Proteomic and immunoproteomic insights into the exoproteome of Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia.

Porcine pleuropneumonia caused by Actinobacillus pleuropneumoniae affects pig health status and the swine industry worldwide. Despite the extensive number of studies focused on A. pleuropneumoniae infection and vaccine development, a thorough analysis of the A. pleuropneumoniae exoproteome is still missing. Using a complementary approach of quantitative proteomics and immunoproteomics we gained an in-depth insight into the A. pleuropneumoniae serotype 2 exoproteome, which provides the basis for future functional studies. Label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed 593 exoproteins, of which 104 were predicted to be virulence factors. The RTX toxins ApxIIA and ApxIIIA -were found to be the most abundant proteins in the A. pleuropneumoniae serotype 2 exoproteome. Furthermore, the ApxIVA toxin was one of the proteins showing the highest abundance, although ApxIVA is commonly assumed to be expressed exclusively in vivo. Our study revealed several antigens, including proteins with moonlight functions, such as the elongation factor (EF)-Tu, and proteins linked to specific metabolic traits, such as the maltodextrin-binding protein MalE, that warrant future functional characterization and might present potential targets for novel therapeutics and vaccines. Our Ig-classes specific serological proteome analysis (SERPA) approach allowed us to explore the development of the host humoral immune response over the course of the infection. These SERPAs pinpointed proteins that might play a key role in virulence and persistence and showed that the immune response to the different Apx toxins is distinct. For instance, our results indicate that the ApxIIIA toxin has properties of a thymus-independent antigen, which should be studied in more detail.

Glia maturation factor-γ is involved in S1P-induced marginal zone B-cell chemotaxis and optimal IgM production to type II T-independent antigen.

Marginal zone B cells (MZBs) represent a unique B-cell sub-population that rapidly differentiate into IgM-secreting plasma cells in response to T-independent (T-I) antigen. Sphingosine 1-phosphate (S1P) promotes MZB localization to the marginal zone. However, intracellular molecules involved in MZB localization and migration remain largely unknown. Here, we show that MZBs lacking the glia maturation factor-γ (GMFG) are impaired in chemotaxis toward S1P under both in vitro and in vivo conditions, suggesting that GMFG is an effector downstream of S1P receptors. GMFG undergoes serine phosphorylation upon S1P stimulation and is required for S1P-induced desensitization of S1P receptor 1 (S1PR1). Compared with wild-type mice, Gmfg-/- mice produce elevated levels of 4-hydroxy-3-nitrophenyl-acetyl (NP)-specific IgM against a T-I type II antigen, NP-Ficoll, accompanied by dysregulated MZB localization. These results identify GMFG as a regulator of S1P-induced MZB chemotaxis and reveal a role for MZB localization in the marginal zone for optimal IgM production against a T-I antigen.

Enhanced IgG1 -mediated antibody response towards thymus-dependent immunization in CXCR1-deficient mice.

BACKGROUND: Chemokine receptors and their corresponding ligands are key players of immunity by regulation of immune cell differentiation and migration. CXCR1 is a high-affinity receptor for CXCL8. Differential expression of CXCR1 is associated with a variety of human pathologies including cancer and inflammatory diseases. While various studies have highlighted the importance of CXCR1-mediated CXCL8-sensing for neutrophil trafficking and function, its role in B-cell responses remains unsolved. Therefore, our aim was to investigate innate and adaptive antibody responses in CXCR1-deficient mice. METHODS: Cell populations of the spleen and the peritoneal cavity were identified and quantified via flow cytometry. To investigate thymus-independent (TI) and thymus-dependent (TD) antibody responses, mice were immunized intraperitoneally with TNP-Ficoll, Pneumovax23, and TNP-Chicken Gamma Globulin. Mice were bled before as well as 7 and 14 days after vaccination to collect serum. Serum antibody levels overtime were analyzed according to their specificity by enzyme-linked immunosorbent assay. B-1 cell functionality was examined by IL-5/IL-5Rα-dependent stimulation of peritoneal and splenic cells in vitro. To analyze CXCR1/2-expression, CD19+ splenocytes were enriched by magnetic-activated cell sorting before isolation of total RNA contents, followed by reverse transcription and real-time polymerase chain reaction. RESULTS: The distribution of natural B-1 cell populations was disturbed in the absence of CXCR1, while their responsiveness towards TI antigens and in vitro stimulation remained functional. Besides, CXCR1-deficiency was accompanied by increased frequencies of follicular B-2 cells in the spleen. Interestingly, these mice produced elevated levels of antigen-specific IgG1 upon TD immunization and harbored a significantly enlarged proportion of CXCR5-expressing T helper (H) cells. CXCR1-expression was detectable in CD19+ splenocytes derived from wild-type, but not CXCR1-deficient mice. CONCLUSION: Our data demonstrate a previously unknown relevance of CXCR1 for the production of specific IgG1 in response to vaccination. These findings identify CXCR1 as a promising candidate for future studies on the regulation of adaptive antibody responses.

Extracellular polysaccharides produced by bacteria of the Leuconostoc genus.

Structurally diverse biopolymers, including extracellular polysaccharides (EPS), synthesized by bacteria can possess physicochemical and functional properties that make them important products of microbial synthesis with a broad and versatile biotechnological potential. Leuconostoc spp. belongs to the group of lactic acid bacteria as one of the predominant members and are relevant not only in varied food fermentations, but also can be employed in the production of extracellular homopolysaccharides (HoPS) such as α-glucans (dextran, alternan) and ß-fructans (levan,inulin) from the sucrose-containing substrates. EPS are synthesized by specific Leuconostoc spp. extracellular glycosyltransferases [dextran sucrase, alternansucrase (ASR)] and fructosyltransferases (levansucrase, inulosucrase) and enzymatic reactions can be performed in whole culture systems as well as using cell-free enzymes. Both α-glucans and ß-fructans have a wide range of properties, mostly depending on their pattern of linkages, which, although differing in some respects, make suitable prerequisites for their versatile application in many fields, especially in the food industry and biomedicine. As a rule, these properties (polymer type, molecular mass, rheological parameters), as well as the overall EPS yield, are strain-specific for the selected producers and depend to a large extent on the nutritional and growth conditions used, which in many cases remain not sufficiently optimized for Leuconostoc spp. This review summarizes the current knowledge on the potential of Leuconostoc spp. to produce commercially relevant EPS, including information on their applications in various fields, producer strains, production methods and techniques used, selected conditions, the productivity of bioprocesses as well as the possible use of renewable resources for their development.

B Cell Subsets Differentially Contribute to the T Cell-Independent Memory Pool.

The roles distinct B cell subsets play in clonal expansion, isotype switching, and memory B cell differentiation in response to T cell-independent type 2 Ags (TI-2 Ags) has been understudied. Using sorted B cells from VHB1-8 knock-in mice, we evaluated B-1b, marginal zone, and follicular B cell responses to the TI-2 Ag, NP-Ficoll. All subsets extensively divided in response to NP-Ficoll. Nonetheless, B-1b cells exhibited significantly increased IgG switching and differentiation into Ab-secreting cells (ASC)-a finding that coincided with increased AgR signaling capacity and Blimp1 expression by B-1b cells. All subsets formed memory cells and expressed markers previously identified for T cell-dependent memory B cells, including CD80, PDL2, and CD73, although B-1b cells generated the greatest number of memory cells with higher frequencies of IgG- and CD80-expressing cells. Despite memory formation, secondary immunization 4 wk after primary immunization did not increase NP-specific IgG. However, boosting occurred in B-1b cell-recipient mice when IgG levels declined. CD80+ memory B-1b cells divided, class switched, and differentiated into ASC in response to Ag in vivo, but this was inhibited in the presence of NP-specific IgG. Furthermore, CD80 blockade significantly increased memory B-1b cell division and differentiation to ASC upon Ag restimulation. Collectively, these findings demonstrate B-1b, marginal zone B, and follicular B subsets significantly contribute to the TI-2 Ag-specific memory B cell pool. In particular, we show B-1b cells generate a functional CD80-regulated memory population that can be stimulated to divide and differentiate into ASC upon Ag re-encounter when Ag-specific IgG levels decline.

Discovery of BIIB068: A Selective, Potent, Reversible Bruton's Tyrosine Kinase Inhibitor as an Orally Efficacious Agent for Autoimmune Diseases.

Autoreactive B cell-derived antibodies form immune complexes that likely play a pathogenic role in autoimmune diseases. In systemic lupus erythematosus (SLE), these antibodies bind Fc receptors on myeloid cells and induce proinflammatory cytokine production by monocytes and NETosis by neutrophils. Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase that signals downstream of Fc receptors and plays a transduction role in antibody expression following B cell activation. Given the roles of BTK in both the production and sensing of autoreactive antibodies, inhibitors of BTK kinase activity may provide therapeutic value to patients suffering from autoantibody-driven immune disorders. Starting from an in-house proprietary screening hit followed by structure-based rational design, we have identified a potent, reversible BTK inhibitor, BIIB068 (1), which demonstrated good kinome selectivity with good overall drug-like properties for oral dosing, was well tolerated across preclinical species at pharmacologically relevant doses with good ADME properties, and achieved >90% inhibition of BTK phosphorylation (pBTK) in humans.

Rainbow trout mount a robust specific immune response upon anal administration of thymus-independent antigens.

Despite the strong demand for orally-delivered fish vaccines and the deficient response of those currently available in the market, little is known about how teleost B cells differentiate to antibody secreting cells (ASCs) in response to antigens delivered to the intestinal mucosa. To fill this gap, in the current study, we have studied the dynamics of B cell differentiation in spleen and kidney of rainbow trout (Oncorhynchus mykiss) anally immunized with antigens catalogued in mammals as thymus dependent (TD) or thymus-independent (TI). Our results show that, in the absence of additional adjuvants, rainbow trout preferentially responded to a model TI antigen such as TNP-LPS (2,4,6-trinitrophenyl hapten conjugated to lipopolysaccharide). The anal administration of TNP-LPS elicited TNP-specific serum antibodies, and a significant increase in the number of total and TNP-specific ASCs in both spleen and kidney, being the kidney the site where most ASCs are found at later time points. In the spleen, a proliferative response of both IgM+ B and T cells was also clearly visible, while the proliferative response was weaker in the kidney. Finally, TNP-LPS also provoked a transcriptional regulation of some immune genes in the spleen and the intestine, including a decreased transcription of foxp3a and foxp3b in intestine that suggests a breach in tolerogenic responses in response to TI stimulation. These results contribute to a better understanding of how intestinal immunity is regulated in teleost and will aid in the future design of effective oral strategies for aquaculture.

RNA-binding protein Ptbp1 is essential for BCR-mediated antibody production.

The RNA-binding protein polypyrimidine tract-binding protein-1 (Ptbp1) binds to the pyrimidine-rich sequence of target RNA and controls gene expression via post-transcriptional regulation such as alternative splicing. Although Ptbp1 is highly expressed in B lymphocytes, its role to date is largely unknown. To clarify the role of Ptbp1 in B-cell development and function, we generated B-cell-specific Ptbp1-deficient (P1BKO) mice. B-cell development in the bone marrow, spleen and peritoneal cavity of the P1BKO mice was nearly normal. However, the P1BKO mice had significantly lower levels of natural antibodies in serum compared with those of the control mice. To investigate the effect of Ptbp1 deficiency on the immune response in vivo, we immunized the P1BKO mice with T-cell-independent type-2 (TI-2) antigen NP-Ficoll and T-cell-dependent (TD) antigen NP-CGG. We found that B-cell-specific Ptbp1 deficiency causes an immunodeficiency phenotype due to defective production of antibody against both TI-2 and TD antigen. This immunodeficiency was accompanied by impaired B-cell receptor (BCR)-mediated B-cell activation and plasmablast generation. These findings demonstrate that Ptbp1 is essential for the humoral immune response.

Influence of CD4-1+, CD4-2+ and CD8+ T lymphocytes subpopulations on the immune response of B lymphocytes in flounder (Paralichthys olivaceus) immunized with thymus-dependent or thymus-independent antigen.

In order to elucidate the influence of T lymphocytes subpopulations on B lymphocytes immune response, in this paper, CD4-1+, CD4-2+, CD8+ T lymphocytes and B lymphocytes responses to thymus-independent (TI) or thymus-dependent (TD) antigen plus immunosuppressant were investigated in flounder (Paralichthys olivaceus). The results showed that in LPS-immunized group, the percentages of CD4-1+, CD4-2+, CD8ß+ T (PCD4-1+ T, PCD4-2+ T and PCD8ß+ T) lymphocytes in peripheral blood leucocytes (PBLs) had no significant variations, the percentages of IgM+ B (PIgM+ B) lymphocytes and LPS-specific antibodies (LA) significantly increased and peaked at 3rd or 4th week post-injection; CsA had no inhibition on both T/B lymphocytes and LA; RaPa only suppressed the PIgM+ B lymphocytes and LA, and the inhibition maximum (Imax) were about 35% and 20%, respectively. In KLH-immunized group, the PCD4-1+, PCD4-2+ and PCD8ß+ T lymphocytes significantly increased and peaked at 3rd or 5th day, successively the PIgM+ B lymphocytes and KLH-specific antibodies (KA) significantly increased to the peak at 5th week; the PCD4-1+, PCD4-2+ T and PIgM+ B lymphocytes and LA were inhibited significantly by both CsA and RaPa, and the Imax on them were 13%-33%, 11%-25%, 19%-34%, 22%-26%, respectively, while the PCD8ß+ T lymphocytes showed no significant suppression. The results indicated that the suppression of PIgM+ B lymphocytes in KLH + CsA group was not directly derived from CsA, but due to the suppression of T lymphocytes, especially CD4+ T lymphocytes subpopulations. The results showed for the first time that, similar to higher vertebrates, T lymphocytes didn't respond to TI antigen, moreover, T lymphocyte subpopulations had a regulation on the immune response of B lymphocyte for TD antigen in flounder.

Characterization of Thymus-dependent and Thymus-independent Immunoglobulin Isotype Responses in Mice Using Enzyme-linked Immunosorbent Assay.

Antibodies, also termed as immunoglobulins (Ig), secreted by differentiated B lymphocytes, plasmablasts/plasma cells, in humoral immunity provide a formidable defense against invading pathogens via diverse mechanisms. One major goal of vaccination is to induce protective antigen-specific antibodies to prevent life-threatening infections. Both thymus-dependent (TD) and thymus-independent (TI) antigens can elicit robust antigen-specific IgM responses and can also induce the production of isotype-switched antibodies (IgG, IgA and IgE) as well as the generation of memory B cells with the help provided by antigen presenting cells (APCs). Here, we describe a protocol to characterize TD and TI Ig isotype responses in mice using enzyme-linked immunosorbent assay (ELISA). In this protocol, TD and TI Ig responses are elicited in mice by intraperitoneal (i.p.) immunization with hapten-conjugated model antigens TNP-KLH (in alum) and TNP-polysaccharide (in PBS), respectively. To induce TD memory response, a booster immunization of TNP-KLH in alum is given at 3 weeks after the first immunization with the same antigen/adjuvant. Mouse sera are harvested at different time points before and after immunization. Total serum Ig levels and TNP-specific antibodies are subsequently quantified using Ig isotype-specific Sandwich and indirect ELISA, respectively. In order to correctly quantify the serum concentration of each Ig isotype, the samples need to be appropriately diluted to fit within the linear range of the standard curves. Using this protocol, we have consistently obtained reliable results with high specificity and sensitivity. When used in combination with other complementary methods such as flow cytometry, in vitro culture of splenic B cells and immunohistochemical staining (IHC), this protocol will allow researchers to gain a comprehensive understanding of antibody responses in a given experimental setting.

Progressive Upregulation of Oxidative Metabolism Facilitates Plasmablast Differentiation to a T-Independent Antigen.

Transitioning from a metabolically quiescent naive B cell to an antibody-secreting plasmablast requires division-dependent cellular differentiation. Though cell division demands significant ATP and metabolites, the metabolic processes used for ATP synthesis during plasmablast formation are not well described. Here, the metabolic requirements for plasmablast formation were determined. Following T-independent stimulation with lipopolysaccharide, B cells increased expression of the oxidative phosphorylation machinery in a stepwise manner. Such activated B cells have increased capacity to perform oxidative phosphorylation but showed dependency on glycolysis. Plasmablasts displayed higher oxidative metabolism to support antibody secretion, as inhibiting oxidative ATP production resulted in decreased antibody titers. Differentiation by Blimp1 was required for this increase in oxidative metabolism, as Blimp1-deficient cells proliferate but do not upregulate oxidative phosphorylation. Together, these findings identify a shift in metabolic pathways as B cells differentiate, as well as the requirement for increased metabolic potential to support antibody production.

CD22 Promotes B-1b Cell Responses to T Cell-Independent Type 2 Antigens.

CD22 (Siglec-2) is a critical regulator of B cell activation and survival. CD22-/- mice generate significantly impaired Ab responses to T cell-independent type 2 (TI-2) Ags, including haptenated Ficoll and pneumococcal polysaccharides, Ags that elicit poor T cell help and activate BCR signaling via multivalent epitope crosslinking. This has been proposed to be due to impaired marginal zone (MZ) B cell development/maintenance in CD22-/- mice. However, mice expressing a mutant form of CD22 unable to bind sialic acid ligands generated normal TI-2 Ab responses, despite significantly reduced MZ B cells. Moreover, mice treated with CD22 ligand-binding blocking mAbs, which deplete MZ B cells, had little effect on TI-2 Ab responses. We therefore investigated the effects of CD22 deficiency on B-1b cells, an innate-like B cell population that plays a key role in TI-2 Ab responses. B-1b cells from CD22-/- mice had impaired BCR-induced proliferation and significantly increased intracellular Ca2+ concentration responses following BCR crosslinking. Ag-specific B-1b cell expansion and plasmablast differentiation following TI-2 Ag immunization was significantly impaired in CD22-/- mice, consistent with reduced TI-2 Ab responses. We generated CD22-/- mice with reduced CD19 levels (CD22-/-CD19+/-) to test the hypothesis that augmented B-1b cell BCR signaling in CD22-/- mice contributes to impaired TI-2 Ab responses. BCR-induced proliferation and intracellular Ca2+ concentration responses were normalized in CD22-/-CD19+/- B-1b cells. Consistent with this, TI-2 Ag-specific B-1b cell expansion, plasmablast differentiation, survival, and Ab responses were rescued in CD22-/-CD19+/- mice. Thus, CD22 plays a critical role in regulating TI-2 Ab responses through regulating B-1b cell signaling thresholds.

Altered Marginal Zone B Cell Selection in the Absence of IκBNS.

Marginal zone (MZ) B cells reside in the splenic MZ and play important roles in T cell-independent humoral immune responses against blood-borne pathogens. IκBNS-deficient bumble mice exhibit a severe reduction in the MZ B compartment but regain an MZ B population with age and, thus, represent a valuable model to examine the biology of MZ B cells. In this article, we characterized the MZ B cell defect in further detail and investigated the nature of the B cells that appear in the MZ of aged bumble mice. Flow cytometry analysis of the splenic transitional B cell subsets demonstrated that MZ B cell development was blocked at the transitional-1 to transitional-2-MZ precursor stage in the absence of functional IκBNS. Immunohistochemical analysis of spleen sections from wild-type and bumble mice revealed no alteration in the cellular MZ microenvironment, and analysis of bone marrow chimeras indicated that the MZ B cell development defect in bumble mice was B cell intrinsic. Further, we demonstrate that the B cells that repopulate the MZ in aged bumble mice were distinct from age-matched wild-type MZ B cells. Specifically, the expression of surface markers characteristic for MZ B cells was altered and the L chain Igλ+ repertoire was reduced in bumble mice. Finally, plasma cell differentiation of sorted LPS-stimulated MZ B cells was impaired, and aged bumble mice were unable to respond to NP-Ficoll immunization. These results demonstrate that IκBNS is required for an intact MZ B cell compartment in C57BL/6 mice.

Reactivity of IgM antibodies elicited by PEGylated liposomes or PEGylated lipoplexes against auto and foreign antigens.

Polyethylene glycol (PEG) is an attractive tool for the development of nanoparticle-based cancer therapy since it endows nanoparticles with extended-circulation properties. Nevertheless, recent reports have revealed that intravenous injection of either PEGylated liposomes (SLs) or PEGylated lipoplex (PLpx) could elicit an anti-PEG immunoglobulin (IgM) response in a T cell-independent (TI) manner that would substantially compromise the in vivo fate of PEGylated products upon repeated administration. In the same context, viral or bacterial infections trigger the production of polyreactive IgM that binds both self and foreign antigens. The polyreactivity of IgM elicited by SLs or PLpx, to bacteria and other polymers, however, is yet to be elucidated. In this study, the polyreactivity of IgM elicited by SLs or PLpx was challenged against different bacteria (TI antigens) and against synthetic polymer composed of repetitive structures (PVP-360 or FITC-dextran). Results demonstrated that anti-PEG IgM elicited by either SLs or PLpx showed no reactivity to various bacteria examined, while the IgM showed remarkable reactivity to both PVP-360 and FITC-dextran. In addition, interestingly, anti-PEG IgM elicited by either SLs or PLpx showed no antinuclear antibody-like immune reactivity, and, therefore, treatment with either SLs or PLpx was not expected to exacerbate autoimmune diseases such as systemic lupus erythematosus. Collectively, our findings could provide information supporting the safety of PEGylated nanoparticle-based pharmaceutics, particularly in patients with autoimmune diseases.

Ageing adversely affects the migration and function of marginal zone B cells.

Marginal zone (MZ) B cells are positioned within the spleen to capture blood-borne antigen and immune complexes and deliver them to follicular dendritic cells in the B-cell follicles. We show that within the spleens of aged mice antigen capture by MZ B cells, and their ability to shuttle between the follicle and MZ, were impaired. The ability of aged MZ B cells to migrate towards the MZ chemoattractant sphingosine-1-phosphate was increased, suggesting that aged MZ B cells had a greater propensity to be retained within the MZ. An extrinsic impairment in aged B-cell migration towards the MZ was demonstrated using bone marrow chimeras. The follicular shuttling of MZ B cells derived from either young or aged bone marrow was similarly reduced in aged recipient spleens, showing that ageing effects on splenic stromal cells were responsible for the impaired follicular shuttling of MZ B cells. MZ B cells rapidly mount T-cell-independent (TI) antibody-responses to microbial polysaccharide antigen. In aged mice the ability to produce immunoglobulins in response to the TI type 1 antigen TNP-LPS was impaired. These ageing-related changes to the MZ and MZ B cells have implications for the clearance of blood-borne pathogens. Indeed elderly people have increased susceptibility to Streptococcus pneumoniae, a TI antigen, and decreased responses to vaccination. A thorough analysis of the mechanisms that underpin the ageing-related decline in the status of the MZ and MZ B cells will help the design of novel treatments to improve immunity in the elderly.

Breaking Hepatitis B Virus Tolerance and Inducing Protective Immunity Based on Mimicking T Cell-Independent Antigen.

There are over 350 million chronic carriers of hepatitis B virus (HBV) in the world, of whom about a third eventually develop severe HBV-related complications. HBV contributes to liver cirrhosis and hepatocellular carcinoma development. Remarkable progress has been made in selective inhibition of HBV replication by nucleoside analogs. However, how to generate protective antibody of HBsAb in HBV-infected patients after HBV-DNA becomes negative still remains a challenge for scientists. In this study, we show that OmpC-HBsAg 'a' epitope chimeric protein vaccine can break HBV tolerance and induce protective immunity in HBV transgenic mice based on mimicking T cell-independent antigen to bypass T cells from the adaptive immune system. The antibodies induced by the vaccine have the ability to prevent HBV virion infection of human hepatocytes.

B Cell-Intrinsic IDO1 Regulates Humoral Immunity to T Cell-Independent Antigens.

Humoral responses to nonproteinaceous Ags (i.e., T cell independent [TI]) are a key component of the early response to bacterial and viral infection and a critical driver of systemic autoimmunity. However, mechanisms that regulate TI humoral immunity are poorly defined. In this study, we report that B cell-intrinsic induction of the tryptophan-catabolizing enzyme IDO1 is a key mechanism limiting TI Ab responses. When Ido1(-/-) mice were immunized with TI Ags, there was a significant increase in Ab titers and formation of extrafollicular Ab-secreting cells compared with controls. This effect was specific to TI Ags, as Ido1 disruption did not affect Ig production after immunization with protein Ags. The effect of IDO1 abrogation was confined to the B cell compartment, as adoptive transfer of Ido1(-/-) B cells to B cell-deficient mice was sufficient to replicate increased TI responses observed in Ido1(-/-) mice. Moreover, in vitro activation with TLR ligands or BCR crosslinking rapidly induced Ido1 expression and activity in purified B cells, and Ido1(-/-) B cells displayed enhanced proliferation and cell survival associated with increased Ig and cytokine production compared with wild-type B cells. Thus, our results demonstrate a novel, B cell-intrinsic, role for IDO1 as a regulator of humoral immunity that has implications for both vaccine design and prevention of autoimmunity.

Anti-CD83 promotes IgG1 isotype switch in marginal zone B cells in response to TI-2 antigen.

CD83 is a transmembrane glycoprotein that is rapidly up-regulated on activated B cells. Although CD83 itself is incapable to transduce intracellular signaling, it acts as a negative regulator of B cell function. We have recently described that a single application of anti-CD83 antibody results in dramatically enhanced production of antigen-specific IgG1 but not other isotypes upon immunization of mice with the TI-2 model antigen (Ag) NIP-Ficoll. This effect was mediated by the binding of anti-CD83 to CD83 on the surface of B cells themselves. In the current study we show that administration of anti-CD83 enhances IgG1-production independent of IL-4. Application of anti-CD83 does not alter the proliferation and general expansion of NIP-specific B cells. In the presence of anti-CD83, immunized mice develop normal frequencies of plasmablasts in response to NIP-Ficoll of which an increased number produces IgG1. These cells localize in extrafollicular foci in the spleen of immunized mice and originate from the marginal zone B cell pool. Taken together, our results indicate that CD83 engagement in vivo does not generally enhance B cell activation but selectively promotes IgG1 class switch in marginal zone B cells in response to TI-2 Ag.

MAVS, cGAS, and endogenous retroviruses in T-independent B cell responses.

Multivalent molecules with repetitive structures including bacterial capsular polysaccharides and viral capsids elicit antibody responses through B cell receptor (BCR) crosslinking in the absence of T cell help. We report that immunization with these T cell-independent type 2 (TI-2) antigens causes up-regulation of endogenous retrovirus (ERV) RNAs in antigen-specific mouse B cells. These RNAs are detected via a mitochondrial antiviral signaling protein (MAVS)-dependent RNA sensing pathway or reverse-transcribed and detected via the cGAS-cGAMP-STING pathway, triggering a second, sustained wave of signaling that promotes specific immunoglobulin M production. Deficiency of both MAVS and cGAS, or treatment of MAVS-deficient mice with reverse transcriptase inhibitors, dramatically inhibits TI-2 antibody responses. These findings suggest that ERV and two innate sensing pathways that detect them are integral components of the TI-2 B cell signaling apparatus.