CD28 Induces Mitochondrial Respiration Dependent Reactive Oxygen Species (ROS) Signaling for Metabolic Fitness and Survival in Long-Lived Plasma Cells

Abstract Durable humoral immunity depends upon bone marrow resident long-lived plasma cell (LLPC) survival. We have shown that CD28, the canonical T-cell costimulatory molecule, is required for LLPC survival; however, the mechanistic basis for this is unclear. We have published that of two binding motifs in the CD28 cytoplasmic tail, only the Grb2/Vav moiety, and not the direct PI3k binding motif, supports CD28-mediated survival. CD28 is known to induce glycolysis in T-cells. In contrast, CD28 activation in both LLPCs and human PCs induces glucose metabolism through mitochondrial respiration. A major byproduct of mitochondrial respiration is the production of reactive oxygen species (ROS). Interestingly, the mitochondrial respiration-dependent production of ROS is required for CD28-mediated survival. Previously published data demonstrate the ability of ROS to signal through both NFkB and PI3k. CD28-induced ROS signal through PI3k in LLPCs for increased glucose uptake and the inhibition of PI3k prevents CD28-mediated survival. The ROS-PI3k signal drives the phosphorylation of both S6k and rpS6, downstream targets of mTORC1, and this regulation is independent of Blimp1 expression. Irf4, a critical PC survival factor, is upregulated by CD28-dependent NFkB activation. To our great surprise, ROS inhibition prevented the induction of mitochondrial respiration itself, demonstrating that signals emanating downstream of ROS govern the metabolic program for survival in LLPCs. Taken together, these data suggest a model wherein CD28 signals directly through Grb2/Vav to induce mitochondrial respiration and ROS production. ROS then reinforce signaling through PI3k and NFkB to increase Irf4 expression and metabolic fitness for LLPC survival.