Sicong Ma

CD4 ⁺ regulatory T (T _reg ) cells accumulate in the tumor microenvironment (TME) and suppress the immune system. Whether and how metabolite availability in the TME influences T _reg cell differentiation is not understood. Here, we measured 630 metabolites in the TME and found that serine and palmitic acid, substrates required for the synthesis of sphingolipids, were enriched. A serine-free diet or a deficiency in Sptlc2, the rate-limiting enzyme catalyzing sphingolipid synthesis, suppressed T _reg cell accumulation and inhibited tumor growth. Sphinganine, an intermediate metabolite in sphingolipid synthesis, physically interacted with the transcription factor c-Fos. Sphinganine c-Fos interactions enhanced the genome-wide recruitment of c-Fos to regions near the transcription start sites of target genes including Pdcd1 (encoding PD-1), which promoted Pdcd1 transcription and increased inducible T _reg cell differentiation in vitro in a PD-1–dependent manner. Thus, Sptlc2-mediated sphingolipid synthesis translates the extracellular information of metabolite availability into nuclear signals for T _reg cell differentiation and limits antitumor immunity.

T cells become functionally exhausted in tumors, limiting T cell–based immunotherapies. Although several transcription factors regulating the exhausted T (T _ex ) cell differentiation are known, comparatively little is known about the regulators of T _ex cell survival. Here, we reported that the regulator of G protein signaling 16 (Rgs-16) suppressed T _ex cell survival in tumors. By performing lineage tracing using reporter mice in which mCherry marked Rgs16-expressing cells, we identified that Rgs16 ⁺ CD8 ⁺ tumor-infiltrating lymphocytes (TILs) were terminally differentiated, expressed low levels of T cell factor 1 (Tcf1), and underwent apoptosis as early as 6 days after the onset of Rgs16 expression. Rgs16 deficiency inhibited CD8 ⁺ T cell apoptosis and promoted antitumor effector functions of CD8 ⁺ T cells. Furthermore, Rgs16 deficiency synergized with programmed cell death protein 1 (PD-1) blockade to enhance antitumor CD8 ⁺ T cell responses. Proteomics revealed that Rgs16 interacted with the scaffold protein IQGAP1, suppressed the recruitment of Ras and B-Raf, and inhibited Erk1 activation. Rgs16 deficiency enhanced antitumor CD8 ⁺ TIL survival in an Erk1-dependent manner. Loss of function of Erk1 decreased antitumor functions of Rgs16 -deficient CD8 ⁺ T cells. RGS16 mRNA expression levels in CD8 ⁺ TILs of patients with melanoma negatively correlated with genes associated with T cell stemness, such as SELL , TCF7 , and IL7R , and predicted low responses to PD-1 blockade. This study uncovers Rgs16 as an inhibitor of T _ex cell survival in tumors and has implications for improving T cell–based immunotherapies.