AMPKα1 in B cells promotes memory B cell longevity and mitochondrial homeostasis while dampening antibody synthesis

Emerging evidence indicates that metabolic programs regulate B cell activation and antibody responses. However, the metabolic mediators that support the durability of the memory B cell and long-lived plasma cell populations are not fully elucidated. Adenosine monophosphate-activated protein kinase (AMPK) is an evolutionary conserved serine/threonine kinase that integrates cellular energy status and nutrient availability to intracellular signaling and metabolic pathways. Here, we use genetic mouse models to show that B cell restricted loss of the catalytic subunit, AMPKα1, leads to a defect in the long-term survival of the memory B cell population. AMPKα1-deficient memory B lymphocytes exhibited aberrant mitochondrial activity including impaired mitophagy and decreased mitochondrial spare respiratory capacity. In contrast, AMPKα1 was dispensable for the stability of the bone marrow-resident long-lived plasma cell population yet its absence led to increased rates of Ig synthesis and elevated serum antibody concentrations elicited by immunization. Collectively, our findings fit a model in which AMPKα1 supports longevity in the memory B cell compartment by promoting mitochondrial homeostasis but restrains rates of immunoglobulin production by plasma cells.

Shifts in disease dynamics in a tropical amphibian assemblage are not due to pathogen attenuation

Infectious diseases rarely end in extinction. Yet the mechanisms that explain how epidemics subside are difficult to pinpoint. We investigated host-pathogen interactions after the emergence of a lethal fungal pathogen in a tropical amphibian assemblage. Some amphibian host species are recovering, but the pathogen is still present and is as pathogenic today as it was almost a decade ago. In addition, some species have defenses that are more effective now than they were before the epidemic. These results suggest that host recoveries are not caused by pathogen attenuation and may be due to shifts in host responses. Our findings provide insights into the mechanisms underlying disease transitions, which are increasingly important to understand in an era of emerging infectious diseases and unprecedented global pandemics.

mTORC1 in B cells regulates antibody responses and promotes mitochondrial and metabolic fitness

B lymphocytes migrate among different micro-anatomic sites for diversification, selection, and eventual differentiation into antibody-secreting plasma cells. Emerging evidence supports the premise that aspects of the nutrient milieu vary within lymphoid micro-environments. However, the role of B cell-intrinsic metabolic programs in regulating B cell differentiation and antibody response quality remain unclear. We now show that the amino acid-sensing mTOR complex 1 (mTORC1) is essential for induction of Bcl6 and IRF4, key transcriptional regulators of germinal center and plasma cell fates. mTORC1 also enhances B cell proliferation upon exposure to antigen, increases the rate of somatic hyper-mutation, and is essential for generating high-affinity class switched antibodies. We also find that AMP-activated kinase (AMPK), an intracellular energy sensor, promotes plasma cell differentiation and antibody production. Collectively, these findings suggest mechanisms by which mTORC1 activity is critical for the germinal center reaction and producing class-switched, high-affinity antibodies, and indicate that AMPK can regulate the development and functional properties of terminally differentiated plasma cells.

Supported by NIH R01 AI113292, HL106812, and NCI T32CA009592-29

LAMP-2C Inhibits MHC Class II Presentation of Cytoplasmic Antigens by Disrupting Chaperone-Mediated Autophagy

Cells use multiple autophagy pathways to sequester macromolecules, senescent organelles, and pathogens. Several conserved isoforms of the lysosome-associated membrane protein-2 (LAMP-2) regulate these pathways influencing immune recognition and responses. LAMP-2A is required for chaperone-mediated autophagy (CMA), which promotes Ag capture and MHC class II (MHCII) presentation in B cells and signaling in T cells. LAMP-2B regulates lysosome maturation to impact macroautophagy and phagocytosis. Yet, far less is known about LAMP-2C function. Whereas LAMP2A and LAMP2B mRNA were broadly detected in human tissues, LAMP2C expression was more limited. Transcripts for the three LAMP2 isoforms increased with B cell activation, although specific gene induction varied depending on TLR versus BCR engagement. To examine LAMP-2C function in human B cells and specifically its role in Ag presentation, we used ectopic gene expression. Increased LAMP-2C expression in B cells did not alter MHCII expression or invariant chain processing, but did perturb cytoplasmic Ag presentation via CMA. MHCII presentation of epitopes from exogenous and membrane Ags was not affected by LAMP-2C expression in B cells. Similarly, changes in B cell LAMP-2C expression did not impact macroautophagy. The gene expression of other LAMP2 isoforms and proteasome and lysosomal proteases activities were unperturbed by LAMP-2C ectopic expression. LAMP-2C levels modulated the steady-state expression of several cytoplasmic proteins that are targeted for degradation by CMA and diminished peptide translocation via this pathway. Thus, LAMP-2C serves as a natural inhibitor of CMA that can selectively skew MHCII presentation of cytoplasmic Ags.

Hyper-responsive Toll-like receptor 7 and 9 activation in NADPH oxidase-deficient B lymphoblasts

Chronic granulomatous disease (CGD) is an inherited immunodeficiency linked with mutations in the multi-subunit leucocyte NADPH oxidase. Myeloid-derived phagocytic cells deficient in NADPH oxidase fail to produce sufficient levels of reactive oxygen species to clear engulfed pathogens. In this study we show that oxidase also influences B-cell functions, including responses to single-stranded RNA or unmethylated DNA by endosomal Toll-like receptors (TLRs) 7 and 9. In response to TLR7/9 ligands, B-cell lines derived from patients with CGD with mutations in either the NADPH oxidase p40(phox) or p47(phox) subunits produced only low levels of reactive oxygen species. Remarkably, cytokine secretion and p38 mitogen-activated protein kinase activation by these oxidase-deficient B cells was significantly increased upon TLR7/9 activation when compared with oxidase-sufficient B cells. Increased TLR responsiveness was also detected in B cells from oxidase-deficient mice. NADPH oxidase-deficient patient-derived B cells also expressed enhanced levels of TLR7 and TLR9 mRNA and protein compared with the same cells reconstituted to restore oxidase activity. These data demonstrate that the loss of oxidase function associated with CGD can significantly impact B-cell TLR signalling in response to nucleic acids with potential repercussions for auto-reactivity in patients.

Hyper-responsive TLR7 and TLR9 activation in NADPH oxidase-deficient human B cells (HUM1P.272)

Chronic Granulomatous Disease (CGD) is an inherited immunodeficiency linked with mutations in the multi-component leukocyte NADPH oxidase. Phagocytic cells deficient in NADPH oxidase fail to produce sufficient levels of the reactive oxygen species (ROS) necessary to clear engulfed pathogens. Though the hyperactive immune response associated with CGD is widely described in phagocytes, the effects of oxidase-deficiency on B cell functions has not been well studied. In B cells, recognition of microbial single-stranded RNA or unmethylated DNA by intracellular toll-like receptors (TLR) 7 and 9, respectively leads to NADPH oxidase derived ROS production. Here, studies demonstrate hyperactive responses to endosomal TLR stimulation in B cell lines derived from patients with mutations in either the p40phox or p47phox subunits of the NADPH oxidase which compromise its function. NADPH oxidase-deficient B cell lines expressed enhanced levels of TLR7 and TLR9 mRNA compared to cells reconstituted to restore oxidase activity. TLR7/9 stimulation of oxidase-deficient B cells resulted in greater cytokine production and p38 MAP kinase activation than oxidase-sufficient B cells. These data are consistent with the autoimmune predisposition associated with CGD and suggest that B cells may also contribute to hyperactive immune responses detected in individuals with CGD.

A role for NADPH oxidase in antigen presentation

The nicotinamide adenine dinucleotide phosphate (NADPH) oxidase expressed in phagocytes is a multi-subunit enzyme complex that generates superoxide (O₂^.−). This radical is an important precursor of hydrogen peroxide (H₂O₂) and other reactive oxygen species needed for microbicidal activity during innate immune responses. Inherited defects in NADPH oxidase give rise to chronic granulomatous disease (CGD), a primary immunodeficiency characterized by recurrent infections and granulomatous inflammation. Interestingly, CGD, CGD carrier status, and oxidase gene polymorphisms have all been associated with autoinflammatory and autoimmune disorders, suggesting a potential role for NADPH oxidase in regulating adaptive immune responses. Here, NADPH oxidase function in antigen processing and presentation is reviewed. NADPH oxidase influences dendritic cell (DC) crosspresentation by major histocompatibility complex class I molecules through regulation of the phagosomal microenvironment, while in B lymphocytes, NADPH oxidase alters epitope selection by major histocompatibility complex class II molecules.