Bovine meat and milk factor protein expression in tumor‐free mucosa of colorectal cancer patients coincides with macrophages and might interfere with patient survival

Bovine milk and meat factors (BMMFs) are plasmid‐like DNA molecules isolated from bovine milk and serum, as well as the peritumor of colorectal cancer (CRC) patients. BMMFs have been proposed as zoonotic infectious agents and drivers of indirect carcinogenesis of CRC, inducing chronic tissue inflammation, radical formation and increased levels of DNA damage. Data on expression of BMMFs in large clinical cohorts to test an association with co‐markers and clinical parameters were not previously available and were therefore assessed in this study. Tissue sections with paired tumor‐adjacent mucosa and tumor tissues of CRC patients [individual cohorts and tissue microarrays (TMAs) (n = 246)], low‐/high‐grade dysplasia (LGD/HGD) and mucosa of healthy donors were used for immunohistochemical quantification of the expression of BMMF replication protein (Rep) and CD68/CD163 (macrophages) by co‐immunofluorescence microscopy and immunohistochemical scoring (TMA). Rep was expressed in the tumor‐adjacent mucosa of 99% of CRC patients (TMA), was histologically associated with CD68+/CD163+ macrophages and was increased in CRC patients when compared to healthy controls. Tumor tissues showed only low stromal Rep expression. Rep was expressed in LGD and less in HGD but was strongly expressed in LGD/HGD‐adjacent tissues. Albeit not reaching statistical significance, incidence curves for CRC‐specific death were increased for higher Rep expression (TMA), with high tumor‐adjacent Rep expression being linked to the highest incidence of death. BMMF Rep expression might represent a marker and early risk factor for CRC. The correlation between Rep and CD68 expression supports a previous hypothesis that BMMF‐specific inflammatory regulations, including macrophages, are involved in the pathogenesis of CRC. BMMFs are a group of infectious agents related to indirect carcinogenesis. Macrophage expression of the conserved BMMF Rep protein in tumor‐adjacent mucosa of CRC patients is increased compared with expression in the tumor and mucosa of healthy donors, indicating that BMMF is involved in inflammation‐driven, indirect carcinogenesis of CRC (zur Hausen 2009). Even after resection and therapy, Rep expression persists and might favor ongoing chronic inflammation, tumor recurrence and metastasis. Rep might, therefore, serve as prognostic marker for CRC.