Background
Although antiretroviral therapy (ART) effectively inhibits viral replication, it does not fully mitigate the immunosenescence instigated by HIV infection. Cellular metabolism regulates cellular differentiation, survival, and senescence. Serine hydroxymethyltransferase 2 (SHMT2) is the first key enzyme for the entry of serine into the mitochondria from the de novo synthesis pathway that orchestrates its conversion glutathione (GSH), a key molecule in neutralising ROS and ensuring the stability of the immune system. It remains incompletely understood whether SHMT2 is involved in the senescence of CD8+ T cells, crucial for immune vigilance against HIV.
Methods
HIV-infected individuals receiving antiretroviral therapy were enrolled in our study. SHMT2-siRNA was electroporated into T cells to disrupt the gene expression of SHMT2, followed by the quantification of mRNA levels of crucial serine metabolism enzymes using real-time PCR. Immunophenotyping, proliferation, cellular and mitochondrial function, and senescence-associated signalling pathways were examined using flow cytometry in CD8+ T cell subsets. Findings: Our findings revealed that CD8+ T cells in HIV-infected subjects are inclined towards senescence, and we identified that SHMT2, a key enzyme in serine metabolism, plays a role in CD8+ T cell senescence. SHMT2 can regulate glutathione (GSH) synthesis and protect mitochondrial function, thus effectively controlling intracellular reactive oxygen species (ROS) levels. Moreover, SHMT2 significantly contributes to averting immunosenescence and sustaining CD8+ T cell competence by modulating downstream DNA damage and phosphorylation cascades in pathways intricately linked to cellular senescence. Additionally, our study identified glycine can ameliorate CD8+ T cell senescence in HIV-infected individuals. Interpretation: Decreased SHMT2 levels in HIV-infected CD8+ T cells affect ROS levels by altering mitochondrial function and GSH content. Increased ROS levels activate senescence-related signalling pathways in the nucleus. However, glycine supplementation counteracts these effects and moderates senescence. Funding: This study was supported by grants from the National Key R&D Program of China (2021YFC2301900-2021YFC2301901), National Natural Science Foundation of China (82372240), and Department of Science and Technology of Liaoning Province Project for the High-Quality Scientific and Technological Development of China Medical University (2022JH2/20200074).