The diversity of the plasmablast signature across species and experimental conditions: A meta-analysis

Antibody-secreting cells (ASC) are divided into two principal subsets, including the long-lived plasma cell (PC) subset residing in the bone marrow and the short-lived subset, also called plasmablast (PB). PB are described as a proliferating subset circulating through the blood and ending its differentiation in tissues. Due to their inherent heterogeneity, the molecular signature of PB is not fully established. The purpose of this study was to decipher a specific PB signature in humans and mice through a comprehensive meta-analysis of different data sets exploring the PB differentiation in both species and across different experimental conditions. The present study used recent analyses using whole RNA sequencing in prdm1-GFP transgenic mice to define a reliable and accurate PB signature. Next, we performed similar analysis using current data sets obtained from human PB and PC. The PB-specific signature is composed of 155 and 113 genes in mouse and human being, respectively. Although only nine genes are shared between the human and mice PB signature, the loss of B-cell identity such as the down-regulation of PAX5, MS4A1, (CD20) CD22 and IL-4R is a conserved feature across species and across the different experimental conditions. Additionally, we observed that the IRF8 and IRF4 transcription factors have a specific dynamic range of expression in human PB. We thus demonstrated that IRF4/IRF8 intranuclear staining was useful to define PB in vivo and in vitro and able to discriminate between atypical PB populations and transient states.

Molecular Mechanisms Driving IL-10- Producing B Cells Functions: STAT3 and c-MAF as Underestimated Central Key Regulators?

Regulatory B cells (Bregs) have been highlighted in very different pathology settings including autoimmune diseases, allergy, graft rejection, and cancer. Improving tools for the characterization of Bregs has become the main objective especially in humans. Transitional, mature B cells and plasma cells can differentiate into IL-10 producing Bregs in both mice and humans, suggesting that Bregs are not derived from unique precursors but may arise from different competent progenitors at unrestricted development stages. Moreover, in addition to IL-10 production, regulatory B cells used a broad range of suppressing mechanisms to modulate the immune response. Although Bregs have been consistently described in the literature, only a few reports described the molecular aspects that control the acquisition of the regulatory function. In this manuscript, we detailed the latest reports describing the control of IL-10, TGFβ, and GZMB production in different Breg subsets at the molecular level. We focused on the understanding of the role of the transcription factors STAT3 and c-MAF in controlling IL-10 production in murine and human B cells and how these factors may represent an important crossroad of several key drivers of the Breg response. Finally, we provided original data supporting the evidence that MAF is expressed in human IL-10- producing plasmablast and could be induced in vitro following different stimulation cocktails. At steady state, we reported that MAF is expressed in specific human B-cell tonsillar subsets including the IgD⁺ CD27⁺ unswitched population, germinal center cells and plasmablast.