Vascularized tumor-on-a-chip to investigate immunosuppression of CAR-T cells

Chimeric antigen receptor (CAR)-T cell immunotherapy, effective in blood cancers, shows limited success in solid tumors, such as prostate, pancreatic, and brain cancers due, in part, to an immunosuppressive tumor microenvironment (TME). Immunosuppression affects various cell types, including tumor cells, macrophages, and endothelial cells. Conventional murine-based models offer limited concordance with human immunology and cancer biology. Therefore, we have developed a human "tumor-on-a-chip" (TOC) platform to model elements of immunosuppression at high spatiotemporal resolution. Our TOC features an endothelial cell-lined channel that mimics features of an <i>in vivo</i> capillary, such as cell attachment and extravasation across the endothelium and into the TME. Using 70 kDa dextran and fluorescence-recovery-after-photobleaching (FRAP), we confirmed physiologic interstitial flow velocities (0.1-1 μm s^-1). Our device demonstrates that tumor-derived factors can diffuse in the opposite direction of interstitial flow to reach the endothelium up to 200 μm away, and at concentrations as high as 20% of those at the tumor margin. M2-like immunosuppressive macrophages and endothelial cells affect prostate tumor cell growth, clustering, and migration. M2-like macrophages also induce PD-L1 and inhibit ICAM-1 gene expression on the adjacent endothelium in a pattern that limits CAR-T cell extravasation and effector function. This observation is abrogated in the presence of the anti-PD-L1 drug atezolizumab. These results provide mechanistic insight for <i>in vivo</i> observations showing limited CAR-T cell extravasation and effector function in solid tumors. Furthermore, they point to a specific role of M2 macrophages in driving CAR-T cell migration into and within the TME and could prove useful in the development of novel therapies to improve solid tumor CAR-T cell therapies.