Targeting the CBM complex causes Treg cells to prime tumors for immune checkpoint therapy

Solid tumors are infiltrated by effector T cells (Teff) with the potential to control or reject them, as well as by regulatory T cells (Treg) that restrict the function of Teff and thereby promote tumor growth.(1) The anti-tumor activity of Teff can be therapeutically unleashed and is now being exploited for the treatment of some forms of human cancer. However, weak tumor-associated inflammatory responses and the immune-suppressive function of Treg remain major hurdles to broader effectiveness of tumor immunotherapy.(2) Here we show that upon disruption of the CARMA1-BCL10-MALT1 (CBM) signalosome complex, the majority of tumor-infiltrating Treg produce IFN-γ, followed by stunted tumor growth. Remarkably, genetic deletion of both or even just one allele of Carma1 in only a fraction of Treg, which avoided systemic autoimmunity, was sufficient to produce this anti-tumor effect, showing that not mere loss of suppressive function, but gain of effector activity by Treg initiates tumor control. Treg-production of IFN-γ was accompanied by macrophage activation and up-regulation of MHC-I on tumor cells. However, tumor cells also up-regulated expression of PD-L1, indicating activation of adaptive immune resistance.(3) Consequently, PD-1 blockade concomitant with CARMA1-deletion caused rejection of tumors that otherwise do not respond to anti-PD-1 monotherapy. This effect was reproduced by pharmacological inhibition of the CBM protein MALT1. Our results demonstrate that partial disruption of the CBM complex and induction of IFN-γ-secretion in the preferentially self-reactive Treg pool does not cause systemic autoimmunity but is sufficient to prime the tumor environment for successful immune checkpoint therapy.