Dual CTLA-4 and PD-L1 Blockade Inhibits Tumor Growth and Liver Metastasis in a Highly Aggressive Orthotopic Mouse Model of Colon Cancer

Immune checkpoint inhibitors have shown clinical benefit in several cancer entities including metastatic microsatellite instable colorectal carcinomas. However, for the majority of metastatic colorectal carcinomas the potential and limitations of immune checkpoint inhibition is not fully understood. In this study, the effects of sole and dual CTLA-4 and PD-L1 blockade were investigated in a microsatellite stable highly aggressive orthotopic mouse model of colon cancer. Dual CTLA-4 and PD-L1 inhibition resulted in tumor growth stagnation and completely blocked liver metastasis. Sole CTLA-4 and PD-L1 inhibition only moderately reduced metastatic spread of the colon cancer cells, though CTLA-4 blockade being superior to PD-L1 inhibition. Dual immune checkpoint blockade and sole CTLA-4 inhibition significantly increased intratumoral CD8+ and CD4+ T cells and reduced FOXP3+/CD4+ Treg cells. This was associated with increased expression levels of the pro-inflammatory Th1/M1-related cytokines IFN-γ, IL-1α, IL-2, and IL-12. Moreover, tumors treated with combined immune checkpoint blockade showed the strongest increase in intratumoral iNOS+ macrophages, reduction of PD-L1+ and Tie2+ macrophages and the lowest expression of M2/Th2-related IL-4, TARC and COX-2. The assessment of further microenvironmental changes by DCE-MRI and immunohistology revealed no alterations in functional tumor vascularization upon combined immune checkpoint blockade, but a significant increase in intratumoral fibroblasts and collagen I deposition. Thus, the synergistic inhibitory effects of dual immune checkpoint inhibition can be explained by anti-tumorigenic T cell responses mediated by CTLA-4 inhibition and M1 macrophage polarization predominantly induced by PD-L1 blockade. This was accompanied by pronounced fibroblast activation highlighting the interconnection between immunogenicity and desmoplasia.