SummaryB lymphocytes provide adaptive immunity by generating antigen-specific antibodies and supporting the activation of T cells. Little is known about how global metabolism supports naive B cell activation to enable an effective immune response. By coupling RNA sequencing (RNA-seq) data with glucose isotopomer tracing, we show that stimulated B cells increase programs for oxidative phosphorylation (OXPHOS), the tricarboxylic acid (TCA) cycle, and nucleotide biosynthesis, but not glycolysis. Isotopomer tracing uncovered increases in TCA cycle intermediates with almost no contribution from glucose. Instead, glucose mainly supported the biosynthesis of ribonucleotides. Glucose restriction did not affect B cell functions, yet the inhibition of OXPHOS or glutamine restriction markedly impaired B cell growth and differentiation. Increased OXPHOS prompted studies of mitochondrial dynamics, which revealed extensive mitochondria remodeling during activation. Our results show how B cell metabolism adapts with stimulation and reveals unexpected details for carbon utilization and mitochondrial dynamics at the start of a humoral immune response.

Ampk is an energy gatekeeper that responds to decreases in ATP by inhibiting energy-consuming anabolic processes and promoting energy-generating catabolic processes. Recently, we showed that Lkb1, an understudied kinase in B lymphocytes and a major upstream kinase for Ampk, had critical and unexpected roles in activating naïve B cells and in germinal center formation. Therefore, we examined whether Lkb1 activities during B cell activation depend on Ampk and report surprising Ampk activation with in vitro B cell stimulation in the absence of energy stress, coupled to rapid biomass accumulation. Despite Ampk activation and a controlling role for Lkb1 in B cell activation, Ampk knockout did not significantly affect B cell activation, differentiation, nutrient dynamics, gene expression, or humoral immune responses. Instead, Ampk loss specifically repressed the transcriptional expression of IgD and its regulator, Zfp318 . Results also reveal that early activation of Ampk by phenformin treatment impairs germinal center formation but does not significantly alter antibody responses. Combined, the data show an unexpectedly specific role for Ampk in the regulation of IgD expression during B cell activation.

T-cell-dependent antigenic stimulation drives the differentiation of B cells into antibody-secreting plasma cells and memory B cells, but how B cells regulate this process is unclear. We show that LKB1 expression in B cells maintains B-cell quiescence and prevents the premature formation of germinal centers (GCs). Lkb1-deficient B cells (BKO) undergo spontaneous B-cell activation and secretion of multiple inflammatory cytokines, which leads to splenomegaly caused by an unexpected expansion of T cells. Within this cytokine response, increased IL-6 production results from heightened activation of NF-jB, which is suppressed by active LKB1. Secreted IL-6 drives T-cell activation and IL-21 production, promoting T follicular helper (T FH ) cell differentiation and expansion to support a~100-fold increase in steady-state GC B cells. Blockade of IL-6 secretion by BKO B cells inhibits IL-21 expression, a known inducer of T FH -cell differentiation and expansion. Together, these data reveal cell intrinsic and surprising cell extrinsic roles for LKB1 in B cells that control T FH -cell differentiation and GC formation, and place LKB1 as a central regulator of T-celldependent humoral immunity.

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Defense from infection or cancer is provided by coordinated actions of the innate and adaptive immune system, including B lymphocytes that generate antibodies (Abs) against pathogens or tumor antigens. B cells establish immune synapses with antigen-presenting cells during activation to initiate Ab-production, which may include entrance into a germinal center (GC) reaction to generate very high-affinity Abs. Establishment of an immune synapse requires polarity proteins to coordinate uptake and processing of external antigens. For this, there is a highly conserved family of partitioning defective (Par) proteins that mediates cell polarity in multiple cell types. In particular, Par-3, a member of the Par polarity complex (Par-3, aPKC, Par-6), is a key protein that localizes to the immune synapse and is required for B cell polarization and immune synapse formation. Recently, we showed that LKB1, the mammalian homolog of Par-4 and a regulator of energy metabolism, controls B cell activation, GC formation and differentiation into Ab-producing plasma cells. However, roles for LKB1 in B cell polarity, immune synapse formation and anti-tumor immunity are unknown. Based on previous studies showing that LKB1 induces cell polarity and co-localizes with Par-3, we hypothesize that LKB1 is required for Par complex assembly and controls immune synapse formation during B cell activation. Subcellular analysis of LKB1 by immunofluorescence microscopy in mouse primary B cells reveals LKB1 co-localization with aPKC in membrane protrusions required for immune synapse formation. Interestingly, B cell specific LKB1 knock out (BKO) mice exhibit mislocalization of aPKC/Par complex, loss of lymphoid tissue polarity and spontaneous B cell activation that results in the formation of giant GCs and secretion of proinflammatory cytokines and chemokines. Similar inflammatory profiles with T cell chemoattractants are known to recruit CD8+ cytotoxic T lymphocytes (CTLs) into solid tumors, such as melanoma. These similarities suggest that LKB1 inactivated B cells in the tumor microenvironment could stimulate T cell infiltrates into solid tumors. Our studies suggest that physiologic inactivation of LKB1 in B cells controls B cell activation, GC formation and potentially infiltration of T cells in the tumor microenvironment during anti-tumor responses. We are currently utilizing an exciting genetic mouse melanoma model to examine the role of LKB1 in promoting anti-tumor immunity. Together, our studies assess LKB1 as a target for augmenting adaptive immunity against cancer. Citation Format: Laura Jimenez, Lynnea Waters, Diane N.H. Kim, Nicole Walsh, Michael A. Teitell. Roles for LKB1 at the immune synapse during B-cell activation and antitumor responses [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr B46.