Macrophages as Key Mediators of Trp-1-Specific CD4+ T Cell Therapy for Murine Melanoma

There is a growing body of evidence indicating that CD4+ T cells, alongside CD8+ T cells, can effectively combat cancer. However, the mechanisms underlying the anti-tumor properties of CD4+ T cells are complex and not yet fully understood. To investigate these mechanisms, we utilized a murine model of tyrosinase-related protein 1 (Trp1)-specific CD4+ T cell adoptive transfer therapy for treating melanoma. By employing single-cell RNA sequencing (scRNA-seq), we analyzed the immune cells present in the tumors of mice that received adoptive transfer of Trp1-specific CD4+ T cells. Unexpectedly, within the tumor-infiltrating immune cells, the Trp1 CD4+ T cell population was relatively small, displaying characteristics indicative of exhaustion. In contrast, the most prominent cell cluster comprised macrophages, expressing high levels of the T cell inhibitory receptor ligand PD-L1 and the pro-inflammatory cytokine IL-1beta, suggesting a distinct M1 phenotype. Systemic depletion of macrophages following Trp1 CD4+ T cell transfer therapy compromised the antitumor effectiveness and resulted in tumor recurrence. These findings highlight the crucial role of innate macrophages as an effector cell population in Trp1-specific CD4+ T cell adoptive cell transfer therapy. ### Competing Interest Statement The authors have declared no competing interest.