Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disease of unknown etiology. It is characterised by inflammation in the synovial joints and is known to affect other organs as well. For over 60 years, glucocorticoids have been used to treat RA, because of their powerful anti-inflammatory properties. However, the mechanism by which glucocorticoids modulate immune responses is not fully understood. Potently induced by glucocorticoids, glucocorticoid-induced leucine zipper (GILZ) is a protein with the unique ability to mimic the therapeutic effects of glucocorticoids. The immuno-suppressive effects of GILZ on key signalling pathways relevant to RA, represents a strategy to reduce pharmacological use of glucocorticoids in RA by harnessing the downstream effects of GILZ. Studies presented in this thesis examine the effect of GILZ in regulating inflammatory processes relevant to RA.
The findings in Chapter 3 of this thesis show that endogenous GILZ acts as an anti-inflammatory mediator in collagen-induced arthritis (CIA). In this study, in vivo silencing of endogenous GILZ, using small-interfering RNA during the effector phase of CIA, resulted in the exacerbation of clinical and histological disease. The expression of GILZ in human rheumatoid synovial tissue, and the robust induction of GILZ by dexamethasone in RA fibroblast-like synoviocytes (FLS), suggests an important role for GILZ in human RA. Notably, inhibition of tumor necrosis factor (TNF)-induced cytokines was observed in GILZ overexpressing RA FLS. These findings suggest that modulation of GILZ expression may be therapeutically beneficial in RA.
Chapter 4 demonstrates a therapeutic effect of exogenous GILZ in arthritis. In this study, overexpressing GILZ using a recombinant adeno-associated virus (rAAV) vector attenuated CIA severity, comparable to the effects of dexamethasone treatment. The inhibition of NF-κB luciferase activity in vitro by GILZ-rAAV in endothelial cells and macrophages demonstrates a mechanism by which GILZ suppress inflammation. In conjunction, the lack of effect on blood glucose concentrations in response to GILZ-rAAV, suggests anti-inflammatory and disease-modifying effects of GILZ may be achieved without the adverse effects that accompany glucocorticoid therapy.
The findings in Chapter 5 report an unexpected role for GILZ in male fertility discovered during the generation of GILZ deficient (GILZ-/-) mice in the course of my PhD. GILZ-/- mice male mice exhibit a loss of germ cells in their testes, which resulted in the failure of spermatogenesis. Characterization of a normal blood-testis barrier and the absence of CD45 leukocytes in the testis demonstrate that this infertility phenotype was not of an autoimmune origin. These data suggest a role for GILZ in the process of spermatogenesis and development of the testes.
In Chapter 6, data are presented which indicate that deficiency of physiological GILZ is without impact on the effector pathways of joint inflammation. GILZ-/- mice displayed increased delayed-type hypersensitivity (DTH) responses and increased accompanying ex vivo T cell responses to various antigens. This suggests a role for endogenous GILZ in mediating T cell activation. However, no enhancement of antigen-induced arthritis (AIA), CIA or K/BxN serum transfer arthritis susceptibility or severity was observed in GILZ-/- mice. A lack of effect of GILZ deletion on LPS-induced cytokinemia suggests that GILZ is not involved in mediating the myeloid effector responses of joint disease. Furthermore, contrary to what has been suggested by studies published without access to a GILZ-/- mouse, the results presented in Chapter 6 demonstrate that GILZ is not required for glucocorticoid effects in inflammation. This suggests a redundant effect of physiological GILZ in glucocorticoid suppression of inflammation, not withstanding the clear inhibitory effects of GILZ overexpression.
This thesis extends the current understanding of the relationship between glucocorticoid function and GILZ and demonstrates the significant impacts that GILZ may have in the regulation of inflammation
