T-cell–B-cell cooperation

The role of B cells and humoral immunity in Mycobacterium tuberculosis infection

Tuberculosis (TB) remains a serious threat to public health, causing 2 million deaths annually world-wide. The control of TB has been hindered by the requirement of long duration of treatment involving multiple chemotherapeutic agents, the increased susceptibility to Mycobacterium tuberculosis infection in the HIV-infected population, and the development of multi-drug resistant and extensively resistant strains of tubercle bacilli. An efficacious and cost-efficient way to control TB is the development of effective anti-TB vaccines. This measure requires thorough understanding of the immune response to M. tuberculosis. While the role of cell-mediated immunity in the development of protective immune response to the tubercle bacillus has been well established, the role of B cells in this process is not clearly understood. Emerging evidence suggests that B cells and humoral immunity can modulate the immune response to various intracellular pathogens, including M. tuberculosis. These lymphocytes form conspicuous aggregates in the lungs of tuberculous humans, non-human primates, and mice, which display features of germinal center B cells. In murine TB, it has been shown that B cells can regulate the level of granulomatous reaction, cytokine production, and the T cell response. This chapter discusses the potential mechanisms by which specific functions of B cells and humoral immunity can shape the immune response to intracellular pathogens in general, and to M. tuberculosis in particular. Knowledge of the B cell-mediated immune response to M. tuberculosis may lead to the design of novel strategies, including the development of effective vaccines, to better control TB.

Major Histocompatibility Class II Pathway Is Not Required for the Development of Nonalcoholic Fatty Liver Disease in Mice

Single-nucleotide polymorphisms within major histocompatibility class II (MHC II) genes have been associated with an increased risk of drug-induced liver injury. However, it has never been addressed whether the MHC II pathway plays an important role in the development of nonalcoholic fatty liver disease, the most common form of liver disease. We used a mouse model that has a complete knockdown of genes in the MHC II pathway (MHCII(Δ/Δ)). Firstly we studied the effect of high-fat diet-induced hepatic inflammation in these mice. Secondly we studied the development of carbon-tetra-chloride- (CCl4-) induced hepatic cirrhosis. After the high-fat diet, both groups developed obesity and hepatic steatosis with a similar degree of hepatic inflammation, suggesting no impact of the knockdown of MHC II on high-fat diet-induced inflammation in mice. In the second study, we confirmed that the CCl4 injection significantly upregulated the MHC II genes in wild-type mice. The CCl4 treatment significantly induced genes related to the fibrosis formation in wild-type mice, whereas this was lower in MHCII(Δ/Δ) mice. The liver histology, however, showed no detectable difference between groups, suggesting that the MHC II pathway is not required for the development of hepatic fibrosis induced by CCl4.

Isolation of carbohydrate-specific CD4(+) T cell clones from mice after stimulation by two model glycoconjugate vaccines

Here we describe how to isolate carbohydrate-specific T cell clones (for which we propose the designation 'Tcarbs') after stimulation by two glycoconjugate vaccines. We describe how to prepare, purify and characterize two model glycoconjugate vaccines that can be used to generate Tcarbs. These glycoconjugate vaccines (GBSIII-OVA and GBSIII-TT) are synthesized by conjugation of type III group B streptococcal polysaccharide (GBSIII) to ovalbumin (OVA) or tetanus toxoid (TT). Upon immunization of mice with GBSIII-OVA, carbohydrate epitopes are presented to and recognized by CD4(+) T cells. Subsequently, polysaccharide-recognizing CD4(+) T cells are expanded in vitro by stimulating splenic CD4(+) T cells with GBSIII-TT. The sequential use of two distinct glycoconjugate vaccines containing the same polysaccharide conjugated to heterologous carrier proteins selects for and expands carbohydrate-specific T cells. This protocol can readily be adapted to study the stimulation of the immune system by alternative glycoconjugate vaccines. This protocol takes 1-2 years to complete.

A mechanism for glycoconjugate vaccine activation of the adaptive immune system and its implications for vaccine design

Although glycoconjugate vaccines have provided enormous health benefits globally, they have been less successful in significant high-risk populations. Exploring novel approaches to the enhancement of glycoconjugate effectiveness, we investigated molecular and cellular mechanisms governing the immune response to a prototypical glycoconjugate vaccine. In antigen-presenting cells, a carbohydrate epitope is generated upon endolysosomal processing of group B streptococcal type III polysaccharide coupled to a carrier protein. In conjunction with a carrier protein-derived peptide, this carbohydrate epitope binds to major histocompatibility class II (MHCII) and stimulates carbohydrate-specific CD4+ T-cell clones to produce interleukins 2 and 4—cytokines essential for providing T-cell help to antibody-producing B cells. An archetypical glycoconjugate vaccine constructed to maximize the presentation of carbohydrate epitopes recognized by T cells is 50–100 times more potent and significantly more protective in an animal model of infection than is a currently used vaccine construct.

Long-term effects of rituximab added to cyclophosphamide in refractory patients with vasculitis

BACKGROUND

Current therapies have changed systemic vasculitis from a disease with a high rate of mortality to a chronic curable condition. A limited percentage of patients either remains refractory to conventional treatment or experiences dose-limiting side effects.

METHODS

11 patients (4 affected by idiopathic systemic microscopic polyangiitis, 5 by Wegener's granulomatosis, and 2 by Churg-Strauss syndrome) intolerant or refractory to conventional therapies including cyclophosphamide were enrolled. All patients received rituximab as a rescue therapy and were followed for 30-54 months. Following rituximab administration, immunosuppressive drugs were rapidly tapered and no immunosuppressive maintenance therapy was given.

RESULTS

Significant decreases in levels of serum creatinine, proteinuria, erythrocyte sedimentation rate, C-reactive protein, and ANCA titers were observed during the follow-up (at least 30 months after rituximab administration). Arthralgia and weakness rapidly disappeared in all patients. Out of 7 patients, 5 reported a decrease in the degree of paresthesia and in the electrophysiologic parameters. Six months after rituximab administration the mean dose of prednisone was 5.5 mg/day.

CONCLUSION

In this sample of patients with systemic vasculitis who were refractory or intolerant to more conventional treatment, rituximab proved to be safe and effective in a long-term follow-up, and showed steroid- and immunosuppressive-sparing effects allowing the persistence of long-lasting remissions without maintenance therapy.

CD4(+) T cells from MHC II-dependent thymocyte-thymocyte interaction provide efficient help for B cells

Recently, a novel CD4⁺ T-cell developmental pathway was reported that generates thymocyte–thymocyte (T–T) CD4⁺ T cells. We established a mouse system (CIITA^tgCIITApIV^−/−) where thymic positive selection occurred only by major histocompatibility complex (MHC) class II⁺ thymocytes. T–T CD4⁺ T cells selected via MHC class II-dependent T–T interaction are comprised of PLZF-negative and innate PLZF-positive populations. Until recently, the functional role of the PLZF-negative population was unclear. In this study, we demonstrate that naïve T–T CD4⁺ T cells provide B-cell help to a level comparable with that of naïve conventional CD4⁺ T cells. Considering the absence of PLZF expression in naïve T–T CD4⁺ T cells, these results suggest that PLZF-negative naïve T–T CD4⁺ T cells are functionally equivalent to conventional naïve CD4⁺ T cells in terms of B-cell help.

Programming the magnitude and persistence of antibody responses with innate immunity

Many successful vaccines induce persistent antibody responses that can last a lifetime. The mechanisms by which they do so remain unclear, but emerging evidence indicates that they activate dendritic cells via Toll-like receptors (TLRs). For example, the yellow fever vaccine YF-17D, one of the most successful empiric vaccines ever developed, activates dendritic cells via multiple TLRs to stimulate proinflammatory cytokines. Triggering specific combinations of TLRs in dendritic cells can induce synergistic production of cytokines, which results in enhanced T-cell responses, but its impact on antibody responses remain unknown. Learning the critical parameters of innate immunity that program such antibody responses remains a major challenge in vaccinology. Here we demonstrate that immunization of mice with synthetic nanoparticles containing antigens plus ligands that signal through TLR4 and TLR7 induces synergistic increases in antigen-specific, neutralizing antibodies compared to immunization with nanoparticles containing antigens plus a single TLR ligand. Consistent with this there was enhanced persistence of germinal centres and of plasma-cell responses, which persisted in the lymph nodes for >1.5 years. Surprisingly, there was no enhancement of the early short-lived plasma-cell response relative to that observed with single TLR ligands. Molecular profiling of activated B cells, isolated 7 days after immunization, indicated that there was early programming towards B-cell memory. Antibody responses were dependent on direct triggering of both TLRs on B cells and dendritic cells, as well as on T-cell help. Immunization protected completely against lethal avian and swine influenza virus strains in mice, and induced robust immunity against pandemic H1N1 influenza in rhesus macaques.

The yin and yang of B cells in graft rejection and tolerance

Various lineages of B cells are being increasingly recognized as important players in the etiology and prognosis of both acute and chronic graft rejection. The role of immature, chronically activated B cells, as efficient antigen-presenting cells, supporting recalcitrant cell-mediated graft rejection and late lineage B cells driving humoral rejections, is being increasingly recognized. This review captures the recent literature on this subject and discusses the various roles of the B cell in renal graft rejection and conversely, also in graft tolerance, both in animal and human studies. In addition, novel therapies targeting specific B-cell lineages in graft rejection are also discussed, with a view to developing more targeted therapies for graft rejection.

Clonal CD27+ CD19+ B cell expansion through inhibition of FC gammaIIR in HCV(+) cryoglobulinemic patients

Persistent HCV infection may be associated with extrahepatic manifestations such as type II mixed cryoglobulinemia (II-MC), a clonal B cell proliferative disorder. In persistent HCV infection without II-MC, an increase in serum immunoglobulins (Ig) is commonly observed. This increase is polyclonal and is determined primarily by increased levels of IgG which include both HCV-specific and nonspecific antibodies. Nonetheless, memory CD27(+) B cells do not accumulate. This paradoxical phenomenon depends on heightened sensitivity of memory B cells to BCR-independent noncognate T cell help, which speeds up their terminal differentiation into antibody-secreting cells and makes them more prone to apoptosis. In persistent HCV infection with II-MC, serum Ig elevation is also a general occurrence, and characteristically includes IgM antibodies with rheumatoid factor activity, which are essential for the development of circulating, cryoprecipitable immune complexes. Hypergammaglobulinemia is sustained by a peripheral expansion of IgM(+)k(+)IgD(low/neg)CD21(low)CD27(+) B cells. These cells exhibit marked V(H), J(H), and V(K) gene segment usage restriction, indicating that a limited number of antigens drive their proliferation through BCR interaction. Recently, two epitopes, one of the human IgG and the second of the HCV(NS3) protein, had been identified and demonstrated able to link the BCR exposed on II-MC subjects. Based on the above findings, we propose a model whereby BCR binding the IgM/IgG/HCV(NS3) immune complexes deprives Fc gammaIIR of its natural ligand. This takes the brake off RF(+)CD27(+) B cell proliferation and promotes their selective accumulation, which is otherwise prevented by increased apoptosis susceptibility in persistent HCV infection without II-MC.

Molecular mechanism and function of CD40/CD40L engagement in the immune system

SUMMARY

During the generation of a successful adaptive immune response, multiple molecular signals are required. A primary signal is the binding of cognate antigen to an antigen receptor expressed by T and B lymphocytes. Multiple secondary signals involve the engagement of costimulatory molecules expressed by T and B lymphocytes with their respective ligands. Because of its essential role in immunity, one of the best characterized of the costimulatory molecules is the receptor CD40. This receptor, a member of the tumor necrosis factor receptor family, is expressed by B cells, professional antigen-presenting cells, as well as non-immune cells and tumors. CD40 binds its ligand CD40L, which is transiently expressed on T cells and other non-immune cells under inflammatory conditions. A wide spectrum of molecular and cellular processes is regulated by CD40 engagement including the initiation and progression of cellular and humoral adaptive immunity. In this review, we describe the downstream signaling pathways initiated by CD40 and overview how CD40 engagement or antagonism modulates humoral and cellular immunity. Lastly, we discuss the role of CD40 as a target in harnessing anti-tumor immunity. This review underscores the essential role CD40 plays in adaptive immunity.

Dynamics of membrane trafficking downstream of B and T cell receptor engagement: impact on immune synapses

The onset of an adaptive immune response requires the activation of T and B lymphocytes by antigen-presenting cells, through a specialized form of intercellular communication, known as the immunological synapse (IS). In B lymphocytes the IS promotes efficient recognition and acquisition of membrane-bound Ags, while in T cells, it modulates the T cell response upon exposure to peptide-major histocompatibility complexes. In this review, we highlight the similarities that determine B and T cell activation, focusing on immune receptor downstream signaling events that lead to synapse formation. We stress the notion that polarization of T and B lymphocytes characterized by global changes in cytoskeleton and membrane trafficking modulates synapse structure and function, thus determining lymphocyte effector functions and fate.

Virus-like particle vaccine activates conventional B2 cells and promotes B cell differentiation to IgG2a producing plasma cells

We have previously shown that immunization with SIV-, SHIV-, or HA (influenza hemagglutinin)-virus-like particles (VLPs) elicits a strong humoral immune response in mice. However, little is known about the action VLPs exert on immune effector cells, including B cells. In this study, we found that all three types of VLPs could directly bind and activate B cells in vitro. VLPs stimulated the proliferation of B220(+)IgM(+)CD43(-)CD5(-) B2 cells and their differentiation to plasma cells that preferentially produce IgG2a antibodies. Up-regulation of Blimp-1, XBP-1, IRF4, and AID genes, which are responsible for class-switch recombination and somatic hypermutation, was observed in VLP-activated B2 cells. Stimulation of naïve splenocytes with VLPs led to a high expression of IL-12, RANTES and MIP, the cytokine milieu that favors B cell differentiation into IgG2a secreting cells. VLP immunization of C57BL/6 mice corroborated our in vitro data showing enlarged germinal centers and expanded conventional B2 cells, but no enlarged marginal zone B1 cells, in the spleen. Enhanced antigen-specific plasma cell formation, antibody production, and IgG2a class switching were found in VLP-immunized groups. The current study details the interaction between VLPs and B cells which result in preferential IgG2a antibody production following VLP immunization.

Chemotherapy induced transient B-cell depletion boosts antibody-forming cells expansion driven by an epidermal growth factor-based cancer vaccine

Cancer vaccines efficacy may improve inducing a rapid and persistent immune response, early at diagnosis along with standard therapies. EGF chemically conjugated to the carrier protein P64k from Neisseria meningitidis in montanide ISA 51 adjuvant is under evaluation, aiming to stimulate a B-cell response. High-dose cyclophosphamide and doxorubicin after priming enhanced the long-term frequency of EGF-specific antibody-forming cells (AFC) of IgM and IgG isotypes, but not the P64k response. Resulting combination, limitedly operational in Btk deficient xid mice, suggests that preferential B-cell lymphocyte space promoted by cyclophosphamide facilitates remaining EGF-specific AFC undergo homeostatic proliferation driven by boosting, amplifying the response.

T cell responses to whole SARS coronavirus in humans

Effective vaccines should confer long-term protection against future outbreaks of severe acute respiratory syndrome (SARS) caused by a novel zoonotic coronavirus (SARS-CoV) with unknown animal reservoirs. We conducted a cohort study examining multiple parameters of immune responses to SARS-CoV infection, aiming to identify the immune correlates of protection. We used a matrix of overlapping peptides spanning whole SARS-CoV proteome to determine T cell responses from 128 SARS convalescent samples by ex vivo IFN-gamma ELISPOT assays. Approximately 50% of convalescent SARS patients were positive for T cell responses, and 90% possessed strongly neutralizing Abs. Fifty-five novel T cell epitopes were identified, with spike protein dominating total T cell responses. CD8(+) T cell responses were more frequent and of a greater magnitude than CD4(+) T cell responses (p < 0.001). Polychromatic cytometry analysis indicated that the virus-specific T cells from the severe group tended to be a central memory phenotype (CD27(+)/CD45RO(+)) with a significantly higher frequency of polyfunctional CD4(+) T cells producing IFN-gamma, TNF-alpha, and IL-2, and CD8(+) T cells producing IFN-gamma, TNF-alpha, and CD107a (degranulation), as compared with the mild-moderate group. Strong T cell responses correlated significantly (p < 0.05) with higher neutralizing Ab. The serum cytokine profile during acute infection indicated a significant elevation of innate immune responses. Increased Th2 cytokines were observed in patients with fatal infection. Our study provides a roadmap for the immunogenicity of SARS-CoV and types of immune responses that may be responsible for the virus clearance, and should serve as a benchmark for SARS-CoV vaccine design and evaluation.

A fundamental role for interleukin-21 in the generation of T follicular helper cells

T cell help to B cells is a fundamental property of adaptive immunity, yet only recently have many of the cellular and molecular mechanisms of T cell help emerged. T follicular helper (Tfh) cells are the CD4(+) T helper cells that provide cognate help to B cells for high-affinity antibody production in germinal centers (GC). Tfh cells produce interleukin-21 (IL-21), and we show that IL-21 was necessary for GC formation. However, the central role of IL-21 in GC formation reflected its effects on Tfh cell generation rather than on B cells. Expression of the inducible costimulator (ICOS) was necessary for optimal production of IL-21, indicative of interplay between these two Tfh cell-expressed molecules. Finally, we demonstrate that IL-21's costimulatory capacity for T helper cell differentiation operated at the level of the T cell receptor signalosome through Vav1, a signaling molecule that controls T cell helper function. This study reveals a previously unappreciated role for Tfh cells in the formation of the GC and isotype switching through a CD4(+) T cell-intrinsic requirement for IL-21.

Actin-binding protein 1 regulates B cell receptor-mediated antigen processing and presentation in response to B cell receptor activation

The BCR serves as both signal transducer and Ag transporter. Binding of Ags to the BCR induces signaling cascades and Ag processing and presentation, two essential cellular events for B cell activation. BCR-initiated signaling increases BCR-mediated Ag-processing efficiency by increasing the rate and specificity of Ag transport. Previous studies showed a critical role for the actin cytoskeleton in these two processes. In this study, we found that actin-binding protein 1 (Abp1/HIP-55/SH3P7) functioned as an actin-binding adaptor protein, coupling BCR signaling and Ag-processing pathways with the actin cytoskeleton. Gene knockout of Abp1 and overexpression of the Src homology 3 domain of Abp1 inhibited BCR-mediated Ag internalization, consequently reducing the rate of Ag transport to processing compartments and the efficiency of BCR-mediated Ag processing and presentation. BCR activation induced tyrosine phosphorylation of Abp1 and translocation of both Abp1 and dynamin 2 from the cytoplasm to plasma membrane, where they colocalized with the BCR and cortical F-actin. Mutations of the two tyrosine phosphorylation sites of Abp1 and depolymerization of the actin cytoskeleton interfered with BCR-induced Abp1 recruitment to the plasma membrane. The inhibitory effect of a dynamin proline-rich domain deletion mutant on the recruitment of Abp1 to the plasma membrane, coimmunoprecipitation of dynamin with Abp1, and coprecipitation of Abp1 with GST fusion of the dyanmin proline-rich domain demonstrate the interaction of Abp1 with dynamin 2. These results demonstrate that the BCR regulates the function of Abp1 by inducing Abp1 phosphorylation and actin cytoskeleton rearrangement, and that Abp1 facilitates BCR-mediated Ag processing by simultaneously interacting with dynamin and the actin cytoskeleton.

NK T cells provide lipid antigen-specific cognate help for B cells

The mechanisms of T cell help for production of antilipid antibodies are largely unknown. This study shows that invariant NK T cells (iNK T cells) and B cells cooperate in a model of antilipid antigen-specific antibody responses. We use a model haptenated lipid molecule, 4-hydroxy-3-nitrophenyl-alphaGalactosylCeramide (NP-alphaGalCer), to demonstrate that iNK T cells provide cognate help to lipid-antigen-presenting B cells. B cells proliferate and IgG anti-NP is produced from in vivo-immunized mice and in vitro cocultures of B and NK T cells after exposure to NP-alphaGalCer, but not closely related control glycolipids. This B cell response is absent in CD1d(-/-) and Jalpha18(-/-) mice but not CD4(-/-) mice. The antibody response to NP-alphaGalCer is dominated by the IgM, IgG3, and IgG2c isotypes, and marginal zone B cells stimulate better in vitro lipid antigen-driven proliferation than follicular B cells, suggesting an important role for this B cell subset. iNK T cell help for B cells is shown to involve cognate help from CD1d-instructed lipid-specific iNK T cells, with help provided via CD40L, B7-1/B7-2, and IFN-gamma, but not IL-4. This model provides evidence of iNK T cell help for antilipid antibody production, an important aspect of infections, autoimmune diseases, and vaccine development. Our findings also now allow prediction of those microbial antigens that would be expected to elicit cognate iNKT cell help for antibody production, namely those that can stimulate iNKT cells and at the same time have a polar moiety that can be recognized by antibodies.

Immunoproteomic identification of bovine pericardium xenoantigens

Bovine pericardium is an important biomaterial with current application in glutaraldehyde-fixed bioprosthetic heart valves and possible future application as an unfixed biological scaffold for tissue engineering. The importance of both humoral and cell-mediated rejection responses toward fixed and unfixed xenogeneic tissues has become increasingly apparent. However, the full scope and specific identities of bovine pericardium proteins that can elicit an immune response remain largely unknown. In this study, an immunoproteomic approach was used to survey bovine pericardium proteins for their ability to elicit a humoral immune response in rabbits. A two-stage protein extraction protocol was used to separate bovine pericardium proteins into water- and lipid-soluble fractions. Two-dimensional (2-D) gel electrophoresis was performed to separate the proteins from each fraction. Western blots were generated from 2-D gels of both bovine pericardium protein fractions. These blots were probed with serum from rabbits immunized with bovine pericardium and a secondary antibody was used to assess for IgG positivity. Western blots were compared to duplicate 2-D gels and proteins in matched spots were identified by tandem mass spectrometry. Thirty-one putative protein antigens were identified, eight of which are known to be antigenic from previous studies. All of the putative antigens demonstrated progressive staining intensity with increasing days of post-exposure serum. Identified antigenic proteins represented a variety of functional and structural protein types, and included both cellular and matrix proteins. The results of this study have implications for the use of bovine pericardium as a biomaterial in bioprostheses and tissue engineering applications, as well as xenotransplantation in general.

Tpl2 and ERK transduce antiproliferative T cell receptor signals and inhibit transformation of chronically stimulated T cells

The protein kinase encoded by the Tpl2 protooncogene plays an obligatory role in the transduction of Toll-like receptor and death receptor signals in macrophages, B cells, mouse embryo fibroblasts, and epithelial cells in culture and promotes inflammatory responses in animals. To address its role in T cell activation, we crossed the T cell receptor (TCR) transgene 2C, which recognizes class I MHC presented peptides, into the Tpl2(-/-) genetic background. Surprisingly, the TCR2C(tg/tg)/Tpl2(-/-) mice developed T cell lymphomas with a latency of 4-6 months. The tumor cells were consistently TCR2C(+)CD8(+)CD4(-), suggesting that they were derived either from chronically stimulated mature T cells or from immature single positive (ISP) cells. Further studies showed that the population of CD8(+) ISP cells was not expanded in the thymus of TCR2C(tg/tg)/Tpl2(-/-) mice, making the latter hypothesis unlikely. Mature peripheral T cells of Tpl2(-/-) mice were defective in ERK activation and exhibited enhanced proliferation after TCR stimulation. The same cells were defective in the induction of CTLA4, a negative regulator of the T cell response, which is induced by TCR signals via ERK. These findings suggest that Tpl2 functions normally in a feedback loop that switches off the T cell response to TCR stimulation. As a result, Tpl2, a potent oncogene, functions as a tumor suppressor gene in chronically stimulated T cells.

Identification of an antigenic and immunogenic motif expressed by two 7-mer rituximab-specific cyclic peptide mimotopes: implication for peptide-based active immunotherapy

Two 7-mer cyclic peptides-Rp15-C and Rp13-C-which bear the antigenic motif recognized by the anti-CD20 mAb rituximab, but have different motif-surrounding amino acids, show a comparable avidity for rituximab and inhibit the binding of rituximab to raft-associated CD20 and rituximab-induced membrane ceramide on human lymphoid Daudi cells. Their immunogenic profiles differed: Abs recognizing CD20 were induced in two and five of five BALB/c mice immunized with Rp15-C and Rp13-C, respectively. Analysis of immunogenic motif, performed by panning a 7-mer phage-display peptide library with purified anti-peptide IgGs, showed that the motif defined by anti-Rp15-C mostly included amino acids surrounding the rituximab-specific antigenic motif <aNPS>, whereas that defined by anti-Rp13-C was <NPS>. These data indicate that their motif-surrounding amino acids can markedly influence the specificity of Abs, even when elicited with a short 7-mer peptide. Because these anti-peptide Abs are of IgG isotype, their specificity is likely to reflect how peptides are processed at the T cell level and suggest that, within a short peptide, the motifs defined by T cells during the initial phase and upon their stimulation may be different. Our findings may account for the failure of most forms of peptide-based immunotherapy in cancer and autoimmune diseases in which anti-mimotope Abs are expected to play a relevant therapeutic effect. They also suggest strategies to implement the specificity of peptide-induced Abs against the target Ag.

Syk-dependent actin dynamics regulate endocytic trafficking and processing of antigens internalized through the B-cell receptor

Antigen binding to the B-cell receptor (BCR) induces multiple signaling cascades that ultimately lead to B lymphocyte activation. In addition, the BCR regulates the key trafficking events that allow the antigen to reach endocytic compartments devoted to antigen processing, i.e., that are enriched for major histocompatibility factor class II (MHC II) and accessory molecules such as H2-DM. Here, we analyze the role in antigen processing and presentation of the tyrosine kinase Syk, which is activated upon BCR engagement. We show that convergence of MHC II- and H2-DM-containing compartments with the vesicles that transport BCR-uptaken antigens is impaired in cells lacking Syk activity. This defect in endocytic trafficking compromises the ability of Syk-deficient cells to form MHC II-peptide complexes from BCR-internalized antigens. Altered endocytic trafficking is associated to a failure of Syk-deficient cells to properly reorganize their actin cytoskeleton in response to BCR engagement. We propose that, by modulating the actin dynamics induced upon BCR stimulation, Syk regulates the positioning and transport of the vesicles that carry the molecules required for antigen processing and presentation.

The actin-based motor protein myosin II regulates MHC class II trafficking and BCR-driven antigen presentation

Antigen (Ag) capture and presentation onto major histocompatibility complex (MHC) class II molecules by B lymphocytes is mediated by their surface Ag receptor (B cell receptor [BCR]). Therefore, the transport of vesicles that carry MHC class II and BCR–Ag complexes must be coordinated for them to converge for processing. In this study, we identify the actin-associated motor protein myosin II as being essential for this process. Myosin II is activated upon BCR engagement and associates with MHC class II–invariant chain complexes. Myosin II inhibition or depletion compromises the convergence and concentration of MHC class II and BCR–Ag complexes into lysosomes devoted to Ag processing. Accordingly, the formation of MHC class II–peptides and subsequent CD4 T cell activation are impaired in cells lacking myosin II activity. Therefore, myosin II emerges as a key motor protein in BCR-driven Ag processing and presentation.

Antigen presentation by B lymphocytes: how receptor signaling directs membrane trafficking

Antigen capture and presentation onto MHC class II molecules by B lymphocytes is mediated by their surface antigen receptor - the B-cell receptor (BCR). The BCR must therefore coordinate the transport of MHC class II- and antigen-containing vesicles for them to converge and ensure efficient processing. Recently, progress has been made in understanding which and how these vesicular transport events are molecularly linked to BCR signaling. In particular, recent studies have emphasized the key roles of membrane microdomains and the actin cytoskeleton in regulation of membrane trafficking upon BCR engagement.

Influence of the nature of the antigen on the boosting of responses to mycobacteria in M. bovis-BCG vaccinated cattle

Mycobacterium bovis is the causative agent of bovine tuberculosis (TB) and of a proportion of human TB. Protection against TB requires Th1 responses and worsening of disease is associated with Th2 responses. To help clarify the nature of the response to mycobacteria, the responses from M. bovis-BCG vaccinated cattle boosted with live mycobacteria (BCG), bacterial soluble antigens (PPD) or PBS were evaluated. The results indicated that macrophages may be the major cell population ingesting and presenting mycobacteria in BCG boosted animals, while B-cells seem able to ingest and present PPD to T-cells in PPD boosted animals.

CD4+ T-cell responses are required for clearance of West Nile virus from the central nervous system

Although studies have established that innate and adaptive immune responses are important in controlling West Nile virus (WNV) infection, the function of CD4(+) T lymphocytes in modulating viral pathogenesis is less well characterized. Using a mouse model, we examined the role of CD4(+) T cells in coordinating protection against WNV infection. A genetic or acquired deficiency of CD4(+) T cells resulted in a protracted WNV infection in the central nervous system (CNS) that culminated in uniform lethality by 50 days after infection. Mice surviving past day 10 had high-level persistent WNV infection in the CNS compared to wild-type mice, even 45 days following infection. The absence of CD4(+) T-cell help did not affect the kinetics of WNV infection in the spleen and serum, suggesting a role for CD4-independent clearance mechanisms in peripheral tissues. WNV-specific immunoglobulin M (IgM) levels were similar to those of wild-type mice in CD4-deficient mice early during infection but dropped approximately 20-fold at day 15 postinfection, whereas IgG levels in CD4-deficient mice were approximately 100- to 1,000-fold lower than in wild-type mice throughout the course of infection. WNV-specific CD8(+) T-cell activation and trafficking to the CNS were unaffected by the absence of CD4(+) T cells at day 9 postinfection but were markedly compromised at day 15. Our experiments suggest that the dominant protective role of CD4(+) T cells during primary WNV infection is to provide help for antibody responses and sustain WNV-specific CD8(+) T-cell responses in the CNS that enable viral clearance.

Lymphocytes: not useless after all

Half a century ago, an immunologist named N. Avrion Mitchison showed that lymphocytes--then thought to be useless--triggered tumor rejection in mice.

Antibody-mediated organ-allograft rejection

Recent studies show that alloantibodies mediate a substantial proportion of graft-rejection episodes, contributing to both early and late graft loss. Rejection that is caused by antibody is mediated by different mechanisms from rejection that is caused by T cells, thereby requiring other approaches to treatment and prevention. Antibody induces rejection acutely through the fixation of complement, resulting in tissue injury and coagulation. In addition, complement activation recruits macrophages and neutrophils, causing additional endothelial injury. Antibody and complement also induce gene expression by endothelial cells, which is thought to remodel arteries and basement membranes, leading to fixed and irreversible anatomical lesions that permanently compromise graft function.

Malaria-specific transgenic CD4+ T cells protect immunodeficient mice from lethal infection and demonstrate requirement for a protective threshold of antibody production for parasite clearance

T cells are important in the immune response to malaria, both for their cytokines and their help for antibody production. To look at the relative importance of these roles, a T-cell receptor (TCR) transgenic mouse has been generated carrying a TCR specific for an epitope of the merozoite surface protein 1 (MSP-1) of the malaria parasite, Plasmodium chabaudi. In adoptive transfer experiments, malaria-specific CD4+ T cells expand and produce interferon γ (IFN-γ) early in infection, but the population contracts quickly despite prolonged persistence of the parasite. MSP-1-specific CD4+ cells can protect immunodeficient mice from lethal infection; however, the parasite is only completely cleared in the presence of B cells showing that T helper cells are critical. Levels of malaria-specific antibody and the speed of their production clearly correlate with the time of resolution of infection, indicating that a critical threshold of antibody production is required for parasite clearance. Furthermore, T cells specific for a shed portion of MSP-1 are able to provide help for antibody to the protective region, which remains bound to the infected erythrocyte, suggesting that MSP-1 has all of the components necessary for a good vaccine. (Blood. 2005;106:1676-1684)

Mice deficient in OX40 and CD30 signals lack memory antibody responses because of deficient CD4 T cell memory

Recently, we reported that a CD4(+)CD3(-)CD11c(-) accessory cell provided OX40-dependent survival signals to follicular T cells. These accessory cells express both OX40 ligand and CD30 ligand, and the receptors, OX40 and CD30, are both expressed on Th2-primed CD4 T cells. OX40 and CD30 signals share common signaling pathways, suggesting that CD30 signals might substantially compensate in OX40-deficient mice. In this report we have dissected the signaling roles of CD30 alone and in combination with OX40. CD30-deficient mice showed an impaired capacity to sustain follicular germinal center responses, and recall memory Ab responses were substantially reduced. Deficiencies in OX40 and CD30 signals were additive; secondary Ab responses were ablated in double-deficient mice. Although the initial proliferation of OX40/CD30 double-knockout OTII transgenic T cells was comparable to that of their normal counterparts, they failed to survive in vivo, and this was associated with reduced T cell numbers associated with CD4(+)CD3(-) cells in B follicles. Finally, we show that OX40/CD30 double-knockout OTII transgenic T cells fail to survive compared with normal T cells when cocultured with CD4(+)CD3(-) cells in vitro.

Molecular case report: IgVH analysis in acute humoral and cellular liver allograft rejection suggests a selected accumulation of effector B cells and plasma cells

Acute cellular (CLR) and humoral liver allograft rejection (HLR) are the most important immunological obstacles to successful liver transplantation. In HLR, serum antibodies play the central pathogenetic role. In CLR, CD3+ T lymphocytes drive the destructive immune response. Although CLR and HLR show different clinical symptoms and can be kept apart in most cases, they share histomorphological similarities. In CLR, hepatic B lymphocytes and plasma cells as well as B-cell-activating cytokines have recently been described, indicating that, in addition to T cells, antibody-mediated mechanisms might be involved. To analyze the impact of hepatic B cells in CLR and HLR, the immunoglobulin (Ig) variable (V)-region gene repertoire was determined from tissue of one case of CLR and one case of HLR. Complement deposits and lymphocytic infiltrate were determined using immunohistochemistry. T cells, B lymphocytes and plasma cells could be detected in both cases, whereas C3c and C4d deposits could only be demonstrated in the HLR case. The molecular analysis of 63 V-region genes showed that B cells in both allografts expressed selected V-gene repertoires. All sequences differed from the putative germline sequences by multiple somatic mutations. This suggests a clonal expansion of selected effector B cells in the portal tracts of liver allografts. Locally accumulated B cells and their antibodies might be involved in IgG-mediated complement activation in CLR and HLR.

T and B Cell Cooperation: A Dance of Life and Death

In order for the body to develop a good antibody response, B cells need to react intimately with antigen specific T cells. Experimental evidence using hapten-carriers revealed that T and B cells do not recognize the same epitope and this led to the view that the physical contact is mediated by the antigen. Although the modern concept of antigen presentation has changed our perception on how the antigen can bridge both cells, the basic virtues of earlier bridging models remain. Over the past few years, a number of surface ligand-receptor pathways have been described, most of them belonging either to the CD28/B7 Ig or to the TNF/TNFR-like families. These act in concert, whether they are agonist or antagonist, in a timely and spatially organized manner. They form cascades of successive induction and recruitment to ensure that T-B cooperation is closely controlled at all stages of antibody induction.