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.

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.

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.

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.

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.

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.

B-1a transitional cells are phenotypically distinct and are lacking in mice deficient in IκBNS.

B-1 cells mediate early protection against infection by responding to T cell-independent (TI) antigens found on the surface of various pathogens. Mice with impaired expression of the atypical IκB protein IκBNS have markedly reduced frequencies of B-1 cells. We used a mouse strain with dysfunctional IκBNS derived from an N-ethyl-N-nitrosourea (ENU) screen, named bumble, to investigate the point in the development of B-1 cells where IκBNS is required. The presence of wild-type (wt) peritoneal cells in mixed wt/bumble chimeras did not rescue the development of bumble B-1 cells, but wt peritoneal cells transferred to bumble mice restored natural IgM levels and response to TI antigens. The bumble and wt mice displayed similar levels of fetal liver B-1 progenitors and splenic neonatal transitional B (TrB) cells, both of which were previously shown to give rise to B-1 cells. Interestingly, we found that a subset of wt neonatal TrB cells expressed common B-1a markers (TrB-1a) and that this cell population was absent in the bumble neonatal spleen. Sorted TrB-1a (CD93(+)IgM(+)CD5(+)) cells exclusively generated B-1a cells when adoptively transferred, whereas sorted CD93(+)IgM(+)CD5(-) cells gave rise to B-2 cells and, to a lesser extent, B-1b and B-1a cells. This study identifies a phenotypically distinct splenic population of TrB-1a cells and establishes that the development of B-1a cells is blocked before this stage in the absence of IκBNS.

Innate lymphoid cells integrate stromal and immunological signals to enhance antibody production by splenic marginal zone B cells.

Innate lymphoid cells (ILCs) regulate stromal cells, epithelial cells and cells of the immune system, but their effect on B cells remains unclear. Here we identified RORγt(+) ILCs near the marginal zone (MZ), a splenic compartment that contains innate-like B cells highly responsive to circulating T cell-independent (TI) antigens. Splenic ILCs established bidirectional crosstalk with MAdCAM-1(+) marginal reticular cells by providing tumor-necrosis factor (TNF) and lymphotoxin, and they stimulated MZ B cells via B cell-activation factor (BAFF), the ligand of the costimulatory receptor CD40 (CD40L) and the Notch ligand Delta-like 1 (DLL1). Splenic ILCs further helped MZ B cells and their plasma-cell progeny by coopting neutrophils through release of the cytokine GM-CSF. Consequently, depletion of ILCs impaired both pre- and post-immune TI antibody responses. Thus, ILCs integrate stromal and myeloid signals to orchestrate innate-like antibody production at the interface between the immune system and circulatory system.

NF-κB2/p100 deficiency impairs immune responses to T-cell-independent type 2 antigens.

Formation of the splenic marginal zone (MZ) depends on the alternative NF-κB signaling pathway. Recently, we reported that unrestricted activation of this pathway in NF-κB2/p100-deficient (p100(-/-) ) knock-in mice alters the phenotype of MZ stroma and B cells. Here, we show that lack of the p100 inhibitor resulted in an expansion of both MZ B and peritoneal B-1 cells. However, these cells failed to generate proliferating blasts in response to T-cell-independent type 2 (TI-2) Ags, correlating with dampened IgM and absent IgG3 responses. This phenotype was in part due to increased activity of the NF-κB subunit RelB. Moreover, p100(-/-) →B6 BM chimeras were more susceptible to infection by encapsulated Streptococcus pneumoniae bacteria, pathogens that induce TI-2 responses. In contrast to the TI-2 defect, p100 deficiency did not impair immune responses to the TI-1 Ag LPS and p100(-/-) MZ B cells showed normal Ag transportation into B-cell follicles. Furthermore, p100(-/-) MZ B and B-1 cells failed to respond to TI-2 Ags in the presence of WT accessory cells. Thus, NF-κB2/p100 deficiency caused a predominant B-cell-intrinsic TI-2 defect that could largely be attributed to impaired proliferation of plasmablasts. Importantly, p100 was also necessary for efficient defense against clinically relevant TI-2 pathogens.

Differential compartmentalization of memory B cells versus plasma cells in salmonid fish.

The disposition of teleost memory and plasma cells (PCs) has essentially been un-explored. As the organization of the teleost immune system differs significantly from that of mammals (i.e. no bone marrow or lymph nodes, hematopoietic anterior kidney), this disposition could be essential in understanding how comparable functions are achieved. To address this question, the primary and secondary antibody-secreting cell, B memory cell, and antibody responses to T-independent and T-dependent antigens were analyzed in trout. Although the TI and TD antibody responses did not differ substantively from one another, the secondary responses to both were significantly prolonged compared with the primary responses. Logarithmic increases in titer and affinity were achieved for both antigens during the primary, with only modest increases during the secondary response. Antibody-secreting cells, both PCs and plasmablasts, quantitatively paralleled antibody production, with antibody-secreting cells skewing to the hematopoietic anterior kidney for both antigens. However, the enhanced antigen-inducible response of immune fish (indicative of the memory pool) skewed to the peripheral blood and spleen. This pattern of memory versus PC disposition parallels that observed in mammals even though the organization of the respective immune systems differs considerably.

Blk haploinsufficiency impairs the development, but enhances the functional responses, of MZ B cells.

Blk was identified two decades ago as a B-cell-specific member of the Src family of tyrosine kinases. Recent studies, however, have discovered that Blk is expressed in many cell types outside of the B lineage, including early thymic precursors, interleukin-17-producing γδ T cells and pancreatic ß-cells. In light of these recent discoveries, we performed a more comprehensive analysis of Blk expression patterns in hematopoietic cells and found that Blk is differentially expressed in mature B-cell subsets, with marginal zone (MZ) B cells expressing high levels, B1 B cells expressing intermediate-to-high levels and follicular (FO) B cells expressing low levels of Blk. To determine whether these differences in Blk expression levels reflected differential requirements for Blk in MZ, B1 and FO B-cell development, we analyzed the effects of reducing and eliminating Blk expression on B-cell development. We report that both Blk haploinsufficiency and Blk deficiency impaired the generation of MZ B cells. Moreover, although there were fewer MZ B cells in Blk(+/-) and Blk(-/-) mice as compared with Blk(+/+) mice, Blk-mutant MZ B cells were hyper-responsive to B-cell receptor stimulation, both in vitro and in vivo. Thus, this study has revealed a previously unappreciated role for Blk in the development and activation of MZ B cells.

Programmed cell death 1 suppresses B-1b cell expansion and long-lived IgG production in response to T cell-independent type 2 antigens.

B-1b cells play a key role in producing Abs against T cell-independent type 2 Ags. However, the factors regulating Ab production by this unique B cell subset are not well understood. In this study, a detailed analysis of the B cell response to 2,4,6-trinitrophenol (TNP)-Ficoll was performed using normal mice. TNP-Ficoll delivered i.p. or i.v. induced rapid Ag-specific B-1b cell activation, expansion, isotype switching, and plasmablast/plasma cell differentiation. Ag-specific B-1b cell numbers peaked at day 5 and then gradually declined in the spleen but remained elevated in the peritoneal cavity beyond 40 d postimmunization. In addition to expressing CD43, CD44, and CD86, Ag-activated B-1b cells transiently expressed programmed cell death 1 (PD-1), which functionally suppressed BCR-induced B-1b cell in vitro proliferation when additional costimulatory signals were lacking. Inhibiting PD-1:PD-1 ligand interactions during TNP-Ficoll immunization significantly enhanced Ag-specific B-1b cell expansion and the frequency of IgG isotype switching and plasmablast/plasma cell differentiation. Remarkably, PD-1 mAb blockade during the first week following immunization resulted in significantly increased numbers of both splenic and bone marrow Ag-specific IgG3-secreting cells, but not IgM-secreting cells, at both early (day 5) and late (week 6) time points. Moreover, Ag-specific serum IgG3 levels, as well as IgG2c, IgG2b, and IgA levels, remained significantly elevated in PD-1 mAb-treated mice relative to control Ab-treated mice for ≥6 wk postimmunization. Thus, PD-1:PD-1 ligand interactions occurring shortly after initial T cell-independent type 2 Ag encounter play a critical role in suppressing Ag-specific B-1b cell expansion and the development of long-term IgG-producing bone marrow and spleen cells.

Lentiviral-mediated gene therapy leads to improvement of B-cell functionality in a murine model of Wiskott-Aldrich syndrome.

BACKGROUND: Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency characterized by thrombocytopenia, eczema, infections, autoimmunity, and lymphomas. Transplantation of hematopoietic stem cells from HLA-identical donors is curative, but it is not available to all patients. We have developed a gene therapy (GT) approach for WAS by using a lentiviral vector encoding for human WAS promoter/cDNA (w1.6W) and demonstrated its preclinical efficacy and safety. OBJECTIVE: To evaluate B-cell reconstitution and correction of B-cell phenotype in GT-treated mice. METHODS: We transplanted Was(-/-) mice sublethally irradiated (700 rads) with lineage marker-depleted bone marrow wild-type cells, Was(-/-) cells untransduced or transduced with the w1.6W lentiviral vector and analyzed B-cell reconstitution in bone marrow, spleen, and peritoneum. RESULTS: Here we show that WAS protein(+) B cells were present in central and peripheral B-cell compartments from GT-treated mice and displayed the strongest selective advantage in the splenic marginal zone and peritoneal B1 cell subsets. After GT, splenic architecture was improved and B-cell functions were restored, as demonstrated by the improved antibody response to pneumococcal antigens and the reduction of serum IgG autoantibodies. CONCLUSION: WAS GT leads to improvement of B-cell functions, even in the presence of a mixed chimerism, further validating the clinical application of the w1.6W lentiviral vector.

Rapid reconstitution of antibody responses following transplantation of purified allogeneic hematopoietic stem cells.

Allogeneic hematopoietic cell transplantation has broad clinical applications extending from the treatment of malignancies to induction of immunologic tolerance. However, adaptive cellular and humoral immunity frequently remain impaired posttransplantation. Here, recovery of T-dependent and T-independent Ab responses was evaluated in mice transplanted with purified hematopoietic stem cells (HSCs) devoid of the mature immune cells believed to hasten immune recovery. Mixed and full donor chimeras were created by conditioning recipients with sublethal or lethal irradiation, respectively, across different donor/host genetic disparities. By 6 wk posttransplantation, all animals demonstrated robust T-independent Ab responses, and all mixed chimeras and recipients of MHC-matched or haploidentical HSCs with a shared MHC haplotype had T-dependent Ab responses equivalent to those of untransplanted controls. Full chimeras that received fully MHC-disparate HSCs showed delayed T-dependent Ab responses that recovered by 12 wk. This delay occurred despite early reconstitution and proper migration to germinal centers of donor-derived T(follicular helper) (T(FH)) cells. Congenic transplants into T(FH)-deficient CD4(-/-) mice revealed restoration of T-dependent Ab responses by 6 wk, leading us to conclude that MHC disparity caused delay in humoral recovery. These findings, together with our previous studies, show that, contrary to the view that depletion of graft lymphocytes results in poor posttransplant immunity, elimination of immune-suppressing graft-versus-host reactions permits superior immune reconstitution. This study also provides insight into the regeneration of T(FH) cells and humoral immunity after allogeneic HSC transplantation.

SIGN-R1, a C-type lectin, binds to Bip/GRP78 and this interaction mediates the regurgitation of T-cell-independent type 2 antigen dextran through the endoplasmic reticulum.

Capsular polysaccharides of Streptococcus pneumoniae are representative T-cell-independent type 2 (TI-2) antigens, frequently causing serious infections in children, the elderly, and immunocompromised patients. However, the detailed mechanism of this immune escape by CPSs is poorly understood. To pursue this question, polysaccharide dextran, ligand of SIGN-R1 as well as an appropriate model of the immunogenicity of many TI-2 polysaccharide antigens was used. SIGN-R1 bound to binding immunoglobulin protein (BiP), a well-characterized endoplasmic reticulum (ER) chaperone, primarily in non-ER compartments. Interestingly, SIGN-R1(+) macrophages in the MZ showed high expression of BiP, implying an important role of SIGN-R1 binding to BiP in vivo. To our surprise, dextran is rapidly transported into the ER and subsequently regurgitated out of cells in vitro or in vivo. BiP down-regulation in SIGN-R1 transfectant reduced the regurgitation of dextran, causing the accumulation of dextran in the ER. Therefore, these results demonstrated the first example to describe the intracellular trafficking and the regurgitation of TI-2 antigen dextran, suggesting the novel pathway of TI-2 antigen presentation to immune cells.

Follicular B cell trafficking within the spleen actively restricts humoral immune responses.

Follicular (FO) and marginal zone (MZ) B cells are maintained in distinct locations within the spleen, but the genetic basis for this separation is still enigmatic. We now report that B cell sequestration requires lineage-specific regulation of migratory receptors by the transcription factor Klf2. Moreover, using gene-targeted mice we show that altered splenic B cell migration confers a significant in vivo gain-of-function phenotype to FO B cells, including the ability to quickly respond to MZ-associated antigens and pathogens in a T cell-dependent manner. This work demonstrates that in wild-type animals, naive FO B cells are actively removed from the MZ, thus restricting their capacity to respond to blood-borne pathogens.