511 Tumor immune microenvironment in adult mice asynchronously cross-fostered as pups

Background Immune responses to cancer are highly variable and influenced by genetic and environmental factors.1 Syngeneic tumor models in mice with intact immune systems are required to study anti-tumor immune responses but are unable to adequately model varied immune responses. Classically, different mouse strain backgrounds have been used to compare different immune responses to cancer immunotherapy, but this approach is limited by the inability to administer identical tumor cell lines, keeping constant the tumor while experimentally varying the immune response. Proper establishment of the immune system begins in early life and is regulated by environmental cues from maternal breast milk and the developing microbiota. To disrupt these cues prior to weaning, newborn pups can be cross-fostered to dams that delivered their litters asynchronously, either 2 weeks earlier or later, a model referred to as asynchronous cross-foster (ACF).2 We previously demonstrated that ACF can profoundly skew the immune profile of genetically identical offspring.2 Young ACF mice exhibited enhanced Th2 immunologic skewing and reduced peripheral tolerance in response to antigen, which resulted from impaired development of peripherally-induced regulatory T cells (pTreg). Adult mice that underwent ACF also exhibited altered systemic cytokine expression even in the absence of immunologic stimuli, suggesting that ACF has lasting impact on the immune system. Because peripheral tolerance and immune skewing directly impact anti-tumor immunity,3 we hypothesized that ACF would also impact the immune response to tumor growth. Methods To measure impact of ACF on tumor growth and tumor infiltration, we introduced EL4 lymphoma cells into 7-week-old mice with the following foster schemes: conventionally reared mice, 1-day-old pups cross-fostered with 10-day post-partum dam (ACF1 to ppd10), and 13-day-old pups cross-fostered with 1-day post-partum dams (ACF13 to ppd1). Immune infiltration at tumor endpoint was measured using flow cytometry. Results EL4 tumor growth was increased in ACF mice compared to conventionally-reared controls. Further, the immune infiltrate at endpoint was altered, with ACF mice having fewer natural killer (NK) cells, dendritic cells, and activated cytotoxic CD8+ T cells in the tumor microenvironment. Conclusions Our observations support the hypothesis that ACF impacts tumor growth and immune infiltration. Future directions include phenotyping the immune infiltrate with finer resolution, the study of additional tumor models, and investigation of the effects of ACF on spontaneous tumor incidence and immunotherapy efficacy. Development of this novel model could provide valuable insight into early life factors that influence anti-tumor immunity. Ethics Approval The study was approved by Mayo Clinic’s IACUC approved all uses in this study, approval number A00004845. Reference Radiloff DR, Rinella ES, and Threadgill DW. Modeling cancer patient populations in mice: complex genetic and environmental factors. Drug Discov Today Dis Models 2008; 4:83–88. Knoop KA, McDonald KG, Coughlin PE, et al. Synchronization of mothers and offspring promotes tolerance and limits allergy. JCI Insight. 2020; 5. Hedge UP, Jellison ER, and Chakraborty NG. pTregs or iTregs are the potent tolerance inducer for the growth and Metastasis of cancer. Int J Immunol Immunother2018:5.