Abstract
Small molecular PD-1 inhibitors are lacking in current immuno-oncology clinic. PD-1/PD-L1 antibody inhibitors currently approved for clinical usage block interaction between PD-L1 and PD-1 to enhance cytotoxicity of CD8+ cytotoxic T lymphocyte (CTL). Whether other steps along the PD-1 signaling pathway can be targeted remains to be determined. Here, we report that methylene blue (MB), an FDA-approved chemical for treating methemoglobinemia, potently inhibits PD-1 signaling. MB enhances the cytotoxicity, activation, cell proliferation, and cytokine-secreting activity of CTL inhibited by PD-1. Mechanistically, MB blocks interaction between Y248-phosphorylated immunoreceptor tyrosine-based switch motif (ITSM) of human PD-1 and SHP2. MB enables activated CTL to shrink PD-L1 expressing tumor allografts and autochthonous lung cancers in a transgenic mouse model. MB also effectively counteracts the PD-1 signaling on human T cells isolated from peripheral blood of healthy donors. Thus, we identify an FDA-approved chemical capable of potently inhibiting the function of PD-1. Equally important, our work sheds light on a novel strategy to develop inhibitors targeting PD-1 signaling axis.