Membrane-spanning 4 domains subfamily A member 4A inhibition promotes M1 macrophage polarization and enhances anti-gastric cancer immune response.

Objective: The immunosuppressive tumor microenvironment (TME) limits the treatment effectiveness of immunotherapy in gastric cancer (GC). This study investigated the role of membrane-spanning four domains subfamily A member 4A (MS4A4A) in the regulation of macrophage polarization and its effect on the immune response in GC, with the aim of enhancing the effectiveness of immunotherapy by addressing the immunosuppressive TME. Material and Methods: A GC ectopic tumor model was initiated in C57BL/6 mice with subcutaneous MKN-45 injection. Five groups were formed by randomly dividing the mice: model, MS4A4A recombinant protein, MS4A4A antibody, programmed cell death protein 1 (PD-1) antibody, and MS4A4A recombinant protein + PD-1 antibody groups. MKN-45 cells and bone marrow-derived macrophages (BMDMs) were cocultured with the MS4A4A protein or antibody. Macrophage polarization was analyzed through flow cytometry, gene expression through quantitative real-time polymerase chain reaction (qRT-PCR), cytokine levels through enzyme-linked immunosorbent assay, protein expression through Western blot, and tumor morphology through hematoxylin and eosin staining. Results: In the GC mouse model, the MS4A4A recombinant protein markedly enhanced tumor growth (P < 0.001), and the MS4A4A antibody exhibited an inhibitory effect (P < 0.001). MS4A4A recombinant protein decreased the levels of inflammatory cytokine concentrations and increased those of anti-inflammatory mediator concentrations (P < 0.001). By contrast, the MS4A4A antibody treatment group displayed the opposite effect. Inhibition of MS4A4A enhanced the accumulation of macrophages, CD4+ T cells, and CD8+ T cells in the tumor (P < 0.001). Flow cytometry and qRT-PCR analyses showed that MS4A4A promoted M2 macrophage polarization, and MS4A4A antibody induced M1 polarization (P < 0.001). MS4A4A played a key role in inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells subunit p50 (p50) during M1 polarization. Furthermore, the PD-1 antibody reversed the pro-tumor effects of MS4A4A, which reestablished pro-inflammatory cytokine levels while lowering anti-inflammatory cytokines (P < 0.001). Conclusion: This study shows that MS4A4A promotes tumor growth by inducing M2 macrophage polarization and suppressing immune responses, while MS4A4A antibody enhances anti-tumor immunity by inducing M1 polarization. PD-1 antibody reverses MS4A4A's pro-tumor effects, restoring anti-tumor immunity. MS4A4A inhibitors or their combination with PD-1 antibodies may offer a promising strategy for GC immunotherapy.