RNA-based therapeutics hold great promise for treating diseases and lipid nanoparticles (LNPs) represent the most advanced platform for RNA delivery. However, the fate of the LNP-mRNA after endosome-engulfing and escape from the autophagy-lysosomal pathway remains unclear. To investigate this, mRNA (encoding human erythropoietin) was delivered to cells using LNPs, which shows, for the first time, a link between LNP-mRNA endocytosis and its packaging into extracellular vesicles (endo-EVs: secreted after the endocytosis of LNP-mRNA). Endosomal escape of LNP-mRNA is dependent on the molar ratio between ionizable lipids and mRNA nucleotides. Our results show that fractions of ionizable lipids and mRNA (1:1 molar ratio of hEPO mRNA nucleotides:ionizable lipids) of endocytosed LNPs were detected in endo-EVs. Importantly, these EVs can protect the exogenous mRNA during in vivo delivery to produce human protein in mice, detected in plasma and organs. Compared to LNPs, endo-EVs cause lower expression of inflammatory cytokines.

Summary The complement receptor 2 (CR2, CD21) is part of a complex (CD21/CD19/CD81) acting as a co‐receptor to the B cell receptor (BCR). Simultaneous triggering of the BCR and CD21 lowers the threshold for B cell activation. Although CD21 is important, B cells that express low amounts or lack surface CD21 (CD21–/low) are increased in conditions with chronic inflammation, e.g. autoimmune diseases. However, little is known about the CD21–/low B cell subset in peripheral blood from healthy donors. Here, we show that CD21–/low cells represent approximately 5% of B cells in peripheral blood from adults but are barely detectable in cord blood, after excluding transitional B cells. The CD21–/low subset can be divided into CD38–24+ and CD38–24low cells, where most of the CD38–24+ are CD27+immunoglobulin (Ig)M+IgD+ and the CD38–24low are switched CD27–. Expression levels of additional markers, e.g. CD95 and CD62L, are similar to those on classical memory B cells. In contrast to naive cells, the majority of CD21–/low cells lack expression of the ABCB1 transporter. Stimulation with a combination of BCR, Toll‐like receptor (TLR)−7/8 and interleukin (IL)−2 induces proliferation and differentiation of the CD21–/low B cells comparable to CD21+CD27+ memory B cells. The response excluding BCR agonist is not on par with that of classical memory B cells, although clearly above that of naive B cells. This is ascribed to a weaker response by the CD38–24low subset, implying that some memory B cells require not only TLR but also BCR triggering. We conclude that the CD21–/low cells in healthy donors are memory B cells.

a These authors contributed equally. AbstractB cells represent one of the cellular components of the immune system that protects the individual from invading pathogens. In response to the invader, these cells differentiate into plasma cells and produce large amounts of antibodies that bind to and eliminate the pathogen. A hallmark of autoimmune diseases is the production of autoantibodies i.e. antibodies that recognize self. Those that are considered pathogenic can damage tissues and organs, either by direct binding or when deposited as immune complexes. For decades, B cells have been considered to play a major role in autoimmune diseases by antibody production. However, as pathogenic autoantibodies appear to derive mainly from T cell dependent responses, T cells have been the focus for many years. The successful treatment of patients with autoimmune diseases with either B cell depletion therapy (rituximab) or inhibition of B cell survival (belimumab), suggested that not only the autoantibodies but also other B cell features are important. This has caused a surge of interest in B cells and their biology resulting in the identification of various subsets e.g. regulatory B cells, several memory B cell subsets etc. Also, in other conditions such as chronic viral infections and primary immunodeficiency, several B cell subsets with unique characteristics have been identified. In this review, we will discuss one of these subsets, a subset that is expanded in conditions characterized by chronic immune stimulation. This B cell subset lacks, or expresses low, surface levels of the complement receptor 2 (CD21) and has therefore been termed CD21 À/low B cells.

BackgroundRheumatoid arthritis (RA) preferentially affects women, with the peak incidence coinciding with estrogen decrease in menopause. Estrogen (E2) may therefore have intrinsic immune-regulatory properties that vanish with menopause. Fc sialylation is a crucial factor determining the inflammatory effector function of antibodies. We therefore analyzed whether E2 affects immunoglobulin G (IgG) sialylation.MethodsPostmenopausal (ovariectomized) mice were immunized with ovalbumin and treated with E2 or vehicle. Total and ovalbumin-specific IgG concentrations, sialylation, and Fcγ receptor expression were analyzed. Postmenopausal women with RA receiving hormone replacement therapy, including E2, or no treatment were analyzed for IgG sialylation. Furthermore, effects of E2 on the expression of the sialylation enzyme β-galactoside α2,6-sialyltransferase 1 (St6Gal1) were studied in mouse and human antibody-producing cells.ResultsE2 treatment significantly increased Fc sialylation of total and ovalbumin-specific IgG in postmenopausal mice. Furthermore, E2 led to increased expression of inhibitory Fcγ receptor IIb on bone marrow leukocytes. Treatment with E2 also increased St6Gal1 expression in mouse and human antibody-producing cells, providing a mechanistic explanation for the increase in IgG-Fc sialylation. In postmenopausal women with RA, treatment with E2 significantly increased the Fc sialylation of IgG.ConclusionsE2 induces anti-inflammatory effector functions in IgG by inducing St6Gal1 expression in antibody-producing cells and by increasing Fc sialylation. These observations provide a mechanistic explanation for the increased risk of RA in conditions with low estrogen levels such as menopause.Electronic supplementary materialThe online version of this article (10.1186/s13075-018-1586-z) contains supplementary material, which is available to authorized users.

Testosterone deficiency in men is associated with increased risk for autoimmunity and increased B cell numbers through unknown mechanisms. Here we show that testosterone regulates the cytokine BAFF, an essential survival factor for B cells. Male mice lacking the androgen receptor have increased splenic B cell numbers, serum BAFF levels and splenic Baff mRNA. Testosterone deficiency by castration causes expansion of BAFF-producing fibroblastic reticular cells (FRCs) in spleen, which may be coupled to lower splenic noradrenaline levels in castrated males, as an α-adrenergic agonist decreases splenic FRC number in vitro. Antibody-mediated blockade of the BAFF receptor or treatment with the neurotoxin 6-hydroxydopamine revert the increased splenic B cell numbers induced by castration. Among healthy men, serum BAFF levels are higher in men with low testosterone. Our study uncovers a previously unrecognized regulation of BAFF by testosterone and raises important questions about BAFF in testosterone-mediated protection against autoimmunity.

Depletion of B cells is beneficial in rheumatoid arthritis (RA) patients with autoantibodies to citrullinated proteins (ACPA) and/or the Fc portion of immunoglobulins (rheumatoid factor [RF]), suggesting a role for B cells in disease pathogenesis. To date, however, the identity of specifically pathogenic B cell subsets has not been discovered. One candidate population is identified by the low expression or absence of complement receptor 2 (CD21 −/low B cells). In this study, we sought to determine whether there was any correlation between CD21 −/low B cells and clinical outcome in patients with established RA, either ACPA + /RF + (n = 27) or ACPA − /RF − (n = 10).Healthy donors (n = 17) were included as controls. The proportion of the CD21 −/ low CD27 − IgD − memory B cell subset in peripheral blood (PB) was significantly increased in ACPA + /RF + RA patients compared with healthy donors, and the frequency of this subset correlated with joint destruction (r = 0.57, P < 0.04). The levels of the chemokines CXCL-9 and CXCL-10 were higher in synovial fluid than in plasma, and PB CD21 −/low cells expressed the receptor, CXCR3. In synovial fluid, most of the B cells were CD21 −/low , approximately 40% of that population was CD27 − IgD − , and a third of those expressed the pro-osteoclastogenic factor receptor activator of the nuclear factor κB ligand (RANKL). This subset also secreted RANKL, in addition to other factors such as IL-6, even in the absence of stimulation. We interpret these data as reason to propose the hypothesis that the CD27 − IgD − subset of CD21 −/ low B cells may mediate joint destruction in patients with ACPA + /RF + RA.

Random recombination of antibody heavy-and light-chain genes results in a diverse B-cell receptor (BCR) repertoire including self-reactive BCRs. However, tolerance mechanisms that prevent the development of self-reactive B cells remain incompletely understood. The absence of the surrogate light chain, which assembles with antibody heavy chain forming a pre-BCR, leads to production of antinuclear antibodies (ANAs). Here we show that the naive follicular B-cell pool is enriched for cells expressing prototypic ANA heavy chains in these mice in a non-autoimmune background with a broad antibody repertoire. This results in the spontaneous formation of T-cell-dependent germinal centres that are enriched with B cells expressing prototypic ANA heavy chains. However, peripheral tolerance appears maintained by selection thresholds on cells entering the memory B-cell and plasma cell pools, as exemplified by the exclusion of cells expressing the intrinsically self-reactive V H 81X from both pools.

Age-associated B cells (ABCs) represent a distinct cell population expressing low levels of CD21 (CD21 ). The Ig repertoire expressed by ABCs in aged mice is diverse and exhibits signs of somatic hypermutation (SHM). A CD21 B-cell population is expanded in autoimmune diseases, e.g. systemic lupus erythematosus, as well as in lupus-prone NZB/W mice and in mice lacking a pre-B cell receptor (SLC ). However, the nature of the CD21 B cells (hereafter ABCs) in autoimmunity is not well understood. Here we show that in young SLC mice, the vast majority of the ABCs express memory B-cell (MBC) markers in contrast to wild-type controls. A similar population is present in lupus-prone MRL mice before and at disease onset. In SLC mice, a majority of the ABCs are IgM , their V genes have undergone SHM, show clonal diversification and clonal restriction at the H-CDR3 level. ABC hybridomas, established from SLC mice, secrete typical lupus autoantibodies, e.g. anti-Smith antigen, and some of those that bind to DNA comprise a H-CDR3 that is identical to previously described IgM anti-DNA antibodies from lupus-prone mice. Together, these results reveal that ABCs in autoimmune mice are comprised of autoreactive MBCs expressing highly restricted H-CDR3 repertoires.