Splenic IgM Research Articles

Immunomodulatory properties of Bacillus subtilis extracellular vesicles on rainbow trout intestinal cells and splenic leukocytes.

Extracellular vesicles (EVs) are cell-derived membrane-surrounded vesicles that carry bioactive molecules. Among EVs, outer membrane vesicles (OMVs), specifically produced by Gram-negative bacteria, have been extensively characterized and their potential as vaccines, adjuvants or immunotherapeutic agents, broadly explored in mammals. Nonetheless, Gram-positive bacteria can also produce bilayered spherical structures from 20 to 400 nm involved in pathogenesis, antibiotic resistance, nutrient uptake and nucleic acid transfer. However, information regarding their immunomodulatory potential is very scarce, both in mammals and fish. In the current study, we have produced EVs from the Gram-positive probiotic Bacillus subtilis and evaluated their immunomodulatory capacities using a rainbow trout intestinal epithelial cell line (RTgutGC) and splenic leukocytes. B. subtilis EVs significantly up-regulated the transcription of several pro-inflammatory and antimicrobial genes in both RTgutGC cells and splenocytes, while also up-regulating many genes associated with B cell differentiation in the later. In concordance, B. subtilis EVs increased the number of IgM-secreting cells in splenocyte cultures, while at the same time increased the MHC II surface levels and antigen-processing capacities of splenic IgM+ B cells. Interestingly, some of these experiments were repeated comparing the effects of B. subtilis EVs to EVs obtained from another Bacillus species, Bacillus megaterium, identifying important differences. The data presented provides evidence of the immunomodulatory capacities of Gram-positive EVs, pointing to the potential of B. subtilis EVs as adjuvants or immunostimulants for aquaculture.

The Structural Characterization and Immunomodulatory Activity of Polysaccharides from Pleurotus abieticola Fruiting Bodies.

Polysaccharides obtained from mushrooms have been reported to possess immunomodulatory properties. In this study, a water-soluble polysaccharide was purified from the fruiting bodies of Pleurotus abieticola, entitled PAPS1. After its composition and structural analysis, the immunomodulatory activity was investigated in immunosuppressed mice induced by cyclophosphamide (CTX) at a dosage of 70 mg/kg by intraperitoneal injection for 7 days. After 28 days of intragastric administration, PAPS1 alleviated cyclophosphamide (CTX)-induced histopathological damage and increased the expressions of splenic CD4, CD8, CD56 and IgM in the serums of immunosuppressed mice. PAPS1 suppressed the oxidative stress indicated by preventing the increases in ROS and MDA levels. According to the intestinal microflora analysis, PAPS1 regulated 11 bacteria at the gene level, including Helicobacter and Paraprevotella, which are related to immunity and oxidative capacity. Compared with CTX-treated mice, significant increases in immune-related cytokines, such as interleukin (IL)-2, IL-6 and IL-12 in the serums of mice treated with PAPS1, were observed. Finally, PAPS1 can strongly increase the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream proteins. In conclusion, PAPS1-boosted immunity may be related to its suppression on oxidative stress via enhancing the activity of Nrf2 signaling. Thus, PAPS1 can be investigated as a candidate for immunomodulatory therapy.

Myosin 18A is a novel checkpoint regulator in B cell differentiation and antibody-mediated immunity

Abstract The B cell antibody response is tightly regulated to facilitate pathogen-specific immunity and prevent self-reactivity, with actin cytoskeleton dynamics playing an important role in regulating B cell activation. The newly discovered unconventional myosin family protein Myosin 18A (Myo18A) is most closely related to Myo2A, and regulates important cellular processes in non-lymphoid cells. Myo18A is expressed in both precursor and mature B cells, and interacts with ezrin, Myo2A and tyrosine phosphorylated proteins suggesting that it may regulate physiological functions of B cells. We investigated the function of Myo18A in antibody-mediated immunity by generating B cell-conditional Myo18A-deficient mice. Basally, Myo18A deficiency led to expansion of both bone marrow progenitor B cells, and mature B cells in secondary lymphoid organs. Myo18A-deficient mice displayed serum IgM hyperglobulinemia with greater levels of splenic IgM secreting cells, with older mice switching to IgG1 hyperglobulinemia and autoantibody development. Immunization of Myo18A-deficient mice with inactivated influenza virus led to development of more potent neutralizing antibodies against the major antigen hemagglutinin, associated with early expansion and persistent accumulation of antigen-specific germinal center B cells. In vitro stimulation with TLR7 and BCR ligands revealed a greater ability of Myo18A-deficient B cells to differentiate into antibody secreting cells, and induce Blimp-1 expression. Overall, our study demonstrates that Myo18A is a novel negative regulator of B cell homeostasis, differentiation, self-tolerance and humoral immunity. Supported by an NIH grant (R21 AI117350) to Neetu Gupta

Plasmablasts induced by chitosan oligosaccharide secrete natural IgM to enhance the humoral immunity in grass carp

Chitosan oligosaccharide (COS) is an attractive immunopotentiator capable of driving humoral immunity in vertebrates, but its cellular and molecular mechanisms still require elucidation. In this study, COS induced the proliferation and differentiation of splenic IgM+ B cells into IgMlo and IgMhi B cell subsets in grass carp (Ctenopharyngodon idella). The IgMlo B cells were further identified as short-lived plasmablasts that secreted natural IgM with binding-abilities to lipopolysaccharide (LPS) and peptidoglycan (PGN). Moreover, the mannose receptor (MR) and integrins were discovered and identified as the binding-receptors of COS on IgMlo plasmablasts. The MR synergized with integrins to trigger intracellular signal transduction to boost plasmablast generation and expansion. Notably, IgMlo plasmablasts originally generated in spleen but they migrated into blood to secrete natural IgM, which augmented the serum bactericidal activity. Taken together, this study revealed the cellular and molecular mechanisms of COS-triggered humoral immunity in fish.

Single-cell transcriptome profiling and the use of AID deficient mice reveal that B cell activation combined with antibody class switch recombination and somatic hypermutation do not benefit the control of experimental trypanosomosis.

Salivarian trypanosomes are extracellular protozoan parasites causing infections in a wide range of mammalian hosts, with Trypanosoma evansi having the widest geographic distribution, reaching territories far outside Africa and occasionally even Europe. Besides causing the animal diseases, T. evansi can cause atypical Human Trypanosomosis. The success of this parasite is attributed to its capacity to evade and disable the mammalian defense response. To unravel the latter, we applied here for the first time a scRNA-seq analysis on splenocytes from trypanosome infected mice, at two time points during infection, i.e. just after control of the first parasitemia peak (day 14) and a late chronic time point during infection (day 42). This analysis was combined with flow cytometry and ELISA, revealing that T. evansi induces prompt activation of splenic IgM+CD1d+ Marginal Zone and IgMIntIgD+ Follicular B cells, coinciding with an increase in plasma IgG2c Ab levels. Despite the absence of follicles, a rapid accumulation of Aicda+ GC-like B cells followed first parasitemia peak clearance, accompanied by the occurrence of Xbp1+ expressing CD138+ plasma B cells and Tbx21+ atypical CD11c+ memory B cells. Ablation of immature CD93+ bone marrow and Vpreb3+Ly6d+Ighm+ expressing transitional spleen B cells prevented mature peripheral B cell replenishment. Interestingly, AID-/- mice that lack the capacity to mount anti-parasite IgG responses, exhibited a superior defense level against T. evansi infections. Here, elevated natural IgMs were able to exert in vivo and in vitro trypanocidal activity. Hence, we conclude that in immune competent mice, trypanosomosis associated B cell activation and switched IgG production is rapidly induced by T. evansi, facilitating an escape from the detrimental natural IgM killing activity, and resulting in increased host susceptibility. This unique role of IgM and its anti-trypanosome activity are discussed in the context of the dilemma this causes for the future development of anti-trypanosome vaccines.

High-dimensional single cell mass cytometry analysis of the murine hematopoietic system reveals signatures induced by ageing and physiological pathogen challenges

BackgroundImmune ageing is a result of repetitive microbial challenges along with cell intrinsic or systemic changes occurring during ageing. Mice under ‘specific-pathogen-free’ (SPF) conditions are frequently used to assess immune ageing in long-term experiments. However, physiological pathogenic challenges are reduced in SPF mice. The question arises to what extent murine experiments performed under SPF conditions are suited to analyze immune ageing in mice and serve as models for human immune ageing. Our previous comparisons of same aged mice with different microbial exposures, unambiguously identified distinct clusters of immune cells characteristic for numerous previous pathogen encounters in particular in pet shop mice.ResultsWe here performed single cell mass cytometry assessing splenic as secondary and bone marrow as primary lymphoid organ-derived leukocytes isolated from young versus aged SPF mice in order to delineate alterations of the murine hematopoietic system induced during ageing. We then compared immune clusters from young and aged SPF mice to pet shop mice in order to delineate alterations of the murine hematopoietic system induced by physiological pathogenic challenges and those caused by cell intrinsic or systemic changes during ageing. Notably, distinct immune signatures were similarly altered in both pet shop and aged SPF mice in comparison to young SPF mice, including increased frequencies of memory T lymphocytes, effector-cytokine producing T cells, plasma cells and mature NK cells. However, elevated frequencies of CD4+ T cells, total NK cells, granulocytes, pDCs, cDCs and decreased frequencies of naïve B cells were specifically identified only in pet shop mice. In aged SPF mice specifically the frequencies of splenic IgM+ plasma cells, CD8+ T cells and CD4+ CD25+ Treg were increased as compared to pet shop mice and young mice.ConclusionsOur study dissects firstly how ageing impacts both innate and adaptive immune cells in primary and secondary lymphoid organs. Secondly, it partly distinguishes murine intrinsic immune ageing alterations from those induced by physiological pathogen challenges highlighting the importance of designing mouse models for their use in preclinical research including vaccines and immunotherapies.

Aeromonas salmonicida activates rainbow trout IgM+ B cells signalling through Toll like receptors

As B cells are singularly equipped with a B cell receptor (BCR) and a range of innate receptors, they are able to integrate both antigen-specific and innate signals, with the latter being essential to reach an adequate level of activation. Whether teleost B cells sense pathogens through innate mechanisms has not yet been explored, despite the fact that fish B cells display a wider array of innate receptors than many mammalian B cell subsets. Hence, in the current study, we have investigated the effects of inactivated Aeromonas salmonicida, a Gram negative rainbow trout pathogen, on trout splenic IgM+ B cells in vitro in the presence or absence of different inhibitors of Toll-like receptor (TLR) signalling, to establish to what degree innate signals are contributing to the activation of B cells in teleosts. Our results demonstrate that most of the effects that A. salmonicida exerts on trout IgM+ B cells are significantly blocked in the presence of inhibitors of MyD88 and TRIF, important nodes in TLR signal pathways. Thus, the data presented demonstrates that, also in teleost, TLR signalling is essential for the activation of IgM+ B cells. These results will be useful for the future optimization of novel vaccines and adjuvants.

Repertoire characterization and validation of gB-specific human IgGs directly cloned from humanized mice vaccinated with dendritic cells and protected against HCMV.

Human cytomegalovirus (HCMV) causes serious complications to immune compromised hosts. Dendritic cells (iDCgB) expressing granulocyte-macrophage colony-stimulating factor, interferon-alpha and HCMV-gB were developed to promote de novo antiviral adaptive responses. Mice reconstituted with a human immune system (HIS) were immunized with iDCgB and challenged with HCMV, resulting into 93% protection. Immunization stimulated the expansion of functional effector memory CD8+ and CD4+ T cells recognizing gB. Machine learning analyses confirmed bone marrow T/CD4+, liver B/IgA+ and spleen B/IgG+ cells as predictive biomarkers of immunization (≈87% accuracy). CD8+ and CD4+ T cell responses against gB were validated. Splenic gB-binding IgM-/IgG+ B cells were sorted and analyzed at a single cell level. iDCgB immunizations elicited human-like IgG responses with a broad usage of various IgG heavy chain V gene segments harboring variable levels of somatic hypermutation. From this search, two gB-binding human monoclonal IgGs were generated that neutralized HCMV infection in vitro. Passive immunization with these antibodies provided proof-of-concept evidence of protection against HCMV infection. This HIS/HCMV in vivo model system supported the validation of novel active and passive immune therapies for future clinical translation.

Distinct modes of action of CD40L and adaptive cytokines IL-2, IL-4/13, IL-10 and IL-21 on rainbow trout IgM+ B cells

In mammals, conventional B (B2) cells are activated within lymphoid follicles through a close relationship with T follicular helper (Tfh) cells. The interaction between CD40 expressed on B cells and its ligand (CD40L) expressed on Tfh cells is a key signal that regulates the formation of germinal centers (GCs), B cell survival, proliferation and differentiation to plasma cells (PCs) or memory cells. Additionally, certain soluble cytokines produced by T cells also strongly condition the outcome of this interaction. Despite the many differences found between fish B cells and mammalian B2 cells, and the lack of conventional GCs, rainbow trout IgM+ B cells have been shown to be stimulated by CD40L, however, whether cytokines commonly produced by T cells can further modulate this response has never been addressed to date. Thus, in this study, we determined the effects of recombinant rainbow trout adaptive cytokines interleukin 2B (IL-2B), IL-4/13A, IL-4/13B, IL-10 and IL-21 (cytokines known to activate B cells in mammals) on splenic IgM+ B cells alone or in combination with CD40L. We studied how these cytokines and CD40L cooperated to promote IgM+ B cell survival, proliferation and IgM secretion. The results obtained provide valuable information for the first time in teleost fish on how different T cell signals cooperate to activate B cells in the absence of GCs.

IDENTIFICATION OF PRIMORDIAL ORGANIZED LYMPHOID STRUCTURE IN THE SPLEEN OF TELEOST FISH

Abstract Induction of adaptive immune responses in higher vertebrate species occur within organized lymphoid structures (e.g. lymph nodes, Peyer’s patches). It has been proposed that such structures emerged throughout evolutionary time with the goal to maximize encounters between antigens, antigens-presenting cells and B-T lymphocytes. Fish lack such structures and thus, it remains unknown how and where antigen-specific immunoglobulin responses are induced in these species. To understand how systemic immune responses are induced in teleost lymphoid organs, Rainbow Trout were immunized with several soluble protein antigens. Overall, our results identified the spleen as the major site for CD4+ T and IgM+ B cell proliferation in systemic lymphoid organs upon immunization. The proliferating splenic IgM+ B cells were frequently observed as clusters in the vicinity of melano-macrophage centers. Moreover, in these areas we observed aggregates of B and T lymphocytes with a loose organized structure reminiscent of the cellular architecture frequently associated with tertiary lymphoid organs. Laser dissection microdissection of these areas enabled us to evaluating the immunoglobulin IgM repertoires within these structures upon immunization. Critically, repertoire analysis identified processes of antigen-specific B cell clonal expansion. In conclusion, these data points to the previously unrecognized existence of primordial semi-organized lymphoid tissue in the spleen of teleost fish in which adaptive IgM immune responses are induced. Our findings provide strong evidence that the induction of antigen-specific immune responses in all bony vertebrates requires the formation of organized or semi-organized lymphoid structures.

PF462 SPECIAL AT‐RICH SEQUENCE BINDING PROTEIN 1 (SATB1) IS MARKEDLY EXPRESSED IN THE DARK ZONE GERMINAL CENTER B CELLS ON MURINE SPLEEN

Background:Special AT‐rich Sequence Binding Protein 1 (SATB1) is known as a protein that binds to the AT‐rich region near the enhancer of various genes. SATB1 is expressed predominantly in the thymus, and controls T lineage‐specific gene transcription by organizing chromatin configurations in the cell nucleus. In addition, we previously reported that SATB1 plays important roles in the early differentiation of hematopoietic stem cells (HSCs) (Immunity, 2013; Cell Reports, 2018). Indeed, exogenous induction of SATB1 in HSCs enhances both T and B lymphopoietic potentials, whereas SATB1 deficiency skews HSC differentiation toward myeloid lineage at the expense of lymphocyte production. Furthermore, SATB1 also associates with the enhancer of immunoglobulin heavy chain gene. Thus, SATB1 plays a pivotal role in various aspects of lymphopoiesis, but its significance in the B lineage development has yet to be determined.Aims:We performed this experiment to know the role of SATB1 in the B lineage development.Methods:We used SATB1/Tomato reporter mice to monitor the precise SATB1 expression during B cell maturation in vivo. We also generated B cell‐specific SATB1 conditional knock out mice using Cre‐loxP‐mediated inactivation of SATB1. We crossed SATB1‐flox mice with mice expressing Cre recombinase under the control of the Mb1 gene promotor.Results:At first, we analyzed the B lineage development of SATB1/Tomato reporter mice. In bone marrow (BM), while markedly increasing with early lymphoid differentiation, SATB1/Tomato expression decreased in pro‐B and pre‐B stages and almost diminished in surface IgM+ immature B cells. However, SATB1/Tomato expression was revived after immature B cells migrated to the spleen. The expression of SATB1/Tomato increased with the maturation from T1 to T3 transitional splenic B cells. The up‐regulation of SATB1/Tomato in developing B cells positively correlated with IgD expression. Indeed, while transcripts for SATB1 gene were barely detected in the splenic IgM+IgD− stage, they were actively induced in IgM+IgD+ cells and became abundant in IgM−IgD+ B cells. When we incubated murine splenic B cells with an anti‐IgM antibody in vitro, SATB1/Tomato+ IgD+ population was significantly induced. Incubation of murine splenic B cells with anti‐IgM and anti‐CD40 also induced the same population. These results suggest that B cell receptor signaling induces SATB1 expression along with the transition of IgM to IgD. The acquisition of antigens by naive B cells triggers the formation of germinal centers (GCs). The mature GC is divided into two compartments, called the light zone (LZ) and the dark zone (DZ). The naïve B cells located in LZ receive the positive selection of higher‐affinity B cells by CD4+ helper T cells and migrate to DZ by expressing the DZ‐homing receptor CXCR4. The B cells in DZ are highly proliferative. The analysis of SATB1/Tomato reporter mice showed that SATB1+ IgD+ splenic B cells had a significantly higher expression of CXCR4 than the SATB1− IgD+ cells. In addition, CD83, the activation marker of LZ B cells, decreased in IgD+ B cells with increased expression of SATB1. The above results imply that the SATB1 protein might be involved in the modulation of Ig heavy chain and the formation of DZ in GC during splenic B cell maturation. To test this hypothesis, we analyzed Mb1‐Cre:SATB1 Flox/Flox mice. The histological analysis showed that the follicles of the spleen in Mb1‐Cre: SATB1 Flox/Flox mice are enlarged compared to those in control SATB1 Flox/Flox mice.Summary/Conclusion:With these results, we conclude that SATB1 plays some role in the formation of GCs.

Participation of the spleen in the IgA immune response in the gut.

The role of the spleen in the induction of an immune response to orally administered antigens is still under discussion. Although it is well known that after oral antigen administration specific germinal centres are not only formed in the Peyers patches (PP) and the mesenteric lymph nodes (mLN) but also in the spleen, there is still a lack of functional data showing a direct involvement of splenic B cells in an IgA immune response in the gut. In addition, after removal of mLN a high level of IgA+ B cells was observed in the gut. Therefore, in this study we analysed the role of the spleen in the induction of IgA+ B cells in the gut after mice were orally challenged with antigens. Here we have shown that antigen specific splenic IgM+ B cells after in vitro antigen stimulation as well as oral immunisation of donor mice were able to migrate into the gut of recipient mice, where they predominantly switch to IgA+ plasma cells. Furthermore, stimulation of recipient mice by orally administered antigens enhanced the migration of the splenic B cells into the gut as well as their switch to IgA+ plasma cells. Removal of the mLN led to a higher activation level of the splenic B cells. Altogether, our results imply that splenic IgM+ B cells migrate in the intestinal lamina propria, where they differentiate into IgA+ plasma cells and subsequently proliferate. In conclusion, we demonstrated that the spleen plays a major role in the gut immune response serving as a reservoir of immune cells that migrate to the site of antigen entrance.

A splenic IgM memory subset with antibacterial specificities is sustained from persistent mucosal responses.

To what extent immune responses against the gut flora are compartmentalized within mucosal tissues in homeostatic conditions remains a much-debated issue. We describe here, based on an inducible AID fate-mapping mouse model, that systemic memory B cell subsets, including mainly IgM+ B cells in spleen, together with IgA+ plasma cells in spleen and bone marrow, are generated in mice in the absence of deliberate immunization. While the IgA component appears dependent on the gut flora, IgM memory B cells are still generated in germ-free mice, albeit to a reduced extent. Clonal relationships and renewal kinetics after anti-CD20 treatment reveal that this long-lasting splenic population is mainly sustained by output of B cell clones persisting in mucosal germinal centers. IgM-secreting hybridomas established from splenic IgM memory B cells showed reactivity against various bacterial isolates and endogenous retroviruses. Ongoing activation of B cells in gut-associated lymphoid tissues thus generates a diversified systemic compartment showing long-lasting clonal persistence and protective capacity against systemic bacterial infections.

Profiling Atlantic salmon B cell populations: CpG-mediated TLR-ligation enhances IgM secretion and modulates immune gene expression

While TLR-activated pathways are key regulators of B cell responses in mammals, their impact on teleost B cells are scarcely addressed. Here, the potential of Atlantic salmon B cells to respond to TLR ligands was shown by demonstrating a constitutive expression of nucleic-acid sensing TLRs in magnetic sorted IgM+ cells. Of the two receptors recognizing CpG in teleosts, tlr9 was the dominating receptor with over ten-fold higher expression than tlr21. Upon CpG-stimulation, IgM secretion increased for head kidney (HK) and splenic IgM+ cells, while blood B cells were marginally affected. The results suggest that CpG directly affects salmon B cells to differentiate into antibody secreting cells (ASCs). IgM secretion was also detected in the non-treated controls, again with the highest levels in the HK derived population, signifying that persisting ASCs are present in this tissue. In all tissues, the IgM+ cells expressed high MHCII levels, suggesting antigen-presenting functions. Upon CpG-treatment the co-stimulatory molecules cd83 and cd40 were upregulated, while cd86 was down-regulated under the same conditions. Finally, ifna1 was upregulated upon CpG-stimulation in all tissues, while a restricted upregulation was evident for ifnb, proposing that salmon IgM+ B cells exhibit a type I IFN-response.

Short-term consumption of n-3 PUFAs increases murine IL-5 levels, but IL-5 is not the mechanistic link between n-3 fatty acids and changes in B-cell populations

N-3 polyunsaturated fatty acids (PUFAs) exert immunomodulatory effects on B cells. We previously demonstrated that n-3 PUFAs enhanced the relative percentage and/or frequency of select B2 cell subsets. The objectives here were to determine if n-3 PUFAs (a) could boost cytokines that target B-cell frequency, (b) enhance the frequency of the B1 population and (c) to identify the mechanism by which n-3 PUFAs modify the proportion of B cells. Administration of n-3 PUFAs as fish oil to C57BL/6 mice enhanced secretion of the Th2 cytokine IL-5 but not IL-9 or IL-13. N-3 PUFAs had no influence on the percentage or frequency of peritoneal B1 or B2 cells. Subsequent experiments with IL-5−/− knockout mice showed n-3 PUFAs decreased the percentage of bone marrow B220loIgMhi cells and increased the proportion and number of splenic IgM+IgDloCD21lo cells compared to the control. These results, when compared with our previous findings with wild-type mice, suggested IL-5 had no role in mediating the effect of n-3 PUFAs on B-cell populations. To confirm this conclusion, we assayed IL-5 secretion in a diet-induced obesity model in which n-3 PUFAs enhanced the frequency of select B-cell subsets. N-3 PUFA supplementation as ethyl esters to obesogenic diets did not alter circulating IL-5 levels. Altogether, the data establish that n-3 PUFAs as fish oil can increase circulating IL-5 in lean mice, which has implications for several disease end points, but this increase in IL-5 is not the mechanistic link between n-3 PUFAs and changes in B-cell populations.

Advax delta inulin adjuvant overcomes immune immaturity in neonatal mice thereby allowing single–dose influenza vaccine protection

Neonates are at high risk for influenza morbidity and mortality due to immune immaturity and lack of priming by prior influenza virus exposure. Inactivated influenza vaccines are ineffective in infants under six months and to provide protection in older children generally require two doses given a month apart. This leaves few options for rapid protection of infants, e.g. during an influenza pandemic. We investigated whether Advax™, a novel polysaccharide adjuvant based on delta inulin microparticles could help overcome neonatal immune hypo-responsiveness. We first tested whether it was possible to use Advax to obtain single-dose vaccine protection of neonatal pups against lethal influenza infection. Inactivated influenza A/H1N1 vaccine (iH1N1) combined with Advax™ adjuvant administered as a single subcutaneous immunization to 7-day-old mouse pups significantly enhanced serum influenza-specific IgM, IgG1, IgG2a and IgG2b levels and was associated with a 3–4 fold increase in the frequency of splenic influenza-specific IgM and IgG antibody secreting cells. Pups immunized with Advax had significantly higher splenocyte influenza-stimulated IFN-γ, IL-2, IL-4, and IL-10 production by CBA and a 3–10 fold higher frequency of IFN-γ, IL-2, IL-4 or IL-17 secreting T cells by ELISPOT. Immunization with iH1N1+Advax induced robust protection of pups against virus challenge 3 weeks later, whereas pups immunized with iH1N1 antigen alone had no protection. Protection by Advax-adjuvanted iH1N1 was dependent on memory B cells rather than memory T cells, with no protection in neonatal μMT mice that are B-cell deficient. Hence, Advax adjuvant overcame neonatal immune hypo-responsiveness and enabled single-dose protection of pups against otherwise lethal influenza infection, thereby supporting ongoing development of Advax™ as a neonatal vaccine adjuvant.

Molecular characterization of CD9 and CD63, two tetraspanin family members expressed in trout B lymphocytes

Tetraspanins are a family of membrane-organizing proteins, characterized by the presence of four highly conserved transmembrane regions that mediate diverse physiological functions. In the current study, we have identified two novel tetraspanin members in rainbow trout (Oncorhynchus mykiss), homologs to mammalian CD9 and CD63. Both genes were expressed in muscle, skin, gills, hindgut, gonad, liver, spleen, head kidney, thymus and peripheral blood leukocytes. Throughout the early life cycle stages, CD9 mRNA levels significantly increased after first feeding, whereas CD63 transcription remained constant during all the developmental stages analyzed. In response to an experimental bath infection with viral hemorrhagic septicemia virus (VHSV), CD9 transcription was down-regulated in the gills, while CD63 mRNA levels were down-regulated in the head kidney. Instead, when the virus was intraperitoneally injected, the transcription of both genes was significantly up-regulated in peritoneal cells at several days post-infection. Additionally, both genes were transcriptionally up-regulated in the muscle of trout injected with a VHSV DNA vaccine. To gain insight on the relation of these tetraspanins with B cell activity we determined their constitutive expression in naive IgM+ populations from different sources and observed that both molecules were being transcribed by IgM+ cells in different tissues. Furthermore, CD9 transcription was significantly down-regulated in splenic IgM+ cells in response to in vitro VHSV exposure. Our results provide insights on the potential role of these tetraspanins on teleost B cell and antiviral immunity.

Developmental propagation of V(D)J recombination-associated DNA breaks and translocations in mature B cells via dicentric chromosomes.

Mature IgM(+) B-cell lymphomas that arise in certain ataxia telangiectasia-mutated (ATM)-deficient compound mutant mice harbor translocations that fuse V(D)J recombination-initiated IgH double-strand breaks (DSBs) on chromosome 12 to sequences downstream of c-myc on chromosome 15, generating dicentric chromosomes and c-myc amplification via a breakage-fusion-bridge mechanism. As V(D)J recombination DSBs occur in developing progenitor B cells in the bone marrow, we sought to elucidate a mechanism by which such DSBs contribute to oncogenic translocations/amplifications in mature B cells. For this purpose, we applied high-throughput genome-wide translocation sequencing to study the fate of introduced c-myc DSBs in splenic IgM(+) B cells stimulated for activation-induced cytidine deaminase (AID)-dependent IgH class switch recombination (CSR). We found frequent translocations of c-myc DSBs to AID-initiated DSBs in IgH switch regions in wild-type and ATM-deficient B cells. However, c-myc also translocated frequently to newly generated DSBs within a 35-Mb region downstream of IgH in ATM-deficient, but not wild-type, CSR-activated B cells. Moreover, we found such DSBs and translocations in activated B cells that did not express AID or undergo CSR. Our findings indicate that ATM deficiency leads to formation of chromosome 12 dicentrics via recombination-activating gene-initiated IgH DSBs in progenitor B cells and that these dicentrics can be propagated developmentally into mature B cells where they generate new DSBs downstream of IgH via breakage-fusion-bridge cycles. We propose that dicentrics formed by joining V(D)J recombination-associated IgH DSBs to DSBs downstream of c-myc in ATM-deficient B lineage cells similarly contribute to c-myc amplification and mature B-cell lymphomas.

Long-lived plasma cells in the spleen (P1472)

Abstract Immunological memory can last a lifetime, in large part due to the antigen-specific antibodies continuously secreted by long-lived plasma cells. Though a great deal of attention has been given to long-lived, IgG-expressing plasma cells within the bone marrow microenvironment, other long-lived plasma cell populations are also found within the secondary lymphoid organs. We sought to characterize the splenic plasma cell populations in comparison to those found in the bone marrow and explore to what extent they contribute to serological memory, hypothesizing that this niche may play a significant role in immune memory. Surprisingly, our initial findings showed a large population of IgM-secreting, long-lived plasma cells dominating the splenic niche in murine models. These plasma cells persisted for the lifetime of the animals. Here we demonstrate how these splenic IgM plasma cells compare to bone marrow resident IgG PCs and explored the factors that contribute to their homing.

The omentum is a site of IgM production during T cell-independent bacterial infection in secondary lymphoid organ-deficient mice (43.6)

Abstract Infection of mice with the bacterium Ehrlichia muris results in a T cell-independent (TI) splenic IgM response, and this antibody is secreted largely by a population of CD11c-expressing B cell plasmablasts in the spleen. However, we found that the TI IgM is maintained in the absence of a spleen; infection of splenectomized mice resulted in a modest decrease in antigen-specific serum IgM, indicating that splenic B cells are not required for the generation of the protective antibody response. To identify the source of the IgM-secreting cells, we infected secondary lymphoid organ (SLO)-deficient mice, which lack both spleen and lymph nodes. SLO-deficient mice were similarly able to produce ehrlichia-specific IgM, and both splenectomized and SLO-deficient mice were protected against challenge infection with a lethal ehrlichial species. These findings led us to investigate the omentum as a source of IgM-secreting cells during infection. We identified the omentum as a large reservoir of antigen-specific IgM-secreting cells; these B cells expressed CD11c, but lacked expression of CD138. CD11c-expressing B cells were elicited by intraperitoneal, but not intravenous infection, revealing a route-dependent response to peritoneal antigen. Our data reveal the omentum as an important site of TI IgM production against peritoneal antigens, in the absence of conventional secondary lymphoid organs.