In a mouse mesothelioma model, Murakami et al. found that local radiotherapy (LRT) increased CD8+ T cell tumor infiltration, slowed tumor growth, and generated systemic memory, but also expanded intratumoral Tregs, which highly expressed GITR. Combining LRT with an IL-15 superagonist (IL-15SA), which increased CD8+ T cell and NK cell proliferation and activation, and a GITR agonist (DTA-1), which depleted intratumoral Tregs, improved tumor control and survival in mesothelioma-bearing mice. Treatment with LRT/IL-15SA/DT-1 elevated the intratumoral CD8/Treg ratio and induced an abscopal effect, slowing growth of a distant tumor.
Contributed by Alex Najibi
ABSTRACT: Malignant pleural mesothelioma (MPM) is an intractable disease with an extremely poor prognosis. Our clinical protocol for MPM of subablative radiotherapy (RT) followed by radical surgery achieved better survival compared to other multimodal treatments, but local relapse and metastasis remain a problem. This subablative RT elicits an antitumoral immune response that is limited by the immunosuppressive microenvironment generated by regulatory T (T(reg)) cells. The antitumor effect of immunotherapy to simultaneously modulate the immune activation and the immune suppression after subablative RT has not been investigated in MPM. Herein, we demonstrated a rationale to combine interleukin-15 (IL-15) superagonist (IL-15SA) and glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) agonist (DTA-1) with subablative RT in mesothelioma. IL-15SA boosted the systemic expansion of specific antitumoral memory CD8(+) T cells that were induced by RT in mice. Their effect, however, was limited by the up-regulation and activation of T(reg) cells in the radiated tumor microenvironment. Hence, selective depletion of intratumoral T(reg) cells through DTA-1 enhanced the benefit of subablative RT in combination with IL-15SA. The addition of surgical resection of the radiated tumor in combination with IL-15SA and DTA-1 maximized the benefit of RT and was accompanied by a reproducible abscopal response in a concomitant tumor model. These data support the development of clinical trials in MPM to test such treatment options for patients with locally advanced or metastatic tumors.