Indirect CD4+ T-cell-mediated elimination of MHC IINEG tumor cells is spatially restricted and fails to prevent escape of antigen-negative cells

Tumor-specific Th1 cells can activate tumor-infiltrating macrophages that eliminateMHC class II negative (MHC IINEG) tumor cells. Activated M1-like macrophages lack antigen (Ag) receptors, and are presumably unable to discriminate and thus kill both Ag-positive (Ag-POS) and Ag-negative (Ag-NEG) tumor cells (bystander killing). The lack of specificity of macrophage-mediated cytotoxicity might be of clinical importance as it could provide a means of avoiding tumor escape. Here, we have tested this idea using mixed populations of Ag-POS and Ag-NEG tumor cells in a TCR-transgenic model in which CD4(+) T cells recognize a secreted tumor-specific antigen. Surprisingly, while Ag-POS tumor cells were recognized and rejected, Ag-NEG cells grew unimpeded and formed tumors. We further demonstrated that macrophage-mediated cytotoxicity was spatially restricted to areas dominated by Ag-POS tumor cells, sparing Ag-NEG tumor cells in the vicinity. As a consequence, macrophage tumoricidal activity did not confer bystander killing in vivo. The present results offer novel insight into the mechanisms of indirect Th1-mediated elimination of MHC IINEG tumor cells.