Abstract C68: Decreased antitumor cytotoxic immunity in African American colon cancer
Authors: Khan Mohammad W. , Tian MengXi , Carethers John M. , McGuire Kathleen L. 
Published: May 2016

Background: African Americans (AA) have a higher incidence of colorectal cancer (CRC) and more aggressive disease than patients of other ethnicities. Several studies have suggested that biological/genetic factors contribute to this health disparity. Recent work has shown the importance of the immune system to cancer patient prognosis, for CRC in particular. Anti-tumor T helper 1 (TH1) cell activation of cytotoxic T lymphocytes (CTL) increases CRC patient disease-free survival while deficiencies in these responses correlate with a worse prognosis. We hypothesize that differences in anti-tumor immunity by race contribute to the disparity and lead to more aggressive CRC in AA patients.

Methods: Over 250 human colon tumors samples were stained using immunohistochemistry to evaluate in situ immune cell infiltration (CD8 and CD57), CTL activity [granzyme B (GzmB)], and IL-17 production. Also, using GzmB immunostaining, we analyzed the histological location of CTL/Natural Killer (NK) cell activity at intratumoral vs. invasive border sites of the CRC samples. In addition, a colon cancer mouse model is being used to identify mechanisms that may alter TH1 and CTL activity in developing tumors. The mouse model is the TS4Cre x APCfl-468 conditional Adenomatous Polyposis Coli (APC) knockout. Exons 11 and 12 are deleted from one APC gene in the colon and distal ileum due to the expression of CRE under a Fatty acid binding protein promoter leading to polyposis. These colonic tumors are being evaluated using qPCR on immune and inflammatory gene products.

Results: Our studies on human microsatellite stable colon cancer samples showed that the number of CD8+ and CD57+ tumor-infiltrating cells doesn't differ by race (p=0.83 and 0.32, respectively). The level of GzmB+ cells, i.e. CTLs and activated NK cells, however, does vary significantly by race (p=0.01), particularly at the invasive border (p=0.0006). The numbers of IL-17A producing cells do not vary (p=0.63) but IL-17 may modulate GzmB correlation with CD8 and CD57 by race. For example, in AA, the correlation between GzmB and CD8 drops from 0.80 in IL-17Ahigh samples to 0.53 in IL-17Alow samples. For GzmB and CD57, the correlation level drops from 0.51 to 0.28 between IL-17A-high and -low samples, respectively. No such drop in correlation is observed in Caucasian American samples. In our mouse model, our preliminary studies show the TS4CRExAPCfl-468 mice initiate colonic polyposis at ~18 weeks of age and invasive cancer is observed in 20-25% of mice older than 31 weeks. The mean number of colonic polyps significantly increases between 18 and 24 weeks (from 2.3±0.9 to 8.0±1.9 tumors/mouse). Interestingly, the number declines by 36 weeks of age (5.5±0.5 tumors/mouse). However, the mean size of the colonic polyps significantly increases from 18 weeks to 36 weeks of age (from 1.54mm±0.55 to 4.23mm±0.56). To determine what mechanisms might result in lower anti-tumor CTL/NK activity, mouse tumors at different stages of development are currently being evaluated for IL-27, GzmA, IL-23, ƔIFN, CD8, Perforin, CX3CL1, IL-17a, H2DMB2, and Tbet gene expression using qPCR. Primers sets are also being developed to include IL-4, IL-6, IL-10, IL-12p40, and FoxP3 in our analyses.

Conclusion: Our data to date indicate that the number of AA patients with high numbers of GzmB expressing cells infiltrating their tumors is significantly lower than Caucasians. In addition, IL-17 may have a modulatory effect on anti-tumor cytotoxicity by race. These observations support our hypothesis that immune processes contribute to the racial disparities observed in CRC. Current studies in our mouse model of CRC should identify mechanisms decreasing anti-tumor CTL/NK cell activity that will be tested in our Caucasian and AA colon cancer patient tumor samples in future studies.

Note: This abstract was not presented at the conference.

Citation Format: Mohammad W. Khan, MengXi Tian, John M. Carethers, Kathleen L. McGuire. Decreased antitumor cytotoxic immunity in African American colon cancer. [abstract]. In: Proceedings of the Eighth AACR Conference on The Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; Nov 13-16, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2016;25(3 Suppl):Abstract nr C68.