(Charles River) Enhanced anti-tumor lymphocyte function and frequencies measured by multichromatic flow cytometry in human CTLA-4 knock-in mice in a colorectal carcinoma model after treatment with ipilimumab

Abstract

Targeting CTLA-4 has shown remarkable long-term benefits and thus remains a valuable approach for combating cancers of many types. A number of preclinical models have been developed over the years to evaluate the efficacy of immune checkpoint blockade in promoting anti-tumor immunity. In particular, knock-in (KI) humanized mouse models offer the possibility to study clinical grade immune checkpoint inhibitors (ICI) in the context of a fully functional immune system. Here we show the response to ipilimumab in a newly developed hCTLA-4 KI humanized mouse model. Our results demonstrate significant tumor growth inhibition as well as complete tumor regressions in the MC38 colorectal cancer model following treatment with ipilimumab. We have extended these studies by re-challenging the tumor-free surviving animals with tumors cells implanted opposite to the original tumor site. We established that all re-challenged hCTLA-4 KI mice remained tumor free suggesting potent T cell memory was maintained. Comprehensive multichromatic phenotyping and functional intracellular cytokine staining (ICS) using validated 18-color flow cytometry panels showed a significant increase in CD8+ T cell frequencies when mice were treated with anti-hCTLA-4 but not with the mouse counterpart or the isotype control. Importantly, hCTLA-4 blockade reduced the regulatory T cell (FoxP3⁺ Treg) frequency and increased leukocyte infiltration into the tumor in the hCTLA-4 treated mice but not in the other two treated groups. Furthermore, treatment of MC38 tumor-bearing mice with ipilimumab enhanced significantly the secretion of IFNg and TNFa from CD8⁺ and CD4⁺ T cells. Altogether, the data presented here demonstrates that hCTLA-4 KI humanized mice are a robust model for evaluating the immune-modulatory effects and the activity of clinical grade ICI against tumors.

‍