The two T cell inhibitory receptors PD-1 and TIM-3 are co-expressed during exhausted T cell differentiation, and recent evidence suggests that their crosstalk regulates T cell exhaustion and immunotherapy efficacy; however, the molecular mechanism is unclear. Here we show that PD-1 contributes to the persistence of PD-1⁺TIM-3⁺ T cells by binding to the TIM-3 ligand galectin-9 (Gal-9) and attenuates Gal-9/TIM-3-induced cell death. Anti-Gal-9 therapy selectively expands intratumoral TIM-3⁺ cytotoxic CD8 T cells and immunosuppressive regulatory T cells (T_reg cells). The combination of anti-Gal-9 and an agonistic antibody to the co-stimulatory receptor GITR (glucocorticoid-induced tumor necrosis factor receptor-related protein) that depletes T_reg cells induces synergistic antitumor activity. Gal-9 expression and secretion are promoted by interferon β and γ, and high Gal-9 expression correlates with poor prognosis in multiple human cancers. Our work uncovers a function for PD-1 in exhausted T cell survival and suggests Gal-9 as a promising target for immunotherapy.

Immunotherapies targeting programmed cell death protein 1 (PD-1) and programmed cell death 1 ligand 1 (PD-L1) immune checkpoints represent a major breakthrough in cancer treatment. PD-1 is an inhibitory receptor expressed on the surface of activated T cells that dampens T-cell receptor (TCR)/CD28 signaling by engaging with its ligand PD-L1 expressed on cancer cells. Despite the clinical success of PD-1 blockade using mAbs, most patients do not respond to the treatment, and the underlying regulatory mechanisms of PD-1 remain incompletely defined. Here we show that PD-1 is extensively N-glycosylated in T cells and the intensities of its specific glycoforms are altered upon TCR activation. Glycosylation was critical for maintaining PD-1 protein stability and cell surface localization. Glycosylation of PD-1, especially at the N58 site, was essential for mediating its interaction with PD-L1. The mAb STM418 specifically targeted glycosylated PD-1, exhibiting higher binding affinity to PD-1 than FDA-approved PD-1 antibodies, potently inhibiting PD-L1/PD-1 binding, and enhancing antitumor immunity. Together, these findings provide novel insights into the functional significance of PD-1 glycosylation and offer a rationale for targeting glycosylated PD-1 as a potential strategy for immunotherapy. Significance: These findings demonstrate that glycosylation of PD-1 is functionally significant and targeting glycosylated PD-1 may serve as a means to improve immunotherapy response.

Programmed cell death 1 (PD-1) is widely expressed in tumor-infiltrating lymphocytes (TILs) of triple-negative breast cancer (TNBC). As a dominant inhibitory immune checkpoint (ICP) receptor, cell surface PD-1 is well-known to transduce negative signaling of effector T cell activity during cell–cell contact. However, despite its well-documented inhibitory effects, higher PD-1 expression in TILs is significantly associated with longer survival in TNBC patients. This phenomenon raises an interesting question whether PD-1 harbors positive activity to enhance anti-tumor immunity. Here, we show that PD-1 is secreted in an exosomal form by activated T cells and can remotely interact with either cell surface or exosomal programmed death-ligand 1 (PD-L1), induce PD-L1 internalization via clathrin-mediated endocytosis, and thereby prevent subsequent cellular PD-L1: PD-1 interaction, restoring tumor surveillance through attenuating PD-L1-induced suppression of tumor-specific cytotoxic T cell activity. Our results, through revealing an anti-PD-L1 function of exosomal PD-1, provide a positive role to enhance cytotoxic T cell activity and a potential therapeutic strategy of modifying the exosome surface with membrane-bound inhibitory ICP receptors to attenuate the suppressive tumor immune microenvironment.

Targeting immune checkpoints such as programmed cell death 1 (PD-1) and programmed cell death ligand 1 (PD-L1) has transformed cancer treatment, with durable clinical responses across a wide range of tumor types. However, a high percentage of patients fail to respond to anti–PD-1/PD-L1 treatment. A greater understanding of PD-L1 regulation is critical to improving the clinical response rate of PD-1/PD-L1 blockade. Here, we demonstrate that PD-L1 is phosphorylated and stabilized by casein kinase 2 (CK2) in cancer and dendritic cells (DC). Phosphorylation of PD-L1 at Thr285 and Thr290 by CK2 disrupted PD-L1 binding with speckle-type POZ protein, an adaptor protein of the cullin 3 (CUL3) ubiquitin E3 ligase complex, protecting PD-L1 from CUL3-mediated proteasomal degradation. Inhibition of CK2 decreased PD-L1 protein levels by promoting its degradation and resulted in the release of CD80 from DC to reactivate T-cell function. In a syngeneic mouse model, combined treatment with a CK2 inhibitor and an antibody against T-cell immunoglobulin mucin-3 (Tim-3) suppressed tumor growth and prolonged survival. These findings uncover a mechanism by which PD-L1 is regulated and suggest a potential antitumor treatment option to activate DC function by blocking the CK2–PD-L1 pathway and inhibiting Tim-3. Significance: This work identifies a role for CK2 in immunosuppression by phosphorylation and stabilization of PD-L1, identifying CK2 inhibition as an immunotherapeutic approach for treating cancer.

Antibodies that target immune checkpoint proteins such as programmed cell death protein 1, programmed death ligand 1, and cytotoxic T-lymphocyte–associated antigen 4 in human cancers have achieved impressive clinical success; however, a significant proportion of patients fail to respond to these treatments. Galectin-9 (Gal-9), a β-galactoside-binding protein, has been shown to induce T-cell death and facilitate immunosuppression in the tumor microenvironment by binding to immunomodulatory receptors such as T-cell immunoglobulin and mucin domain–containing molecule 3 and the innate immune receptor dectin-1, suggesting that it may have potential as a target for cancer immunotherapy. Here, we report the development of two novel Gal-9-neutralizing antibodies that specifically react with the N-carbohydrate-recognition domain of human Gal-9 with high affinity. We also show using cell-based functional assays that these antibodies efficiently protected human T cells from Gal-9-induced cell death. Notably, in a T-cell/tumor cell coculture assay of cytotoxicity, these antibodies significantly promoted T cell-mediated killing of tumor cells. Taken together, our findings demonstrate potent inhibition of human Gal-9 by neutralizing antibodies, which may open new avenues for cancer immunotherapy.

Galectin-12, a member of the galectin family of β-galactoside-binding animal lectins, is preferentially expressed in adipocytes and required for adipocyte differentiation in vitro. This protein was recently found to regulate lipolysis, whole body adiposity, and glucose homeostasis in vivo. Here we identify VPS13C, a member of the VPS13 family of vacuolar protein sorting-associated proteins highly conserved throughout eukaryotic evolution, as a major galectin-12-binding protein. VPS13C is upregulated during adipocyte differentiation, and is required for galectin-12 protein stability. Knockdown of Vps13c markedly reduces the steady-state levels of galectin-12 by promoting its degradation through primarily the lysosomal pathway, and impairs adipocyte differentiation. Our studies also suggest that VPS13C may have a broader role in protein quality control. The regulation of galectin-12 stability by VPS13C could potentially be exploited for therapeutic intervention of obesity and related metabolic diseases.