Tumor-released autophagosomes induces CD4 T cell-mediated immunosuppression via a TLR2-IL-6 cascade.

Background: CD4 T cells are critical effectors of anti-tumor immunity, but how tumor cells influence CD4 T cell effector function is not fully understood. Tumor cell-released autophagosomes (TRAPs) are being recognized as critical modulators of host anti-tumor immunity during tumor progression. Here, we explored the mechanistic aspects of TRAPs in the modulation of CD4 T cells in the tumor microenvironment.

Methods: TRAPs isolated from tumor cell lines and pleural effusions or ascites of cancer patients were incubated with CD4 T cells to examine the function and mechanism of TRAPs in CD4 T cell differentiation and function. TRAPs-elicited CD4 T cells were tested for their suppression of effector T cell function, induction of regulatory B cells, and promotion of tumorigenesis and metastasis in a mouse model.

Results: Heat shock protein 90α (HSP90α) on the surface of TRAPs from malignant effusions of cancer patients and tumor cell lines stimulated CD4 T cell production of IL-6 via a TLR2-MyD88-NF-κB signal cascade. TRAPs-induced autocrine IL-6 further promoted CD4 T cells secretion of IL-10 and IL-21 via STAT3. Notably, TRAPs-elicited CD4 T cells inhibited CD4 and CD8 effector T cell function in an IL-6- and IL-10-dependent manner and induced IL-10-producing regulatory B cells (Bregs) via IL-6, IL-10 and IL-21, thereby promoting tumor growth and metastasis. Consistently, inhibition of tumor autophagosome formation or IL-6 secretion by CD4 T cells markedly retarded tumor growth. Furthermore, B cell or CD4 T cell depletion impeded tumor growth by increasing effector T cell function.

Conclusions: HSP90α on the surface of TRAPs programs the immunosuppressive functions of CD4 T cells to promote tumor growth and metastasis. TRAPs or their membrane-bound HSP90α represent important therapeutic targets to reverse cancer-associated immunosuppression and improve immunotherapy.