Cytotoxic T lymphocyte-associated antigen 4 (CTLA4) is a well-established immune checkpoint for antitumor immune responses. The pro-tumorigenic function of CTLA4 is believed to be limited to T cell inhibition by countering the activity of the T cell co-stimulating receptor CD28. However, as we demonstrate here, there are two additional roles for CTLA4 in cancer, including via CTLA4 overexpression in diverse B cell lymphomas and in melanoma-associated B cells. CTLA4-CD86 ligation recruited and activated the JAK family member Tyk2, resulting in STAT3 activation and expression of genes critical for cancer immunosuppression and tumor growth and survival. CTLA4 activation resulted in lymphoma cell proliferation and tumor growth, whereas silencing or antibody-blockade of CTLA4 in B cell lymphoma tumor cells in the absence of T cells inhibits tumor growth. This inhibition was accompanied by reduction of Tyk2/STAT3 activity, tumor cell proliferation, and induction of tumor cell apoptosis. The CTLA4-Tyk2-STAT3 signal pathway was also active in tumor-associated non-malignant B cells in mouse models of melanoma and lymphoma. Overall, our results show how CTLA4 induced immune suppression occurs primarily via an intrinsic STAT3 pathway and that CTLA4 is critical for B cell lymphoma proliferation and survival.
Funding sources: This work was funded by the Deutsche Forschungsgemeinschaft (DFG) project number 394046768 -SFB1366 projects C4 and Z2 (to A.F., C.M.), DFG project number 419966437 (to J.R.V.), the Cooperation Program in Cancer Research of the Deutsches Krebsforschungszentrum (DKFZ) and Israel's Ministry of Science and Technology (MOST) Ca 178 (to A.F. and R.M.) and the Helmholtz Association (to A.F.).
Tumor progression, therapy resistance and metastasis are profoundly controlled by the tumor microenvironment. The contribution of endothelial cells to tumor progression was initially only attributed to the formation of new blood vessels (angiogenesis). Research in the last decade has revealed however that endothelial cells control their microenvironment through the expression of membrane-bound and secreted factors. Such angiocrine functions are frequently hijacked by cancer cells, which deregulate the signaling pathways controlling the expression of angiocrine factors. Here, we review the crosstalk between cancer cells and endothelial cells and how this contributes to the cancer stem cell phenotype, epithelial to mesenchymal transition, immunosuppression, remodeling of the extracellular matrix and intravasation of cancer cells into the bloodstream. We also address the long-distance crosstalk of a primary tumor with endothelial cells at the pre-metastatic niche and how this contributes to metastasis.
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