Lymphocyte activation gene-3 induces tumor regression and antitumor immuneresponses

The lymphocyte activation gene-3 (LAG-3) product is an MHC class II ligand related to CD4. We investigated whether LAG-3 could be used in vivo to stim ulate MHC class II+ antigen-presenting cells (APC), such as resident macrop hages or dendritic cells known to play a crucial role in processing and pre senting of antigens to the immune system. We first introduced human (h) LAG -3 or mouse LAG-3 into three types of tumor cells (MCA 205, TS/A and RENCA) to evaluate its capacity to stimulate a tumor-specific immune response in vivo. In contrast to the progressive growth of wild-type cells in syngeneic mice, LAG-3-transfected tumors completely regressed or their growth was ma rkedly reduced. Mice were significantly to completely protected against a r echallenge with parental tumor cells. Protection induced by hLAG-3(+) tumor cells involved recruitment of a CD8(+) T cell response since nu/nu mice an d CD8-depleted mice did not reject tumors, and a systemic tumor-specific CT L activity was induced. Co-administration of soluble LAG-3 with wild-type t umor cells also markedly reduced primary tumor growth. Interestingly, immun ization with LAG-3(+) tumor cells or co-administration of soluble LAG-3 wit h irradiated wild-type tumor cells reduced the growth of pre-established tu mors. We therefore suggest that LAG-3 could be used as a vaccine adjuvant f or its ability to trigger APC via MHC class II molecules.