We have previously reported that direct injection of dendritic cells (DC) engineered to express the Type-1 transactivator Tbet (i.e., DC.Tbet) into murine tumors results in antitumor efficacy in association with the development of structures resembling tertiary lymphoid organs (TLO) in the tumor microenvironment (TME). These TLO contained robust infiltrates of B cells, DC, NK cells, and T cells in proximity to PNAd blood vessels; however, they were considered incomplete, since the recruited B cells failed to organize into classic germinal center-like structures. We now report that antitumor efficacy and TLO-inducing capacity of DC.Tbet-based i.t. therapy is operational in peripheral lymph node-deficient LTA mice, and that it is highly dependent upon a direct Tbet target gene product, IL-36γ/IL-1F9. Intratumoral DC.Tbet fails to provide protection to tumor-bearing IL-36R hosts, or to tumor-bearing wild-type recipient mice co-administered rmIL-1F5/IL-36RN, a natural IL-36R antagonist. Remarkably, the injection of tumors with DC engineered to secrete a bioactive form of mIL-36γ (DC.IL36γ) also initiated therapeutic TLO and slowed tumor progression . Furthermore, DC.IL36γ cells strongly upregulated their expression of Tbet, suggesting that Tbet and IL-36γ cooperate to reinforce each other's expression in DC, rendering them competent to promote TLO formation in an "immunologically normalized," therapeutic TME.
Tertiary lymphoid structures (TLS) form at sites of chronic inflammation, and locally prime the immune system independently of secondary lymphoid organs. The intratumoral presence of TLS has been associated with a positive prognostic outcome for patients with solid tumors, including colorectal carcinoma. Thus, the therapeutic induction of TLS would be hypothesized to confer clinical benefit to patients. Indeed, the therapeutic introduction of the IL-1 family cytokine IL-36 gamma into the tumor microenvironment (TME) of murine MC38 colon carcinoma leads to delayed tumor progression and the development of TLS at 5 days post-treatment. The TLS present with characteristics of secondary lymphoid organs, including PNAd+ high endothelial venules (HEV) surrounded by CD3+ T cells and CD11c+ dendritic cells, but are considered as “non-classical” due to the absence of a defined B cell zone. The formation of TLS is dependent upon the adaptive immune response, as IL-36 gamma-treated tumors grown in Rag2−/− animals fail to present with PNAd+ vasculature within the tumor. Thus, crosstalk between the immune system and cells of the vasculature is key to the efficacy of the IL-36 gamma-induced anti-tumor immune response. In human colorectal carcinoma, IL-36 gamma was also observed to promote the local anti-tumor immune response. Expression of IL-36 gamma by vascular endothelial cells of HEV was associated with an increased density of CD20+ B cells in TLS. Specific IL-36 gamma production by CD68+ macrophages was associated with an increased infiltration of CD4+ central memory T cells into the TME. Therefore, IL-36 gamma is a pro-inflammatory mediator involved in the anti-tumor immune response in both murine and human colorectal cancer.
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