M2 macrophage increase in prostate adenocarcinoma after high-dose rate brachytherapy.

Abstract

271 Background: Brachytherapy delivers high doses of hypofractionated radiation (RT) internally within the prostate tumor. While radiation has been shown to exhibit immunomodulatory effects, the success of combining radiation and immunotherapy in prostate cancer has been limited. M2 macrophages can contribute to an immunosuppressive tumor environment, and we therefore hypothesize that radiation may increase the abundance of M2 macrophage in the prostate tumor immune infiltrate. Methods: 10 Patients with intermediate risk prostate cancer (age 51-78; 7 GS 3+4, 3 GS 4+3) underwent high-dose rate brachytherapy (2 implants, 13.5Gy per implant, 1 week apart). Patients consented to biopsies before each fraction at the first and second implant. Bulk RNA-seq gene expression was performed on formalin-fixed, paraffin-embedded pre- vs. post-RT tumor samples, and then analyzed with a machine learning deconvolution model to resolve immune cell abundances. CD68+CD163+ M2 macrophages were analyzed with multiplex immunohistochemistry and quantified with tissue analysis software. Paired t-test was used to compare M2 macrophage abundance pre- versus post-RT. Results: Bulk RNA-seq deconvolution demonstrated that the most prevalent immune cell types in the pre-RT samples were resting CD4 memory T cells (39.7%), resting mast cells (38.5%), and M2 macrophages (30.4%), while those in the post-RT samples were M2 Macrophages (78.7%), resting mast cells (58.1%), and resting CD4 memory T cells (48%). M2 macrophages demonstrated the greatest increase, which was corroborated by multiplex IHC: average CD68+CD163+ cell counts were significantly elevated after RT (pre-RT 63.7 vs. post-RT 200.1 per mm2, p=0.015). Conclusions: Hypofractionated radiation increases the immunosuppressive M2 macrophage population in prostate cancer, which may present a therapeutic target to augment the efficacy of radiation and immunotherapy combinations.