Regulation of proteoglycan-specific immune responses in arthritis

Abstract

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic synovial inflammation. Many cells of the immune system are involved in this chronic inflammatory process. The primary target organs are the joints although as the disease progresses systemic manifestations often also appear. As a consequence of the inflammatory process, RA patients generally develop a progressive loss of cartilage and bone in joints, resulting in painful joints and impaired functional status. Understanding the immune-pathogenic development of RA is a crucial step toward improving its management. In this thesis we focused on two major points: gaining more insight into the immune-pathogenic mechanisms underlying RA and investigating the possible anti-inflammatory role of HSP70.

In order to study arthritis, we made use of the proteoglycan-induced arthritis (PGIA) model in BALB/c mice, which is a T cell-dependent, autoantibody-mediated progressive relapsing autoimmune model resembling many characteristics of RA. Originally PGIA was induced by four PG immunizations of which the first and fourth injection were in complete Freund's adjuvant (CFA) and the second and third PG injection in incomplete Freund's adjuvant (IFA). In chapter 2 (published in Arthritis & Rheumatism 2004;50(5):1665-76) we described that by replacing the Freund's adjuvants by dimethyldioctadecylammonium bromide (DDA), arthritis can be induced by only two PG immunizations. Although the clinical and immunological features of the “classic” form of PGIA are preserved, the onset of the disease is faster and the severity is increased. An additional advantage is that hereby all undesired side effects of the Freund's adjuvants, such as the role of interfering immune responses induced by microbial HSPs present in CFA, are eliminated. DDA exerted a strong stimulatory effect via the activation of innate immunity and forced the immune response toward Th1 dominance (chapter 2). In the PGIA model using DDA as adjuvant, we explored the anti-inflammatory capacity of immune responses to HSP70 in arthritis and searched for the mechanisms involved (chapter 3, manuscript submitted). HSP70 pretreatment dramatically suppressed the development of inflammation and subsequent tissue damage in PGIA. Moreover, HSP70 pretreatment resulted in a regulatory immune response not only to HSP70 but also to PG. To better understand the role of T cells in arthritis we generated a T cell receptor transgenic (TCR-Tg) mouse (chapter 4, accepted Journal of Autoimmunity and chapter 5, manuscript submitted). The T cells of this mouse are specific for an epitope (designated 5/4E8) in the G1 domain of human cartilage PG (amino acids 70-84). These TCR-5/4E8-Tg mice are a unique source of naïve antigen-specific T cells that are capable of inducing progressive chronic arthritis, both spontaneously (<15%), and after immunization with PG (100%) and upon adoptive transfer to BALB/c.SCID mice. Based upon the clinical and histopathological features and the autoimmune aspects shared between PGIA and RA, we believe that these TCR-5/4E8-Tg mice are a valuable tool for further analysis of the mechanisms associated with the initiation and pathogenesis of autoimmune arthritis; and more specifically, for the analysis of the role of antigen specific T cells in disease development.

As in PGIA, T cell responses against cartilage PG do also occur in RA, at least in a subset of patients. In chapter 6 (manuscript in preparation) we tested for responses to several PG epitopes in longstanding RA patients compared to osteoarthritis (OA) patients and healthy controls. In particular, two epitopes (amino acids 16-39 and 263-282; both located in the G1 domain of human PG) appeared to be frequently recognized RA and OA patients as shown by proliferation and preferential Th1 cytokine production. This indicates that PG-specific responses might be involved in RA immune pathogenesis either primarily or, more likely, as a consequence of cartilage destruction.

Taken together, PG-specific immune responses were studied in PGIA as well as in RA and showed that the PGIA model provides a unique opportunity to study possible autoimmune mechanisms. Pretreatment with HSP70 showed the potential to modulate (suppress) the arthritogenic effect of PG via IL-10 mediated response and thus to treat arthritis.

RA is a complex autoimmune disease controlled by various cells of innate and adaptive immunities, environmental factors and genetic components. A successful treatment of RA will depend on the early diagnosis and a better understanding of pathological processes. The PGIA model enabled the investigation of several in vivo mechanisms in a complex system of inflammation and autoimmunity. The analysis of PGs and HSPs, both present at the site of inflammation, provided insight into the pathogenesis of autoimmunity and arthritis and suggests that both proteins may be targets of immunomodulation by different mechanisms.