PD-L2⁺ wound zone macrophage-like cells display M1/M2-mixed activation and restrain the effector Th1 responses

Depending on the microenvironment conditions, macrophages display phenotypic and functional heterogeneity. This study characterized the programmed cell death-ligand 2 (PD-L2)-expressing macrophage-like cells drained from surgical wound zones, and investigated their influence on helper T (Th) cell responses. Although all CD14⁺ myeloid cells possessed macrophage-like features, CD206⁺ and CD163⁺ cells constituted a specific subpopulation with high PD-L2 expression. There was a modest correlation between the PD-L2 levels on CD206⁺ macrophages and the amount of interferon (IFN)-γ in the drainage fluid. The adhesion-independent macrophages simultaneously presented both classically-activated M1 and alternatively-activated M2 characteristics. CD206⁺ and PD-L2⁺ cells were identified with high granularity and size, expressed arginase-1 and costimulatory molecules, had enhanced phagocytic activity and produced reactive oxygen species. The genes associated with macrophage differentiation (MERTK, AXL and TYRO3) were also upregulated. These cells provided costimulation to Th cells; yet, when PD-L2 was blocked, T-cell proliferation and IFNγ production were enhanced. Under defined conditions devoid of activation stimuli and matrix adhesion, ex vivo-generated monocyte-derived macrophages displayed limited capacity to stimulate T cells. Upon exposure to IFNγ, they significantly upregulated programmed death 1 ligands, especially PD-L2. These cells did not completely abrogate T-cell differentiation; however, PD-L2 checkpoint blockade restored Th1 proliferation and secretion of interleukin-2, tumor necrosis factor-α and IFNγ. In conclusion, upregulation of PD-L2 on the wound zone macrophages may constitute a negative feedback loop that restrains the Th1 effector responses and avoids exacerbation of inflammation during tissue healing.