Abstract B017: Bacterial minicells decrease tumor development and modulate immunity in a mouse mouse model of colon cancer
Authors: Tian Mengxi , Khan Mohammad W. , Grenier Shea , Tsuji Shingo , Giacalone Matthew A. , McGuire Kathleen L. 
Published: November 2016

Background: Colorectal cancer is the second leading cause of cancer deaths in the US. Genetic predisposition occurs in ∼30% of cases, suggesting that treatments with the ability to decrease lesion formation could significantly improve disease-free survival in potential patients with a strong family history. Genetically-engineered bacterial minicells (MC) are noninfectious, nano-sized bacterial particles derived from E. coli that can display immunomodulatory and antitumor activity in animal models of cancer. The expression of Invasin on the MC surface targets them to cancer-associated integrin heterodimers exposed on malignant cells, where they deliver Perfringolysin O, a unique tumorlytic bacterial toxin, that destroys these cells. We hypothesize that these MC can reduce colon lesions in genetically predisposed animals and that they have potential to be developed as a ‘probiotic’ therapy for individuals at high risk to develop colorectal cancer.

Methods: The TS4CRE mouse line expresses the Cre recombinase specifically in the distal ileum and colon under the control of the fatty acid binding protein-4 promoter. The Apcfl-468 mouse line has lox-P recombination sites flanking exons 11-12 of the Apc gene. Cross-breeding results in a deletion in one allele of the Apc gene in a tissue-specific manner, resulting in lesion formation by 14-18 weeks of age. Cross-bred mice were treated intra-rectally with 1.5×109 MC or PBS, once a week for 6 doses, at either 8-13 or 14-19 weeks of age and harvested at various times after treatment is complete to evaluate tumor number and size. Tumor and tumor adjacent tissue is evaluated for immune biomarkers using qPCR and immunohistochemistry studies. Mann-Whitney (two-tailed) tests are used to determine statistical significance.

Results: MC treatment during lesion development (14-19 weeks of age) significantly decreases tumor number (p = 0.007) and size (p = 0.03) in mice harvested at 6 months of age as compared to controls. Similarly, in animals treated from 8-13 weeks of age, when lesions are absent or only in the earliest stages, a significant decrease in tumor number (p = 0.04) was observed. MC can reduce tumor load through direct killing of developing lesions but they also have the potential to modulate immune responses. Since immune/inflammatory mediators are known to significantly contribute to tumor progression in colorectal cancer, decreased tumors in MC treated mice could be the result, at least in part, of a change in the immune/inflammatory environment of the colon. Chloracetate Esterase (CAE) staining for inflammatory granulocytes and mast cells showed that numbers of CAE+ cells in tumors and normal adjacent tissue from MC treated mice (at 14-19 weeks of age) were significantly lower than PBS-treated controls (p = 0.02). Moreover we saw that cells expressing CD11b, a biomarker for inflammatory, myeloid-lineage cells, were also significantly decreased in MC treated mice (p = 0.01). In addition, using qPCR we have determined that both the CD8 and IFNγ biomarkers were significantly increased in the tumor tissue of MC treated mice (p = 0.03 for each) while perforin and Tbet were increased in the tumor-adjacent tissue in these same mice (p = 0.04 for each), when compared to controls.

Conclusions: Taken together, these data strongly suggest that MC reduce colon tumor burden in genetically predisposed mice, most likely both by directly eliminating tumor cells and changing the immune environment of the lesions. The immunomodulatory effects appear to consist of both increased anti-tumor immunity and decreased inflammatory cell infiltration. Future studies will continue to access the actions of these MC and the possibility that they can be developed as a ‘probiotic’ therapeutic approach against colorectal cancer in genetically predisposed individuals.

Citation Format: Mengxi Tian, Mohammad W. Khan, Shea Grenier, Shingo Tsuji, Matthew A. Giacalone, Kathleen L. McGuire. Bacterial minicells decrease tumor development and modulate immunity in a mouse mouse model of colon cancer [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B017.