Abstract

Prostate cancer is the second leading cause of cancer-related deaths in American men. While prostate cancer patients typically respond to androgen-deprivation therapy and/or taxane chemotherapy, patients inevitably develop resistance, and the disease progresses to an untreatable and lethal form known as castrate resistant prostate cancer (CRPC). Immunotherapy has changed the treatment paradigm for many types of cancer; however, immune checkpoint inhibitors have not shown clinical benefits in CRPC. Tumor associated macrophages (TAMs) secrete factors that promote tumor progression and suppress antitumor immunity, making them a promising therapeutic target. The Proviral Integration site for Moloney murine leukemia virus (PIM) kinases are serine/threonine kinases that are overexpressed in prostate cancer. PIM regulates many signaling pathways that promote cell survival. However, how PIM kinase alters the prostate tumor immune microenvironment and impacts immunotherapy resistance is not well understood.

We analyzed primary prostate and metastatic lymph node samples from treatment-naive metastatic hormone-sensitive prostate cancer patients. Prostate cancer samples with PIM1/PIM2/PIM3-high and -low expression were classified by top and bottom quartile, respectively. PIM high tumors had higher expression of immunostimulatory genes (IL1β, TNF, and TNFSF13), increased infiltration of M2 macrophages, B cells, and NK Cells, and a higher T cell inflamed score compared to PIM-low tumors. Due to the changes observed in inflammation, we hypothesized that PIM kinase may regulate macrophage inflammatory signaling. We showed that PIM inhibition suppresses inflammasome signaling and the release of the pro-inflammatory cytokine, IL-1β. Chronic inflammation can lead to the recruitment of TAMs, further promoting resistance to immune checkpoint inhibitors. Utilizing a syngeneic mouse model of prostate cancer, we demonstrated that PIM inhibition in combination with immune checkpoint blockade synergistically decreased tumor growth. Furthermore, immunoprofiling demonstrated that combination treatment enhances T cell activity. Overall, our results suggest that PIM kinase plays an important role in regulating the prostate tumor immune microenvironment and that PIM kinase may be a potential target to enhance the efficacy of immunotherapy for the treatment of prostate cancer.