triggering receptor expressed on myeloid cells-2 (TREM-2)restrains dendritic cells (DCs) and macrophages (MΦs) phagocytosis, as well asreduces pro-inflammatory cytokines release through DNAX-activation protein12 (DAP12) signaling. However, the role of TREM-2 signaling in cancer has neverbeen elucidated. In the current study, we found that TREM-2 was up-regulated onperipheral blood monocytes in tumor-bearing host. More TREM-2+DCs were detectedin the lung of 3LL tumor-bearing mice. On the other hand, the level of TREM-2 onpulmonary MΦs positively correlated with the pathological staging of lung cancer.However, surgical or chemotherapeutic reduction of tumor burden led to the obviousdecline of TREM-2. In vitro, TREM-2 expression of bone marrow (BM)-derived DCsand MΦs was induced by conditional medium (CM) containing the supernatant of3LL cells. TREM-2+DCs from CM and/or tumor-bearing mice held altered phenotypes(CD80LowCD86LowMHCIILow) and impaired functions, such as, reduced interleukin (IL)-12secretion, increased IL-10 production, and weakened ovalbumin (OVA)-endocyticcapacity; also developed potent inhibitory effect on T cell proliferation that couldbe partially reversed by TREM-2 blockage. Moreover, spleen tyrosine kinase (Syk)inhibitor restrained IL-10 production of TREM-2+DC. Remarkably, IL-10 neutralizingantibody and Syk inhibitor both lowered the suppressive potential of TREM-2+DCs inT cell proliferation. Also, adoptive transfer of this TREM-2+DCs accelerated the tumorgrowth rather than jeopardized survival in lung cancer-bearing mice. In conclusion,these results indicate that TREM-2 might act as a negative immuno-regulatorymolecule through Syk pathway in an IL-10 dependent manner and partially predictsprognosis in lung cancer patients.