Chimeric antigen receptor (CAR) T-cell therapy engineers T-cells to express a synthetic receptor which redirects effector function to the tumor, to improve efficacy and reduce toxicities associated with conventional treatments, such as radiotherapy and chemotherapy. This approach has proved effective in treating hematological malignancies; however, the same effects have not been observed in solid tumors. The immunosuppressive tumor microenvironment (TME) creates a significant barrier to solid tumor efficacy and reduces the anti-cancer activity of endogenous tumor-resident immune cells, enabling cancer progression. In recent years, researchers have attempted to enhance CAR T-cell function in the TME by engineering the cells to express various proteins alongside the CAR. Examples of this engineering include inducing CAR T-cells to secrete cytokines or express cytokine receptors to modulate the cytokine milieu of the TME. Alternatively, the CAR T-cell may secrete antibody-like proteins to target a range of tumor antigens. Collectively, these methods are termed armored CAR T-cell therapy, and in this review, we will discuss the range of armored CAR T-cell approaches which have been investigated to date.

T cell-based cancer immunotherapy has typically relied on membrane-bound cytotoxicity enhancers such as chimeric antigen receptors expressed in autologous αβT cells. These approaches are limited by tonic signalling of synthetic constructs and costs associated with manufacture of bespoke patient products. γδT cells are an emerging alternative chassis for cellular therapy, possessing innate anti-tumour activity, potent antibody-dependent cytotoxicity (ADCC) and minimal alloreactivity. We present an immunotherapeutic platform technology built around the Vγ9Vδ2 γδT cell chassis, harnessing specific characteristics of this cell type and offering an allo-compatible means of delivering cellular therapy that recruits bystander immunity. We engineered γδT cells to secrete synthetic opsonins and stabilized IL15 (stIL15). Using GD2 as a model antigen we show how opsonin-secreting Vγ9Vδ2 (OPS-γδ) have enhanced cytotoxicity and also confer this benefit on lymphoid and myeloid bystander cells. Reflecting the secreted nature of the engineered efficacy modules, the entire product rather than just the gene-modified fraction exhibited enhanced activation and cytotoxic profiles, superior persistence and proliferative capacity even upon repeated tumour challenge. Secretion of stIL15 abrogated the need for exogenous cytokine supplementation during expansion and further mediated functional licensing of bystander NK cells. Compared to unmodified γδT cells, stIL15-OPS-γδ cells exhibited superiorin-vivocontrol of subcutaneous tumour and persistence in the blood. stIL15-OPS-γδ cells were further efficacious in 3D patient-derived osteosarcoma models, where efficacy could be boosted with the addition of immunomodulatory aminobisphosphonate drug, zoledronic acid. Together the data identify stIL15-OPS-γδ cells as a novel allogeneic platform combining direct cytolysis with bystander activation to effect solid tumour control.One Sentence SummaryArmoured, opsonin-secreting OPS-γδ cell immunotherapy is built on the innate strengths of the Vγ9Vδ2 cell chassis for allogeneic solid tumour targeting.

Malignant mesothelioma (MM) is a rare, aggressive solid tumor with limited therapeutic options and poor therapeutic response. The role of immunotherapy in MM is now well established and therapeutic options, such as checkpoint inhibitors, are increasingly being approved. Chimeric antigen receptor (CAR)-T cell therapy is successfully implemented in several hematologic cancers, but currently has inadequate effect in solid tumors, owing to several limitations, such as trafficking and infiltration, limited T cell persistence and exhaustion, the immunosuppressive TME and tumor antigen heterogeneity. The lack of uniform and universal expression of tumor-associated antigens (TAAs) on tumor cells, as well as TAA heterogeneity following tumor editing post-therapy, are issues of significant importance to CAR-T cell and associated antigen-targeting therapies. Our review discusses the concept of tumor antigen heterogeneity in MM, the consequences for CAR-T cell therapies and the strategies to overcome it.