Dynamics and magnitude of virus-induced polyclonal B cell activation mediated by BCR-independent presentation of viral antigen

Extrafollicular Plasmablasts Present in the Acute Phase of Infections Express High Levels of PD-L1 and Are Able to Limit T Cell Response

During infections with protozoan parasites or some viruses, T cell immunosuppression is generated simultaneously with a high B cell activation. It has been described that, as well as producing antibodies, plasmablasts, the differentiation product of activated B cells, can condition the development of protective immunity in infections. Here, we show that, in T. cruzi infection, all the plasmablasts detected during the acute phase of the infection had higher surface expression of PD-L1 than other mononuclear cells. PD-L1^hi plasmablasts were induced in vivo in a BCR-specific manner and required help from Bcl-6⁺CD4⁺T cells. PD-L1^hi expression was not a characteristic of all antibody-secreting cells since plasma cells found during the chronic phase of infection expressed PD-L1 but at lower levels. PD-L1^hi plasmablasts were also present in mice infected with Plasmodium or with lymphocytic choriomeningitis virus, but not in mice with autoimmune disorders or immunized with T cell-dependent antigens. In vitro experiments showed that PD-L1^hi plasmablasts suppressed the T cell response, partially via PD-L1. Thus, this study reveals that extrafollicular PD-L1^hi plasmablasts, whose peaks of response precede the peak of germinal center response, may have a modulatory function in infections, thus influencing T cell response.

Cutaneous Dengue Virus Inoculation Triggers Strong B Cell Reactions but Contrastingly Poor T Cell Responses

Dengue is a global health problem without current specific treatment nor safe vaccines available. While severe dengue is related to pre-existing non-neutralizing dengue virus (DENV) antibodies, the role of T cells in protection or pathology is unclear. Using cutaneous DENV infection in immunocompetent mice we previously showed the generation of PNA+ germinal centers (GCs), now we assessed the activation and proliferation of B and T cells in draining lymph nodes (DLNs). We found a drastic remodelling of DLN compartments from 7 to 14 days post-infection (dpi) with greatly enlarged B cell follicles, occupying almost half of the DLN area compared to ~24% in naïve conditions. Enormous clusters of proliferating (Ki-67+) cells inside B follicles were found 14 dpi, representing ~33% of B cells in DLNs but only ~2% in non-infected mice. Inside GCs, we noticed an important recruitment of tingle body macrophages removing apoptotic cells. In contrast, the percentage of paracortex area and total T cells decreased by 14–16 dpi, compared to controls. Scattered randomly distributed Ki-67+ T cells were found, similar to non-infected mice. CD69 expression by CD4+ and CD8+ T cells was minor, while it was remarkable in B cells, representing 1764.7% of change from basal levels 3 dpi. The apparent lack of T cell responses cannot be attributed to apoptosis since no significant differences were observed compared to non-infected mice. This study shows massive B cell activation and proliferation in DLNs upon DENV infection. In contrast, we found very poor, almost absent CD4+ and CD8+ T cell responses.

LCMV-specific CD4 T cell dependent polyclonal B-cell activation upon persistent viral infection is short lived and extrafollicular

Persistent virus infections with non‐ or poorly cytopathic viruses are commonly associated with B cell dysregulations. These include the induction of hypergammaglobulinemia and the emergence of virus‐unspecific antibodies. These seemingly unspecific antibody responses interfere with the virus‐specific humoral immunity and contribute to delayed virus control. Whether these virus‐unspecific antibodies are induced in the B cell follicle or at extrafollicular sites and whether one specific CD4 T cell subset is involved in the polyclonal B cell activation is unclear. Here we studied virus‐unrelated IgG antibody responses against self or foreign antigens in the context of persistent lymphocytic choriomeningitis virus (LCMV) infection. We found that the LCMV‐unspecific antibody response is short‐lived and induced predominantly at extrafollicular sites and depends on the presence of LCMV‐specific CD4 T cells. Our data support a scenario in which activated, virus‐specific CD4 T cells provide help to non‐specific B cells at extrafollicular sites, supporting the production of virus unspecific IgG antibodies during persistent viral infection.

The Janus Face of Follicular T Helper Cells in Chronic Viral Infections

Chronic infections with non-cytopathic viruses constitutively expose virus-specific adaptive immune cells to cognate antigen, requiring their numeric and functional adaptation. Virus-specific CD8 T cells are compromised by various means in their effector functions, collectively termed T cell exhaustion. Alike CD8 T cells, virus-specific CD4 Th1 cell responses are gradually downregulated but instead, follicular T helper (T_FH) cell differentiation and maintenance is strongly promoted during chronic infection. Thereby, the immune system promotes antibody responses, which bear less immune-pathological risk compared to cytotoxic and pro-inflammatory T cell responses. This emphasis on T_FH cells contributes to tolerance of the chronic infection and is pivotal for the continued maturation and adaptation of the antibody response, leading eventually to the emergence of virus-neutralizing antibodies, which possess the potential to control the established chronic infection. However, sustained high levels of T_FH cells can also result in a less stringent B cell selection process in active germinal center reactions, leading to the activation of virus-unspecific B cells, including self-reactive B cells, and to hypergammaglobulinemia. This dispersal of B cell help comes at the expense of a stringently selected virus-specific antibody response, thereby contributing to its delayed maturation. Here, we discuss these opposing facets of T_FH cells in chronic viral infections.

Stochasticity enables BCR-independent germinal center initiation and antibody affinity maturation

Two immunoglobulin (Ig) diversification mechanisms collaborate to provide protective humoral immunity. Combinatorial assembly of IgH and IgL V region exons from gene segments generates preimmune Ig repertoires, expressed as B cell receptors (BCRs). Secondary diversification occurs when Ig V regions undergo somatic hypermutation (SHM) and affinity-based selection toward antigen in activated germinal center (GC) B cells. Secondary diversification is thought to only ripen the antigen-binding affinity of Igs that already exist (i.e., cognate Igs) because of chance generation during preimmune Ig diversification. However, whether stochastic activation of noncognate B cells can generate new affinity to antigen in GCs is unclear. Using a mouse model whose knock-in BCR does not functionally engage with immunizing antigen, we found that chronic immunization induced antigen-specific serological responses with diverse SHM-mediated antibody affinity maturation pathways and divergent epitope targeting. Thus, intrinsic GC B cell flexibility allows for somatic, noncognate B cell evolution, permitting de novo antigen recognition and subsequent antibody affinity maturation without initial preimmune BCR engagement.

IL-4-producing B cells regulate T helper cell dichotomy in type 1- and type 2-controlled diseases

Interleukin-4 (IL-4)-induced T helper (Th) 2 cells promote susceptibility to the protozoan parasite Leishmania major, while conferring immunity to the intestinal trematode Schistosoma mansoni Here, we report that abrogation of IL-4 receptor alpha (IL-4Rα) signaling on B cells in BALB/c mice (mb1^creIL-4Rα^-/lox) transformed nonhealer BALB/c to a healer phenotype with an early type 1 and dramatically reduced type 2 immune response and an absence of ulceration and necrosis during cutaneous leishmaniasis. From adoptive reconstitution and mixed bone-marrow chimera studies in B cell-deficient (µMT) mice, we reveal a central role for B cell-derived IL-4 and IL-4Rα in the optimal induction of the susceptible type 2 phenotype to L. major infection. We further demonstrate that the absence of IL-4Rα signaling on B cells exacerbated S. mansoni-induced mortality and pathology in BALB/c mice, due to a diminished type 2 immune response. In both disease models, IL-4Rα-responsive B cells displayed increased IL-4 production as early as day 1 after infection. Together, these results demonstrate that IL-4-producing and IL-4Rα-responsive B cells are critical in regulating and assisting early T helper dichotomy toward Th2 responses, which are detrimental in cutaneous leishmaniasis but beneficial in acute schistosomiasis.

Serological markers of autoimmunity in children with hepatitis A: relation to acute and fulminant presentation

OBJECTIVES

Hepatitis A virus (HAV) infection tends to be a self-limiting disease without serious sequelae, but fulminant hepatitis, with a high mortality, develops in 0.1-0.2% of the cases. Sometimes, HAV infection precipitates autoimmune hepatitis (AIH). We aimed to assess the frequency and clinical significance of serologic markers of autoimmunity during hepatitis A infection with an acute or fulminant presentation compared with those in AIH.

METHODS

The study included 126 children: 46 with HAV infection (33 with acute and 13 with fulminant presentation), 53 with AIH, and 27 healthy controls. In all, we measured autoantibodies titer (antinuclear antibody, antismooth muscle antibody, and liver kidney microsomal antibody-1) and serum gammaglobulins.

RESULTS

Autoantibodies were detected in the majority of HAV (63.1%) and AIH (79.2%) groups, but in none of the controls. Gammaglobulins were significantly higher in the HAV group (1.93±0.57 g/dl) than in the controls (1.32±0.29 g/dl), but lower than that in the AIH group (2.93±1.2 g/dl) (P<0.0001 for all). In the HAV group, gammaglobulins were significantly higher in those with fulminant (2.21±0.46 g/dl) than in those with acute presentation (1.82±0.57 g/dl) (P=0.019), but comparable with that in AIH (P=0.095). Gammaglobulins correlated significantly with disease severity in both HAV and AIH groups.

CONCLUSION

Hypergammaglobulinemia and a high occurrence of autoantibodies are encountered in HAV infection. This may support the immunological basis of its pathogenesis. Moreover, the higher gammaglobulins in fulminant HAV, with an insignificant difference from that in AIH, suggest that a more aggressive immunological reaction is related to this presentation.

Toll-like receptor 2-mediated innate immune responses against Junín virus in mice lead to antiviral adaptive immune responses during systemic infection and do not affect viral replication in the brain

Successful adaptive immunity to virus infection often depends on the initial innate response. Previously, we demonstrated that Junín virus, the etiological agent responsible for Argentine hemorrhagic fever (AHF), activates an early innate immune response via an interaction between the viral glycoprotein and Toll-like receptor 2 (TLR2). Here we show that TLR2/6 but not TLR1/2 heterodimers sense Junín virus glycoprotein and induce a cytokine response, which in turn upregulates the expression of the RNA helicases RIG-I and MDA5. NF-κB and Erk1/2 were important in the cytokine response, since both proteins were phosphorylated as a result of the interaction of virus with TLR2, and treatment with an Erk1/2-specific inhibitor blocked cytokine production. We show that the Junín virus glycoprotein activates cytokine production in a human macrophage cell line as well. Moreover, we show that TLR2-mediated immune response plays a role in viral clearance because wild-type mice cleared Candid 1 (JUNV C1), the vaccine strain of Junín virus, more rapidly than did TLR2 knockout mice. This clearance correlated with the generation of Junín virus-specific CD8(+) T cells. However, infected wild-type and TLR2 knockout mice developed TLR2-independent blocking antibody responses with similar kinetics. We also show that microglia and astrocytes but not neurons are susceptible to infection with JUNV C1. Although JUNV C1 infection of the brain also triggered a TLR2-dependent cytokine response, virus levels were equivalent in wild-type and TLR2 knockout mice. Importance: Junín virus is transmitted by rodents native to Argentina and is associated with both systemic disease and, in some patients, neurological symptoms. Humans become infected when they inhale aerosolized Junín virus. AHF has a 15 to 30% mortality rate, and patients who clear the infection develop a strong antibody response to Junín virus. Here we investigated what factors determine the immune response to Junín virus. We show that a strong initial innate immune response to JUNV C1 determines how quickly mice can clear systemic infection and that this depended on the cellular immune response. In contrast, induction of an innate immune response in the brain had no effect on virus infection levels. These findings may explain how the initial immune response to Junín virus infection could determine different outcomes in humans.

IL-4Rα-associated antigen processing by B cells promotes immunity in Nippostrongylus brasiliensis infection

In this study, B cell function in protective T_H2 immunity against N. brasiliensis infection was investigated. Protection against secondary infection depended on IL-4Rα and IL-13; but not IL-4. Protection did not associate with parasite specific antibody responses. Re-infection of B cell-specific IL-4Rα^−/− mice resulted in increased worm burdens compared to control mice, despite their equivalent capacity to control primary infection. Impaired protection correlated with reduced lymphocyte IL-13 production and B cell MHC class II and CD86 surface expression. Adoptive transfer of in vivo N. brasiliensis primed IL-4Rα expressing B cells into naïve BALB/c mice, but not IL-4Rα or IL-13 deficient B cells, conferred protection against primary N. brasiliensis infection. This protection required MHC class II compatibility on B cells suggesting cognate interactions by B cells with CD4⁺ T cells were important to co-ordinate immunity. Furthermore, the rapid nature of these protective effects by B cells suggested non-BCR mediated mechanisms, such as via Toll Like Receptors, was involved, and this was supported by transfer experiments using antigen pulsed Myd88^−/− B cells. These data suggest TLR dependent antigen processing by IL-4Rα-responsive B cells producing IL-13 contribute significantly to CD4⁺ T cell-mediated protective immunity against N. brasiliensis infection.

Parasitic nematode infections are an extremely important global public health problem. Infections by hookworms and roundworms for example cause anemia, widespread developmental problems and devalued immunity against bacterial infections such as salmonella and tuberculosis. Although treatable with drugs, parasitic nematode re-infections occur as humans do not develop protective immunity. Ultimately, the public health burden caused by these infections will be best controlled by the development of vaccines against nematode infections. For these to be effective, it is important to understand how the various components of the immune system can respond to infection. In this study, we show that B cells, which typically protect against infection by producing antibodies, can also protect against an experimental hookworm like nematode infection by additional mechanisms. This form of protection instead depended on B cells producing cytokines associated with parasitic nematode expulsion and also by providing T cells with specific instruction. Together, these B cell driven responses lead to a rapid resolution of the infection. These important findings indicate that vaccination strategies against nematode parasites such as hookworms need to understand immune responses other than antibody to be optimally protective.

High efficiency of antiviral CD4(+) killer T cells

The destruction of infected cells by cytotxic T lymphocytes (CTL) is integral to the effective control of viral and bacterial diseases, and CTL function at large has long been regarded as a distinctive property of the CD8(+)T cell subset. In contrast, and despite their first description more than three decades ago, the precise contribution of cytotoxic CD4(+)T cells to the resolution of infectious diseases has remained a matter of debate. In particular, the CTL activity of pathogen-specific CD4(+) "helper" T cells constitutes a single trait among a diverse array of other T cell functionalities, and overall appears considerably weaker than the cytolytic capacity of CD8(+) effector T cells. Here, using an in vivo CTL assay, we report that cytotoxic CD4(+)T cells are readily generated against both viral and bacterial pathogens, and that the efficiency of MHC-II-restricted CD4(+)T cell killing adjusted for effector:target cell ratios, precise specificities and functional avidities is comparable in magnitude to that of CD8(+)T cells. In fact, the only difference between specific CD4(+) and CD8(+)T cells pertains to the slightly delayed killing kinetics of the former demonstrating that potent CTL function is a cardinal property of both antiviral CD8(+) and CD4(+)T cells.

Control of B cells expressing naturally occurring autoantibodies

Naturally occurring autoantibodies (NAbs) are typically polyreactive, bind with low affinity to a discrete set of autoantigens and are encoded by variable region genes in germline configuration. They differ from disease-associated autoantibodies (autoAb), which are mostly monoreactive, somatically mutated and of high affinities. Structure-function studies have shown that polyreactivity of NAbs relies on the somatically generated complementarity determining region, CDR3, of the heavy chain. This finding suggested that NAb-producing B cells were positively selected from the pre-immune B-cell repertoire. The biological significance of this selection remains, however, unclear. Data originating mainly from transgenic mice have shown that mature NAb-producing B cells are frequently ignorant toward their antigen, possibly due to their low affinity, though active tolerance mechanisms are not excluded. An important issue is whether NAb-producing B cells constitute the pool from which pathologic auto Ab emerge after autoantigen-driven maturation. We summarize results obtained in mouse models, showing that some infectious agents are able to induce an autoantigen-driven activation of certain NAb-producing B cells. However direct proof that selection by autoantigen may lead to somatic hypermutation are still lacking. Other data tend to suggest that pathologic auto Abs may derive from non-autoimmune B cells that have diversified by somatic hypermutation of their variable region genes.

Adaptive immunity to Lymphocytic choriomeningitis virus: new insights into antigenic determinants

Lymphocytic choriomeningitis virus (LCMV) is one of the most studied infectious disease models in mice. Human infection with LCMV can result in severe disease, ranging from aseptic meningitis in immunocompetent individuals, hydrocephalus, chorioretinitis or microcephaly in fetal infection, or to a highly lethal outcome in immunosuppressed individuals. This review examines recent advances in our understanding of the adaptive immune response to LCMV and how the cell-mediated and humoral immune responses contribute to protective immunity. New insights into the antigenicity of the LCMV proteome and the complexity of the cell-mediated immune response are addressed. We also discuss state-of-the-art approaches for T-cell epitope discovery in murine and human backgrounds and their recent application to LCMV. New findings regarding CD4+ T-cell dysregulation during chronic LCMV infection, and potential avenues for the treatment of chronic viral infection through modulation of the programmed cell death-1 receptor and/or IL-10 signaling pathways, are also evaluated.