Impaired germinal centre formation and humoral immune response in the absence of CD28 and interleukin-4

Follicular dendritic cells restrict interleukin-4 availability in germinal centers and foster memory B cell generation

B cells within germinal centers (GCs) enter cycles of antibody affinity maturation or exit the GC as memory cells or plasma cells. Here, we examined the contribution of interleukin (IL)-4 on B cell fate decisions in the GC. Single-cell RNA-sequencing identified a subset of light zone GC B cells expressing high IL-4 receptor-a (IL4Ra) and CD23 and lacking a Myc-associated signature. These cells could differentiate into pre-memory cells. B cell-specific deletion of IL4Ra or STAT6 favored the pre-memory cell trajectory, and provision of exogenous IL-4 in a wild-type context reduced pre-memory cell frequencies. IL-4 acted during antigen-specific interactions but also influenced bystander cells. Deletion of IL4Ra from follicular dendritic cells (FDCs) increased the availability of IL-4 in the GC, impaired the selection of affinity-matured B cells, and reduced memory cell generation. We propose that GC FDCs establish a niche that limits bystander IL-4 activity, focusing IL-4 action on B cells undergoing selection and enhancing memory cell differentiation.

HIV-1 infection of CD4 T cells impairs antigen-specific B cell function

Failures to produce neutralizing antibodies upon HIV‐1 infection result in part from B‐cell dysfunction due to unspecific B‐cell activation. How HIV‐1 affects antigen‐specific B‐cell functions remains elusive. Using an adoptive transfer mouse model and ex vivo HIV infection of human tonsil tissue, we found that expression of the HIV‐1 pathogenesis factor NEF in CD4 T cells undermines their helper function and impairs cognate B‐cell functions including mounting of efficient specific IgG responses. NEF interfered with T cell help via a specific protein interaction motif that prevents polarized cytokine secretion at the T‐cell–B‐cell immune synapse. This interference reduced B‐cell activation and proliferation and thus disrupted germinal center formation and affinity maturation. These results identify NEF as a key component for HIV‐mediated dysfunction of antigen‐specific B cells. Therapeutic targeting of the identified molecular surface in NEF will facilitate host control of HIV infection.

T Follicular Regulatory Cell Suppression of T Follicular Helper Cell Function Is Context-Dependent in vitro

The production of antibody-secreting plasma cells and memory B cells requires the interaction of T follicular helper (Tfh) cells with B cells in the follicle and is modulated by T follicular regulatory (Tfr) cells. We compare the effects of Tfr cells in an in vitro model of bystander Tfh function in the absence of BCR engagement and in a model in which mimics cognate T-B interactions in which the BCR is engaged. In the absence of Tfr cells, Tfh cells from primed mice induce naive B cell differentiation into GC B cells and class switch recombination (CSR) in the presence of anti-CD3 alone or anti-CD3/IgM in a contact-dependent manner. Addition of primed Tfr cells efficiently suppressed GC B cell proliferation, differentiation and CSR in the anti-CD3 alone cultures, but only moderately suppressed BCR-stimulated B cells. When stimulated with anti-CD3 alone, IL-4 is critical for the induction of GC B cells and CSR. IL-21 plays a minimal role in GC B cell differentiation, but a greater role in switching. When the BCR is engaged, IL-4 is primarily required for switching and IL-21 only modestly affects switching. CD40L expression was critical for Tfh-mediated B cell proliferation/differentiation in the absence of B cell engagement. When the BCR was engaged, proliferation of CD40 deficient B cells was partially restored, but was susceptible to suppression by Tfr. These studies suggest that in vitro Tfr suppressor function is complex and is modulated by BCR signaling and CD40-CD40L interactions.

Oral Vaccination with a DNA Vaccine Encoding Capsid Protein of Duck Tembusu Virus Induces Protection Immunity

The emergence of duck tembusu virus (DTMUV), a new member of the Flavivirus genus, has caused great economical loss in the poultry industry in China. Since the outbreak and spread of DTMUV is hard to control in a clinical setting, an efficient and low-cost oral delivery DNA vaccine SL7207 (pVAX1-C) based on the capsid protein of DTMUV was developed and evaluated in this study. The antigen capsid protein was expressed from the DNA vaccine SL7207 (pVAX1-C), both in vitro and in vivo. The humoral and cellular immune responses in vivo were observed after oral immunization with the SL7207 (pVAX1-C) DNA vaccine. High titers of the specific antibody against the capsid protein and the neutralizing antibody against the DTMUV virus were both detected after inoculation. The ducks were efficiently protected from lethal DTMUV exposure by the SL7207 (pVAX1-C) vaccine in this experiment. Taken together, we demonstrated that the capsid protein of DTMUV possesses a strong immunogenicity against the DTMUV infection. Moreover, an oral delivery of the DNA vaccine SL7207 (pVAX1-C) utilizing Salmonella SL7207 was an efficient way to protect the ducks against DTMUV infection and provides an economic and fast vaccine delivery strategy for a large scale clinical use.

Differing Requirements for MALT1 Function in Peripheral B Cell Survival and Differentiation

During a T cell-dependent immune response, formation of the germinal center (GC) is essential for the generation of high-affinity plasma cells and memory B cells. The canonical NF-κB pathway has been implicated in the initiation of GC reaction, and defects in this pathway have been linked to immune deficiencies. The paracaspase MALT1 plays an important role in regulating NF-κB activation upon triggering of Ag receptors. Although previous studies have reported that MALT1 deficiency abrogates the GC response, the relative contribution of B cells and T cells to the defective phenotype remains unclear. We used chimeric mouse models to demonstrate that MALT1 function is required in B cells for GC formation. This role is restricted to BCR signaling where MALT1 is critical for B cell proliferation and survival. Moreover, the proapoptotic signal transmitted in the absence of MALT1 is dominant to the prosurvival effects of T cell-derived stimuli. In addition to GC B cell differentiation, MALT1 is required for plasma cell differentiation, but not mitogenic responses. Lastly, we show that ectopic expression of Bcl-2 can partially rescue the GC phenotype in MALT1-deficient animals by prolonging the lifespan of BCR-activated B cells, but plasma cell differentiation and Ab production remain defective. Thus, our data uncover previously unappreciated aspects of MALT1 function in B cells and highlight its importance in humoral immunity.

Characteristics of B cell‑associated gene expression in patients with coronary artery disease

The current study aimed to identify differentially expressed B cell‑associated genes in peripheral blood mononuclear cells and observe the changes in B cell activation at different stages of coronary artery disease. Groups of patients with acute myocardial infarction (AMI) and stable angina (SA), as well as healthy volunteers, were recruited into the study (n=20 per group). Whole human genome microarray analysis was performed to examine the expression of B cell‑associated genes among these three groups. The mRNA expression levels of 60 genes associated with B cell activity and regulation were measured using reverse transcription‑quantitative polymerase chain reaction. The mRNA expression of the B cell antigen receptor (BCR)‑associated genes, CD45, NFAM, SYK and LYN, were significantly upregulated in patients with AMI; however, FCRL3, CD79B, CD19, CD81, FYN, BLK, CD22 and CD5 mRNA expression levels were significantly downregulated, compared with patients in the SA and control group. The mRNA levels of the T‑independent B cell‑associated genes, CD16, CD32, LILRA1 and TLR9, were significantly increased in AMI patients compared with SA and control patients. The mRNA expression of genes associated with T‑dependent B cells were also measured: EMR2 and CD97 were statistically upregulated, whereas SLAMF1, LY9, CD28, CD43, CD72, ICOSL, PD1, CD40 and CD20 mRNAs were significantly downregulated in AMI group patients compared with the two other groups. Additionally the gene expression levels of B cell regulatory genes were measured. In patients with AMI, CR1, LILRB2, LILRB3 and VAV1 mRNA expression levels were statistically increased, whereas, CS1 and IL4I1 mRNAs were significantly reduced compared with the SA and control groups. There was no statistically significant difference in B cell‑associated gene expression levels between patients with SA and the control group. The present study identified the downregulation of genes associated with BCRs, B2 cells and B cell regulators in patients with AMI, indicating a weakened T cell‑B cell interaction and reduced B2 cell activation during AMI. Thus, improving B2 cell‑mediated humoral immunity may be a potential target for medical intervention in patients with AMI.

IL-4-secreting secondary T follicular helper (Tfh) cells arise from memory T cells, not persisting Tfh cells, through a B cell-dependent mechanism

Humoral immunity requires cross-talk between T follicular helper (Tfh) cells and B cells. Nevertheless, a detailed understanding of this intercellular interaction during secondary immune responses is lacking. We examined this by focusing on the response to a soluble, unadjuvanted, pathogen-derived Ag (soluble extract of Schistosoma mansoni egg [SEA]) that induces type 2 immunity. We found that activated Tfh cells persisted for long periods within germinal centers following primary immunization. However, the magnitude of the secondary response did not appear to depend on pre-existing Tfh cells. Instead, Tfh cell populations expanded through a process that was dependent on memory T cells recruited into the reactive LN, as well as the participation of B cells. We found that, during the secondary response, IL-4 was critical for the expansion of a population of plasmablasts that correlated with increased SEA-specific IgG1 titers. Additionally, following immunization with SEA (but not with an Ag that induced type 1 immunity), IL-4 and IL-21 were coproduced by individual Tfh cells, revealing a potential mechanism through which appropriate class-switching can be coupled to plasmablast proliferation to enforce type 2 immunity. Our findings demonstrate a pivotal role for IL-4 in the interplay between T and B cells during a secondary Th2 response and have significant implications for vaccine design.

A novel Lyn-protein kinase Cδ/ε-protein kinase D axis is activated in B cells by signalosome-independent alternate pathway BCR signaling

BCR signaling initiates multiple activities critical for B-cell function. Recently, we identified an alternate BCR signaling pathway, induced by IL-4, that is signalosome-independent, unlike the classical signalosome-dependent pathway, and that leads to activation of the MAP kinase, ERK. Here we questioned whether alternate pathway signaling extends to other key downstream events, especially protein kinase D (PKD) activation. We found that in murine spleen-derived B cells the IL-4-induced alternate pathway for BCR signaling results in PKD and PKD substrate phosphorylation, and that alternate pathway phosphorylation of HDAC5/7 and other key substrates requires PKD. Furthermore, we found that tyrosine phosphorylation of PKCδ/ε occurs as a result of alternate but not classical pathway signaling and is required for phosphorylation of PKD and PKD substrates. This result identifies PKCδ/ε tyrosine phosphorylation as a unique outcome of the alternate pathway. The alternate pathway is mediated by Lyn that is not required for classical pathway signaling and we found that Lyn associates directly with PKCδ/ε and is required for phosphorylation of PKCδ/ε and of PKD. These findings indicate that IL-4 influences B-cell activation by inducing a novel signaling pathway from BCR to Lyn to PKCδ/ε to PKD.

Fyn kinase is required for optimal humoral responses

The generation of antigen-specific antibodies and the development of immunological memory require collaboration between B and T cells. T cell-secreted IL-4 is important for B cell survival, isotype switch to IgG1 and IgE, affinity maturation, and the development of germinal centers (GC). Fyn, a member of the Src family tyrosine kinase, is widely expressed in many cell types, including lymphocytes. This kinase is known to interact with both the B cell and T cell receptor (BCR and TCR, respectively). While Fyn deletion does not impair the development of immature T cells and B cells, TCR signaling is altered in mature T cells. The current study demonstrates that Fyn deficient (KO) B cells have impaired IL-4 signaling. Fyn KO mice displayed low basal levels of IgG1, IgE and IgG2c, and delayed antigen-specific IgG1 and IgG2b production, with a dramatic decrease in antigen-specific IgG2c following immunization with a T-dependent antigen. Defects in antibody production correlated with significantly reduced numbers of GC B cells, follicular T helper cells (T_FH), and splenic plasma cells (PC). Taken together, our data demonstrate that Fyn kinase is required for optimal humoral responses.

Extracellular matrix from porcine small intestinal submucosa (SIS) as immune adjuvants

Porcine small intestinal submucosa (SIS) of Cook Biotech is licensed and widely used for tissue remodeling in humans. SIS was shown to be highly effective as an adjuvant in model studies with prostate and ovarian cancer vaccines. However, SIS adjuvanticity relative to alum, another important human-licensed adjuvant, has not yet been delineated in terms of activation of innate immunity via inflammasomes and boosting of antibody responses to soluble proteins and hapten-protein conjugates. We used ovalbumin, and a hapten-protein conjugate, phthalate-keyhole limpet hemocyanin. The evaluation of SIS was conducted in BALB/c and C57BL/6 mice using both intraperitoneal and subcutaneous routes. Inflammatory responses were studied by microarray profiling of chemokines and cytokines and by qPCR of inflammasomes-related genes. Results showed that SIS affected cytokine and chemokines microenvironments such as up-regulation of IL-4 and CD30-ligand and activation of chemotactic factors LIX and KC (neutrophil chemotactic factors), MCP-1 (monocytes chemotactic factors), MIP 1-α (macrophage chemotactic factor) and lymphotactin, as well as, growth factors like M-CSF. SIS also promoted gene expression of Nod-like receptors (NLR) and associated downstream effectors. However, in contrast to alum, SIS had no effects on pro-inflammatory cytokines (IL-6, IL-1β, TNF-α) or NLRP3, but it appeared to promote both Th1 and Th2 responses under different conditions. Lastly, it was as effective as alum in engendering a lasting and specific antibody response, primarily of IgG1 type.

An enigmatic tail of CD28 signaling

CD28 costimulation regulates a wide range of cellular processes, from proliferation and survival to promoting the differentiation of specialized T-cell subsets. Since first being identified over 20 years ago, CD28 has remained a subject of intense study because of its profound consequences on T cell function and its potential for therapeutic manipulation. In this review we highlight the signaling cascades initiated by the major signaling motifs in CD28, focusing on PI-3 kinase-dependent and -independent pathways and how these are linked to specific cellular outcomes. Recent studies using gene targeted knockin mice have clarified the relative importance of these motifs on in vivo immune responses; however, much remains to be elucidated. Understanding the mechanism behind costimulation holds great potential for development of new clinically relevant reagents, a fact beginning to be realized with the advent of drugs that prevent CD28 ligation and signaling.

Th1 cytokine responses fail to effectively control Chlamydia lung infection in ICOS ligand knockout mice

ICOS ligand (ICOSL) plays an important role in controlling specific aspects of T cell activation, differentiation, and function. Th1-type immune responses have been shown to be critical in host defense against chlamydial infections. To assess the role of ICOSL-ICOS interaction in host defense against chlamydial infection, we compared the immune responses and pathological reactions in ICOSL gene knockout (KO) and wild-type (WT) mice following Chlamydia muridarum lung infection. The results showed that ICOSL KO mice exhibited greater body weight loss, higher pathogen burden, and more severe histopathology in their lung than did WT mice. Cytokine analysis revealed that ICOSL KO mice produced lower levels of Th2 (IL-4 and IL-5) and anti-inflammatory (TGF-beta1 and IL-10) cytokines, but higher Th1-related (IFN-gamma and IL-12p40/IL-23) and proinflammatory (IL-6 and TNF-alpha) cytokines. ICOSL KO mice also showed reduced Chlamydia-specific Ab levels in their sera and lung homogenates. In addition, ICOSL KO mice demonstrated significantly lower ICOS expression in T cells and lower Th17 responses than did WT mice. Finally, we showed that ICOS-ICOSL interaction and cell-cell contact are essential for CD4(+) T cells to inhibit chlamydial growth in the cultured lung fibroblasts. The data suggest that ICOSL plays a significant role in immunoregulation and protective immunity against Chlamydia infections and that the Th1 skew in cytokine responses per se is not sufficient for effective control of Chlamydia infections.

Cytokine-secreting follicular T cells shape the antibody repertoire

High-affinity antibodies are critical for host protection and underlie successful vaccines. The generation of such antibodies requires T cell-dependent help, which mediates germinal center reactions in which mutation and selection of B cells occurs. Using an interleukin 4-reporter system, we show here that CD4(+) follicular helper T cells constituted essentially all of the cytokine-secreting T cells in lymph nodes and were functionally distinct from T cells secreting the same cytokine in peripheral tissues. Follicular helper T cells with different cytokine profiles could be isolated as conjugates with B cells undergoing cytokine-specific immunoglobulin class switching with evidence of somatic hypermutation. Our findings support a model in which B cells compete for cytokines produced by follicular helper T cells that shape the affinity and isotype of the antibody response.

Low-dose tolerance is mediated by the microfold cell ligand, reovirus protein sigma1

Mucosal tolerance induction generally requires multiple or large Ag doses. Because microfold (M) cells have been implicated as being important for mucosal tolerance induction and because reovirus attachment protein sigma1 (psigma1) is capable of binding M cells, we postulated that targeting a model Ag to M cells via psigma1 could induce a state of unresponsiveness. Accordingly, a genetic fusion between OVA and the M cell ligand, reovirus psigma1, termed OVA-psigma1, was developed to enhance tolerogen uptake. When applied nasally, not parenterally, as little as a single dose of OVA-psigma1 failed to induce OVA-specific Abs even in the presence of adjuvant. Moreover, the mice remained unresponsive to peripheral OVA challenge, unlike mice given multiple nasal OVA doses that rendered them responsive to OVA. The observed unresponsiveness to OVA-psigma1 could be adoptively transferred using cervical lymph node CD4(+) T cells, which failed to undergo proliferative or delayed-type hypersensitivity responses in recipients. To discern the cytokines responsible as a mechanism for this unresponsiveness, restimulation assays revealed increased production of regulatory cytokines, IL-4, IL-10, and TGF-beta1, with greatly reduced IL-17 and IFN-gamma. The induced IL-10 was derived predominantly from FoxP3(+)CD25(+)CD4(+) T cells. No FoxP3(+)CD25(+)CD4(+) T cells were induced in OVA-psigma1-dosed IL-10-deficient (IL-10(-/-)) mice, and despite showing increased TGF-beta1 synthesis, these mice were responsive to OVA. These data demonstrate the feasibility of using psigma1 as a mucosal delivery platform specifically for low-dose tolerance induction.

Role of CXCR5 and CCR7 in follicular Th cell positioning and appearance of a programmed cell death gene-1high germinal center-associated subpopulation

Th cell access to primary B cell follicles is dependent on CXCR5. However, whether CXCR5 induction on T cells is sufficient in determining their follicular positioning has been unclear. In this study, we find that transgenic CXCR5 overexpression is not sufficient to promote follicular entry of naive T cells unless the counterbalancing influence of CCR7 ligands is removed. In contrast, the positioning of Ag-engaged T cells at the B/T boundary could occur in the absence of CXCR5. The germinal center (GC) response was 2-fold reduced when T cells lacked CXCR5, although these T cells were able to access the GC. Finally, CXCR5(high)CCR7(low) T cells were found to have elevated IL-4 transcript and programmed cell death gene-1 (PD-1) expression, and PD-1(high) cells were reduced in the absence of T cell CXCR5 or in mice compromised in GC formation. Overall, these findings provide further understanding of how the changes in CXCR5 and CCR7 expression regulate Th cell positioning during Ab responses, and they suggest that development and/or maintenance of a PD-1(high) follicular Th cell subset is dependent on appropriate interaction with GC B cells.

Germinal-center organization and cellular dynamics

Germinal centers (GCs) are important sites of antibody affinity maturation. In the classical model, the GC dark zone contains large centroblasts that are rapidly proliferating and undergoing somatic hypermutation of their antibody variable-region genes. Centroblasts give rise to smaller nonproliferating centrocytes in the light zone that compete for binding antigen on follicular dendritic cells. Recently, the approach of real-time imaging of GCs by two-photon microscopy of intact lymph nodes has provided new insights into GC dynamics that both support and challenge fundamental aspects of this model. Here we review recent and older findings on cell migration, proliferation, and interaction dynamics in the GC and discuss a model in which dark- and light-zone cells are morphologically similar, proliferation occurs in both zones, and GC B cells compete for T cell help as well as antigen.

Role of T lymphocyte replicative senescence in vaccine efficacy

Immunological changes associated with aging play a major role in both the blunted responses to infections as well as to vaccines intended to prevent many of these infections. Several independent studies on immune correlates of poor vaccine responsiveness have identified a novel immune biomarker of reduced antibody response to vaccination, namely high proportions of memory CD8 T lymphocytes lacking expression of the CD28 costimulatory molecule. Research on this population of CD8(+)CD28(-) T lymphocytes has documented characteristics suggestive of replicative senescence, including inability to proliferate, reduced telomere length, and altered cytokine profiles. CD8(+)CD28(-) T lymphocytes have also been associated with suppressor functions and with early mortality in the elderly. This article discusses some of the challenges involved in custom-designing vaccines for the elderly, and suggests several immunomodulatory strategies that may enhance vaccine responsiveness in this age group.

A dose-dependent requirement for the proline motif of CD28 in cellular and humoral immunity revealed by a targeted knockin mutant

Activation of naive T cells requires the integration of signals through the antigen receptor and CD28. Although there is agreement on the importance of CD28, there remains controversy on the mechanism by which CD28 regulates T cell function. We have generated a gene-targeted knockin mouse expressing a mutation in the C-terminal proline-rich region of the cytoplasmic tail of CD28. Our analysis conclusively showed that this motif is essential for CD28-dependent regulation of interleukin 2 secretion and proliferation. In vivo analysis revealed that mutation of this motif-dissociated CD28-dependent regulation of cellular and humoral responses in an allergic airway inflammation model. Furthermore, we find an important gene dosage effect on the phenotype of the mutation and provide a mechanistic explanation for the conflicting data on the significance of this motif in CD28 function.

DNA vaccines, combining form of antigen and method of delivery to raise a spectrum of IFN-gamma and IL-4-producing CD4+ and CD8+ T cells

DNA-based immunizations have been used to determine the patterns of type 1 and type 2 cytokines that can be induced in vivo for Ag-specific CD4(+) and CD8(+) T cells. IL-4 was used as a signature cytokine for a type 2 T cell response and IFN-gamma as the signature cytokine for a type 1 response. Gene gun deliveries of secreted Ags were used to bias responses toward type 2 and saline injections of cell-associated Ags to bias responses toward type 1. The studies revealed that gene gun bombardments of DNAs expressing secreted Ags strongly biased responses toward type 2, inducing IL-4-producing CD8(+) as well as CD4(+) T cells. Saline injections of DNAs expressing cell-associated Ags strongly biased responses toward type 1, inducing IFN-gamma-producing CD8(+) and CD4(+) cells. A mixed type 1/type 2 response of IFN-gamma-producing CD8(+) T cells and IL-4-producing CD4(+) T cells was found for gene gun deliveries of cell-associated Ags. Saline injections of secreted Ags raised a weakly type 1-biased response characterized by only slightly higher frequencies of IFN-gamma- than IL-4-producing CD4(+) and CD8(+) T cells. Studies in B cell knockout and hen egg lysozyme Ig transgenic mice revealed that B cells were required for the generation of IL-4-producing CD8(+) T cells.

Costimulation through the inducible costimulator ligand is essential for both T helper and B cell functions in T cell-dependent B cell responses

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell-dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl(-/-) and Icos(-/-) mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl(-/-) mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell-dependent IgG1 production.

Problems and solutions to the development of vaccines in the elderly

Scientists involved in vaccine research and development face the challenge of protecting the ever-increasing elderly population from a broad spectrum of infectious diseases. The optimal vaccine-induced immune response to confer protection is undefined for many pathogens, and the field of vaccine research is undergoing a gradual shift from the original focus on humoral immunity to a focus that incorporates cellular and innate immune components. The age-related changes in various aspects of immune function, including an increase in a population of T cells that shows signs of replicative senescence, underscore the need to enhance research aimed at designing vaccines to meet the unique requirements of the elderly population.