Cd28 Induces Autophagy In Plasma Cells To Enhance Mitochondrial Respiration And Survival

Abstract Durable humoral immunity relies on the persistence of long-lived plasma cells (LLPC) that continuously produce protective antibodies. The prototypical T cell costimulatory molecule CD28, that is also expressed on LLPC, is critical to their survival but the mechanisms involved remain unclear. We found CD28 activation enhances autophagy in LLPC/multiple myeloma as evidenced by increased level of autophagy marker LC3II, elevated autophagosome numbers and decreased level of autophagic cargo receptor P62. Inhibition of autophagy by 3MA or BafA abolished CD28’s pro-survival effects. Mechanistically, CD28 activation increased the protein level of ATG5, a critical autophagy regulator, even with translation blockade, but this was abrogated by proteasome inhibition. Knocking down atg5 eliminated CD28’s pro-survival effects, suggesting CD28 signaling regulates ATG5 degradation to sustain pro-survival autophagy. CD28 activation significantly increased mitochondrial respiration and blocking autophagy prevented this. Free fatty acids (FFA) can be degraded from cellular lipid droplets (LD) by lipophage, a selective form of autophagy, to fuel mitochondrial respiration. We found CD28 activation decreased LD staining and blocking autophagy prevented this. Blocking lipophagic lipase by lalistat eliminated CD28-induced oxidative phosphorylation and pro-survival effects. Addition of FFA (oleic and palmitic acids) boosted oxidative phosphorylation and restored survival with autophagy inhibition but not with fatty acid oxidation blockade. These data suggest CD28 signaling regulates ATG5 protein degradation to enhance autophagy and this in return produces FFA to sustain mitochondrial respiration and support LLPC survival.