Reovirus sensitizes microsatellite stable colorectal cancer to anti-PD-1 treatment via cross-talk in innate and adaptive immune systems

BackgroundMicrosatellite stable (MSS) colorectal cancer (CRC) represents ~85% of all CRCs. These tumors are poorly immunogenic and largely resistant to immunotherapy, necessitating a need to develop new immune enhancing strategies. Oncolytic reovirus has a high propensity to replicate in KRAS mutant tumors which account for ~50% of MSS CRCs. Current study explores the ability of reovirus to potentiate the effect of immune checkpoint inhibition in MSS CRC. MethodsEffectiveness of reovirus infection was quantified through MTT assay for cell viability, and expression of immune-response genes by flow cytometry, RT-qPCR, and microarray. Computational analysis of differentially expressed genes was performed by TAC, DAVID and STRING. Combinatorial approach using anti-PD-1 monoclonal antibody was assessed in ex vivo and in vivo models. Live-cell imaging, tumor volume and survival were measured for quantification of anti-tumor activity. Expression of pattern recognition receptors (PRRs), cell surface and activation markers of immune cells, and PD-1/PD-L1 axis were studied using multi-color flow cytometry, immunoblotting, immunohistochemistry, and immunofluorescence. ResultsReovirus infection exerted growth arrest and expression of immune-response genes in CRCs cell lines in a KRAS-dependent manner. However, microsatellite instability, rather than KRAS status determined immune-repose pathways, functionalities and biological processes post-reovirus infection. Furthermore, reovirus significantly enhanced the anti-tumor activity of anti-human PD-1 [nivolumab] treatment in MSS CRC cell lines ex vivo. Similarly, reovirus increased the activity of anti-mouse PD-1 treatment in the CT26 [MSS, KRASMut], but not the MC38 [MSI, KRASWt] syngeneic mouse model of CRC. Combinatorial treatment has reduced the proliferative index, increased apoptosis and differentially altered PD-L1/PD-1 signaling among CT26 and MC38 tumors. Activation of innate immune system and expression of PRRs and antigen presentation markers were observed under reovirus and anti-PD-1 treatment that additionally reduced immunosuppressive macrophages. This led to an increase in T cell subsets, increase in effector T cell activation, and decrease in exhaustion markers specifically within CT26 microenvironment. ConclusionThe current study systematically evaluates immune characteristics and immune microenvironment of CRC under reovirus/anti-PD-1 combination treatment that proves increased effectiveness among MSS compared to MSI CRCs. This is a promising regimen warranting translation into clinical trials. One Sentence SummaryOncolytic reovirus alters innate and adaptive immune system and potentiates MSS type colorectal cancer to checkpoint inhibition therapy.