Extrafollicular Responses Research Articles

Acute Plasmodium chabaudi Infection Dampens Humoral Responses to a Secondary T-Dependent Antigen but Enhances Responses to a Secondary T-Independent Antigen

High rates of coinfection occur in malaria endemic regions, leading to more severe disease outcomes. Understanding how coinfecting pathogens influence the immune system is important in the development of treatment strategies that reduce morbidity and mortality. Using the Plasmodium chabaudi mouse model of malaria and immunization with model Ags that are either T-dependent (4-hydroxy-3-nitrophenyl [NP]-OVA) or T-independent (NP-Ficoll), we analyzed the effects of acute malaria on the development of humoral immunity to secondary Ags. Total Ig and IgG1 NP-specific Ab responses to NP-OVA were significantly decreased in the P. chabaudi-infected group compared with the uninfected group, whereas NP-specific IgG2c Ab was significantly increased in the P. chabaudi-infected group. In contrast, following injection with T-independent NP-Ficoll, the P. chabaudi-infected group had significantly increased NP-specific total Ig, IgM, and IgG2c Ab titers compared with controls. Treatment with anti-IFN-γ led to an abrogation of the NP-specific IgG2c Ab induced by P. chabaudi infection but did not affect other NP-specific Ab isotypes or titers. IFN-γ depletion also increased the percentage of plasma cells in both P. chabaudi-infected and uninfected groups but decreased the percentage of B cells with a germinal center (GC) phenotype. Using immunofluorescent microscopy, we were able to detect NP(+) GCs in the spleens of noninfected mice, but there were no detectible NP(+) GCs in mice infected with P. chabaudi. These data suggest that during P. chabaudi infection, there is a shift toward an extrafollicular Ab response that could be responsible for decreased Ab responses to secondary T-dependent Ags.

Rheumatoid Factor B Cell Memory Leads to Rapid, Switched Antibody-Forming Cell Responses

B cells are critical in the initiation and maintenance of lupus. Autoreactive B cells clonally expand, isotype switch, and mutate--properties associated with memory B cells (MBCs), which are typically generated via germinal centers. The development and functions of autoreactive MBCs in lupus are poorly understood. Moreover, mounting evidence implicates the extrafollicular (EF) response in the generation of switched and mutated autoantibodies that are driven by BCR and TLR corecognition, raising the question of whether MBCs are generated in this context. In this study, we investigated autoreactive MBC generation associated with this type of response. We transferred B cells from AM14 site-directed BCR transgenic mice into nontransgenic normal recipients and elicited an EF response with anti-chromatin Ab, as in prior studies. By following the fate of the stimulated cells at late time points, we found that AM14 B cells persisted at increased frequency for up to 7 wk. Furthermore, these cells had divided in response to Ag but were subsequently quiescent, with a subset expressing the memory marker CD73. These cells engendered rapid, isotype-switched secondary plasmablast responses upon restimulation. Both memory and rapid secondary responses required T cell help to develop, emphasizing the need for T-B collaboration for long-term self-reactivity. Thus, using this model system, we show that the EF response generated persistent and functional MBCs that share some, but not all, of the characteristics of traditional MBCs. Such cells could play a role in chronic or flaring autoimmune disease.

In this issue

In this issue

Cover Picture: Eur. J. Immunol. 12/11

The cover image shows immunohistological analysis of a spleen section from QMxB6 mice 96 hours after immunization with NP-Ficoll. IgD staining (brown) determines the B-cell follicles and NP-staining (blue) the antibody-producing cells; plasmablasts (dark blue regions) are present in the red pulp, and germinal centres (pale blue) can be seen in the follicles; the T zones (which lack B cells) remain unstained. The image is kindly provided by Jennifer Marshall and relates to the study Marshall et al. (pp. 3506–3512) in which it is shown that Ig class-switch recombination during extrafollicular responses occurs at the B blast rather than the plasmablast stage.

Early B blasts acquire a capacity for Ig class switch recombination that is lost as they become plasmablasts

Rapid production of neutralizing antibody can be critical for limiting the spread of infection. Such early antibody results when B-cell blasts mature directly to plasmablasts without forming germinal centers. These extrafollicular responses can involve Ig class switch recombination (CSR), producing antibody that can readily disseminate through infected tissues. The present study identifies the differentiation stage where CSR occurs in an extrafollicular response induced by 4-hydroxy-3-nitrophenyl acetyl (NP) conjugated to Ficoll (NP-Ficoll). To do this, we took advantage of the antigen dose dependency of CSR in this response. Thus, while both 30 and 1 μg NP-Ficoll induce plasmablasts, only the higher antigen dose induces CSR. Activation-induce cytidine deaminase (AID) is critical for CSR and in keeping with this a proportion of NP-specific B-cell blasts induced by 30 μg NP-Ficoll express AID. None of the B blasts responding to the non-CSR-inducing 1 μg dose of NP-Ficoll express AID. We confirmed that CSR occurs in B blasts by demonstrating the presence of rearranged heavy-chain transcripts in B blasts in the 30 μg response. CSR in this extrafollicular response is confined to B blasts, because NP-specific plasmablasts, identified by expressing CD138 and Blimp-1, no longer express AID and cannot undergo CSR.

Histone Deacetylase 9 Deficiency Protects against Effector T Cell-mediated Systemic Autoimmunity

Co-repressor histone deacetylase 9 (HDAC9) plays a key role in the development and differentiation of many types of cells, including regulatory T cells. However, the biological function of HDAC9 in T effector cells is unknown. Systemic autoimmune diseases like lupus, diabetes, and rheumatoid arthritis have dysfunctional effector T cells. To determine the role of HDAC9 in systemic autoimmunity, we created MRL/lpr mice with HDAC9 deficiency that have aberrant effector T cell function. HDAC9 deficiency led to decreased lympho-proliferation, inflammation, autoantibody production, and increased survival in MRL/lpr mice. HDAC9-deficient mice manifested Th2 polarization, decreased T effector follicular cells positive for inducible co-stimulator, and activated T cells in vivo compared with HDAC9-intact MRL/lpr mice. HDAC9 deficiency also resulted in increased GATA3 and roquin and decreased BCL6 gene expression. HDAC9 deficiency was associated with increased site-specific lysine histone acetylation at H3 (H3K9, H3K14, and H3K18) globally that was localized to IL-4, roquin, and peroxisome proliferator-activated receptor-γ promoters with increased gene expression, respectively. In kidney and spleen, HDAC9 deficiency decreased inflammation and cytokine and chemokine production due to peroxisome proliferator-activated receptor γ overexpression. These findings suggest that HDAC9 acts as an epigenetic switch in effector T cell-mediated systemic autoimmunity.

Plexin-D1 Is a Novel Regulator of Germinal Centers and Humoral Immune Responses

Long-lived humoral immune responses depend upon the generation of memory B cells and long-lived plasma cells during the germinal center (GC) reaction. These memory compartments, characterized by class-switched IgG and high-affinity Abs, are the basis for successful vaccination. We report that a new member of the plexin family of molecules, plexin-D1, controls the GC reaction and is required for secondary humoral immune responses. Plexin-D1 was not required for B cell maturation, marginal zone precursor development, dark and light zone formation, Igλ(+) and Igκ(+) B cell skewing, B1/B2 development, and the initial extrafollicular response. Plexin-D1 expression was increased following B cell activation, and PlxnD1(-/-) mice exhibited defective GC reactions during T-dependent immune activation. PlxnD1(-/-) B cells showed a defect in migration toward the GC chemokines, CXCL12, CXCL13, and CCL19. Accordingly, PlxnD1(-/-) mice exhibited defective production of IgG1 and IgG2b, but not IgG3 serum Ab, accompanied by reductions in long-lived bone marrow plasmacytes and recall humoral memory responses. These data show a new role for immune plexins in the GC reaction and generation of immunologic memory.

Facultative role for T cells in extrafollicular Toll-like receptor-dependent autoreactive B-cell responses in vivo

Extrafollicular (EF) B-cell responses are increasingly being recognized as an alternative pathway of B-cell activation, particularly in autoimmunity. Critical cellular interactions required for the EF B-cell response are unclear. A key question in autoimmunity, in which Toll-like receptor (TLR) signals are costimulatory and could be sufficient for B-cell activation, is whether T cells are required for the response. This is pivotal, because autoreactive B cells are considered antigen-presenting cells for autoreactive T cells, but where such interactions occur has not been identified. Here, using AM14 site-directed transgenic rheumatoid factor (RF) mice, we report that B cells can be activated, differentiate, and isotype-switch independent of antigen-specific T-cell help, αβ T cells, CD40L signaling, and IL-21 signaling to B cells. However, T cells do dramatically enhance the response, and this occurs via CD40L and IL-21 signals. Surprisingly, the response is completely inducible T-cell costimulator ligand independent. These results establish that, although not required, T cells substantially amplify EF autoantibody production and thereby implicate T-independent autoreactive B cells as a potential vector for breaking T-cell tolerance. We suggest that these findings explain why autoreactivity first focuses on self-components for which B cells carry TLR ligands, because these will uniquely be able to activate B cells independently of T cells, with subsequent T-B interactions activating autoreactive T cells, resulting in chronic autoimmunity.

Dendritic cell-B cell interactions in the marginal zone drive extrafollicular antibody responses. (61.4)

Abstract Targeting Ag directly to dendritic cell (DC) subsets through the use of monoclonal antibodies (mAb) is increasingly being employed as a means to generate CD4 and CD8 T cell responses. However, the ability of individual DC subsets to raise primary Ab responses following Ag targeting is not well characterized. Using the hapten 4-hydroxy-3-nitrophenyl (NP) coupled to mAbs that recognize DC-specific DEC205 or DCIR2, we found that MZ-associated DCIR2+ DCs, but not DEC205+ DCs, were able to induce robust T cell- and CD40-dependent anti-NP Ab responses following injection of soluble Ag. Anti-NP Ab responses were restricted to the IgG1 sub-class but failed to either persist over time or undergo affinity maturation, despite the addition of TLR or RLR agonists. Using adoptive transfers with NP-specific B cells, we show that the unique ability of DCIR2+ DCs to induce Ab responses was associated with rapid, MHC class II-independent activation of Ag-specific B cells that occurred specifically upon Ag delivery to DCIR2. Importantly, we found that NP-specific B cells accumulated in MZ-associated bridging channels in close association with CD11c+ cells as early as 24hr following Ag delivery to DCIR2+ DCs. Together, these results reveal a previously unrecognized role for DC-B cell interactions in splenic bridging channels, and provide a novel mechanism for the initiation of T-dependent extrafollicular Ab responses.

B cell extrinsic MyD88 and FcRγ control contraction of the extrafollicular B cell response (61.12)

Abstract The extrafollicular (EF) B cell reaction is emerging as a lynchpin of systemic autoimmune responses. External factors that limit this response remain undefined. AM14 transgenic mice have been useful in elucidating how autoreactive Rheumatoid Factor B cells are activated. On a lupus-prone genetic background, AM14 B cells undergo an isotype-switched EF plasmablast response. A similar response ensues when these cells are exposed in vivo to IgG2aa anti-chromatin antibodies, which presumably form immune complexes (ICs) with chromatin shed from dying cells. ICs have Fc that could also stimulate myeloid cells via FcRγ and TLRs, which in turn could influence the EF response. To investigate this, we transferred AM14 B cells into mice lacking FcRγ, MyD88 or both. Initial expansion and differentiation of AM14 B cells was equivalent in all hosts. However, by day 7, whereas the AM14 B cell response contracted in the WT environment, dysregulated expansion continued in the deficient environments. Furthermore, MyD88 and FcRγ are non-redundant, as responses in mice lacking both proteins were larger and more prolonged compared with mice deficient in either factor alone. By d10, the AFC frequency was 40-fold larger in double-deficient compared to WT recipients. These results reveal novel regulatory roles in the EF B cell response for two receptors that are typically considered proinflammatory. Targeting these pathways may provide a way to curtail dysregulated autoreactive B cell activation.

Dendritic Cells in Lupus Are Not Required for Activation of T and B Cells but Promote Their Expansion, Resulting in Tissue Damage

Dendritic cells (DCs) initiate and control the adaptive immune response against infections. However, their contributions to the anti-self adaptive immune response in autoimmune disorders like systemic lupus erythematosus are uncertain. By constitutively deleting DCs in MRL.Fas(lpr) mice, we show that they have complex roles in murine lupus. The net effect of DC deletion was to ameliorate disease. DCs were crucial for the expansion and differentiation of Tcells but, surprisingly, not required for their initial activation. Correspondingly, kidney interstitial infiltrates developed in the absence of DCs, but failed to progress. DC deletion concomitantly decreased inflammatory and regulatory Tcell numbers. Unexpectedly, plasmablast numbers and autoantibody concentrations depended on DCs, in contrast to total serum immunoglobulin concentrations, suggesting an effect of DCs on extrafollicular humoral responses. These findings reveal that DCs operate in unanticipated ways in murine lupus and validate them as a potential therapeutic target in autoimmunity.

Trypanosoma cruzi infection induces a massive extrafollicular and follicular splenic B-cell response which is a high source of non-parasite-specific antibodies

Acute infection with Trypanosoma cruzi, the aetiological agent of Chagas' disease, results in parasitaemia and polyclonal lymphocyte activation. It has been reported that polyclonal B-cell activation is associated with hypergammaglobulinaemia and delayed parasite-specific antibody response. In the present study we analysed the development of a B-cell response within the different microenvironments of the spleen during acute T. cruzi infection. We observed massive germinal centre (GC) and extrafollicular (EF) responses at the peak of infection. However, the EF foci were evident since day 3 post-infection (p.i.), and, early in the infection, they mainly provided IgM. The EF foci response reached its peak at 11 days p.i. and extended from the red pulp into the periarteriolar lymphatic sheath. The GCs were detected from day 8 p.i. At the peak of parasitaemia, CD138(+) B220(+) plasma cells in EF foci, red pulp and T-cell zone expressed IgM and all the IgG isotypes. Instead of the substantial B-cell response, most of the antibodies produced by splenic cells did not target the parasite, and parasite-specific IgG isotypes could be detected in sera only after 18 days p.i. We also observed that the bone marrow of infected mice presented a strong reduction in CD138(+) B220(+) cells compared with that of normal mice. Hence, in acute infection with T. cruzi, the spleen appears to be the most important lymphoid organ that lodges plasma cells and the main producer of antibodies. The development of a B-cell response during T. cruzi infection shows features that are particular to T. cruzi and other protozoan infection but different to other infections or immunization with model antigens.

B‐cell fate decisions following influenza virus infection

Rapidly induced, specific Ab generated in extrafollicular foci are important components of early immune protection to influenza virus. The signal(s) that prompt B cells to participate in extrafollicular rather than germinal center responses are incompletely understood. To study the regulation of early B-cell differentiation events following influenza infection, we exploited earlier findings of a strong contribution of C12 idiotype-expressing B cells to the primary HA-specific response against influenza A/PR/8/34. Using an idiotype-specific mAb to C12 and labeled HA, in conjunction with multicolor flow cytometry, we followed the fate of C12Id-expressing influenza HA-specific B cells in WT BALB/c mice, requiring neither genetic manipulation nor adoptive cell transfer. Our studies demonstrate that HA-specific C12Id(+) B cells are phenotypically indistinguishable from follicular B cells. While they induced both extrafollicular and germinal center responses, extrafollicular responses were strongly predominant. Provision of increased HA-specific T-cell help increased the magnitude of the extrafollicular response, but did not shift the C12Id(+) response toward germinal center formation. Collectively the data are consistent with the hypothesis that B-cell fate determination following activation is a stochastic process in which infection-induced innate signals might drive the preferential expansion of the early extrafollicular response.

Extrafollicular foci-derived B cells provide long-lived mucosal immunity (84.16)

Abstract Antibodies to influenza are contributed by extrafollicular foci and germinal centers. The latter induce long-lived immunity via bone marrow plasma cells and memory B cells. Extrafollicular foci are thought to only provide early, short-lived responses generated from high-affinity B cells. The selection of the highest affine B cells into the early response might ensure survival from acute infection, but it seems counterintuitive that they would be excluded from the long-lived response. Here we assessed the contributions of the extrafollicular foci response to long-term immunity. We tracked a prototypic early B cell response to hemagglutinin (HA) of influenza A/PR8 in BALB/c mice by multicolor flow cytometry using an idiotype-specific mAb to the C12 idiotype (Id) together with fluorescent-labeled HA. C12Id+ HA-specific antibodies were measured by ELISA and ELISPOT. The results demonstrate the extrafollicular nature of the C12Id response and the lack of C12Id+ bone marrow plasma cells. Nonetheless, following infection HA-specific serum C12Id+ antibodies were measurable for the life of mice, secreted presumably by antibody-secreting C12Id+ plasma cells present in the lung. BrDU-labeling experiments suggested continuous turnover of these cells. Thus, extrafollicular foci induce long-term mucosal antibody responses via short-lived plasma cells. Supported by NIH AI051354 (N.B) and T32 HL007013 (K.R.).

ICOS-dependent extrafollicular helper T cells elicit IgG production via IL-21 in systemic autoimmunity

The role of specialized follicular helper T (TFH) cells in the germinal center has become well recognized, but it is less clear how effector T cells govern the extrafollicular response, the dominant pathway of high-affinity, isotype-switched autoantibody production in the MRL/MpJ-Faslpr (MRLlpr) mouse model of lupus. MRLlpr mice lacking the Icos gene have impaired extrafollicular differentiation of immunoglobulin (Ig) G+ plasma cells accompanied by defects in CXC chemokine receptor (CXCR) 4 expression, interleukin (IL) 21 secretion, and B cell helper function in CD4 T cells. These phenotypes reflect the selective loss of a population of T cells marked by down-regulation of P-selectin glycoprotein ligand 1 (PSGL-1; also known as CD162). PSGL-1lo T cells from MRLlpr mice express CXCR4, localize to extrafollicular sites, and uniquely mediate IgG production through IL-21 and CD40L. In other autoimmune strains, PSGL-1lo T cells are also abundant but may exhibit either a follicular or extrafollicular phenotype. Our findings define an anatomically distinct extrafollicular population of cells that regulates plasma cell differentiation in chronic autoimmunity, indicating that specialized humoral effector T cells akin to TFH cells can occur outside the follicle.

CD4 T Cells That Promote Extrafollicular B Cell Responses

The role of follicular helper T cells (TFH) in development and maintenance of the germinal center is well known; however the role of T cells in the extrafollicular B cell response is less understood. We have described a novel subset of activated IL‐21 producing CD4 T cells that are located at extrafollicular sites in murine lupus. These cells are CXCR5lo and are marked by downregulation of P selectin glycoprotein ligand 1 (PSGL1). To determine if the IL‐21+ CXCR5lo PSGL1lo population exists in a normal immune response, we used adoptive transfer of ovalbumin (ova) specific TCR transgenic CD4 T cells to track their development following ova immunization. We find that PSGL1lo cells arise following ova immunization, with significant expansion when the antigen is administered chronically. Like TFH cells, this population requires ICOS for its development; however, unlike the former, B cells do not appear to be required. In the absence of the PSGL1lo population, isotype switched antibody production to ova is lost. These cells make IL‐21; however, only a subset makes IL‐4 and is CXCR5hi. By microscopy, PSGL1lo cells are found in the B cell follicle and at extrafollicular sites. Based on this data, we hypothesize that IL‐21+ CXCR5lo PSGL1lo cells are important for extrafollicular responses. We have termed this novel population T helper extrafollicular cells or THEF cells.

Levels of autoantibodies, unlike antibodies to all extrinsic antigen groups, fall following B cell depletion with Rituximab

Many autoantibodies have variable-region sequences indicating their production in an affinity-matured antibody response involving germinal centers (GC). Plasma cells from GC can be long-lived, do not express CD20 and thus should not be depleted by a therapeutic monoclonal Ab against human CD20 - Rituximab. Nevertheless, autoantibody titers often fall following Rituximab treatment. To test if this reflects exclusive production by short-lived plasma cells in extrafollicular Ab responses, we monitored, after Rituximab treatment, levels of natural Ab and Ab against extrinsic antigens that do not induce productive GC. Eleven patients with active vasculitis and anti-proteinase-3 (PR3) Ab were assessed before and during 5 months after Rituximab therapy. Blood B cells were undetectable within 2 wk, and all patients achieved clinical remission. Levels of natural Ab - isohemagglutinins and anti-phosphorylcholine Ab - and Ab levels against thymus-independent and thymus-dependent extrinsic antigens were little affected. By contrast, 5 months after Rituximab, IgG autoantibody against PR3 had fallen to a median of 22% of pretreatment values. While the kinetics of this fall do not suggest an intrinsically short lifespan of autoantibody-producing cells, the data are consistent with Rituximab causing loss of sites within inflammatory tissues that selectively sustain autoantibody-producing cells.

MicroRNA-155 Regulates the Generation of Immunoglobulin Class-Switched Plasma Cells

microRNA-155 (miR-155) is expressed by cells of the immune system after activation and has been shown to be required for antibody production after vaccination with attenuated Salmonella. Here we show the intrinsic requirement for miR-155 in B cell responses to thymus-dependent and -independent antigens. B cells lacking miR-155 generated reduced extrafollicular and germinal center responses and failed to produce high-affinity IgG1 antibodies. Gene-expression profiling of activated B cells indicated that miR-155 regulates an array of genes with diverse function, many of which are predicted targets of miR-155. The transcription factor Pu.1 is validated as a direct target of miR155-mediated inhibition. When Pu.1 is overexpressed in wild-type B cells, fewer IgG1 cells are produced, indicating that loss of Pu.1 regulation is a contributing factor to the miR-155-deficient phenotype. Our results implicate post-transcriptional regulation of gene expression for establishing the terminal differentiation program of B cells.

B Cell Activation State-Governed Formation of Germinal Centers following Viral Infection

Germinal centers are structures that promote humoral memory cell formation and affinity maturation, but the triggers for their development are not entirely clear. Activated extrafollicular B cells can form IgM-producing plasmablasts or enter a germinal center reaction and differentiate into memory or plasma cells, mostly of the IgG isotype. Vesicular stomatitis virus (VSV) induces both types of response, allowing events that promote each of these pathways to be studied. In this work, extrafollicular vs germinal center responses were examined at a cellular level, analyzing VSV-specific B cells in infected mice. We show that VSV-specific germinal centers are transiently formed when insufficient proportions of specific T cell help is available and that strong B cell activation in cells expressing high levels of the VSV-specific BCR promoted their differentiation into early blasts, whereas moderate stimulation of B cells or interaction with Th cells restricted extrafollicular responses and promoted germinal center formation.

Salmonella Induces a Switched Antibody Response without Germinal Centers That Impedes the Extracellular Spread of Infection

T-dependent Ab responses are characterized by parallel extrafollicular plasmablast growth and germinal center (GC) formation. This study identifies that, in mice, the Ab response against Salmonella is novel in its kinetics and its regulation. It demonstrates that viable, attenuated Salmonella induce a massive early T-dependent extrafollicular response, whereas GC formation is delayed until 1 mo after infection. The extrafollicular Ab response with switching to IgG2c, the IgG2a equivalent in C57BL/6 mice, is well established by day 3 and persists through 5 wk. Switching is strongly T dependent, and the outer membrane proteins are shown to be major targets of the early switched IgG2c response, whereas flagellin and LPS are not. GC responses are associated with affinity maturation of IgG2c, and their induction is associated with bacterial burden because GC could be induced earlier by treating with antibiotics. Clearance of these bacteria is not a consequence of high-affinity Ab production, for clearance occurs equally in CD154-deficient mice, which do not develop GC, and wild-type mice. Nevertheless, transferred low- and high-affinity IgG2c and less efficiently IgM were shown to impede Salmonella colonization of splenic macrophages. Furthermore, Ab induced during the infection markedly reduces bacteremia. Thus, although Ab does not prevent the progress of established splenic infection, it can prevent primary infection and impedes secondary hemogenous spread of the disease. These results may explain why attenuated Salmonella-induced B cell responses are protective in secondary, but not primary infections.