Unique contribution of IRF-5-Ikaros axis to the B-cell IgG2a response.

IRF-5 is a transcription factor activated by toll like receptor (TLR)7 and TLR9 during innate immune responses. IRF-5 activates not only Type I IFN, but also inflammatory cytokines. Most importantly, a genetic variation in the IRF-5 gene shows a strong association with autoimmune diseases such as Lupus. Here, we report that IRF5-deficient mice have attenuated IgG2a/c responses to T-cell-dependent and -independent antigens and to polyoma virus infection. This defect is due to the intrinsic deletion of IRF-5 in B cells, as SCID mice reconstituted with Irf5-/- B cells show a decrease in IgG2a/c expression after viral infection compared with mice that received wild-type B cells. Irf5-/-B cells in vitro have diminished TLR and cytokine-induced class switching to IgG2a/c. Addressing the molecular mechanism, we show that IRF-5 regulates IgG2a/c expression by decreasing Ikaros expression; reconstitution of IRF-5 in Irf5-/- B cells downregulates Ikaros levels and increases switching to IgG2a/c. The IRF site in ikzf1 promoter binds IRF-5, IRF-4 and IRF-8. We show that IRF-8 but not IRF-4 activates the ikzf1 promoter, and IRF-5 inhibits the transcriptional activity of IRF-8. Collectively, these results identify the IRF-5-Ikaros axis as a critical modulator of IgG2a/c class switching.

Mycoplasma infection induces a scleroderma-like centrosome autoantibody response in mice.

Development of autoantibodies to intracellular molecules is a universal feature of autoimmune diseases and parallels onset of chronic inflammatory pathology. Initiating antigens of disease-specific autoantibody responses are unknown. We previously showed that the major targets of autoantibodies in scleroderma are centrosomes, organelles involved in mitotic spindle organization. Here we show that centrosome autoantibodies are induced in mice by mycoplasma infection. The centrosome-specific antibody response involves class switching of preexisting IgM to IgG isotypes, suggesting a T cell-dependent mechanism. The antibody response spreads to include additional intracellular targets, with newly recruited autoantibody specificities arising as IgM isotypes. Antibiotic treatment of mice prevents autoantibody development. Centrosome autoantibodies may provide an aetiological link between infection and human autoimmunity and suggest novel therapeutic strategies in these disorders.

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.

Innate immunity to viruses: control of vaccinia virus infection by gamma delta T cells.

The existence of gammadelta T cells has been known for over 15 years, but their significance in innate immunity to virus infections has not been determined. We show here that gammadelta T cells are well suited to provide a rapid response to virus infection and demonstrate their role in innate resistance to vaccinia virus (VV) infection in both normal C57BL/6 and beta TCR knockout (KO) mice. VV-infected mice deficient in gammadelta T cells had significantly higher VV titers early postinfection (PI) and increased mortality when compared with control mice. There was a rapid and profound VV-induced increase in IFN-gamma-producing gammadelta T cells in the peritoneal cavity and spleen of VV-infected mice beginning as early as day 2 PI. This rapid response occurred in the absence of priming, as there was constitutively a significant frequency of VV-specific gammadelta T cells in the spleen in uninfected beta TCR KO mice, as demonstrated by limiting dilution assay. Also, like NK cells, another mediator of innate immunity to viruses, gammadelta T cells in uninfected beta TCR KO mice expressed constitutive cytolytic activity. This cytotoxicity was enhanced and included a broader range of targets after VV infection. VV-infected beta TCR KO mice cleared most of the virus by day 8 PI, the peak of the gammadelta T cell response, but thereafter the gammadelta T cell number declined and the virus recrudesced. Thus, gammadelta T cells can be mediators of innate immunity to viruses, having a significant impact on virus replication early in infection in the presence or absence of the adaptive immune response.

Antiviral T-cell-independent type 2 antibody responses induced in vivo in the absence of T and NK cells.

Polyomavirus (PyV) infection induces protective T-cell-independent (TI) IgM and IgG responses in T-cell-deficient (TCR beta x delta-/-) mice. In this study, we show that PyV is a TI -2 antigen: B cells with a mutated Bruton's tyrosine kinase (Xid mutants) do not respond to PyV with antibody secretion in the absence of T cells. We also demonstrate that NK-cell-mediated "help" is not absolutely required for the induction of the TI-2 antibodies to PyV; thus for the first time, we provide evidence for protective IgM and IgG responses against a viral infection induced in mice lacking T and NK cells (CD3Etg). Comparison of the antibody responses observed in T- and NK-cell-deficient mice with those of mice lacking only T cells, however, suggests that NK cells may promote isotype switching to IgG2a. This effect is probably mediated by IFN gamma secretion. In support of this idea, studies on the antibody responses of PyV-infected SCID mice that had been reconstituted with IFN gamma R-/- B cells or wild-type B cells demonstrated the IFN gamma dependence of PyV-specific TI IgG2a secretion and provided evidence that IFN gamma acting directly on B cells plays an important role in TI pathways of isotype switching to IgG2a in vivo.

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

Polyomavirus (PyV) infection elicits protective T cell-independent (TI) IgG responses in T cell-deficient mice. The question addressed in this report is whether CD40 signaling plays a role in this TI antiviral IgG response. Because CD40 ligand (CD40L) can be expressed on numerous cell types in addition to activated T cells, it is possible that cells other than T cells provide CD40L to signal through CD40 on B cells and hence positively influence the antiviral TI IgG responses. In this study we show, by blocking CD40-CD40L interactions in vivo with anti-CD40L Ab treatment in TCR betaxdelta-/- mice and by using SCID mice reconstituted with CD40-/- B cells, that the lack of CD40 signaling in B cells results in a 50% decrease in TI IgG secreted in response to PyV. SCID mice reconstituted with CD40L-/- B cells also responded to PyV infection with diminished IgG secretion compared with that of SCID mice reconstituted with wild-type B cells. This finding suggests that B cells may provide the CD40L for CD40 signaling in the absence of T cell help during acute virus infection. Our studies demonstrate that, although about half of the TI IgG responses to PyV are independent of CD40-CD40L interactions, these interactions occur in T cell-deficient mice and enhance antiviral TI Ab responses.

T-cell-independent antiviral antibody responses.

Recent work has shown that viruses can act in vivo as T-cell-independent antigens, eliciting protective, isotype-switched antibodies in the absence of conventional TCR alpha beta+ T cell help. Inactivated virus or virus-like particles can stimulate IgM production, but factors induced during live virus infection appear to be required to induce the isotype switch that leads to IgG or IgA responses.

T-Cell-independent immunoglobulin G responses in vivo are elicited by live-virus infection but not by immunization with viral proteins or virus-like particles.

Immunoglobulin G (IgG) responses to viruses are generally assumed to be T-cell dependent (TD). Recently, however, polyomavirus (PyV) infection of T-cell-deficient (T-cell receptor beta chain [TCR-beta] -/- or TCR-betaxdelta -/-) mice was shown to elicit a protective, T-cell-independent (TI) antiviral IgM and IgG response. A repetitive, highly organized antigenic structure common to many TI antigens is postulated to be important in the induction of antibody responses in the absence of helper T cells. To test whether the repetitive structure of viral antigens is essential and/or sufficient for the induction of TI antibodies, we compared the abilities of three forms of PyV antigens to induce IgM and IgG responses in T-cell-deficient mice: soluble capsid antigens (VP1), repetitive virus-like particles (VLPs), and live PyV. Immunization with each of the viral antigens resulted in IgM production. VLPs and PyV elicited 10-fold-higher IgM titers than VP1, indicating that the highly organized, repetitive antigens are more efficient in IgM induction. Antigen-specific TI IgG responses, however, were detected only in mice infected with live PyV, not in VP1- or VLP-immunized mice. These results suggest that the highly organized, repetitive nature of the viral antigens is insufficient to account for their ability to elicit TI IgG response and that signals generated by live-virus infection may be essential for the switch to IgG production in the absence of T cells. Germinal centers were not observed in T-cell-deficient PyV-infected mice, indicating that the germinal center pathway of B-cell differentiation is TD even in the context of a virus infection.

T cell-independent antibody-mediated clearance of polyoma virus in T cell-deficient mice.

Polyomavirus (PyV) infection of SCID mice, which lack functional T and B cells, leads to a lethal acute myeloproliferative disease (AMD) and to high levels of virus replication in several organs by two wk after infection. This is in contrast to infection of T cell-deficient athymic nude mice, which are resistant to acute PyV-induced disease and poorly replicate the virus in their organs. This major difference in the virus load and in the outcome of PyV infection between SCID and nude mice suggested that an efficient, T cell-independent antiviral mechanism operates in T cell-deficient, PyV infected mice. To investigate this possibility, mice with different genetically engineered T and/or B cell deficiencies and SCID mice adoptively reconstituted with B and/or T cells were infected with PyV. The results indicated that the presence of B cells in the absence of T cells protected mice from the AMD, and this was accompanied by a major reduction of PyV in all organs tested. Sera from PyV-infected T cell receptor (TCR) alpha beta knockout or TCR alpha beta gamma delta knockout mice contained IgG2a antibodies to PyV. Sera or purified immunoglobulin fractions from PyV-infected TCR alpha beta knockout mice protected SCID mice from the PyV-induced AMD. To our knowledge, this is the first report of an effective T cell-independent antibody response clearing a virus and changing the outcome of infection from 100% mortality to 100% survival.

Acute, lethal, natural killer cell-resistant myeloproliferative disease induced by polyomavirus in severe combined immunodeficient mice.

Infection of severe combined immunodeficient mice, which lack T and B lymphocytes, with polyomavirus (PyV) induced an acute hematological disorder leading to the death of the mice by 2 weeks postinfection. The disease was characterized by a dramatic decrease in megakaryocytes, multiple hemorrhages, anemia, thrombocytopenia, splenomegaly, a massive myeloproliferation and splenic erythroproliferation with a defect in maturation of the myeloid elements similar to that in acute leukemia. This pathology in severe combined immunodeficient mice is very different from that of the well-characterized tumor profiles induced by PyV in normal newborn or nude mice. Viral T and capsid (VP1) antigens and viral genome were detected in some cells in the spleen, but not in the majority of the proliferating myeloid cells. This suggests that the myeloproliferation is induced by some indirect mechanism, such as secretion of growth factors or cytokines by virus-infected cells, rather than by direct transformation by PyV. Neither the spread of PyV, its replication in different organs, nor the pathogenesis or the time of death were altered by depleting natural killer cells in vivo by anti-natural killer cell antibodies. Analysis of the spleen leukocyte population indicated that the cells expressed high levels of class I major histocompatibility complex antigens and were resistant to lysis by activated natural killer cells.