After transplantation of B6RV2 leukemia, initial tumor growth was followed by tumor regression in B6 (CB6F1) female, but not male, mice. This indicated that H-Y antigen is involved in B6RV2 rejection by syngeneic female recipient mice. In the case of another leukemia, BALB.RL male 1, and Ir gene, probably identical to the Rgv-1 gene, is responsible for RL male 1 rejection. Thus, F1 hybrids of BALB/c with certain other strains of mice can reject RL male 1. Using these two different systems of tumor rejection, we investigated the effects of in vivo administration of Lyt and Thy-1 monoclonal antibodies (mAb). Results showed that Lyt-2 and -3 mAb blocked both B6RV2 rejection by B6 female mice and BALB.RL male 1 rejection by CB6F1 mice. The specificity of blocking was confirmed by use of Lyt-2 and -3 mAb to reciprocal alleles and mice from B6 Lyt-congeneic stocks. No blocking was observed with Lyt-1 and Thy-1 mAb. The Lyt phenotype of T cells in lymphoid tissues from mice treated with mAb was then studied. Blocking of the Lyt-2+3+ population was observed in the lymph node and spleen, but not in the thymus. These results indicate the involvement of Lyt-2+3+ cells (or Lyt-2,3 antigen) in tumor rejection. The precise mechanism of blocking is unknown, but it was observed after even a single injection of Lyt-2,3 mAb on day 9 after tumor transplantation, suggesting that effector cells were functionally blocked, rather than that the generation of these cells was inhibited.

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