Using Low Dose Radiation Therapy To Propagate Systemic Response To In Situ Vaccines PDF Download

Radiation is a mainstay of cancer therapy with over half of all patients receiving radiation therapy at some point throughout their clinical course. Though once thought of as primarily a cytotoxic therapy, growing eloquent preclinical work has outlined the complex interplay between radiation therapy and activation of key immune signaling pathways and effector cell populations. Several groups have taken advantage of immunostimulatory effects of radiation to generate an in situ vaccination, converting a patient's own tumor into a nidus for enhanced antigen presentation to drive a systemic immune response against distant sites of disease. However, responses generated against distant sites (i.e. abscopal response) following radiation remain rare. The advent of immunotherapy, particularly immune checkpoint blockade, has enabled propagation of the local immune response generated by radiation with remarkable success in preclinical models. Despite robust preclinical success, clinical translation remains limited. This may be due in part to the observation that the multitude of immune signaling pathways influenced by radiation each have a unique dose range for optimal activation, with no one single dose being able to optimally modulate all pathways. One goal of this thesis work was to investigate whether radiation dose heterogeneity, both temporal as administered via slow decay of a targeted radionuclide (targeted radionuclide therapy, TRT), and spatial as delivered via high dose rate brachytherapy (BT), offered meaningful advantages over homogenous dose radiation delivered via external beam radiation (EBRT). Delivery of TRT via 90Y decay resulted in a similar magnitude of immune activation compared to EBRT but with a protracted time course. This finding paired with the potential for TRT to modulate all sites of disease given its systemic nature likely underscore the capacity for TRT to generate more robust anti-tumor immunity compared to EBRT in a model of systemic disease. Spatial dose heterogeneity delivered via BT resulted in spatial heterogeneity in gene expression and T cell infiltration within the tumor microenvironment. Compared to low, moderate, and high dose EBRT, BT enabled peak activation of several immune pathways whereas each EBRT dose only enabled one. This was further borne out upon single cell RNA sequencing analysis. Each EBRT dose enriched unique immune cell clusters within the tumor microenvironment, whereas each of these clusters was represented in BT treated tumors. Subsequently, BT generated more robust anti-tumor immunity compared to EBRT in both localized and systemic disease models. In complementary work investigating additional methods to enhance in situ vaccination, we demonstrated the capacity of the infectious disease vaccine adjuvant monophosphoryl lipid A (MPL) to augment the anti-tumor effect of combination EBRT and checkpoint blockade. We observed that local delivery of MPL generated production of endogenous anti-tumor antibodies which were required for treatment efficacy. Through genetic knockout models we identified that the anti-tumor effect was dependent on induction of Th1 CD4 T cells and resulted from direct macrophage-mediated tumor cell killing via FcÎđR recognition but interestingly, was independent of CD8 T cells. These results suggest that this combination therapy may be particularly useful in settings of low MHC-1 expression which is associated with resistance to checkpoint blockade. BT as a treatment modality lends itself well to combination with intratumoral agents, given that catheters are placed within the tumor to deliver the radiation. To that end, we designed, created, and validated a multipurpose BT catheter to enable combination BT and intratumoral injection. As we gain further insight into immunosuppressive mechanisms occurring within the tumor microenvironment, it is becoming increasingly clear that combination approaches to treatment will be required to circumvent diverse resistance mechanisms.