B cells are activated by two temporally distinct signals, the first provided by the binding of antigen to the B cell antigen receptor (BCR), and the second provided by helper T cells. Here we found that B cells responded to antigen by rapidly increasing their metabolic activity, including both oxidative phosphorylation and glycolysis. In the absence of a second signal, B cells progressively lost mitochondrial function and glycolytic capacity, which led to apoptosis. Mitochondrial dysfunction was a result of the gradual accumulation of intracellular calcium through calcium response-activated calcium channels that, for approximately 9 h after the binding of B cell antigens, was preventable by either helper T cells or signaling via the receptor TLR9. Thus, BCR signaling seems to activate a metabolic program that imposes a limited time frame during which B cells either receive a second signal and survive or are eliminated.

Key events in T cell-dependent antibody responses, including affinity maturation, are dependent on the B cell’s presentation of antigen to helper T cells at critical check points in germinal center formation in secondary lymphoid organs. Here we show that Toll-like receptor 9 (TLR9) signaling blocked the ability of antigen-specific B cells to capture, process and present antigen and to activate antigen-specific helper T cells in vitro. In a mouse model in vivo and in a human clinical trial the TLR9 agonist, CpG, enhanced the magnitude of the antibody response to a protein vaccine but failed to promote affinity maturation. Thus, TLR9 signaling may enhance antibody titers at the expense of the ability of B cells to engage in germinal center events that are highly dependent on B cells’ antigen capture and presentation.

Activation of CD4+ T cells to proliferate, drives cells towards aerobic glycolysis for energy production while using mitochondria primarily for macromolecular synthesis. In addition, the mitochondria of activated T cells increase production of reactive oxygen species (ROS) providing an important second messenger for intracellular signaling pathways. To better understand the critical changes in mitochondria that accompany prolonged T cell activation, we carried out an extensive analysis of mitochondrial remodeling using a combination of conventional strategies and a novel high-resolution imaging method. We show that for four days following activation, mouse CD4+ T cells sustained their commitment to glycolysis facilitated by increased glucose uptake through increased expression of GLUT transporters. Despite their limited contribution to energy production, mitochondria were active and showed increased ROS production. Moreover, prolonged activation of CD4+ T cells led to increases in mitochondrial content and volume, in the number of mitochondria per cell and in mitochondrial biogenesis. Thus, during prolonged activation, CD4+ T cells continue to obtain energy predominantly from glycolysis but also undergo extensive mitochondrial remodeling resulting in increased mitochondrial activity.