Heterogeneity of Phenotype and Function Reflects the Multistage Development of T Follicular Helper Cells.

T follicular helper cells (Tfh) provide crucial signals for germinal center (GC) formation, but Tfh populations are heterogeneous. While PD1hi Tfh are important in the GC response, the function of the PD1lo Tfh-like subset is unknown. We show that these cells, like the PD1hi GC-Tfh, depend upon B cells; however, their entry to follicles is independent of CXCR5 or cognate interactions with B cells. The differentiation into PD1hi Tfh is dependent on MHC class II interactions with B cells and requires CXCR5. Our data suggest a Tfh differentiation pathway that is initially B cell-independent, then dependent on non-cognate B cell interactions, and finally following cognate interaction with B cells and CXCR5-ligands allows the formation of GC-Tfh. The PD1lo Tfh-like cells make early cytokine responses and may represent precursors of CD4 memory cells.

Macrophage origin limits functional plasticity in helminth-bacterial co-infection.

Rapid reprogramming of the macrophage activation phenotype is considered important in the defense against consecutive infection with diverse infectious agents. However, in the setting of persistent, chronic infection the functional importance of macrophage-intrinsic adaptation to changing environments vs. recruitment of new macrophages remains unclear. Here we show that resident peritoneal macrophages expanded by infection with the nematode Heligmosomoides polygyrus bakeri altered their activation phenotype in response to infection with Salmonella enterica ser. Typhimurium in vitro and in vivo. The nematode-expanded resident F4/80high macrophages efficiently upregulated bacterial induced effector molecules (e.g. MHC-II, NOS2) similarly to newly recruited monocyte-derived macrophages. Nonetheless, recruitment of blood monocyte-derived macrophages to Salmonella infection occurred with equal magnitude in co-infected animals and caused displacement of the nematode-expanded, tissue resident-derived macrophages from the peritoneal cavity. Global gene expression analysis revealed that although nematode-expanded resident F4/80high macrophages made an anti-bacterial response, this was muted as compared to newly recruited F4/80low macrophages. However, the F4/80high macrophages adopted unique functional characteristics that included enhanced neutrophil-stimulating chemokine production. Thus, our data provide important evidence that plastic adaptation of MΦ activation does occur in vivo, but that cellular plasticity is outweighed by functional capabilities specific to the tissue origin of the cell.

Plasma cell homeostasis: the effects of chronic antigen stimulation and inflammation.

Long-lived plasma cells (LLPCs) that maintain humoral immunity to previously encountered Ags occupy a compartment in the bone marrow (BM). The rules and mechanisms by which cells enter (and leave) this compartment are poorly understood. We looked at what happens to the LLPC compartment and to plasma cell lifespan in general, in situations in which Ag stimulation and/or inflammation persist. We find that chronic Ag supply causes the generation of short-lived plasma cells in the local lymphoid organ, at the expense of any LLPC production. Furthermore, we find that inflammation caused by infection (mediated via TNF-α) causes a dramatic mobilization of LLPCs from the BM, with a concomitant reduction in circulating Ab levels against previously immunized Ags. These data are discussed in the context of the capacity of the BM LLPC compartment and competition for entry to it.

Cutting edge: IL-6-dependent autoimmune disease: dendritic cells as a sufficient, but transient, source.

Mice lacking IL-6 are resistant to autoimmune diseases, such as experimental autoimmune encephalomyelitis (EAE), which is driven by CNS-reactive CD4(+) T cells. There are multiple cellular sources of IL-6, but the critical source in EAE has been uncertain. Using cell-specific IL-6 deficiency in models of EAE induced by active immunization, passive transfer, T cell transfer, and dendritic cell transfer, we show that neither the pathogenic T cells nor CNS-resident cells are required to produce IL-6. Instead, the requirement for IL-6 was restricted to the early stages of T cell activation and was entirely controlled by dendritic cell-derived IL-6. This reflected the loss of IL-6R expression by T cells over time. These data explain why blockade of IL-6R only achieves protection against EAE if used at the time of T cell priming. The implications for therapeutic manipulation of IL-6 signaling in human T cell-driven autoimmune conditions are considered.

T(FH) memory: more or less T(FH)?

Follicular T-helper (T(FH) ) cells are a subset of CD4(+) T cells that is specialized to help B cells. Their capacity to give rise to enhanced T(FH) memory responses has not been documented. A study by Weber et al. [Eur. J. Immunol. 2012 42: 1981-1988] in this issue of the European Journal of Immunology addresses this question, and a picture is emerging from this and several other recent studies which suggests that the formation of memory T(FH) and of central memory (T(CM) ) cells are intimately bound and that the heterogeneity of what we currently call "T(FH) '' cells clouds this issue. In this Commentary, we discuss these complexities and ask what memory T(FH) cells are doing given that the germinal centers are a feature of primary, but not secondary immune responses.

B cell depletion therapy ameliorates autoimmune disease through ablation of IL-6-producing B cells.

B cells have paradoxical roles in autoimmunity, exerting both pathogenic and protective effects. Pathogenesis may be antibody independent, as B cell depletion therapy (BCDT) leads to amelioration of disease irrespective of autoantibody ablation. However, the mechanisms of pathogenesis are poorly understood. We demonstrate that BCDT alleviates central nervous system autoimmunity through ablation of IL-6-secreting pathogenic B cells. B cells from mice with experimental autoimmune encephalomyelitis (EAE) secreted elevated levels of IL-6 compared with B cells from naive controls, and mice with a B cell-specific IL-6 deficiency showed less severe disease than mice with wild-type B cells. Moreover, BCDT ameliorated EAE only in mice with IL-6-sufficient B cells. This mechanism of pathogenesis may also operate in multiple sclerosis (MS) because B cells from MS patients produced more IL-6 than B cells from healthy controls, and this abnormality was normalized with B cell reconstitution after Rituximab treatment. This suggests that BCDT improved disease progression, at least partly, by eliminating IL-6-producing B cells in MS patients. Taking these data together, we conclude that IL-6 secretion is a major mechanism of B cell-driven pathogenesis in T cell-mediated autoimmune disease such as EAE and MS.

Chronic infection drives expression of the inhibitory receptor CD200R, and its ligand CD200, by mouse and human CD4 T cells.

Certain parasites have evolved to evade the immune response and establish chronic infections that may persist for many years. T cell responses in these conditions become muted despite ongoing infection. Upregulation of surface receptors with inhibitory properties provides an immune cell-intrinsic mechanism that, under conditions of chronic infection, regulates immune responses and limits cellular activation and associated pathology. The negative regulator, CD200 receptor, and its ligand, CD200, have been shown to regulate macrophage activation and reduce pathology following infection. We show that CD4 T cells also increase expression of inhibitory CD200 receptors (CD200R) in response to chronic infection. CD200R was upregulated on murine effector T cells in response to infection with bacterial, Salmonella enterica, or helminth, Schistosoma mansoni, pathogens that respectively drive predominant Th1- or Th2-responses. In vitro chronic and prolonged stimuli were required for the sustained upregulation of CD200R, and its expression coincided with loss of multifunctional potential in T effector cells during infection. Importantly, we show an association between IL-4 production and CD200R expression on T effector cells from humans infected with Schistosoma haematobium that correlated effectively with egg burden and, thus infection intensity. Our results indicate a role of CD200R:CD200 in T cell responses to helminths which has diagnostic and prognostic relevance as a marker of infection for chronic schistosomiasis in mouse and man.

B cells: programmers of CD4 T cell responses.

B cells are once again gaining prominence as important programmers of CD4 T cell responses. With widespread use of B cell depletion therapy in the clinic, proving effective in treating diseases previously considered T cell-mediated, the time is right for a re-appraisal of the B cell. Though typically considered weak, Th2 driving APC, it is now clear that they are necessary for a robust and long-lived CD4 T cell response in many settings. The sphere of B cell influence extends well beyond that of simply antibody production; antigen presentation, cytokine secretion, costimulation and development of lymphoid tissue architecture are all critical aspects of B cell immunobiology, the absence of which has serious impacts for T cell priming and memory. The aim of this review is to look at non-antibody mediated B cell function and to ask how, where and when do B cells influence the CD4 T cell response?

Schistosomes induce regulatory features in human and mouse CD1d(hi) B cells: inhibition of allergic inflammation by IL-10 and regulatory T cells.

Chronic helminth infections, such as schistosomes, are negatively associated with allergic disorders. Here, using B cell IL-10-deficient mice, Schistosoma mansoni-mediated protection against experimental ovalbumin-induced allergic airway inflammation (AAI) was shown to be specifically dependent on IL-10-producing B cells. To study the organs involved, we transferred B cells from lungs, mesenteric lymph nodes or spleen of OVA-infected mice to recipient OVA-sensitized mice, and showed that both lung and splenic B cells reduced AAI, but only splenic B cells in an IL-10-dependent manner. Although splenic B cell protection was accompanied by elevated levels of pulmonary FoxP3(+) regulatory T cells, in vivo ablation of FoxP3(+) T cells only moderately restored AAI, indicating an important role for the direct suppressory effect of regulatory B cells. Splenic marginal zone CD1d(+) B cells proved to be the responsible splenic B cell subset as they produced high levels of IL-10 and induced FoxP3(+) T cells in vitro. Indeed, transfer of CD1d(+) MZ-depleted splenic B cells from infected mice restored AAI. Markedly, we found a similarly elevated population of CD1d(hi) B cells in peripheral blood of Schistosoma haematobium-infected Gabonese children compared to uninfected children and these cells produced elevated levels of IL-10. Importantly, the number of IL-10-producing CD1d(hi) B cells was reduced after anti-schistosome treatment. This study points out that in both mice and men schistosomes have the capacity to drive the development of IL-10-producing regulatory CD1d(hi) B cells and furthermore, these are instrumental in reducing experimental allergic inflammation in mice.

CD11c depletion severely disrupts Th2 induction and development in vivo.

Although dendritic cells (DCs) are adept initiators of CD4(+) T cell responses, their fundamental importance in this regard in Th2 settings remains to be demonstrated. We have used CD11c-diphtheria toxin (DTx) receptor mice to deplete CD11c(+) cells during the priming stage of the CD4(+) Th2 response against the parasitic helminth Schistosoma mansoni. DTx treatment significantly depleted CD11c(+) DCs from all tissues tested, with 70-80% efficacy. Even this incomplete depletion resulted in dramatically impaired CD4(+) T cell production of Th2 cytokines, altering the balance of the immune response and causing a shift toward IFN-γ production. In contrast, basophil depletion using Mar-1 antibody had no measurable effect on Th2 induction in this system. These data underline the vital role that CD11c(+) antigen-presenting cells can play in orchestrating Th2 development against helminth infection in vivo, a response that is ordinarily balanced so as to prevent the potentially damaging production of inflammatory cytokines.

TLR and B cell receptor signals to B cells differentially program primary and memory Th1 responses to Salmonella enterica.

Protective Th1 responses to Salmonella enterica do not develop in the absence of B cells. Using chimeric mice, we dissect the early (innate) and late (cognate) contributions of B cells to Th programming. B cell-intrinsic MyD88 signaling is required for primary effector Th1 development, whereas Ag-specific BCR-mediated Ag presentation is necessary for the development of memory Th1 populations. Programming of the primary T cell response is BCR/B cell MHC II independent, but requires MyD88-dependent secretion of cytokines by B cells. Chimeras in which B cells lack IFN-gamma or IL-6 genes make impaired Th1 or Th17 responses to Salmonella.

TLR-mediated loss of CD62L focuses B cell traffic to the spleen during Salmonella typhimurium infection.

B cells recognize Ags on microorganisms both with their BCRs and TLRs. This innate recognition has the potential to alter the behavior of whole populations of B cells. We show in this study that in culture and in mice, MyD88-dependent activation of B cells via TLR2 or TLR9 causes the rapid loss of expression of CD62L by metalloproteinase-dependent shedding. Adoptive transfer of in vitro CpG-activated B cells showed them to be excluded from lymph nodes and Peyer's patches, but not the spleen. In vivo, both injection of CpG and systemic infection with Salmonella typhimurium caused the shedding of CD62L and the consequent focusing of B cell migration to the spleen and away from lymph nodes. We propose that wholesale TLR-mediated changes to B cell migration influence the development of immunity to pathogens carrying appropriate ligands.

Quantum dots decorated with pathogen associated molecular patterns as fluorescent synthetic pathogen models.

We attached the pathogen associated molecular pattern Kdo(2)-Lipid A (the lipopolysaccharide (LPS) from Escherichia coli (E. coli)) to QDs by hydrophobic interactions to synthetically mimic the surface of E. coli. QD-LPS conjugates bind, are taken up and activate effectively macrophages in vitro and they have potent immunostimulatory activity in vivo.

Helminth-induced CD19+CD23hi B cells modulate experimental allergic and autoimmune inflammation.

Numerous population studies and experimental models suggest that helminth infections can ameliorate immuno-inflammatory disorders such as asthma and autoimmunity. Immunosuppressive cell populations associated with helminth infections include Treg and alternatively-activated macrophages. In previous studies, we showed that both CD4(+)CD25(+) Treg, and CD4(-) MLN cells from Heligmosomoides polygyus-infected C57BL/6 mice were able to transfer protection against allergic airway inflammation to sensitized but uninfected animals. We now show that CD4(-)CD19(+) MLN B cells from infected, but not naïve, mice are able to transfer a down-modulatory effect on allergy, significantly suppressing airway eosinophilia, IL-5 secretion and pathology following allergen challenge. We further demonstrate that the same cell population can alleviate autoimmune-mediated inflammatory events in the CNS, when transferred to uninfected mice undergoing myelin oligodendrocyte glycoprotein((p35-55))-induced EAE. In both allergic and autoimmune models, reduction of disease was achieved with B cells from helminth-infected IL-10(-/-) donors, indicating that donor cell-derived IL-10 is not required. Phenotypically, MLN B cells from helminth-infected mice expressed uniformly high levels of CD23, with follicular (B2) cell surface markers. These data expand previous observations and highlight the broad regulatory environment that develops during helminth infections that can abate diverse inflammatory disorders in vivo.

Toll-like receptor 4 signalling through MyD88 is essential to control Salmonella enterica serovar typhimurium infection, but not for the initiation of bacterial clearance.

Toll-like receptor-4 (TLR4) is important in protection against lethal Salmonella enterica serovar Typhimurium (S. Typhimurium) infection. Control of the early stages of sublethal S. Typhimurium infection in mice depends on TLR4-dependent activation of macrophages and natural killer (NK) cells to drive an inflammatory response. TLR4 signals through the adapter proteins Mal/MyD88 and TRIF-related adaptor molecule (TRAM)/TIR-domain-containing adaptor-inducing interferon-b (TRIF). In the mouse typhoid model we showed that TLR4 and MyD88, but not Mal or TRIF, are essential for the control of exponential S. Typhimurium growth. TRIF(-/-) mice have a higher bacterial load in comparison with wild-type mice during a sublethal infection because TRIF is important for bacterial killing during the first day of systemic disease. Minimal pro-inflammatory responses were induced by S. Typhimurium infection of macrophages from TLR4(-/-), MyD88(-/-) and TRIF(-/-) mice in vitro. Pro-inflammatory responses from Mal(-/-) macrophages were similar to those from wild-type cells. The pro-inflammatory responses of TRIF(-/-) macrophages were partially restored by the addition of interferon-gamma (IFN-gamma), and TRIF(-/-) mice produced markedly enhanced IFN-gamma levels, in comparison to wild-type mice, probably explaining why bacterial growth can be controlled in these mice. TLR4(-/-), MyD88(-/-), TRIF(-/-) and Mal(-/-) mice all initiated clearance of S. Typhimurium, suggesting that TLR4 signalling is not important in driving bacterial clearance in comparison to its critical role in controlling early bacterial growth in mouse typhoid.

Dying and necrotic neutrophils are anti-inflammatory secondary to the release of alpha-defensins.

Neutrophils are recruited to sites of injury but their timely removal is thought to be vital to prevent exacerbating inflammation. In addition, the recognition of apoptotic cells by cells of the innate immune system provides potent anti-inflammatory and anti-immunogenic signals. In this article, we describe how human neutrophils dying by apoptosis or necrosis release anti-inflammatory peptides, the alpha-defensins. This family of small cationic peptides effectively inhibits the secretion of multiple proinflammatory cytokines and NO from macrophages, the main innate immune cell found at sites of chronic inflammation. In addition, the systemic administration of necrotic neutrophil supernatants and alpha-defensins protects mice from a murine model of peritonitis. Hence. their effects may be far-reaching and serve to kill microbes while regulating a potentially tissue-destructive inflammatory response.

B cell intrinsic MyD88 signals drive IFN-gamma production from T cells and control switching to IgG2c.

The question of whether Ab responses to T-dependent Ags require B cell intrinsic signaling via the main TLR adaptor (MyD88) has become embroiled in confusion. In part this may be related to the methods used to analyze B cell intrinsic signaling. We have used a mixed bone marrow chimera model to generate mice in which the B cell compartment is completely deficient in MyD88 expression, while the other hematopoietic lineages are largely normal. These mice were immunized with T-dependent Ags or infected with Salmonella. We found that the Ag-specific IgG2c primary response was absolutely dependent on MyD88 signaling to B cells, while other Ig classes were not (IgG1 and IgG3) or much less so (IgG2b, IgA). The MyD88(B-/-) chimeric mice exhibited an impairment of development of IFN-gamma effector T cells, a likely contributory factor in the lack of IgG2c. We also found that B cell intrinsic MyD88 signals are required for the production of natural Abs. The data emphasize the nonredundant role of B cells as programmers of T cell differentiation in vivo.

Virulent Salmonella enterica infections can be exacerbated by concomitant infection of the host with a live attenuated S. enterica vaccine via Toll-like receptor 4-dependent interleukin-10 production with the involvement of both TRIF and MyD88.

During systemic disease in mice, Salmonella enterica grows intracellularly within discrete foci of infection in the spleen and liver. In concomitant infections, foci containing different S. enterica strains are spatially separated. We have investigated whether functional interactions between bacterial populations within the same host can occur despite the known spatial separation of the foci and independence of growth of salmonellae residing in different foci. In this study we have demonstrated that bacterial numbers of virulent S. enterica serovar Typhimurium C5 strain in mouse tissues can be increased by the presence of the attenuated aroA S. Typhimurium SL3261 vaccine strain in the same tissue. Disease exacerbation does not require simultaneous coinjection of the attenuated bacteria. SL3261 can be administered up to 48 hr after or 24 hr before the administration of C5 and still determine higher tissue numbers of the virulent bacteria. This indicates that intravenous administration of a S. enterica vaccine strain could potentially exacerbate an established infection with wild-type bacteria. These data also suggest that the severity of an infection with a virulent S. enterica strain can be increased by the prior administration of a live attenuated vaccine strain if infection occurs within 48 hr of vaccination. Exacerbation of the growth of C5 requires Toll-like receptor 4-dependent interleukin-10 production with the involvement of both Toll/interleukin-1 receptor-domain-containing adaptor inducing interferon-beta and myeloid differentiation factor 88.

Innate responses of B cells.

In this review, we describe the non-antibody-mediated functions of B cells within the immune system. In addition to antibody production, B cells also present antigen to T cells, programme T cell differentiation and regulate effector T cell responses and much of this is mediated by the cytokines they make. We focus on the potential of B cells to perform these functions simply as a result of activation via 'innate' receptors (e.g. Toll-like receptors) and often independently of BCR ligation. We feel an appreciation of these broad and often antigen-nonspecific functions is important at a time when there is an increasing use of B cell depletion as a therapy for autoimmune disease.

TLR-mediated stimulation of APC: Distinct cytokine responses of B cells and dendritic cells.

In addition to their role in humoral immunity, B lymphocytes are important antigen-presenting cells (APC). In the same way as other APC, B cells make cytokines upon activation and have the potential to modulate T cell responses. In this study, we investigated which mouse B cell subsets are the most potent cytokine producers, and examined the role of Toll-like receptors (TLR) in the control of secretion of IL-6, IL-10, IL-12 and IFN-gamma by B cells. Production of some cytokines was restricted to particular subsets. Marginal zone and B1 cells were the predominant source of B cell IL-10 in the spleen. Conversely, follicular B cells were found to express IFN-gamma mRNA directly ex vivo. The nature of the activating stimulus dramatically influenced the cytokine made by B cells. Thus, in response to combined TLR stimulation, or via phorbol esters, IFN-gamma was secreted. IL-10 was elicited by T-dependent activation or stimulation through TLR2, 4 or 9. This pattern of cytokine expression contrasts with that elicited from dendritic cells. QRT-PCR array data indicate that this may be due to differential expression of TLR signalling molecules, effectors and adaptors. Our data highlight the potentially unique nature of immune modulation when B cells act as APC.