Previous studies (Leadbetter, E.A., I.R. Rifkin, A.H. Hohlbaum, B. Beaudette, M.J. Shlomchik, and A. Marshak-Rothstein. 2002. Nature. 416:603–607; Viglianti, G.A., C.M. Lau, T.M. Hanley, B.A. Miko, M.J. Shlomchik, and A. Marshak-Rothstein. 2003. Immunity. 19:837–847) established the unique capacity of DNA and DNA-associated autoantigens to activate autoreactive B cells via sequential engagement of the B cell antigen receptor (BCR) and Toll-like receptor (TLR) 9. We demonstrate that this two-receptor paradigm can be extended to the BCR/TLR7 activation of autoreactive B cells by RNA and RNA-associated autoantigens. These data implicate TLR recognition of endogenous ligands in the response to both DNA- and RNA-associated autoantigens. Importantly, the response to RNA-associated autoantigens was markedly enhanced by IFN-α, a cytokine strongly linked to disease progression in patients with systemic lupus erythematosus (SLE). As further evidence that TLRs play a key role in autoantibody responses in SLE, we found that autoimmune-prone mice, lacking the TLR adaptor protein MyD88, had markedly reduced chromatin, Sm, and rheumatoid factor autoantibody titers.

Systemic lupus erythematosus, a prototypical systemic autoimmune disease, is the result of a series of interactions within the immune system that ultimately lead to the loss of self-tolerance to nuclear autoantigens. Here, we present an integrated model that explains how self-tolerance is initially lost and how the loss of tolerance is then amplified and maintained as a chronic autoimmune state. Key to this model are the self-reinforcing interactions of T and B cells, which we suggest lead to perpetuation of autoimmunity as well as its spread to multiple autoantigen targets.

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The Lyme disease spirochete, Borrelia burgdorferi, causes persistent mammalian infection despite the development of vigorous immune responses against the pathogen. To examine spirochetal phenotypes that dominate in the hostile immune environment, the mRNA transcripts of four prototypic surface lipoproteins, decorinbinding protein A (DbpA), outer surface protein C (OspC), BBF01, and VlsE, were analyzed by quantitative reverse transcription-PCR under various immune conditions. We demonstrate that B. burgdorferi changes its surface antigenic expression in response to immune attack. dbpA expression was unchanged while the spirochetes decreased ospC expression by 446 times and increased BBF01 and vlsE expression up to 20 and 32 times, respectively, under the influence of immune pressure generated in immunocompetent mice during infection. This change in antigenic expression could be induced by passively immunizing infected severe combined immunodeficiency mice with specific Borrelia antisera or OspC antibody and appears to allow B. burgdorferi to resist immune attack.

The normal functioning immune system is programmed to attack foreign pathogens and other foreign proteins while maintaining tolerance to self-proteins. The mechanisms by which tolerance is broken in the initiation of autoimmunity are not completely understood. In the present study, mice immunized with the murine cytochrome c peptide 90 -104 showed no response by the B or T cell compartments. However, immunization with the isoaspartyl form of this peptide, where the linkage of Asp 93 to Leu 94 occurs through the ␤-carboxyl group, resulted in strong B and T cell autoimmune responses. Antibodies elicited by immunization with the isoaspartyl form of self-peptide were cross-reactive in binding to both isoforms of cytochrome c peptide and to native cytochrome c self-protein. In a similar manner, immunization of mice with the isoaspartyl form of a peptide autoantigen of human systemic lupus erythematosus (SLE) resulted in strong B and T cell responses while mice maintained tolerance to the normal aspartyl form of self-antigen. Isoaspartyl linkages within proteins are enhanced in aging and stressed cells and arise under physiological conditions. These posttranslationally modified peptides may serve as an early immunologic stimulus in autoimmune disease.The immune system has evolved to be tolerant of self-proteins by the deletion of autoreactive cells in the thymus or bone marrow and by the establishment of B and T lymphocyte anergy in the peripheral circulation (1-6). These mechanisms are based on the presentation of a vast array of self-peptides to the lymphoid repertoire. Despite the efforts to instruct the immune system to ignore self-tissues, the appearance of various autoimmune diseases demonstrates that tolerance to self-antigens is not perfect. Flaws in the development of immune tolerance can be revealed by the immunization of animal models with a variety of self-peptides leading to B and T cell autoimmunity as well as autoimmune-mediated pathology (7-12).How tolerance is broken in the initiation of autoimmunity is not completely understood. The immunization of mice with a single self-peptide, the amino-terminal 11 amino acids of myelin basic protein (MBP) 1 in complete Freund's adjuvant can elicit pathology resembling that of human multiple sclerosis (7). The induction of disease requires a post-translationally acetylated form of MBP peptide 1-11. While this disease can be elicited with a single self-peptide or even with T cells of a single specificity, the autoimmune response diversifies to many sites on the MBP over the course of the disease. T cell responses originate with the dominant single self-peptide but rapidly evolve to include other cryptic peptide epitopes within MBP. Similar observations of determinant spreading have been made in murine models of diabetes and systemic lupus erythematosus (SLE), two diseases arising spontaneously in susceptible strains of mice (8,11,13).Antinuclear autoantibodies specific for double-stranded DNA and the U1/Sm ribonucleoprotein particle (snRNP) are diagnostic markers ...

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West Nile (WN) virus causes fatal meningoencephalitis in laboratory mice, thereby partially mimicking human disease. Using this model, we have demonstrated that mice deficient in γδ T cells are more susceptible to WN virus infection. TCRδ−/− mice have elevated viral loads and greater dissemination of the pathogen to the CNS. In wild-type mice, γδ T cells expanded significantly during WN virus infection, produced IFN-γ in ex vivo assays, and enhanced perforin expression by splenic T cells. Adoptive transfer of γδ T cells to TCRδ−/− mice reduced the susceptibility of these mice to WN virus, and this effect was primarily due to IFN-γ-producing γδ T cells. These data demonstrate a distinct role for γδ T cells in the control of and prevention of mortality from murine WN virus infection.

SummaryA novel mechanism for breaking T cell self tolerance is described . B cells induced to make autoantibody by immunization ofmice with the non-self protein human cytochrome c can present the selfprotein mouse cytochrome c to autoreactive T cells in immunogenic form . This mechanism of breaking T cell self tolerance could account for the role of foreign antigens in breaking not only B cell but also T cell self tolerance, leading to sustained autoantibody production in the absence of the foreign antigen .