In recent years, tumor immunotherapy has achieved remarkable efficacy in the treatment of tumors, especially in solid tumors, with impressive successes from immune checkpoint inhibitor (ICI) therapy. ICI therapy is effective in relieving T cell immunosuppression, restoring T cell immune cytotoxic against tumors, and significantly prolonging patient survival [1].

Programmed death-1 (PD-1), a target of ICI, is an inhibitory receptor of T cells and engaged by its ligands programmed cell death 1 ligand 1 (PD-L1) and programmed cell death 1 ligand 2 (PD-L2), that maintains peripheral organs tolerance and compromises the anti-tumor immunity [2]. Therefore, blocking the immune checkpoint molecule PD-1 or its ligand PD-L1/L2 can relieve tumor immune suppression of T cells and restore T cell cytotoxic activity against tumors, ultimately achieving the cure of tumors. The US Food and Drug Administration (FDA) has approved the PD-1 monoclonal antibodies, e.g. the nivolumab and pembrolizumab, to treat human cancers [3]. Studies have shown that PD-1/PD-L1 blockade therapy can significantly prolong the lifespan of many advanced cancer patients [4], [5], and improve the overall response rate (ORR) [6], [7].

Although the anti-PD-1/PD-L1 ICI immunotherapy has achieved effective anti-tumor results, the overall clinical efficacy of PD-1/PD-L1 monoclonal antibodies for ICI is not optimistic. According to clinical data reported by the FDA, the total ORR of pembrolizumab blockade antibodies treatment on melanoma is only 32.9% [8]. The ORR for liver cancer is even lower, which is less than 20% [1]. Although the indiscriminate blockade of the antibody drugs causes T cells to damage the tumor tissues, it also leads to excessive immune activation of systemic dendritic cells and T cells for normal organs attacking [3], [9].

Epigallocatechin-3-gallate (EGCG) is one of the tea polyphenols with anti-tumor capacity. As a natural active molecule, EGCG is characterized by low toxicity and cheap preparation. Compared with macromolecule antibodies, small molecule drugs are more convenient to take [10], [11]. Therefore, it is extremely important to develop small molecule drugs such as EGCG that target PD-1/PD-L1 as ICIs. The system cytokine storm, autoinflammation and tumor progression are mainly caused by persistent secretion of inflammatory cytokines, for instance the IL-1 family cytokines [12]. Our previous study showed that EGCG could inhibit both STAT/IRF and NLRP3/IL-1β inflammatory pathways, alleviate the auto-inflammation and decrease the expression of PD-L1 in tumors [13]. Moreover, EGCG showed great anti-inflammation capacity and has been found to inhibit IL-1 family cytokine secretion, thus reducing the tumor occurrence caused by auto-inflammation [14], [15], [16]. Our previous study also indicated that nano-formulated EGCG showed a better PD-L1 suppression, which was effective in enhancing the cytotoxic T cells-mediated anti-tumor effects [17], [18], [19]. Therefore, EGCG could be a safer ICI, which may avoid serious inflammatory storm. However, studies of EGCG effect on T cells are limited so far.

The expression of PD-1 is very low in inactivated T cells, which cannot allow us to explore its inhibitory effect. The CD3/CD28 antibodies, phytohaemagglutinin (PHA), or phorbol 12-myristate 13-acetate (PMA) with ionomycin were used for T cell activation and PD-1 over-expression to investigate the effects on PD-1 inhibition [20]. In this work, we explored the effect of EGCG on PD-1 of the activated T cells. This study for the first time discovered the inhibitory effect of EGCG on PD-1 expression in T cells. Additionally, the in vivo experiment furtherly demonstrated that the EGCG treatment could effectively alleviate the PD-1 expression of T cells for effective B16F10 tumor killing in mice. The mechanism study revealed that the EGCG inhibited the phosphorylation and nuclear translocation of NF-κB in T cells. In conclusion, the EGCG inhibits PD-1 expression in T cells via lessening nuclear translocation of NF-κB, resulting in a strong cytotoxic effect of T cells against tumors.