Distinct Tumor Infiltrating Immune Cell Profiles in Mice by Non‐Steroidal Anti‐Inflammatory Drugs (Aspirin and Naproxen) During <i>TMPRSS2‐ERG</i> (Fusion)‐Driven and Non‐Fusion Driven Prostate Cancer

Our previous preclinical studies demonstrated that non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and naproxen significantly inhibited prostate tumorigenesis in a TMPRSS2-ERG fusion-driven model compared to non-fusion models. Since TMPRSS2-ERG fusion-positive tumors display heightened inflammatory signaling and substantial immune infiltration, we hypothesized that the differential efficacy of NSAIDs may arise from their ability to remodel the tumor immune microenvironment. Accordingly, in the present study, we systematically profiled innate and adaptive immune-cell populations: F4/80⁺ macrophages, mast cells, neutrophils, CD3⁺ T cells, CD8⁺ cytotoxic T cells, FoxP3⁺ regulatory T cells, CD20⁺ B cells, IgKC⁺ plasma cells, and Granzyme B⁺ effector cells in highly infiltrated dorso-lateral prostate regions of TMPRSS2-ERG fusion-driven and non-fusion PCa models, with and without NSAID intervention. Our analyses revealed pronounced macrophage infiltration in TMPRSS2-ERG. Pten^flox/flox and Hi-Myc^+/ ^- model, which was further augmented by NSAIDs. Importantly, NSAID intervention shifted macrophage polarization toward an M1-like, pro-inflammatory state, contrasting with the M2-dominant phenotype characteristic of untreated tumors. NSAID treatment reduced mast cell density within the stromal compartment, suggesting suppression of mast cell-mediated tumor-promoting signals. In the fusion model, infiltration of total T cells and CD8⁺ cytotoxic T cells decreased following NSAID exposure, whereas FoxP3⁺ Tregs remained largely unaffected. Both models showed increased B-cell infiltration independent of NSAID efficacy, and no clear correlation was observed between plasma-cell presence and treatment response. Collectively, our findings offer new insight into NSAID-mediated immunomodulation in TMPRSS2-ERG fusion-driven PCa; however, further in-depth immune subtyping and spatial mapping could fully delineate the immunological mechanisms driving NSAID responsiveness.