Mucosa-associated invariant T (MAIT) cells are innate-like T cells with a conserved TCR α-chain recognizing bacterial metabolites presented on the invariant MHC-related 1 molecule. MAIT cells are present in intestinal tissues and liver, and they rapidly secrete IFN-γ and IL-17 in response to bacterial insult. In colon cancer, IL-17–driven inflammation promotes tumor progression, whereas IFN-γ production is essential for antitumor immunity. Thus, tumor-associated MAIT cells may affect antitumor immune responses by their secreted cytokines. However, the knowledge of MAIT cell presence and function in tumors is virtually absent. In this study, we determined the frequency, phenotype, and functional capacity of MAIT cells in colon adenocarcinomas and unaffected colon lamina propria. Flow cytometric analyses showed significant accumulation of MAIT cells in tumor tissue, irrespective of tumor stage or localization. Colonic MAIT cells displayed an activated memory phenotype and expression of chemokine receptors CCR6 and CCR9. Most MAIT cells in unaffected colon tissues produced IFN-γ, whereas only few produced IL-17. Colonic MAIT cells also produced TNF-α, IL-2, and granzyme B. In the tumors, significantly lower frequencies of IFN-γ–producing MAIT cells were seen, whereas there were no differences in the other cytokines analyzed, and in vitro studies showed that secreted factors from tumor tissue reduced IFN-γ production from MAIT cells. In conclusion, MAIT cells infiltrate colon tumors but their ability to produce IFN-γ is substantially reduced. We suggest that MAIT cells have the capacity to promote local immune responses to tumors, but factors in the tumor microenvironment act to reduce MAIT cell IFN-γ production.

Several Butyrophilin (BTN) and Btn‐like (BTNL) molecules control T lymphocyte responses, and are genetically associated with inflammatory disorders and cancer. In this study, we present a comprehensive expression analysis of human and murine BTN and BTNL genes in conditions associated with intestinal inflammation and cancer. Using real‐time PCR, expression of human BTN and BTNL genes was analyzed in samples from patients with ulcerative colitis, irritable bowel syndrome, and colon tumors. Expression of murine Btn and Btnl genes was examined in mouse models of spontaneous colitis (Muc2 −/−) and intestinal tumorigenesis (Apc Min/+). Our analysis indicates a strong association of several of the human genes with ulcerative colitis and colon cancer; while especially BTN1A1, BTN2A2, BTN3A3, and BTNL8 were significantly altered in inflammation, colonic tumors exhibited significantly decreased levels of BTNL2, BTNL3, BTNL8, and BTNL9 as compared to unaffected tissue. Colonic inflammation in Muc2 −/− mice significantly down‐regulated the expression of particularly Btnl1, Btnl4, and Btnl6 mRNA, and intestinal polyps derived from Apc Min/+ mice displayed altered levels of Btn1a1, Btn2a2, and Btnl1 transcripts. Thus, our data present an association of BTN and BTNL genes with intestinal inflammation and cancer and represent a valuable resource for further studies of this gene family.

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Tumor progression in the colon moves from aberrant crypt foci to adenomatous polyps to invasive carcinomas. The composition of the tumor-infiltrating leukocyte population affects the ability of the immune system to fight the tumor. T cell infiltration into colorectal adenocarcinomas, particularly T helper 1 (Th1) type T cells as well as increased regulatory T cell (Treg) frequencies, is correlated with improved prognosis. However, whether Th1 cells and Tregs are already present at the adenoma stage is not known. In this study, the APC(Min/+) mouse model of intestinal adenomatous polyposis was used to investigate tumor-associated lymphocyte subsets and the mechanisms of their accumulation into gastrointestinal adenomas. Compared to unaffected tissue, adenomas accumulated CD4(+)FoxP3(+) putative Treg in parallel with lower frequencies of conventional T cells and B cells. The accumulation of Treg was also observed in human adenomatous polyps. Despite high Treg numbers, the function of conventional T cells present in the APC(Min/+) adenomas was not different from those in the unaffected tissue. Adenomas displayed an altered chemokine balance, with higher CCL17 and lower CXCL11 and CCL25 expression than in the unaffected tissue. In parallel, CXCR3(+) Tregs were largely absent from adenomas. The data indicate that already in early stages of tumor development, the balance of lymphocyte-recruiting chemokines is altered possibly contributing to the observed shift toward higher frequencies of Treg.

Increasing knowledge of the function and regulation of tumor-infiltrating lymphocytes has led to new insights in cancer immunotherapy. Regulatory T cells (Treg) accumulate in colon tumors, and we recently showed that CD39+ Treg from cancer patients inhibit transendothelial migration of conventional T cells. CD39 mediates the hydrolysis of ATP to immunosuppressive adenosine and adds to the immunosuppressive effects of Treg. Here, we further investigated the regulatory features of intratumoral CD39+ Treg in colon cancer. Using flow cytometry analyses of cells from 46 colon cancer patients, we confirm the accumulation of CD39+ Treg in the tumor tissue compared to unaffected colon tissue, and also show that tumor-infiltrating Treg express more CD39 and Foxp3 on a per cell basis. Furthermore, CD39+ Treg in tumors express markers indicating increased turnover and suppressive ability. In particular, tumor-infiltrating CD39+ Treg have high expression of surface molecules related to immunosuppression, such as ICOS, PD-L1 and CTLA-4. Functional suppression assays also indicate potent suppressive capacity of CD39+ Treg on proliferation and IFN-γ secretion by conventional T cells. In conclusion, our results identify tumor-infiltrating CD39+ Treg as a numerous and potentially important immunosuppressive subset, and suggest that immunotherapy aimed at reducing the activity of CD39+ Treg may be particularly useful in the setting of colon cancer.