Gene-targeted mice have recently revealed a role for lymphocytes and interferon-gamma (IFNgamma) in conferring protection against cancer, but the mechanisms remain unclear. Here, we have characterized a successful primary antitumor immune response initiated by naive CD4+ T cells. Major histocompatibility complex class II (MHC-II)-negative myeloma cells injected subcutaneously into syngeneic mice were surrounded within 3 days by macrophages that captured tumor antigens. Within 6 days, naive myeloma-specific CD4+ T cells became activated in draining lymph nodes and subsequently migrated to the incipient tumor site. Upon recognition of tumor-derived antigenic peptides presented on MHC-II by macrophages, the myeloma-specific CD4+ T cells were reactivated and started to secrete cytokines. T cell-derived IFNgamma activated macrophages in close proximity to the tumor cells. Tumor cell growth was completely inhibited by such locally activated macrophages. These data indicate a mechanism for immunosurveillance of MHC-II-negative cancer cells by tumor-specific CD4+ T cells through collaboration with macrophages.

Tumor-specific CD4 + T cells orchestrate the adaptive immune responses against cancer. We have previously shown that CD4 + T cells recognize MHC class II-negative myeloma cells indirectly by collaborating with tumor-infiltrating macrophages. We, here, hypothesize that this critical step may be dependent on secretion of tumor-specific antigens by cancer cells. This was investigated using T-cell receptor-transgenic mice, in which CD4 + T cells mediate rejection of syngeneic MOPC315 myeloma cells. We analyzed the immune response against myeloma cell variants, which either secrete or retain intracellularly a tumor-specific idiotypic (Id) antigen. Our results reveal that CD4 + T cells helped by macrophages are capable of detecting nonsecreted tumor antigens from MHC class II-negative cancer cells. However, Id secretion was required for successful myeloma immunosurveillance. Antigen secretion resulted in stronger priming of naive myelomaspecific CD4 + T cells in tumor-draining lymph nodes. Secretion of antigen by at least some cancer cells within a tumor was shown to facilitate immunosurveillance. Treatment by local injection of purified tumor-specific antigen successfully enhanced immunity against nonsecreting myeloma cells. Collectively, the data indicate that antigen concentration within the tumor extracellular matrix must reach a certain threshold to allow successful cancer immunosurveillance by

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 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. 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. http://cancerres.aacrjournals.org/content/canres/78/16/4573/F1.large.jpg .