M2­TAM subsets altered by lactic acid promote T­cell apoptosis through the PD­L1/PD­1 pathway.

ABSTRACT

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, THP­1, was selected and treated with lactic acid. Immunofluorescence staining, laser confocal microscopy, reverse­transcription polymerase chain reaction (RT­PCR), 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 ligand­1 (PD­L1) as well as those of cytokines, tumor necrosis factor (TNF)­α, interferon (IFN)­Î³, interleukin (IL)­12, and IL­10. THP­1 macrophages and T cells were co­cultured 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, IFN­Î³ and IL­10, secreted by M2­tumor­associated macrophages (TAM, P<0.05), and HIF­1α and PD­L1 (P<0.05), and downregulated the expression of cytokines, TNF­α and IL­12, secreted by M1­TAM (P<0.05). Redistribution of M2­TAM subsets and PD­L1 expression was reversed after further transfection of THP­1 cells with HIF­1α siRNA (P<0.05). After co­culturing, T­cell proliferation was inhibited and apoptosis was promoted. In summary, modulation of lactic acid level can redistribute M2­TAM subsets and upregulate PD­L1 to assist tumor immune escape. The HIF­1α 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.