Germinal center B cells (GCBCs) are critical for generating long-lived humoral immunity. How GCBCs meet the energetic challenge of rapid proliferation is poorly understood. Dividing lymphocytes typically rely on aerobic glycolysis over oxidative phosphorylation for energy. Here we report that GCBCs are exceptional among proliferating B and T cells as they actively oxidize fatty acids (FAs) and conduct minimal glycolysis. In vitro, GCBCs had a very low glycolytic extracellular acidification (ECAR) but consumed oxygen in response to FAs. [ 13 C 6 ]-glucose feeding revealed that GCBCs generate significantly less phosphorylated glucose and little lactate. Further, GCBCs did not metabolize glucose into TCA cycle intermediates. Conversely, [ 13 C 16 ]-palmitic acid labeling demonstrated that GCBCs generate most of their acetyl-CoA and acetylcarnitine from FAs. FA oxidation (FAO) was functionally important, as drug-mediated and genetic dampening of FAO resulted in a selective reduction GCBCs. Hence, GCBCs appear to uncouple rapid proliferation from aerobic glycolysis.

Over the last 15 years, the inducible T cell co-stimulator (ICOS) has been implicated in various immune outcomes, including the induction and regulation of Th1, Th2, and Th17 immunity. In addition to its role in directing effector T cell differentiation, ICOS has also been consistently linked with the induction of thymus-dependent (TD) antibody (Ab) responses and the germinal center (GC) reaction. ICOS co-stimulation, therefore, appears to play a complex role in dictating the course of adaptive immunity. In this article, we summarize the initial characterization of ICOS and its relationship with the related co-stimulatory molecule CD28. We then address the contribution of ICOS in directing an effector T cell response, and ultimately disease outcome, against various bacterial, viral, and parasitic infections. Next, we assess ICOS in the context of TD Ab responses, connecting ICOS signaling to follicular helper T cell differentiation and its role in the GC reaction. Finally, we address the link between ICOS and human autoimmune disorders and evaluate potential therapies aiming to mitigate disease progression by modulating ICOS signaling.

Blood-stage Plasmodium chabaudi chabaudi AS infection requires cell- and antibody-mediated immunity to control acute and persistent infection, respectively. The inducible T cell co-stimulator (ICOS) regulates CD4+ T cell activation and promotes the induction of follicular helper T (TFH) cells—CD4+ T cells that support B cell affinity maturation within germinal centers (GCs), resulting in the production of high-affinity antibodies (Abs). Here, we demonstrate that, in response to P. c. chabaudi AS infection, the absence of ICOS resulted in an enhanced TH1 immune response that reduced peak parasitemia. Despite the absence of ICOS, CD4+ T cells were capable of expressing PD-1, Bcl6, and CXCR5 during early infection, indicating TFH cell development was not impaired. However, by day 21 post-infection, Icos−/− mice accumulated fewer splenic TFH cells compared to Icos+/+ mice, leading to substantially fewer GC B cells and a decrease in affinity, but not production, of parasite-specific isotype-switched Abs. Moreover, treatment of mice with anti-ICOSL Abs to modulate ICOS-ICOSL signaling revealed a requirement for ICOS in TFH cell differentiation only after day 6 post-infection. Ultimately, the quality and quantity of isotype-switched Abs produced in Icos−/− mice declined over time, resulting in impaired control of persistent parasitemia. Collectively, these data suggest ICOS is not required for TFH cell induction during P. c. chabaudi AS infection, or production of isotype-switched Abs, but is necessary for maintenance of a sustained high-affinity, protective Ab response.

Compared to naïve B cells (NBCs), both B cell antigen receptor (BCR) and CD40 signaling are rewired in germinal center (GC) B cells (GCBCs) to optimize selection for high-affinity B cells. The mechanism for BCR reprogramming in GCBCs remains unknown. We describe a GC-specific, AKT kinase-driven negative feedback loop that attenuates BCR signaling. A mass spectrometry proteomic approach revealed that AKT activity was retargeted in GCBCs compared to NBCs. Retargeting was linked to differential AKT T308 and S473 phosphorylation, in turn due to GC-specific upregulation of phosphoinositide-dependent protein kinase PDK1 and the phosphatase PTEN, which retuned phosphatidylinositol-3-OH kinase (PI3K) signals. In GCBCs, AKT preferentially targeted CSK, SHP-1 and HPK1, which are negative regulators of BCR signaling. Phosphorylation results in markedly increased enzymatic activity of these proteins, creating a negative-feedback loop that dampens upstream BCR signaling. Inhibiting AKT substantially enhanced activation of BCR proximal kinase LYN as well as downstream BCR signaling molecules in GCBCs, establishing the relevance of this pathway.

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions, and in response to inflammation due to infection or stress. While the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occurs in the spleen during acute Plasmodium infection –n a critical step in the host immune response. Here, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a Lineage−Sca-1+c-kit− (LSK−) phenotype that proliferates in response to infection with non-lethal Plasmodium yoelii 17X. Infection-derived LSK− cells upon transfer into naïve congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK− cells gave rise to B cells capable of entering into a germinal center reaction, and developed into memory B cells and antibody-secreting cells that were capable of producing parasite-specific antibodies. Differentiation of LSK− cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK− cells expand and differentiate in direct response to the parasite. However, the ability of LSK− cells to differentiate into B cells was not dependent on MyD88 as myd88−/− LSK− cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B-lymphocytes in the spleen, and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.