CD4+ T-cell killing of multiple myeloma cells is mediated by resident bone marrow macrophages.

CD4+ T cells may induce potent antitumor immune responses through interaction with antigen-presenting cells within the tumor microenvironment. Using a murine model of multiple myeloma, we demonstrated that adoptive transfer of idiotype-specific CD4+ T cells may elicit curative responses against established multifocal myeloma in bone marrow. This finding indicates that the myeloma bone marrow niche contains antigen-presenting cells that may be rendered tumoricidal. Given the complexity of the bone marrow microenvironment, the mechanistic basis of such immunotherapeutic responses is not known. Through a functional characterization of antitumor CD4+ T-cell responses within the bone marrow microenvironment, we found that killing of myeloma cells is orchestrated by a population of bone marrow-resident CD11b+F4/80+MHC-IIHigh macrophages that have taken up and present secreted myeloma protein. The present results demonstrate the potential of resident macrophages as powerful mediators of tumor killing within the bone marrow and provide a basis for novel therapeutic strategies against multiple myeloma and other malignancies that affect the bone marrow.

CD4+ T-cell-Mediated Rejection of MHC Class II-Positive Tumor Cells Is Dependent on Antigen Secretion and Indirect Presentation on Host APCs.

Tumor-specific CD4+ T cells have been shown to mediate efficient antitumor immune responses against cancer. Such responses can occur through direct binding to MHC class II (MHC II)-expressing tumor cells, or indirectly via activation of professional antigen-presenting cells (APC) that take up and present the tumor antigen. We have previously shown that CD4+ T cells reactive against an epitope within the Ig light chain variable region of a murine B-cell lymphoma can reject established tumors. Given the presence of MHC II molecules at the surface of lymphoma cells, we investigated whether MHC II-restricted antigen presentation on tumor cells alone was required for rejection. Variants of the A20 B lymphoma cell line that either secreted or intracellularly retained different versions of the tumor-specific antigen revealed that antigen secretion by the MHC II-expressing tumor cells was essential both for the priming and effector phase of CD4+ T-cell-driven antitumor immune responses. Consistent with this, genetic ablation of MHC II in tumor cells, both in the case of B lymphoma and B16 melanoma, did not preclude rejection of tumors by tumor antigen-specific CD4+ T cells in vivo These findings demonstrate that MHC class II expression on tumor cells themselves is not required for CD4+ T-cell-mediated rejection and that indirect display on host APC is sufficient for effective tumor elimination. These results support the importance of tumor-infiltrating APC as mediators of tumor cell killing by CD4+ T cells.Significance: Elimination of tumors by CD4+ T cells recognizing secreted tumor neoantigens can occur in the absence of tumor cell-intrinsic MHC II expression, highlighting the potential clinical relevance of indirect antigen recognition by tumor-infiltrating APC.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/16/4573/F1.large.jpg Cancer Res; 78(16); 4573-85. ©2018 AACR.

Indirect CD4+ T-cell-mediated elimination of MHC II(NEG) tumor cells is spatially restricted and fails to prevent escape of antigen-negative cells.

Tumor-specific Th1 cells can activate tumor-infiltrating macrophages that eliminate MHC class II negative (MHC II(NEG)) 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 II(NEG) tumor cells.