic SJL/J mice. I. Inhibition of tumor growth by passive administration of L3T4 monoclonal antibody before or after tumor inoculation.

Spontaneously arising reticulum cell sarcoma (RCS) tumors in SJL/J mice stimulate syngeneic host T lymphocytes to proliferate and are dependent on host T cells for maintenance and growth. Tumor-associated Ia antigens have been implicated in the proliferative response both in vivo and in vitro, and the responding T cells are predominantly Lyt-1+2- L3T4+. We hypothesized that elimination or depletion of the responding L3T4 subpopulation in vivo should inhibit growth of transplantable RCS tumors, and continued RCS growth may be dependent on the continued presence of L3T4 cells. This hypothesis was tested experimentally by examining the effect of passive administration of L3T4 monoclonal antibody (mAb) into SJL/J mice either before or at different times after tumor inoculation. The tumor inoculum used killed all mice 15 to 30 days after injection. Administration of a single dose of L3T4 mAb 4 days before tumor inoculation resulted in complete depletion of L3T4 cells and complete inhibition of tumor growth. The antibody-treated mice survived with no sign of tumor growth even after complete recovery of L3T4+ cells. These results demonstrate that initiation of tumor growth is dependent on host L3T4+ cells. Administration of mAb as late as 7 days after tumor inoculation resulted in inhibition of tumor growth, and administration of mAb at day 10 resulted in significant inhibition of tumor growth. Compared with the kinetics of tumor growth in normal control mice, administration of L3T4 after tumor inoculation results in tumor growth arrest. These findings demonstrate that continued tumor growth in vivo is dependent on the presence of L3T4+ cells. In the RCS system, the present studies show that administration of mAb to L3T4+ cells is therapeutic in that it inhibits the induction of tumor growth, and it also prevents tumor growth in tumor-bearing animals.