M2-TAM subsets altered by lactic acid promote T-cell apoptosis through the PD-L1/PD-1 pathway

The aim of the study was to investigate the effects of lactic acid on the phenotypic polarization and immune function of macrophages. The human monocyte/macrophage cell line, THP-1, was selected and treated with lactic acid. Immunofluorescence staining, laser confocal microscopy, reverse-transcription polymerase chain reaction (RT-PCR), western blot, siRNA, and ELISA analyses were used to observe changes in the levels of cluster of differentiation (CD)68, CD163, hypoxia inducible factor (HIF)-1α, and programmed death ligand-1 (PD-L1) as well as those of cytokines, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, interleukin (IL)-12, and IL-10. THP-1 macrophages and T cells were co-cultured in vitro to observe the changes in proliferation and apoptosis of T cells. The results showed that, lactic acid (15 mmol/l) significantly upregulated the expression of the macrophage M2 marker CD163 (P<0.05), cytokines, IFN-γ and IL-10, secreted by M2-tumor-associated macrophages (TAM, P<0.05), and HIF-1α and PD-L1 (P<0.05), and downregulated the expression of cytokines, TNF-α and IL-12, secreted by M1-TAM (P<0.05). Redistribution of M2-TAM subsets and PD-L1 expression was reversed after further transfection of THP-1 cells with HIF-1α siRNA (P<0.05). After co-culturing, T-cell proliferation was inhibited and apoptosis was promoted. In summary, modulation of lactic acid level can redistribute M2-TAM subsets and upregulate PD-L1 to assist tumor immune escape. The HIF-1α signaling pathway may participate in this process, revealing that macrophages, as 'checkpoints' in organisms, are links that connect the immune status and tumor evolution, and can be used as a target in tumor treatment.