Deficient CD4+ T cell priming and regression of CD8+ T cell functionality in virus-infected mice lacking a normal B cell compartment

Interferon α facilitates anti-HBV cellular immune response in a B cell-dependent manner

OBJECTIVES

Dissecting the underlying mechanism of T cells remodeling mediated by interferon α (IFN-α) is indispensable for achieving an optimum therapeutic response in chronic hepatitis B (CHB) patients. However, little is known about B cells in this process. This study aims to elucidate the roles of B cells in IFN-α-mediated anti-hepatitis B virus (HBV) cellular immunity.

METHOD

The effects of B cells on IFN-α-mediated T cell response were investigated in B cell-deficient mouse model with HBV and IFN-α plasmid hydrodynamic injection. Single-cell RNA sequencing was performed to dissect the crosstalk among B cell and T cell subsets and the underlying molecule and pathway signatures on longitudinal blood samples from IFN-α-treated CHB patients.

RESULTS

B cell depletion impaired the functional T cell subsets, including HBV-specific CD8⁺ T cells, and engendered a delayed HBV clearance. IFN-α treatment boosted the response of HBV-specific CD8⁺ T cells, whereas such effects disappeared in B cell-deficient mice. The underlying mechanisms were associated with IFN-α-reinforced connections of B cells toward T cells as mediated by the antigen presentation and costimulatory functions in B cells.

CONCLUSION

IFN-α orchestrates protective HBV-specific cellular immunity in a B cell-dependent manner.

B lymphocytes contribute to indirect pathway T cell sensitization via acquisition of extracellular vesicles

B cells have been implicated in transplant rejection via antibody-mediated mechanisms and more recently by presenting donor antigens to T cells. We have shown in patients with chronic antibody-mediated rejection that B cells control the indirect T cell alloresponses. To understand more about the role of B cells as antigen-presenting cells for CD4⁺ T cell with indirect allospecificity, B cells were depleted in C57BL/6 mice, using an anti-CD20 antibody, prior to receiving MHC class I-mismatched (K^d ) skin. The absence of B cells at the time of transplantation prolonged skin graft survival. To study the mechanisms behind this observation, T cells with indirect allospecificity were transferred in mice receiving a K^d skin transplant. T cell proliferation was markedly inhibited in the absence of recipient B cells, suggesting that B cells contribute to indirect pathway sensitization. Furthermore, we have shown that a possible way in which B cells present alloantigens is via acquisition of MHC-peptide complexes. Finally, we demonstrate that the addition of B cell depletion to the transfer of regulatory T cells (Tregs) with indirect alloresponse further prolonged skin graft survival. This study supports an important role for B cells in indirect T cell priming and further emphasizes the advantage of combination therapies in prolonging transplant survival.

Complexities of Type I Interferon Biology: Lessons from LCMV

Over the past decades, infection of mice with lymphocytic choriomeningitis virus (LCMV) has provided an invaluable insight into our understanding of immune responses to viruses. In particular, this model has clarified the central roles that type I interferons play in initiating and regulating host responses. The use of different strains of LCMV and routes of infection has allowed us to understand how type I interferons are critical in controlling virus replication and fostering effective antiviral immunity, but also how they promote virus persistence and functional exhaustion of the immune response. Accordingly, these discoveries have formed the foundation for the development of novel treatments for acute and chronic viral infections and even extend into the management of malignant tumors. Here we review the fundamental insights into type I interferon biology gained using LCMV as a model and how the diversity of LCMV strains, dose, and route of administration have been used to dissect the molecular mechanisms underpinning acute versus persistent infection. We also identify gaps in the knowledge regarding LCMV regulation of antiviral immunity. Due to its unique properties, LCMV will continue to remain a vital part of the immunologists' toolbox.

Mucosal Vaccination with Heterologous Viral Vectored Vaccine Targeting Subdominant SIV Accessory Antigens Strongly Inhibits Early Viral Replication

Conventional HIV T cell vaccine strategies have not been successful in containing acute peak viremia, nor in providing long-term control. We immunized rhesus macaques intramuscularly and rectally using a heterologous adenovirus vectored SIV vaccine regimen encoding normally weakly immunogenic tat, vif, rev and vpr antigens fused to the MHC class II associated invariant chain. Immunizations induced broad T cell responses in all vaccinees. Following up to 10 repeated low-dose intrarectal challenges, vaccinees suppressed early viral replication (P = 0.01) and prevented the peak viremia in 5/6 animals. Despite consistently undetectable viremia in 2 out of 6 vaccinees, all animals showed evidence of infection induced immune responses indicating that infection had taken place. Vaccinees, with and without detectable viremia better preserved their rectal CD4 + T cell population and had reduced immune hyperactivation as measured by naïve T cell depletion, Ki-67 and PD-1 expression on T cells. These results indicate that vaccination towards SIV accessory antigens vaccine can provide a level of acute control of SIV replication with a suggestion of beneficial immunological consequences in infected animals of unknown long-term significance.

In conclusion, our studies demonstrate that a vaccine encoding subdominant antigens not normally associated with virus control can exert a significant impact on acute peak viremia.

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Mucosal heterologousvirus-vectored vaccine used with MHC class II associated invariant chain linked SIV accessory antigens

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Mucosal vaccination targeting subdominant antigens delay SIV mac251 replication in rhesus macaques.

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Longterm reduction of immune hyperactivation following SIV infection of vaccinated macaques.

Mucosal immunization is used with heterologous viral vectors and a genetic adjuvant to raise responses against poorly immunogenic SIV antigens. Following repeated low-dose challenge we observed delayed establishment of chronic phase viremia and reduced immune hyperactivation 6 months after established infection. Vaccination was found to strongly reduce viremia at early, but not late time points, after detected infection and in 2 out of 6 animals infection could only observed as virus induced T cell responses. Subdominant antigen vaccines may thus be used to delay SIV mac251 infection and can enable control of chronic viremia in a minority of cases.

Dendritic Cells Are Dispensable for T Cell Priming and Control of Acute Lymphocytic Choriomeningitis Virus Infection

Dendritic cells (DCs) are considered to be the major APCs with potent activity for priming of naive CD4 and CD8 T cells. However, T cell priming can also be achieved by other APCs including macrophages, B cells, or even nonhematopoietic cell types. Systemic low-dose infection of mice with lymphocytic choriomeningitis virus (LCMV) results in massive expansion of virus-specific CD4 and CD8 T cells. To determine the role of DCs as APCs and source of type I IFNs in this infection model, we used ΔDC mice in which DCs are constitutively ablated because of expression of the diphtheria toxin α subunit within developing DCs. ΔDC mice showed lower serum concentrations of IFN-β and IL-12p40, but normal IFN-α levels during the first days postinfection. No differences were found for proliferation of transferred TCR-transgenic cells during the early phase of infection, suggesting that T cell priming occurred with the same efficiency in wild-type and ΔDC mice. Expansion and cytokine expression of endogenous LCMV-specific T cells was comparable between wild-type and ΔDC mice during primary infection and upon rechallenge of memory mice. In both strains of infected mice the viral load was reduced below the limit of detection with the same kinetic. Further, germinal center formation and LCMV-specific Ab responses were not impaired in ΔDC mice. This indicates that DCs are dispensable as APCs for protective immunity against LCMV infection.

B Cell Depletion With an Anti-CD20 Antibody Enhances Alloreactive Memory T Cell Responses After Transplantation

Alloreactive memory T cells mediate accelerated allograft rejection and transplant tolerance resistance. Recent studies have shown that B cell deficient-μMT mice fail to mount donor-specific memory T cell responses after transplantation. At the same time, other studies showed that pretransplant B cell depletion using rituximab (IgG1 anti-CD20 mAb) combined with cyclosporine A promoted the survival of islet allografts in monkeys. In this study, we investigated the effect of anti-CD20 antibody-mediated B cell depletion on the memory T cell alloresponse in mice. Wild-type and anti-OVA TCR transgenic mice were treated with an IgG2a anti-CD20 monoclonal antibody, which depleted nearly all B cells in the peripheral blood and secondary lymphoid organs but spared some B cells in the bone marrow. B cell depletion did not affect the direct alloresponse but resulted in a marked increase of indirect alloresponse after skin transplantation of naïve mice. Furthermore, in allosensitized mice, anti-CD20 mAb treatment enhanced the reactivation of allospecific memory T cells and accelerated second set rejection of skin allografts. This suggests that the effect of anti-CD20 antibodies on alloimmunity and allograft rejection might vary upon the nature of the antibodies as well as the circumstances under which they are delivered.

Immunogenicity of HLA Class I and II Double Restricted Influenza A-Derived Peptides

The aim of the present study was to identify influenza A-derived peptides which bind to both HLA class I and -II molecules and by immunization lead to both HLA class I and class II restricted immune responses. Eight influenza A-derived 9-11mer peptides with simultaneous binding to both HLA-A*02:01 and HLA-DRB1*01:01 molecules were identified by bioinformatics and biochemical technology. Immunization of transgenic HLA-A*02:01/HLA-DRB1*01:01 mice with four of these double binding peptides gave rise to both HLA class I and class II restricted responses by CD8 and CD4 T cells, respectively, whereas four of the double binding peptides did result in HLA-A*02:01 restricted responses only. According to their cytokine profile, the CD4 T cell responses were of the Th2 type. In influenza infected mice, we were unable to detect natural processing in vivo of the double restricted peptides and in line with this, peptide vaccination did not decrease virus titres in the lungs of intranasally influenza challenged mice. Our data show that HLA class I and class II double binding peptides can be identified by bioinformatics and biochemical technology. By immunization, double binding peptides can give rise to both HLA class I and class I restricted responses, a quality which might be of potential interest for peptide-based vaccine development.

The application of anti-Toso antibody enhances CD8(+) T cell responses in experimental malaria vaccination and disease

Toso is a molecule highly expressed on B cells. It influences their survival and was identified as an IgM binding molecule. B cells and natural antibodies play a role in vaccination-induced CD8(+) T cell responses. We investigated the impact of an anti-Toso antibody on vaccination efficiency in a malaria vaccination model. In this model, CD8(+) T cells exert antiparasitic functions on infected hepatocytes in the liver stage of the disease. In vaccinated anti-Toso treated mice, more antigen-specific CD8(+) T cells were induced than in control mice and after infection with Plasmodium berghei ANKA (PbA) sporozoites, the liver parasite burden was lower. In B cell deficient mice, the anti-Toso antibody did not stimulate the CD8(+) T cell response, indicating that B cells were mediating this effect. Furthermore, we analyzed the influence of anti-Toso treatment on non-vaccinated mice in the PbA infection model, in which CD8(+) T cells cause brain pathology. Anti-Toso treatment increased cerebral pathology and the accumulation of CD8(+) T cells in the brain. Thus, anti-Toso treatment enhanced the CD8(+) T cell response against PbA in a vaccination and in an infection model. Our findings indicate that Toso may be a novel target to boost vaccine-induced CD8(+) T cell responses.

PD-L1 mediated the differentiation of tumor-infiltrating CD19+ B lymphocytes and T cells in Invasive breast cancer

Accumulating evidence suggests that B cells play important roles in inhibiting the immune response in autoimmune disorders and human tumors as well as murine tumor models. In an effort to explore the role of B cells in human breast cancer etiology, we examined the presence of CD19⁺ B lymphocytes in 134 cases of invasive breast carcinoma (IBCa) and 31 breast fibroadenoma, and assessed its relationship with PD-L1 (programmed death-ligand 1) expression in breast cancer. We found that the density of CD19⁺ B lymphocytes was higher in IBCa compared with fibroadenoma, and significantly associated with increasing tumor grade, negative estrogen status. Similar findings were observed for the expression of IL-10 in IBCa. Meanwhile, CD19⁺ B lymphocytes were shown to be highly coincident with PD-L1 and IL-10 in IBCa. We further demonstrated that CD19⁺ B cells can differentiate into CD19⁺CD24⁺CD38⁺ B cells when co-cultured with PD-L1^hi MDA-MB231 cells. In addition, the percentage of CD19⁺CD24⁺CD38⁺ B cells was higher in breast tissue and peripheral blood cells of IBCa patients than that of benign tumor and health individuals. And CD19⁺CD24⁺CD38⁺ B cells were found to be IL-10 secreting B cells. Finally, we showed that CD19⁺ B cells from IBCa patients but not healthy individuals induced formation of CD4⁺CD25⁺Foxp3⁺ T cells when co-cultured with T cells from IBCa patients and healthy subjects (80.4% and 30.8% respectively). The induction of CD4⁺CD25⁺Foxp3⁺ T cells by CD19⁺ B cells was further shown to be mediated by PD-L1. Together, these results are suggestive of a role for CD19⁺ B lymphocytes in immune suppression and tumor evasion via PD-L1 in breast cancer.

Targeting of non-dominant antigens as a vaccine strategy to broaden T-cell responses during chronic viral infection

In this study, we compared adenoviral vaccine vectors with the capacity to induce equally potent immune responses against non-dominant and immunodominant epitopes of murine lymphocytic choriomeningitis virus (LCMV). Our results demonstrate that vaccination targeting non-dominant epitopes facilitates potent virus-induced T-cell responses against immunodominant epitopes during subsequent challenge with highly invasive virus. In contrast, when an immunodominant epitope was included in the vaccine, the T-cell response associated with viral challenge remained focussed on that epitope. Early after challenge with live virus, the CD8+ T cells specific for vaccine-encoded epitopes, displayed a phenotype typically associated with prolonged/persistent antigenic stimulation marked by high levels of KLRG-1, as compared to T cells reacting to epitopes not included in the vaccine. Notably, this association was lost over time in T cells specific for the dominant T cell epitopes, and these cells were fully capable of expanding in response to a new viral challenge. Overall, our data suggests a potential for broadening of the antiviral CD8+ T-cell response by selecting non-dominant antigens to be targeted by vaccination. In addition, our findings suggest that prior adenoviral vaccination is not likely to negatively impact the long-term and protective immune response induced and maintained by a vaccine-attenuated chronic viral infection.

CD8+ T cells complement antibodies in protecting against yellow fever virus

The attenuated yellow fever (YF) vaccine (YF-17D) was developed in the 1930s, yet little is known about the protective mechanisms underlying its efficiency. In this study, we analyzed the relative contribution of cell-mediated and humoral immunity to the vaccine-induced protection in a murine model of YF-17D infection. Using different strains of knockout mice, we found that CD4(+) T cells, B cells, and Abs are required for full clinical protection of vaccinated mice, whereas CD8(+) T cells are dispensable for long-term survival after intracerebral challenge. However, by analyzing the immune response inside the infected CNS, we observed an accelerated T cell influx into the brain after intracerebral challenge of vaccinated mice, and this T cell recruitment correlated with improved virus control in the brain. Using mice deficient in B cells we found that, in the absence of Abs, YF vaccination can still induce some antiviral protection, and in vivo depletion of CD8(+) T cells from these animals revealed a pivotal role for CD8(+) T cells in controlling virus replication in the absence of a humoral response. Finally, we demonstrated that effector CD8(+) T cells also contribute to viral control in the presence of circulating YF-specific Abs. To our knowledge, this is the first time that YF-specific CD8(+) T cells have been demonstrated to possess antiviral activity in vivo.

Prolonged antigen presentation by immune complex-binding dendritic cells programs the proliferative capacity of memory CD8 T cells

Antibodies can regulate the quality and functionality of a subset of antiviral CD8⁺ T cell memory responses to influenza by promoting sustained DC antigen presentation during the contraction phase of primary responses.

The commitment of naive CD8 T cells to effector or memory cell fates can occur after a single day of antigenic stimulation even though virus-derived antigens (Ags) are still presented by DCs long after acute infection is resolved. However, the effects of extended Ag presentation on CD8 T cells are undefined and the mechanisms that regulate prolonged Ag presentation are unknown. We showed that the sustained presentation of two different epitopes from influenza virus by DCs prevented the premature contraction of the primary virus-specific CD8 T cell response. Although prolonged Ag presentation did not alter the number of memory CD8 T cells that developed, it was essential for programming the capacity of these cells to proliferate, produce cytokines, and protect the host after secondary challenge. Importantly, prolonged Ag presentation by DCs was dependent on virus-specific, isotype-switched antibodies (Abs) that facilitated the capture and cross-presentation of viral Ags by FcγR-expressing DCs. Collectively, our results demonstrate that B cells and Abs can regulate the quality and functionality of a subset of antiviral CD8 T cell memory responses and do so by promoting sustained Ag presentation by DCs during the contraction phase of the primary T cell response.

B cell depletion curtails CD4+ T cell memory and reduces protection against disseminating virus infection

Dynamic interactions between CD4(+) T cells and B cells are needed for humoral immunity and CD4(+) T cell memory. It is not known whether B cells are needed early on to induce the formation of memory precursor cells or are needed later to sustain memory cells. In this study, primary and memory CD4(+) T cells responses were followed in wild-type mice that were depleted of mature B cells by anti-CD20 before or different times after acute lymphocytic choriomeningitis virus infection. The Ab treatment led to a 1000-fold reduction in B cell number that lasted 6 wk. Primary virus-specific CD4(+) Th1 cells were generated in B cell-depleted mice; however, there was a decrease in the CD4(+)Ly6C(lo)Tbet(+) memory precursor population and a corresponding 4-fold reduction in CD4(+) memory cell number. Memory T cells showed impaired cytokine production when they formed without B cells. B cell depletion had no effect on established memory populations. During disseminating virus infection, B cell depletion led to sustained weight loss and functional exhaustion of CD4(+) and CD8(+) T cells, and prevented mice from resolving the infection. Thus, B cells contribute to the establishment and survival of memory CD4(+) T cells post-acute infection and play an essential role in immune protection against disseminating virus infection.

Comparison of vaccine-induced effector CD8 T cell responses directed against self- and non-self-tumor antigens: implications for cancer immunotherapy

It is generally accepted that CD8 T cells play a major role in tumor control, yet vaccination aimed at eliciting potent CD8 T cell responses are rarely efficient in clinical trials. To try and understand why this is so, we have generated potent adenoviral vectors encoding the endogenous tumor Ags (TA) tyrosinase-related protein-2 (TRP-2) and glycoprotein 100 (GP100) tethered to the invariant chain (Ii). Using these vectors, we sought to characterize the self-TA-specific CD8 T cell response and compare it to that induced against non-self-Ags expressed from a similar vector platform. Prophylactic vaccination with adenoviral vectors expressing either TRP-2 (Ad-Ii-TRP-2) or GP100 (Ad-Ii-GP100) had little or no effect on the growth of s.c. B16 melanomas, and only Ad-Ii-TRP-2 was able to induce a marginal reduction of B16 lung metastasis. In contrast, vaccination with a similar vector construct expressing a foreign (viral) TA induced efficient tumor control. Analyzing the self-TA-specific CD8 T cells, we observed that these could be activated to produce IFN-γ and TNF-α. In addition, surface expression of phenotypic markers and inhibitory receptors, as well as in vivo cytotoxicity and degranulation capacity matched that of non-self-Ag-specific CD8 T cells. However, the CD8 T cells specific for self-TAs had a lower functional avidity, and this impacted on their in vivo performance. On the basis of these results and a low expression of the targeted TA epitopes on the tumor cells, we suggest that low avidity of the self-TA-specific CD8 T cells may represent a major obstacle for efficient immunotherapy of cancer.

Nucleoprotein-specific nonneutralizing antibodies speed up LCMV elimination independently of complement and FcγR

CD8(+) T cells have an essential role in controlling lymphocytic choriomeningitis virus (LCMV) infection in mice. Here, we examined the contribution of humoral immunity, including nonneutralizing antibodies (Abs), in this infection induced by low virus inoculation doses. Mice with impaired humoral immunity readily terminated infection with the slowly replicating LCMV strain Armstrong but showed delayed virus elimination after inoculation with the faster replicating LCMV strain WE and failed to clear the rapidly replicating LCMV strain Docile, which is in contrast to the results obtained with wild-type mice. Thus, the requirement for adaptive humoral immunity to control the infection was dependent on the replication speed of the LCMV strains used. Ab transfers further showed that LCMV-specific IgG Abs isolated from LCMV immune serum accelerated virus elimination. These Abs were mainly directed against the viral nucleoprotein (NP) and completely lacked virus neutralizing activity. Moreover, mAbs specific for the LCMV NP were also able to decrease viral titers after transfer into infected hosts. Intriguingly, neither C3 nor Fcγ receptors were required for the antiviral activity of the transferred Abs. In conclusion, our study suggests that rapidly generated nonneutralizing Abs specific for the viral NP speed up virus elimination and thereby may counteract T-cell exhaustion.

B cells and autoimmunity

There is a growing appreciation for the role for B cells in autoimmune disorders in which inflammation is driven by T cells, in addition to the well-established role for B cells in autoimmune disorders characterized by pathogenic auto-antibodies. Current information on tolerance checkpoints in B cells, B cell depletion, BAFF blockade, regulatory B cells and clonal ignorance mediated by the SIAE/Siglec pathway will be reviewed.

Vaccination against lymphocytic choriomeningitis virus infection in MHC class II-deficient mice

The impact of prophylactic vaccination against acute and chronic infection in a Th-deficient host has not been adequately addressed because of difficulties in generating protective immunity in the absence of CD4(+) T cell help. In this study, we demonstrated that a broad CD8(+) T cell immune response could be elicited in MHC class II-deficient mice by vaccination with adenovirus encoding lymphocytic choriomeningitis virus (LCMV) glycoprotein tethered to MHC class II-associated invariant chain. Moreover, the response induced conferred significant cytolytic CD8(+) T cell-mediated protection against challenge with a high dose of the invasive clone 13 strain of LCMV. In contrast, vaccination with adenovirus encoding unlinked LCMV glycoprotein induced weak virus control in the absence of CD4(+) T cells, and mice may die of increased immunopathology associated with incomplete protection. Acute mortality was not observed in any vaccinated mice following infection with the less-invasive Traub strain. However, LCMV Traub infection caused accelerated late mortality in unvaccinated MHC class II-deficient mice; in this case, we observed a strong trend toward delayed mortality in vaccinated mice, irrespective of the nature of the vaccine. These results indicated that optimized vaccination may lead to efficient protection against acute viral infection, even in Th-deficient individuals, but that the duration of such immunity is limited. Nevertheless, for select immunodeficiencies in which CD4(+) T cell deficiency is incomplete or transient, these results are very encouraging.

B cells help alloreactive T cells differentiate into memory T cells

B cells are recognized as effector cells in allograft rejection that are dependent upon T cell help to produce alloantibodies causing graft injury. It is not known if B cells can also help T cells differentiate into memory cells in the alloimmune response. We found that in B-cell-deficient hosts, differentiation of alloreactive T cells into effectors was intact whereas their development into memory T cells was impaired. To test if B cell help for T cells was required for their continued differentiation into memory T cells, activated T cells were sorted from alloimmunized mice and transferred either with or without B cells into naïve adoptive hosts. Activated T cells cotransferred with B cells gave rise to more memory T cells than those transferred without B cells and upon recall, mediated accelerated rejection of skin allografts. Cotransfer of B cells led to increased memory T cells by enhancing activated CD4 T-cell proliferation and activated CD8 T-cell survival. These results indicate that B cells help alloreactive T-cell differentiation, proliferation and survival to generate optimal numbers of functional memory T cells.

Adenoviral vaccination combined with CD40 stimulation and CTLA-4 blockage can lead to complete tumor regression in a murine melanoma model

Therapeutic vaccination with replication deficient adenovirus expressing a viral antigen linked to invariant chain was recently found to markedly delay the growth of B16.F10 melanomas expressing the same antigen; however, complete regression of the tumors was never observed. Here we show that the delay in tumor growth can be converted to complete regression and long-term survival in 30-40% of the mice by a booster vaccination plus combinational treatment with agonistic anti-CD40 monoclonal antibodies (mAb) and anti-CTLA-4 mAb. Regarding the mechanism underlying the improved clinical effect, analysis of the tumor-specific response revealed a significantly prolonged tumor-specific CD8 T cell response in spleens of the mice receiving the combinational treatment compared with mice receiving either treatment individually. Matching this, CD8 T cell depletion completely prevented tumor control. These results indicate that even with a strong tumor vaccine candidate, combinatorial treatment may be required to obtain clinically relevant results.

Effector and regulatory B cells: modulators of CD4+ T cell immunity

B cells are essential for humoral immunity, but the role that they have in regulating CD4(+) T cell responses remains controversial. However, new data showing that the transient depletion of B cells potently influences the induction, maintenance and reactivation of CD4(+) T cells, with the recent identification of antibody-independent functions of B cells, have reinvigorated interest in the many roles of B cells in both infectious and autoimmune diseases. In this Review, we discuss recent data showing how effector and regulatory B cells modulate CD4(+) T cell responses to pathogens and autoantigens.

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.

Impaired antibody response causes persistence of prototypic T cell-contained virus

CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor gamma chain or Fc gamma receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell-controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

Fusion of a viral antigen to invariant chain leads to augmented T-cell immunity and improved protection in gene-gun DNA-vaccinated mice

It has recently been demonstrated that a recombinant replication-deficient human adenovirus 5 (Ad5) vector expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) fused to the p31 invariant (Ii) chain confers broad, long-lasting T-cell immunity that completely protects C57BL/6 mice against lethal peripheral challenge. The current study questioned whether the same strategy, i.e. linkage of GP to an Ii chain, could be applied to a naked DNA vaccine. Following gene-gun immunization with the linked construct (DNA–IiGP), GP-specific CD4+ T cells could not be detected by flow cytometry. However, inclusion of the Ii chain augmented the priming of GP-specific CD8+ T cells directed towards both immunodominant (GP33–41) and subdominant (GP276–286 and GP92–101) epitopes, and vaccination with DNA–IiGP conferred significantly improved protection against systemic LCMV infection compared with the unlinked construct. In contrast, substantial protection against peripheral challenge was not observed. Additional experiments with T-cell subset-depleted or perforin-deficient mice revealed that virus control in vaccinated mice depends critically on cytotoxic CD8+ T cells. Finally, priming with the naked DNA vaccine was shown to augment the immune response raised by subsequent immunization with the Ad5 vector. In conclusion, this study showed that the immunoenhancing effect of Ii chain linkage is not limited to the Ad5 vector, but is also relevant with a DNA platform. Furthermore, given the fact that the Ii chain enhances the presentation of more than one epitope, this suggests that Ii-chain-based DNA vaccines may be promising candidates for various heterologous prime–boost regimes.

B cell antigen presentation promotes Th2 responses and immunopathology during chronic allergic lung disease

Background

The role of B cells in allergic asthma remains undefined. One mechanism by which B cells clearly contribute to allergic disease is via the production of specific immunoglobulin, and especially IgE. Cognate interactions with specific T cells result in T cell help for B cells, resulting in differentiation and immunoglobulin secretion. Proximal to (and required for) T cell-dependent immunoglobulin production, however, is antigen presentation by B cells. While interaction with T cells clearly has implications for B cell function and differentiation, this study investigated the role that B cells have in shaping the T cell response during chronic allergic lung disease.

Methodology/Principal Findings

In these studies, we used a clinically relevant mouse model of chronic allergic lung disease to study the role of B cells and B cell antigen presentation in this disease. In these studies we present several novel findings: 1) Lung B cells from chronically allergen challenged mice up-regulated MHC II and costimulatory molecules CD40, CD80 and CD86. 2) Using in vitro studies, B cells from the lungs of allergen challenged mice could present antigen to T cells, as assessed by T cell proliferation and the preferential production of Th2 cytokines. 3) Following chronic allergen challenge, the levels of Th2 cytokines IL-4 and IL-5 in the lungs and airways were significantly attenuated in B cell −/− mice, relative to controls. 4) B cell driven Th2 responses and mucus hyper secretion in the lungs were dependent upon MHC II expression by B cells.

Conclusions/Significance

Collectively, these results provide evidence for antigen presentation as a novel mechanism by which B cells contribute to chronic allergic disease. These findings give new insight into the mechanisms by which B cells promote asthma and other chronic diseases.

MHC class II-associated invariant chain linkage of antigen dramatically improves cell-mediated immunity induced by adenovirus vaccines

The ideal vaccine induces a potent protective immune response, which should be rapidly induced, long-standing, and of broad specificity. Recombinant adenoviral vectors induce potent Ab and CD8+ T cell responses against transgenic Ags within weeks of administration, and they are among the most potent and versatile Ag delivery vehicles available. However, the impact of chronic infections like HIV and hepatitis C virus underscore the need for further improvements. In this study, we show that the protective immune response to an adenovirus-encoded vaccine Ag can be accelerated, enhanced, broadened, and prolonged by tethering of the rAg to the MHC class II-associated invariant chain (Ii). Thus, adenovirus-vectored vaccines expressing lymphocytic choriomeningitis virus (LCMV)-derived glycoprotein linked to Ii increased the CD4+ and CD8+ T cell stimulatory capacity in vitro and in vivo. Furthermore, mice vaccinated with a single dose of adenovirus-expressing LCMV-derived glycoprotein linked to Ii were protected against lethal virus-induced choriomeningitis, lethal challenge with strains mutated in immunodominant T cell epitopes, and systemic infection with a highly invasive strain. In therapeutic tumor vaccination, the vaccine was as efficient as live LCMV. In comparison, animals vaccinated with a conventional adenovirus vaccine expressing unmodified glycoprotein were protected against systemic infection, but only temporarily against lethal choriomeningitis, and this vaccine was less efficient in tumor therapy.

B cell regulation of CD4+CD25+ T regulatory cells and IL-10 via B7 is essential for recovery from experimental autoimmune encephalomyelitis

CD4(+)CD25(+) T regulatory (Treg) cells expressing the Foxp3 transcription factor have been shown to be present in the CNS during the autoimmune disease experimental autoimmune encephalomyelitis (EAE) and can inhibit EAE clinical disease by an IL-10-dependent mechanism. In addition, IL-10 expression in the CNS late in the EAE disease course has been attributed to recovery. However, it is not known how Treg cells and IL-10 expressions are regulated during EAE. We have previously shown a requirement for B cells in recovery from EAE and here investigated whether this was due to a deficiency in Treg cells and IL-10 in the CNS. We found that B cell deficiency resulted in a delay in the emergence of Foxp3-expressing Treg cells and IL-10 in the CNS during EAE, but not in the periphery. Reconstitution with wild-type B cells resulted in disease recovery and normalized IL-10 and Foxp3 expression. However, reconstitution with B7-deficient B cells did not. Furthermore, we show that IL-10 and Foxp3 expression is enhanced in CNS nonencephalitogenic T cells. These data suggest a novel mechanism whereby B cells regulate CD4(+)CD25(+) Treg cells via B7 and subsequently enter the CNS and suppress autoimmune inflammation, mediating recovery.

Presence of distinct subsets of cytolytic CD8+ T cells in chronic HIV infection

Cytolytic T lymphocytes (CTL) play an important role in the control of HIV infection. The eventual failure to contain HIV-1 infection may arise because of a functional impairment of HIV-specific CTL. We evaluated Gag-specific cytotoxicity in HIV-1-positive Ugandans. Expression of CD107, a marker for cytolytic activity, was present in CD45RA(bright) and CD45RA(dim) CD8(+) T cell populations in HIV-infected individuals. The frequency of Gag-specific CD107(+)CD45RA(bright)CD28(-)CCR7(-) CD8(+) T cells decreased with CD4 cell depletion and correlated with the presence of Gag-specific T helper response. In contrast, the frequency of Gag-specific CD107(+)CD45RA(dim)CD28(-)CCR7(-) CD8(+) T cells within the same individuals has no significant association with viral load or CD4 cell count. The ratio of CD45RA(bright) to CD45RA(dim) CTL correlates significantly with CD4 cell count. This positive association decreases with antiretroviral treatment (ARV), indicating that suppression of viral replication alters the balance of circulating Gag-specific CD8(+) effector T cells. Subsets of cytolytic T cells may have distinct antiviral functions and further characterization of these effector CD8(+) T cells may yield important information on T cell regulation and dysfunction in HIV infection.

Antibody-independent control of gamma-herpesvirus latency via B cell induction of anti-viral T cell responses

B cells can use antibody-dependent mechanisms to control latent viral infections. It is unknown whether this represents the sole function of B cells during chronic viral infection. We report here that hen egg lysozyme (HEL)-specific B cells can contribute to the control of murine gamma-herpesvirus 68 (gammaHV68) latency without producing anti-viral antibody. HEL-specific B cells normalized defects in T cell numbers and proliferation observed in B cell-/- mice during the early phase of gammaHV68 latency. HEL-specific B cells also reversed defects in CD8 and CD4 T cell cytokine production observed in B cell-/- mice, generating CD8 and CD4 T cells necessary for control of latency. Furthermore, HEL-specific B cells were able to present virally encoded antigen to CD8 T cells. Therefore, B cells have antibody independent functions, including antigen presentation, that are important for control of gamma-herpesvirus latency. Exploitation of this property of B cells may allow enhanced vaccine responses to chronic virus infection.

B cells are required for generation of protective effector and memory CD4 cells in response to Pneumocystis lung infection

B cell-deficient mice are susceptible to infection by Pneumocystis carinii f. sp. muris (PC). To determine whether this susceptibility is due to a requirement for B cells to prime T cells, we compared CD4 T cell responses to PC in bone marrow chimeric mice that express MHC class II (MHCII) on all APCs (wild-type (WT) chimeras) and in bone marrow chimeric mice that express MHCII on all APCs except B cells (MHCII(-/-) chimeras). Although PC was rapidly cleared by WT chimeric mice, PC levels remained high in chimeric mice that lacked MHCII on B cells. In addition, although T cells were primed in the draining lymph nodes of MHCII(-/-) chimeric mice, the number of activated CD4 T cells infiltrating the lungs of these mice was reduced relative to the number in the lungs of WT chimeras. We also adoptively transferred purified CD4 T cells from the draining lymph nodes of PC-infected normal or B cell-deficient mice into SCID mice. Mice that received CD4 cells from normal mice were able to mount a response to infection in the lungs and clear PC. However, mice that received CD4 cells from B cell-deficient mice had a delayed T cell response in the lungs and failed to control the infection. These data indicate that B cells play a vital role in generation of CD4(+) memory T cells in response to PC infection in the lungs.

Prohormone convertase 1/3 is essential for processing of the glucose-dependent insulinotropic polypeptide precursor

The physiology of the incretin hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), and their role in type 2 diabetes currently attract great interest. Recently we reported an essential role for prohormone convertase (PC) 1/3 in the cleavage of intestinal proglucagon, resulting in formation of GLP-1, as demonstrated in PC1/3-deficient mice. However, little is known about the endoproteolytic processing of the GIP precursor. This study investigates the processing of proGIP in PC1/3 and PC2 null mice and in cell lines using adenovirus-mediated overexpression. Supporting a role for PC1/3 in proGIP processing, we found co-localization of GIP and PC1/3 but not PC2 in intestinal sections by immunohistochemistry, and analysis of intestinal extracts from PC1/3-deficient animals demonstrated severely impaired processing to GIP, whereas processing to GIP was unaltered in PC2-deficient mice. Accordingly, overexpression of preproGIP in the neuroendocrine AtT-20 cell line that expresses high levels of endogenous PC1/3 and negligible levels of PC2 resulted in production of GIP. Similar results were obtained after co-expression of preproGIP and PC1/3 in GH4 cells that express no PC2 and only low levels of PC1/3. In addition, studies in GH4 cells and the alpha-TC1.9 cell line, expressing PC2 but not PC1/3, indicate that PC2 can mediate processing to GIP but also to other fragments not found in intestinal extracts. Taken together, our data indicate that PC1/3 is essential and sufficient for the production of the intestinal incretin hormone GIP, whereas PC2, although capable of cleaving proGIP, does not participate in intestinal proGIP processing and is not found in intestinal GIP-expressing cells.

Virus-induced polyclonal B cell activation improves protective CTL memoryvia retained CD27 expression on memory CTL

Different viruses elicit distinct phenotypes of memory cytotoxic T lymphocytes (CTL). This is reflected in differential expression of homing receptors and costimulatory molecules like CD27. Memory CTL retained CD27 following lymphocytic choriomeningitis virus (LCMV) infection, but not after immunization with recombinant vaccinia virus or tumor cells expressing LCMV glycoprotein. Stable CD27 expression on memory CTL required ligation by CD70 expressed on polyclonally activated B cells during the contraction phase. The functional consequence of CD27 expressed on virus-specific CTL was analyzed in CD27-deficient mice. LCMV infection of CD27(-/-) mice revealed that primary CTL activation and expansion as well as elimination of the virus were independent of CD27 expression. In contrast, ligation of CD27 on memory CTL upon secondary antigen encounter increased clonal expansion and improved protection against re-infection. This points to novel B cell-CTL interactions during viral infection and to a beneficial role of polyclonal B cell activation that represents a characteristic of murine LCMV, human immunodeficiency virus and human hepatitis B and C virus infection.

B cells are not required for T cell priming in low zone tolerance to contact allergens and contact hypersensitivity

Low zone tolerance (LZT) to contact allergens is induced by epicutaneous exposure to haptens in subsensitizing doses resulting in an inhibition of contact hypersensitivity (CHS), which, in contrast, occurs after sensitization with immunogenic doses of allergens. Performing the protocol of tolerance induction resulted in robust LZT to allergens in B cell-deficient mice in vivo, indicating that B cells are not required for the induction and effector phase of LZT. However, CHS reactions in vivo were restricted in B cell-deficient mice as compared to wild-type (WT) mice. In contrast, analysis of hapten-specific T cell activation in vitro revealed a strong proliferative response of T cells derived from both WT and B cell-deficient sensitized mice. Similar to WT animals, T cells obtained from tolerized B cell-deficient mice produced a Tc2 cytokine pattern of LZT with high levels of IL-4 and IL-10, whereas sensitization of B cell-deficient mice resulted in the typical Tc1 cytokine profile of CHS. Adoptive transfer of CD8+ effector T cells from tolerized or sensitized B cell-deficient mice induced significant LZT or CHS reactions, respectively, in WT recipients, demonstrating that the development of hapten-specific effector CD8+ T cells of LZT and CHS is independent of B cells.