HCAR1 and neoplasm: Signaling through GPR81 has been shown to induce the expression of PD-L1, which can lead to suppressed T cell function and immune evasion in the tumor microenvironment, and blocking lactate production can improve PD-1 targeted immune checkpoint therapies (Feng et al., 2017; Daneshmandi et al., 2019; Feichtinger and Lang, 2019).