The thymus-specific serine protease TSSP/PRSS16 is crucial for the antitumoral role of CD4(+) T cells.

In cancer, immune cells can play conflicting roles, either protective, by elimination of tumor cells during immune surveillance, or detrimental, by promoting carcinogenesis during inflammation. We report here that the thymus-specific serine protease (TSSP), which is involved in CD4(+) T cell maturation in the thymus, exerts a tumor suppressor activity. Mice genetically deficient for TSSP are highly prone to spontaneous cancer development. The absence of TSSP also increases the rate of induced colitis-associated colorectal (CAC) tumor formation, through exacerbated colon inflammation. Adoptive transfer of T cells in various combinations (CD4(+) and CD8(+) from wild-type and/or knockout mice) into T cell-deficient mice showed that the TSSP-deficient CD4(+) T cell compartment promotes tumor development, associated with high levels of the cytokine IL-17A. Inhibition of IL-17A during CAC tumor formation prevents the increased carcinogenesis and colic immune disequilibrium observed in TSSP-deficient mice. Therefore, our data demonstrate that antitumoral immune surveillance requires thymic TSSP-driven production of CD4(+) T cells contributing to inflammatory balance.