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CARD9 prevents lung cancer development by suppressing the expansion of myeloid‐derived suppressor cells and IDO production

Caspase recruitment domain‐containing protein 9 (CARD9) is an adaptor protein and highly expressed in myeloid cells. Our previous study demonstrates a critical protective effect of CARD9 in the development of colitis‐associated colon cancer. Nevertheless, the effect of CARD9 in lung cancer remains unclear. Here, using a mouse Lewis lung cancer model, we found the tumor burden of CARD9−/− mice was much heavier than that in wild‐type (WT) mice. More myeloid‐derived suppressor cells (MDSCs) were accumulated and less cytotoxicity T lymphocyte was found in tumor tissues of CARD9−/− mice, compared to WT mice. Depleting MDSCs using anti‐Gr1 antibody can significantly decrease tumor burden in CARD9−/− mice. Furthermore, the noncanonical nuclear factor‐kappaB (NF‐κB) pathway was activated in CARD9−/− mice‐derived MDSCs. Deficiency of CARD9 enhanced expression of indoleamine 2,3‐dioxygenase (IDO) in MDSCs via noncanonical NF‐κB pathway. Moreover, correlations between CARD9 expressions and MDSCs relative genes (IDO, iNOS‐2 and arginase 1 [ARG‐1]) were further confirmed in tumor tissues from lung cancer patients. Taken together, we showed a CARD9‐NF‐κB‐IDO pathway in MDSCs which can inhibit the suppressive function of MDSCs and prevent lung cancer development.