Regulatory T cells selectively preserve immune privilege of self-antigens during viral central nervous system infection

Regulatory T cells (Tregs) are important for the attenuation of immune reactions. During viral CNS infections, however, an indiscriminate maintenance of CNS immune privilege through Treg-mediated negative regulation could prevent autoimmune sequelae but impair the control of viral replication. We analyzed in this study the impact of Tregs on the development of acute viral encephalomyelitis, T cell-mediated antiviral protection, and prevention of CNS autoimmunity following intranasal infection with the gliatropic mouse hepatitis virus strain A59. To assess the contribution of Tregs in vivo, we specifically depleted CD4(+)Foxp3(+) T cells in a diphtheria toxin-dependent manner. We found that depletion of Tregs had no impact on viral distribution and clearance and did not significantly alter virus-specific CD4(+) and CD8(+) T cell responses. However, Treg depletion led to a more severe CNS inflammation associated with neuronal damage. Dissection of the underlying immunopathological mechanisms revealed the elaborate Treg-dependent regulation of self-reactive CD4(+) T cell proliferation within the CNS-draining lymph node and downtuning of CXCR3 expression on T cells. Taken together, these results suggest that Tregs preserve CNS immune privilege through selective control of CNS-specific Th cells while keeping protective antiviral immunity fully operative.

Subversion of innate and adaptive immune activation induced by structurally modified lipopolysaccharide from Salmonella typhimurium

Salmonella are successful pathogens that infect millions of people every year. During infection, Salmonella typhimurium changes the structure of its lipopolysaccharide (LPS) in response to the host environment, rendering bacteria resistant to cationic peptide lysis in vitro. However, the role of these structural changes in LPS as in vivo virulence factors and their effects on immune responses and the generation of immunity are largely unknown. We report that modified LPS are less efficient than wild-type LPS at inducing pro-inflammatory responses. The impact of this LPS-mediated subversion of innate immune responses was demonstrated by increased mortality in mice infected with a non-lethal dose of an attenuated S. typhimurium strain mixed with the modified LPS moieties. Up-regulation of co-stimulatory molecules on antigen-presenting cells and CD4(+) T-cell activation were affected by these modified LPS. Strains of S. typhimurium carrying structurally modified LPS are markedly less efficient at inducing specific antibody responses. Immunization with modified LPS moiety preparations combined with experimental antigens, induced an impaired Toll-like receptor 4-mediated adjuvant effect. Strains of S. typhimurium carrying structurally modified LPS are markedly less efficient at inducing immunity against challenge with virulent S. typhimurium. Hence, changes in S. typhimurium LPS structure impact not only on innate immune responses but also on both humoral and cellular adaptive immune responses.

Type I IFN-mediated protection of macrophages and dendritic cells secures control of murine coronavirus infection

The swift production of type I IFNs is one of the fundamental aspects of innate immune responses against viruses. Plasmacytoid dendritic cell-derived type I IFNs are of prime importance for the initial control of highly cytopathic viruses such as the mouse hepatitis virus (MHV). The aim of this study was to determine the major target cell populations of this first wave of type I IFNs. Generation of bone marrow-chimeric mice expressing the type I IFN receptor (IFNAR) on either hemopoietic or non-bone marrow-derived cells revealed that the early control of MHV depended mainly on IFNAR expression on hemopoietic cells. To establish which cell population responds most efficiently to type I IFNs, mice conditionally deficient for the IFNAR on different leukocyte subsets were infected with MHV. This genetic analysis revealed that IFNAR expression on LysM+ macrophages and CD11c+ dendritic cells was most important for the early containment of MHV within secondary lymphoid organs and to prevent lethal liver disease. This study identifies type I IFN-mediated cross-talk between plasmacytoid dendritic cells on one side and macrophages and conventional dendritic cells on the other, as an essential cellular pathway for the control of fatal cytopathic virus infection.

TLR2 and TLR4 signaling shapes specific antibody responses to Salmonella typhi antigens

TLR directly induce innate immune responses by sensing a variety of microbial components and are critical for the fine-tuning of subsequent adaptive immune responses. However, their impact and mechanism of action on antibody responses against bacterial antigens are not yet fully understood. Salmonella enterica serovar Typhi (S. typhi) porins have been characterized as inducers of long-lasting specific antibody responses in mice. In this report, we show that immunization of TLR4-deficient (TLR4(-/-)), myeloid differentiating gene 88-deficient and Toll/IL-R domain-containing adaptor-inducing IFN-beta-deficient mice with S. typhi porins led to significantly reduced B-cell responses. TLR2(-/-) mice, as well, showed reduced IgG titers with a more pronounced impairment in the production of IgG3 anti-porins antibodies. Adoptive transfer of TLR2(-/-)- or TLR4(-/-)-B cells into B-cell-deficient mice revealed a direct effect of TLR4 on B cells for the primary IgM response, whereas stimulation of B cells via TLR2 was important for IgG production. Furthermore, S. typhi porins were found to efficiently elicit maturation of CD11c(+) conventional DC. Taken together, S. typhi porins represent not only a suitable B-cell antigen for vaccination, but exhibit potent TLR-dependent stimulatory functions on B cells and DC, which help to further enhance and shape the antibody response.

Lack of conventional dendritic cells is compatible with normal development and T cell homeostasis, but causes myeloid proliferative syndrome

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.

Mouse hepatitis virus liver pathology is dependent on ADP-ribose-1''-phosphatase, a viral function conserved in the alpha-like supergroup

Viral infection of the liver can lead to severe tissue damage when high levels of viral replication and spread in the organ are coupled with strong induction of inflammatory responses. Here we report an unexpected correlation between the expression of a functional X domain encoded by the hepatotropic mouse hepatitis virus strain A59 (MHV-A59), the high-level production of inflammatory cytokines, and the induction of acute viral hepatitis in mice. X-domain (also called macro domain) proteins possess poly-ADP-ribose binding and/or ADP-ribose-1''-phosphatase (ADRP) activity. They are conserved in coronaviruses and in members of the "alpha-like supergroup" of phylogenetically related positive-strand RNA viruses that includes viruses of medical importance, such as rubella virus and hepatitis E virus. By using reverse genetics, we constructed a recombinant murine coronavirus MHV-A59 mutant encoding a single-amino-acid substitution of a strictly conserved residue that is essential for coronaviral ADRP activity. We found that the mutant virus replicated to slightly reduced titers in livers but, strikingly, did not induce liver disease. In vitro, the mutant virus induced only low levels of the inflammatory cytokines tumor necrosis factor alpha and interleukin-6 (IL-6). In vivo, we found that IL-6 production, in particular, was reduced in the spleens and livers of mutant virus-infected mice. Collectively, our data demonstrate that the MHV X domain exacerbates MHV-induced liver pathology, most likely through the induction of excessive inflammatory cytokine expression.

Translating innate response into long-lasting antibody response by the intrinsic antigen-adjuvant properties of papaya mosaic virus

Identifying the properties of a molecule involved in the efficient activation of the innate and adaptive immune responses that lead to long-lasting immunity is crucial for vaccine and adjuvant development. Here we show that the papaya mosaic virus (PapMV) is recognized by the immune system as a pathogen-associated molecular pattern (PAMP) and as an antigen in mice (Pamptigen). A single immunization of PapMV without added adjuvant efficiently induced both cellular and specific long-lasting antibody responses. PapMV also efficiently activated innate immune responses, as shown by the induction of lipid raft aggregation, secretion of pro-inflammatory cytokines, up-regulation of co-stimulatory molecules on dendritic cells and macrophages, and long-lasting adjuvant effects upon the specific antibody responses to model antigens. PapMV mixed with Salmonella enterica serovar Typhi (S. typhi) outer membrane protein C increased its protective capacity against challenge with S. typhi, revealing the intrinsic adjuvant properties of PapMV in the induction of immunity. Antigen-presenting cells loaded with PapMV efficiently induced antibody responses in vivo, which may link the innate and adaptive responses observed. PapMV recognition as a Pamptigen might be translated into long-lasting antibody responses and protection observed. These properties could be used in the development of new vaccine platforms.

Coronavirus non-structural protein 1 is a major pathogenicity factor: implications for the rational design of coronavirus vaccines

Attenuated viral vaccines can be generated by targeting essential pathogenicity factors. We report here the rational design of an attenuated recombinant coronavirus vaccine based on a deletion in the coding sequence of the non-structural protein 1 (nsp1). In cell culture, nsp1 of mouse hepatitis virus (MHV), like its SARS-coronavirus homolog, strongly reduced cellular gene expression. The effect of nsp1 on MHV replication in vitro and in vivo was analyzed using a recombinant MHV encoding a deletion in the nsp1-coding sequence. The recombinant MHV nsp1 mutant grew normally in tissue culture, but was severely attenuated in vivo. Replication and spread of the nsp1 mutant virus was restored almost to wild-type levels in type I interferon (IFN) receptor-deficient mice, indicating that nsp1 interferes efficiently with the type I IFN system. Importantly, replication of nsp1 mutant virus in professional antigen-presenting cells such as conventional dendritic cells and macrophages, and induction of type I IFN in plasmacytoid dendritic cells, was not impaired. Furthermore, even low doses of nsp1 mutant MHV elicited potent cytotoxic T cell responses and protected mice against homologous and heterologous virus challenge. Taken together, the presented attenuation strategy provides a paradigm for the development of highly efficient coronavirus vaccines.

Prevention of viral diseases by vaccination aims for controlled induction of protective immune responses against viral pathogens. Live viral vaccines consist of attenuated, replication-competent viruses that are believed to be superior in the induction of broad immune responses, including cell-mediated immunity. The recent proceedings in the area of virus reverse genetics allows for the rational design of recombinant vaccines by targeting, i.e., inactivating, viral pathogenicity factors. For coronaviruses, a major pathogenicity factor has now been identified. The effect of coronavirus non-structural protein 1 on pathogenicity has been analyzed in a murine model of coronavirus infection. By deleting a part of this protein, a recombinant virus has been generated that is greatly attenuated in vivo, while retaining immunogenicity. In particular, the mutant virus retained the ability to replicate in professional antigen-presenting cells and fulfilled an important requirement of a promising vaccine candidate: the induction of a protective long-lasting, antigen-specific cellular immune response. This study has implications for the rational design of live attenuated coronavirus vaccines aimed at preventing coronavirus-induced diseases of veterinary and medical importance, including the potentially lethal severe acute respiratory syndrome.

Salmonella porins induce a sustained, lifelong specific bactericidal antibody memory response

We examined the ability of porins from Salmonella enterica serovar typhi to induce a long-term antibody response in BALB/c mice. These porins triggered a strong lifelong production of immunoglobulin G (IgG) antibody in the absence of exogenous adjuvant. Analysis of the IgG subclasses produced during this antibody response revealed the presence of the subclasses IgG2b, IgG1, IgG2a and weak IgG3. Despite the high homology of porins, the long-lasting anti-S. typhi porin sera did not cross-react with S. typhimurium. Notably, the antiporin sera showed a sustained lifelong bactericidal-binding activity to the wild-type S. typhi strain, whereas porin-specific antibody titres measured by enzyme-linked immunosorbent assay (ELISA) decreased with time. Because our porin preparations contained the outer membrane proteins C and F (OmpC and OmpF), we evaluated the individual contribution of each porin to the long-lasting antibody response. OmpC and OmpF induced long-lasting antibody titres, measured by ELISA, which were sustained for 300 days. In contrast, although OmpC induced sustained high bactericidal antibody titres for 300 days, postimmunization, the bactericidal antibody titre induced by OmpF was not detected at day 180. These results indicate that OmpC is the main protein responsible for the antibody-mediated memory bactericidal response induced by porins. Taken together, our results show that porins are strong immunogens that confer lifelong specific bactericidal antibody responses in the absence of added adjuvant.