PMN-MDSCs are responsible for immune suppression in anti-PD-1 treated TAP1 defective melanoma

The transporter associated with antigen processing (TAP) is a key component of the classical HLA I antigen presentation pathway. Our previous studies have demonstrated that the downregulation of TAP1 contributes to tumor progression and is associated with an increased presence of myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment. However, it remains unclear whether the elevation of MDSCs leads to immune cell exhaustion in tumors lacking TAP1. In this study, we established mouse models of tumors with TAP1 deficiency, and we employed PMN-MDSC depletion to investigate their impact on the immune microenvironment within the tumors. We found that MDSC depletion significantly altered the immune-suppressive effects of TAP1-deficient tumor when anti-PD-1 treatment was administered. Targeting PMN-MDSC may be a promising therapeutic strategy for the treatment of tumors with TAP1 deficiency during ICB treatment.

Immunohistochemistry (IHC) was conducted to assess TAP1 expression in mouse melanoma tissues. Ly6G, F4/80, and NKp46 markers were detected in B16 parental and TAP1 knockout tissues, respectively. To enhance anti-tumor immunity, hyperthermia-treated B16F10 WT cell suspension was injected prior to tumor cell introduction. Subsequently, we established B16F10 TAP1 knockout and WT melanoma mouse models. Tumors were collected, and the immune microenvironment was monitored accordingly. Anti-Ly6G antibody was administered to deplete polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs). Finally, flow cytometry analysis for immune infiltration, quantitative PCR for cytokine levels, and immunofluorescence assays were performed to analyze the immune response.

The level of Ly6G+ cell infiltration was significantly higher in samples exhibiting low TAP1 expression, while no differences were observed in the infiltration of F4/80+ cells or NKp46+ cells. Furthermore, the immune-suppressive effects associated with PMN-MDSCs were reversed following their elimination; this resulted in an increase in CD8+ T cells and a higher ratio of CD8+ T cells to Tregs, while the infiltration of innate immune cells remained unaffected. Functional markers of these immune cells indicated an active anti-tumoral immune response following the removal of PMN-MDSCs. Quantitative PCR analysis indicated elevated levels of TNF-α and IL-6, accompanied by decreased levels of TGF-β in the tumor microenvironment of TAP1.

Our data indicate that myeloid-derived suppressor cells (PMN-MDSCs) play an essential role in creating a tumorigenic immune microenvironment in TAP1 knockout tumors. Therefore, targeting PMN-MDSCs may become a promising therapeutic strategy for the treatment of tumors with TAP1 deficiency during ICB treatment.