The multifunctional RNA-binding protein hnRNPL is implicated in antibody class switching but its broader function in B cells is unknown. Here, we show that hnRNPL is essential for B cell activation, germinal center formation, and antibody responses. Upon activation, hnRNPL-deficient B cells show proliferation defects and increased apoptosis. Comparative analysis of RNA-seq data from activated B cells and another eight hnRNPL-depleted cell types reveals common effects on MYC and E2F transcriptional programs required for proliferation. Notably, while individual gene expression changes are cell type specific, several alternative splicing events affecting histone modifiers like KDM6A and SIRT1, are conserved across cell types. Moreover, hnRNPL-deficient B cells show global changes in H3K27me3 and H3K9ac. Epigenetic dysregulation after hnRNPL loss could underlie differential gene expression and upregulation of lncRNAs, and explain common and cell type-specific phenotypes, such as dysfunctional mitochondria and ROS overproduction in mouse B cells. Thus, hnRNPL is essential for the resting-to-activated B cell transition by regulating transcriptional programs and metabolism, at least in part through the alternative splicing of several histone modifiers.
© 2024. The Author(s).

Short-chain fatty acids (SCFAs) are metabolites from gut microbes involved in the host's inflammatory response and immunity. The aim of this study was to investigate the role of SCFAs in rheumatoid arthritis (RA) and possible mechanisms.
Gut microbiota diversity in mice was analysed by 16S rDNA sequencing. SCFAs levels were analysed by gas chromatography mass spectrometry. T and B cells were analysed by flow cytometry. Bone damage was analysed by micro-CT and X-ray. Histopathological status was analysed by HE staining. Proteins in tissues were analysed by immunohistochemistry and PCR. Mice with CD19+ B cells deficient in FFA2 receptors were used to explore the molecular mechanisms involved.
Levels of acetate, propionate, butyrate, and valerate were decreased in RA patients, and the first three correlated positively with the frequency of Bregs but not Tregs in peripheral blood. Administration of the three SCFAs prior to the onset of collagen-induced arthritis in mice improved arthritic symptoms, increased the Bregs frequency, and decreased transitional B and follicular B cell frequency. However, the preceding phenomena could not be observed in mice with CD19+ B cells deficient in FFA2 receptors. The effects of the three SCFAs in RA were dependent on FFA2 receptors but were independent of the other five B cell receptors (FFA3 receptor, HCA2 receptor, PPARγ, Olfr-78, and AhR).
SCFAs regulate B cells differentiation via FFA2 receptors to alleviate RA. This provides new insights into the treatment of RA from an immunological and microbiological perspective.
© 2022 British Pharmacological Society.

The germinal center (GC) plays a central role in the generation of antigen-specific B cells and antibodies. Tight regulation of the GC is essential due to the inherent risks of tumorigenesis and autoimmunity posed by inappropriate GC B cell processes. Gammaherpesviruses such as Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68) utilize numerous armaments to drive infected naïve B cells, independent of antigen, through GC reactions to expand the latently infected B cell population and establish a stable latency reservoir. We previously demonstrated that the MHV68 microRNA (miRNA) mghv-miR-M1-7-5p represses host EWSR1 (Ewing sarcoma breakpoint region 1) to promote B cell infection. EWSR1 is a transcription and splicing regulator that is recognized for its involvement as a fusion protein in Ewing sarcoma. A function for EWSR1 in B cell responses has not been previously reported. Here, we demonstrate that 1) B cell-specific deletion of EWSR1 had no effect on generation of mature B cell subsets or basal immunoglobulin levels in naïve mice, 2) repression or ablation of EWSR1 in B cells promoted expansion of MHV68 latently infected GC B cells, and 3) B cell-specific deletion of EWSR1 during a normal immune response to nonviral antigen resulted in significantly elevated numbers of antigen-specific GC B cells, plasma cells, and circulating antibodies. Notably, EWSR1 deficiency did not affect the proliferation or survival of GC B cells but instead resulted in the generation of increased numbers of precursor GC B cells. Cumulatively, these findings demonstrate that EWSR1 is a negative regulator of B cell responses.

The production of autoantibodies by autoreactive B cells plays a major role in the pathogenesis of lupus. Increases in memory B cells have been observed in human lupus patients and autoimmune lpr mice. Autophagy is required for the maintenance of memory B cells against viral infections; however, whether autophagy regulates the persistence of autoantigen-specific memory B cells and the development of lupus remains to be determined. Here we show that memory B cells specific for autoantigens can be detected in autoimmune lpr mice and a pristane-induced lupus mouse model. Interestingly, B cell-specific deletion of Atg7 led to significant loss of autoreactive memory B cells and reduced autoantibody production in pristane-treated mice. Autophagy deficiency also attenuated the development of autoimmune glomerulonephritis and pulmonary inflammation after pristane treatment. Adoptive transfer of wild type autoreactive memory B cells restored autoantibody production in Atg7-deficient recipients. These data suggest that autophagy is important for the persistence of autoreactive memory B cells in mediating autoantibody responses. Our results suggest that autophagy could be targeted to suppress autoreactive memory B cells and ameliorate humoral autoimmunity.
Copyright © 2021 Jang, Sharp, Wang, Feng, Wang and Chen.

Human metapneumovirus (hMPV) is an emergent virus, which mainly infects the upper and lower respiratory tract epithelium. This pathogen is responsible for a significant portion of hospitalizations due to bronchitis and pneumonia in infants and the elderly worldwide. hMPV infection induces a pro-inflammatory immune response upon infection of the host, which is not adequate for the clearance of this pathogen. The lack of knowledge regarding the different molecular mechanisms of infection of this virus has delayed the licensing of effective treatments or vaccines. As part of this work, we evaluated whether a single and low dose of a recombinant Mycobacterium bovis Bacillus Calmette-Guérin (BCG) expressing the phosphoprotein of hMPV (rBCG-P) can induce a protective immune response in mice. Immunization with the rBCG-P significantly decreased neutrophil counts and viral loads in the lungs of infected mice at different time points. This immune response was also associated with a modulated infiltration of innate cells into the lungs, such as interstitial macrophages (IM) and alveolar macrophages (AM), activated CD4+ and CD8+ T cells, and changes in the population of differentiated subsets of B cells, such as marginal zone B cells and plasma cells. The humoral immune response induced by the rBCG-P led to an early and robust IgA response and a late and constant IgG response. Finally, we determined that the transfer of cells or sera from immunized and infected mice to naïve mice promoted an efficient viral clearance. Therefore, a single and low dose of rBCG-P can protect mice from the disease caused by hMPV, and this vaccine could be a promising candidate for future clinical trials.
Copyright © 2021 Soto, Gálvez, Pacheco, Canedo-Marroquín, Bueno and Kalergis.