Anti-L-selectin monoclonal antibody treatment in mice enhances tumor growth by preventing CTL sensitization in peripheral lymph nodes draining the tumor area

To examine the in vivo contribution of L-selectin in the sensitization of tumor-specific CTL, we investigated the effects of treatment with the anti-L-selectin monoclonal antibody (MAb) MEL-14 on the immune response to Moloney-murine sarcoma virus (M-MSV)-induced tumors, which exhibit spontaneous regression following generation of a strong virus-specific CTL response. Daily systemic administration of MEL-14 for 10 days to M-MSV-injected mice gave rise to larger sarcomas that persisted for a longer time, compared with those arising in control mice injected with virus only. The enhanced tumor growth could not be attributed to cytotoxic activity on leukocytes by MEL-14 since no reduction in the total cell number was detected in peripheral blood and spleen of MAb-treated mice. Evaluation of the immunological response in MAb-treated animals revealed a strong reduction in the generation of virus-specific CTL precursors (CTLp) in tumor-draining peripheral lymph nodes(PLN) 10 and 15 days after M-MSV injection, while in spleen, where lymphocyte localization is independent of L-selectin expression, CTLp generation was only delayed. By day 20, when tumors had begun to regress, the CTLp number showed a marked increase in both spleen and local PLN, where naive recirculating CTL could now enter because L-selectin was no longer down-regulated or blocked by the injected MAb. Our findings indicate that functional inactivation of L-selectin by MEL-14 treatment prevented migration of naive L-selectin⁺ CTL through high endothelial venules (HEV) and their accumulation in PLN draining the tumor area, thereby precluding the initiation of a tumor-specific CTL response that takes place primarily at this site.