The role of CD40-CD154 interaction in antiviral T cell-independent IgG responses

Use of gene expression profiling to identify candidate genes for pretherapeutic patient classification in acute appendicitis

Background

Phlegmonous and gangrenous appendicitis represent independent pathophysiological entities with different clinical courses ranging from spontaneous resolution to septic disease. However, reliable predictive methods for these clinical phenotypes have not yet been established. In an attempt to provide pathophysiological insights into the matter, a genomewide gene expression analysis was undertaken in patients with acute appendicitis.

Methods

Peripheral blood mononuclear cells were isolated and, after histological confirmation of PA or GA, analysed for genomewide gene expression profiling using RNA microarray technology and subsequent pathway analysis.

Results

Samples from 29 patients aged 7–17 years were included. Genomewide gene expression analysis was performed on 13 samples of phlegmonous and 16 of gangrenous appendicitis. From a total of 56 666 genes, 3594 were significantly differently expressed. Distinct interaction between T and B cells in the phlegmonous appendicitis group was suggested by overexpression of T cell receptor α and β subunits, CD2, CD3, MHC II, CD40L, and the B cell markers CD72 and CD79, indicating an antiviral mechanism. In the gangrenous appendicitis group, expression of genes delineating antibacterial mechanisms was found.

Conclusion

These results provide evidence for different and independent gene expression in phlegmonous and gangrenous appendicitis in general, but also suggest distinct immunological patterns for the respective entities. In particular, the findings are compatible with previous evidence of spontaneous resolution in phlegmonous and progressive disease in gangrenous appendicitis.

Closing the serological gap in the diagnostic testing for COVID-19: The value of anti-SARS-CoV-2 IgA antibodies

During coronavirus disease 2019 (COVID‐19) pandemic, the early diagnosis of patients is a priority. Serological assays, in particular immunoglobulin (Ig)M and IgG anti‐severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), have today several applications but the interpretation of their results remains an open challenge. Given the emerging role of the IgA isotype in the COVID‐19 diagnostics, we aimed to identify the SARS‐CoV‐2 IgA antibodies in a COVID‐19 population seronegative for IgM. A total of 30 patients hospitalized in San Giovanni di Dio Hospital (Florence, Italy) for COVID‐19, seronegative for IgM antibodies, have been studied for anti‐SARS‐CoV‐2 antibodies. They all had a positive oro/nasopharyngeal swab reverse transcription‐polymerase chain reaction result. Assays used were a chemiluminescent assay measuring SARS‐CoV‐2 specific IgM and IgG (S + N) and an ELISA, measuring specific IgG (S1) and IgA antibodies against SARS‐CoV‐2. Among the 30 patients, eight were positive for IgA, seven were positive for IgG (N + S), and two for IgG (S1), at the first point (5‐7 days from the onset of symptoms). The IgA antibodies mean values at the second (9‐13 days) and third (21‐25 days) time points were even more than twice as high as IgG assays. The agreement between the two IgG assays was moderate (Cohen's K = 0.59; SE = 0.13). The inclusion of the IgA antibodies determination among serological tests of the COVID‐19 diagnostic is recommended. IgA antibodies may help to close the serological gap of the COVID‐19. Variations among anti‐SARS‐CoV‐2 IgG assays should be considered in the interpretation of results.

The inclusion of SARS‐CoV‐2 IgA antibodies may increase the diagnostic sensitivity of the serological tests for COVID‐19

The early appearance and high concentrations of SARS‐CoV‐2 IgA antibodies make them good potential markers for identifying COVID‐19 patients.

Variations of anti‐SARS‐CoV‐2 IgG assays exist.

Rainbow Trout IgM+ B Cells Preferentially Respond to Thymus-Independent Antigens but Are Activated by CD40L

In the absence of class switch recombination and germinal centers, the mechanisms through which B cells from teleost fish mount extrafollicular immunoglobulin M (IgM) memory responses remains mostly unexplored. In this report, we demonstrate that teleost IgM⁺ B cells respond to CD40L, a thymus-dependent activation signal, similarly to mammalian B2 cells. However, when stimulated with different types of antigens, fish IgM⁺ B cells only reach a general activation state in response to antigens cataloged as thymus-independent 1 (TI-1) in mammals, as established through both functional assays and RNA sequencing. Interestingly, fish IgM⁺ B cells remained completely unresponsive to TI-2 antigens, suggesting that the engagement of innate receptors provided by TI-1 antigens is required for the activation of teleost B cells. Finally, a synergy between CD40L and TI-1 antigens was also demonstrated, further supporting that there is no clear dichotomy between thymus-dependent and TI responses in teleost fish.

Scaffolded Antigens in Yeast Cell Particle Vaccines Provide Protection against Systemic Polyoma Virus Infection

Background. U65, a self-aggregating peptide scaffold, traps fused protein antigens in yeast cells. Conversion to Yeast Cell Particle (YCP) vaccines by partial removal of surface mannoproteins exposes β-glucan, mediating efficient uptake by antigen-presenting cells (APCs). YCP vaccines are inexpensive, capable of rapid large-scale production and have potential for both parenteral and oral use. Results. YCP processing by alkaline hydrolysis exposes up to 20% of the glucan but converts scaffolded antigen and internal yeast proteins into a common aggregate, preventing selective yeast protein removal. For U65-green fluorescent protein (GFP) or U65-Apolipoprotein A1 (ApoA1) subcutaneous vaccines, maximal IgG responses in mice required 10% glucan exposure. IgG responses to yeast proteins were 5-fold lower. Proteolytic mannoprotein removal produced YCPs with only 6% glucan exposure, insufficiently porous for selective removal of even native yeast proteins. Vaccine efficacy was reduced 10-fold. Current YCP formulations, therefore, are not suitable for human use but have considerable potential for use in feed animal vaccines. Significantly, a YCP vaccine expressing a GFP fusion to VP1, the murine polyoma virus major capsid protein, after either oral or subcutaneous administration, protected mice against an intraperitoneal polyoma virus challenge, reducing viral DNA levels in spleen and liver by >98%.

Direct Interaction of CD40 on Tumor Cells with CD40L on T Cells Increases the Proliferation of Tumor Cells by Enhancing TGF-β Production and Th17 Differentiation

It has recently been reported that the CD40-CD40 ligand (CD40L) interaction is important in Th17 development. In addition, transforming growth factor-beta (TGF-β) promotes tumorigenesis as an immunosuppressive cytokine and is crucial in the development of Th17 cells. This study investigated the role of CD40 in breast cancer cells and its role in immunosuppressive function and tumor progression. CD40 was highly expressed in the breast cancer cell line MDA-MB231, and its stimulation with CD40 antibodies caused the up-regulation of TGF-β. Direct CD40-CD40L interaction between MDA-MB231 cells and activated T cells also increased TGF-β production and induced the production of IL-17, which accelerated the proliferation of MDA-MB231 cells through the activation of STAT3. Taken together, the direct CD40-CD40L interaction of breast tumor cells and activated T cells increases TGF-β production and the differentiation of Th17 cells, which promotes the proliferation of breast cancer cells.

Long-lasting T cell-independent IgG responses require MyD88-mediated pathways and are maintained by high levels of virus persistence

Many viruses induce acute T cell-independent (TI) B cell responses due to their repetitive epitopes and the induction of innate cytokines. Nevertheless, T cell help is thought necessary for the development of long-lasting antiviral antibody responses in the form of long-lived plasma cells and memory B cells. We found that T cell-deficient (T cell receptor β and δ chain [TCRβδ] knockout [KO]) mice persistently infected with polyomavirus (PyV) had long-lasting antiviral serum IgG, and we questioned whether they could generate TI B cell memory. TCRβδ KO mice did not form germinal centers after PyV infection, lacked long-lived IgG-secreting plasma cells in bone marrow, and did not have detectable memory B cell responses. Mice deficient in CD4(+) T cells had a lower persisting virus load than TCRβδ KO mice, and these mice had short-lived antiviral IgG responses, suggesting that a high virus load is required to activate naive B cells continuously, and maintain the long-lasting serum IgG levels. Developing B cells in bone marrow encounter high levels of viral antigens, which can cross-link both their B cell receptor (BCR) and Toll-like receptors (TLRs), and this dual engagement may lead to a loss of their tolerance. Consistent with this hypothesis, antiviral serum IgG levels were greatly diminished in TCRβδ KO/MyD88(-/-) mice. We conclude that high persisting antigen levels and innate signaling can lead to the maintenance of long-lasting IgG responses even in the absence of T cell help.

IMPORTANCE

Lifelong control of persistent virus infections is essential for host survival. Several members of the polyomavirus family are prevalent in humans, persisting at low levels in most people without clinical manifestations, but causing rare morbidity/mortality in the severely immune compromised. Studying the multiple mechanisms that control viral persistence in a mouse model, we previously found that murine polyomavirus (PyV) induces protective T cell-independent (TI) antiviral IgG. TI antibody (Ab) responses are usually short-lived, but T cell-deficient PyV-infected mice can live for many months. This study investigates how protective IgG is maintained under these circumstances and shows that these mice lack both forms of B cell memory, but they still have sustained antiviral IgG responses if they have high levels of persisting virus and intact MyD88-mediated pathways. These requirements may ensure life-saving protection against pathogens even in the absence of T cells, but they prevent the continuous generation of TI IgG against harmless antigens.

Generation of protective T cell-independent antiviral antibody responses in SCID mice reconstituted with follicular or marginal zone B cells

B cells generated in the bone marrow of adult mice enter the periphery as transitional B cells and subsequently differentiate into one of two phenotypically and functionally distinct subsets, marginal zone (MZ) or follicular (Fo) B cells. Recent reports indicate, however, that in response to environmental cues, such as lymphopenia, mature Fo B cells can change to display phenotypic markers characteristic of MZ B cells. Previously, we found that splenic B cells transferred to SCID mice responded to polyoma virus (PyV) infection with T cell-independent (TI) IgM and IgG secretion, reducing the viral load and protecting mice from the lethal effect of the infection. The contribution of MZ and Fo B cell subsets to this antiviral TI-2 response, however, has not been addressed. In this study, we show that both sort-purified MZ and Fo B cells generate protective TI Ab responses to PyV infection when transferred into SCID mice. Moreover, the transferred Fo B cells in the spleens of the PyV-infected SCID mice change phenotype, with many of them displaying MZ B cell characteristics. These findings demonstrate the plasticity of the B cell subsets in virus-infected hosts and show for the first time that B cells derived exclusively from Fo B cells can effectively function in antiviral TI-2 responses.

Immunity to polyomavirus infection: the polyomavirus-mouse model

A ubiquitous clinically silent murine pathogen, polyomavirus has enjoyed long-term co-evolution with the mouse, a highly tractable and genetically and immunologically informative small animal model. Thus, polyomavirus has provided a valuable experimental construct to decipher the host immune mechanisms that come into play to control systemic low-level persistent viral infections. Impaired immunosurveillance for infected cells puts the murine host at risk both to injury resulting from excessive direct virus cytolysis and development of virus-induced tumors. In this review, we present our current understanding of the multifaceted immune response invoked by the mouse to maintain détente with this potentially deleterious persistent natural pathogen, and discuss implications of these studies for therapeutic interventions for human polyomavirus infection.

Allogeneic differences in the dependence on CD4+ T-cell help for virus-specific CD8+ T-cell differentiation

CD4(+) T-cell help enables antiviral CD8(+) T cells to differentiate into fully competent memory cells and sustains CD8(+) T-cell-mediated immunity during persistent virus infection. We recently reported that mice of C57BL/6 and C3H strains differ in their dependence on CD28 and CD40L costimulation for long-term control of infection by polyoma virus, a persistent mouse pathogen. In this study, we asked whether mice of these inbred strains also vary in their requirement for CD4(+) T-cell help for generating and maintaining polyoma virus-specific CD8(+) T cells. CD4(+) T-cell-depleted C57BL/6 mice mounted a robust antiviral CD8(+) T-cell response during acute infection, whereas unhelped CD8(+) T-cell effectors in C3H mice were functionally impaired during acute infection and failed to expand upon antigenic challenge during persistent infection. Using (C57BL/6 x C3H)F(1) mice, we found that the dispensability for CD4(+) T-cell help for the H-2(b)-restricted polyoma virus-specific CD8(+) T-cell response during acute infection extends to the H-2(k)-restricted antiviral CD8(+) T cells. Our findings demonstrate that dependence on CD4(+) T-cell help for antiviral CD8(+) T-cell effector differentiation can vary among allogeneic strains of inbred mice.

The antiviral CD8+ T cell response is differentially dependent on CD4+ T cell help over the course of persistent infection

Although many studies have investigated the requirement for CD4(+) T cell help for CD8(+) T cell responses to acute viral infections that are fully resolved, less is known about the role of CD4(+) T cells in maintaining ongoing CD8(+) T cell responses to persistently infecting viruses. Using mouse polyoma virus (PyV), we asked whether CD4(+) T cell help is required to maintain antiviral CD8(+) T cell and humoral responses during acute and persistent phases of infection. Though fully intact during acute infection, the PyV-specific CD8(+) T cell response declined numerically during persistent infection in MHC class II-deficient mice, leaving a small antiviral CD8(+) T cell population that was maintained long term. These unhelped PyV-specific CD8(+) T cells were functionally unimpaired; they retained the potential for robust expansion and cytokine production in response to Ag rechallenge. In addition, although a strong antiviral IgG response was initially elicited by MHC class II-deficient mice, these Ab titers fell, and long-lived PyV-specific Ab-secreting cells were not detected in the bone marrow. Finally, using a minimally myeloablative mixed bone marrow chimerism approach, we demonstrate that recruitment and/or maintenance of new virus-specific CD8(+) T cells during persistent infection is impaired in the absence of MHC class II-restricted T cells. In summary, these studies show that CD4(+) T cells differentially affect CD8(+) T cell responses over the course of a persistent virus infection.

MyD88 is required for the formation of long-term humoral immunity to virus infection

Development of long-term humoral immunity is a major goal of vaccination, but the mechanisms involved in the formation of long-term Ab responses are still being determined. In this study, we identify a previously unknown requirement for MyD88, an adaptor molecule that mediates signals at most TLRs, for the generation of long-term humoral immunity during live virus infection. Polyoma virus-infected MyD88 knockout mice generated strong acute T cell-dependent antiviral IgM and IgG responses and developed germinal centers. Activation-induced cytidine deaminase, an enzyme required for isotype switching and somatic hypermutation, was also induced in germinal center B cells, similar to wild-type mice. However, MyD88 knockout mice failed to develop bone marrow plasma cells and did not maintain long-term serum antiviral Ab responses. The isotype distribution of antiviral IgG responses was also altered; serum IgG2a and IgG2b levels were diminished, whereas IgG1 responses were not affected. The requirement for MyD88 for the formation of long-term humoral immunity to polyoma virus was intrinsic to B cells and was independent of IL-1R and IL-18R, cytokine receptors that also signal through MyD88. Our findings show that MyD88-dependent signaling pathways in B cells are essential for effectively generating long-term Ab responses and implicate a role for TLR in the formation of long-term humoral immunity.

Polyomavirus-associated nephropathy: update on BK virus-specific immunity

The human polyomavirus type 1, also called BK virus (BKV), causes polyomavirus-associated nephropathy (PVAN) in 1-10% of renal transplant recipients, with graft loss in over 50% of cases. The risk factors for PVAN are not conclusively defined and likely involve complementing determinants of recipient, graft, and virus. A central element seems to be the failing balance between BKV replication and BKV-specific immune control, which can result from intense triple immunosuppression, HLA-mismatches, prior rejection and anti-rejection treatment, or BKV-seropositive donor/seronegative recipient pairs. Consistent with this general hypothesis, the timely reduction of immunosuppression in kidney transplant recipients reduced graft loss to less than 10% of cases. However, the BKV-specific humoral and cellular immune response is not well characterized. Recent work from several groups suggest that changes in antibody titers and BKV-specific CD4+ and CD8+ T cells may help to better define the risk and the course of PVAN in renal transplant patients.

Costimulation requirements for antiviral CD8+ T cells differ for acute and persistent phases of polyoma virus infection

The requirement for costimulation in antiviral CD8+ T cell responses has been actively investigated for acutely resolved viral infections, but it is less defined for CD8+ T cell responses to persistent virus infection. Using mouse polyoma virus (PyV) as a model of low-level persistent virus infection, we asked whether blockade of the CD40 ligand (CD40L) and CD28 costimulatory pathways impacts the magnitude and function of the PyV-specific CD8+ T response, as well as the humoral response and viral control during acute and persistent phases of infection. Costimulation blockade or gene knockout of either CD28 or CD40L substantially dampened the magnitude of the acute CD8+ T cell response; simultaneous CD28 and CD40L blockade severely depressed the acute T cell response, altered the cell surface phenotype of PyV-specific CD8+ T cells, decreased PyV VP1-specific serum IgG titers, and resulted in an increase in viral DNA levels in multiple organs. CD28 and CD40L costimulation blockade during acute infection also diminished the memory PyV-specific CD8+ T cell response and serum IgG titer, but control of viral persistence varied between mouse strains and among organs. Interestingly, we found that CD28 and CD40L costimulation is dispensable for generating and/or maintaining PyV-specific CD8+ T cells during persistent infection; however, blockade of CD27 and CD28 costimulation in persistently infected mice caused a reduction in PyV-specific CD8+ T cells. Taken together, these data indicate that CD8+ T cells primed within the distinct microenvironments of acute vs persistent virus infection differ in their costimulation requirements.

B lymphocyte activation by human papillomavirus-like particles directly induces Ig class switch recombination via TLR4-MyD88

Vaccination with human papillomavirus type 16 (HPV16) L1 virus-like particles (VLP) induces both high titer neutralizing IgG and protective immunity. Because protection from experimental infection by papillomavirus is mediated by neutralizing IgG, we sought the mechanisms that trigger humoral immunity to HPV16 L1 VLP. We find that HPV16 L1 VLP bind to murine B lymphocytes thereby inducing activation-induced cytidine deaminase expression and Ig class switch recombination to cause the generation of IgG. HPV16 L1 VLP also activate production of proinflammatory factors IFN-alpha, IL-6, MIP-1alpha, RANTES, and KC, up-regulate the expression of costimulatory molecules by naive B cells, and increase the B1 B cell subpopulation. These B cell responses to HPV16 L1 VLP are dependent upon MyD88. Although MyD88(-/-) B cells produce only mu transcript after exposure to HPV16 L1 VLP, MyD88(+/+) B cells express alpha, gamma, and mu Ig H chain and activation-induced cytidine deaminase transcripts. Notably, TLR4 mutant C3H/HeJ mice exhibited significantly reduced HPV16 VLP-specific IgG1, IgG2a, IgG2b, and IgG3 titers after vaccination as compared with the control C3H/HeOuJ mice. HPV16 L1 VLP directly activated class switch recombination and costimulatory molecule expression by B cells of C3H/HeOuJ mice but not C3H/HeJ mice. Thus HPV16 L1 VLP directly activate B cells to induce CD4(+) T cell independent humoral immune responses via TLR4- and MyD88-dependent signaling.

Why do B cells produce CD40 ligand?

The CD40-CD40 ligand (CD40L) interaction is one of the most important receptor-ligand interactions that occurs during a T dependent immune response. However, while CD40L is expressed on a range of cell types including activated T cells and B cells, dendritic cells granulocytes, macrophages and platelets, only CD40L on T cells is considered by most immunologists when planning experiments or analysing data. The current theory professes that T cells expressing CD40L can provide signals to B cells that induce proliferation, immunoglobulin class switching, antibody secretion, rescue from apoptosis at different times during the life of a B cell and also has a role in the development of germinal centres and the survival of memory B cells. However, the whole story is more complex than presently understood as human and mouse B cells express CD40L on their surface following activation and can release a soluble form of the ligand. This paper hypothesizes how CD40L on B cells may regulate antibody responses and the development of germinal centres.

Differential requirements for expression of CD80/86 and CD40 on B cells for T-dependent antibody responses in vivo

The CD80/86-CD28 and CD40-CD40 ligand costimulatory pathways are essential for Th cell-dependent B cell responses that generate high-affinity, class-switched Ab in vivo. Disruption of either costimulatory pathway results in defective in vivo humoral immune responses, but it remains unclear to what extent this is due to deficient activation of Th cells and/or of B cells. To address this issue, we generated mixed chimeras in which CD80/86- or CD40-deficient bone marrow-derived cells coexist with wild-type (WT) cells, thereby providing the functional T cell help and accessory cell functions required for fully competent B cell responses. We were then able to assess the requirement for CD80/86 or CD40 expression on B cells producing class-switched Ig in response to a T-dependent Ag. In CD80/86 WT plus CD80/86 double-knockout mixed chimeras, both WT- and CD80/86-deficient B cells produced IgG1 and IgE responses, indicating that direct signaling by CD80/86 is not essential for efficient B cell activation. In marked contrast, only WT IgG1 and IgE responses were detected in the chimeras containing CD40-deficient cells, demonstrating that CD40 expression on B cells is essential for class switching by those B cells. Thus, while disrupting either the CD80/86-CD28 or the CD40-CD40 ligand costimulatory pathway abrogates T-dependent B cell immune responses, the two pathways are nonredundant and mediated by distinct mechanisms.

Immunity to polyoma virus infection and tumorigenesis

Because oncogenic DNA viruses establish persistent infections in humans, continuous immunosurveillance for neoplastic cells is required to prevent virus-induced tumors. Antigen-specific CD8+ T lymphocytes are critical in vivo effectors for eliminating virus-infected and virus-transformed cells. Investigation into the induction, regulation, and maintenance of CD8+ T cells specific for these viruses is hindered by the lack of tractable animal models that mimic natural infection. Resistance to tumors induced by polyoma virus, a persistent natural mouse DNA virus, is mediated by polyoma-specific CD8+ T cells. Mice susceptible to polyoma virus tumorigenesis mount a smaller, albeit still considerable, expansion of anti-polyoma CD8+ T cells; importantly, these antiviral CD8+ T cells lack cytotoxic activity while retaining the phenotype of cytotoxic T lymphocyte (CTL) effectors. In this review, we will discuss potential in vivo mechanisms that regulate the functional competence of anti-polyoma CD8+ T cells, particularly in the context of chronic antigenic stimulation provided by persistent viral infections and tumors.

Essential role for CD40 ligand interactions in T lymphocyte-mediated modulation of the murine immune response to pneumococcal capsular polysaccharides

Protection against infection with pneumococci is provided by anti-capsular polysaccharide (caps-PS) Abs. We investigated whether CD40 ligand (CD40L) plays a role in T lymphocyte-mediated regulation of the immune response to caps-PS, which are considered thymus-independent Ags. Administration of MR1, an antagonist mAb against murine CD40L, in BALB/c mice immunized with Pneumovax resulted in an inhibition of the IgM and IgG Ab response for various caps-PS serotypes. Evidence for the involvement of CD4(+) T lymphocytes in the Ab response to caps-PS was obtained in SCID/SCID mice that, when reconstituted with B lymphocytes and CD4(+) T lymphocytes, mounted a higher specific IgM response compared with SCID/SCID mice reconstituted with only B lymphocytes. This helper effect of CD4(+) T lymphocytes was abrogated by MR1. Blocking CD40L in vitro decreased the IgM response to caps-PS and abolished the helper effect of CD4(+) T lymphocytes. CD8(+) T lymphocyte-depleted murine spleen cells mounted a higher in vivo immune response than total murine spleen cells, which provided evidence for a suppressive role of CD8(+) T lymphocytes on the anti-caps-PS immune response. CD4(+) T lymphocyte-depleted murine spleen cells, leaving a B and CD8(+) T lymphocyte fraction, elicited only a weak in vivo and in vitro Ab response, which was enhanced after MR1 administration. In summary, our data provide evidence that T lymphocytes contribute to the regulation of the anti-caps-PS immune response in a CD40L-dependent manner.

The role of CD154 in organ transplant rejection and acceptance

CD154 plays a critical role in determining the outcome of a transplanted organ. This simple statement is amply supported by experimental evidence demonstrating that anti-CD154 antibodies are potent inhibitors of allograft rejection in many rigorous transplant models. Unfortunately, despite intensive investigation over the past ten years, the precise mechanisms by which antibodies against CD154 exert their anti-rejection effects have remained less obvious. Though originally classified with reference to B-cell function, CD154-CD40 interactions have also been shown to be important in T cell-antigen-presenting cell interactions. Accordingly, CD154 has been classified as a T-cell co-stimulatory molecule. However, mounting data suggest that treatment with anti-CD154 antibodies does not simply block costimulatory signals, but rather that the antibodies appear to induce signalling in receptor-bearing T cells. Other data suggest that anti-CD154 effects may be mediated by endothelial cells and possibly even platelets. In fact, the current literature suggests that CD154 can either stimulate or attenuate an immune response, depending upon the model system under study. CD154 has secured a fundamental place in transplant biology and general immunology that will no doubt be the source of considerable investigation and therapeutic manipulation in the coming decade.

Plasma cell differentiation requires the transcription factor XBP-1

Considerable progress has been made in identifying the transcription factors involved in the early specification of the B-lymphocyte lineage. However, little is known about factors that control the transition of mature activated B cells to antibody-secreting plasma cells. Here we report that the transcription factor XBP-1 is required for the generation of plasma cells. XBP-1 transcripts were rapidly upregulated in vitro by stimuli that induce plasma-cell differentiation, and were found at high levels in plasma cells from rheumatoid synovium. When introduced into B-lineage cells, XBP-1 initiated plasma-cell differentiation. Mouse lymphoid chimaeras deficient in XBP-1 possessed normal numbers of activated B lymphocytes that proliferated, secreted cytokines and formed normal germinal centres. However, they secreted very little immunoglobulin of any isotype and failed to control infection with the B-cell-dependent polyoma virus, because plasma cells were markedly absent. XBP-1 is the only transcription factor known to be selectively and specifically required for the terminal differentiation of B lymphocytes to plasma cells.