The roles of T helper 17 cells and regulatory T cells in experimental glomerulonephritis.

CD4+ T helper (Th) cells are implicated in the pathogenesis of rapidly progressive glomerulonephritis (RPGN), and can be proinflammatory, in the case of Th1 and Th17 subsets, or suppressive, known as regulatory T cells (Tregs). Exploiting the immunoregulatory ability of Tregs and understanding the role of the proinflammatory cytokines Interleukin (IL)-17A and IL-17F, as way of understanding the potential protective effects of systemic blockade of these cytokines, may provide two novel methods treat diseases causing RPGN. I have used two related murine models of RPGN (‘non-accelerated’ and ‘accelerated’ anti-glomerular basement membrane (GBM) disease models), which induce glomerulonephritis through adaptive immune responses to sheep globulin (a foreign antigen) planted within the GBM. I hypothesised that enhancing Treg populations would protect mice from GN and tested this in two ways. Firstly, by expanding endogenous Tregs through the administration of Fms-like tyrosine kinase (FLT) 3-ligand (FL), a growth factor that acts on the FLT3 receptor on haematopoietic progenitors promoting plasmacytoid dendritic cells (pDCs) and Tregs. Secondly, through the adoptive transfer of in vitro induced Tregs (iTregs). I hypothesised that Interleukin (IL)-17 receptor A (IL-17RA), a subunit of the receptor complex for a number of IL-17 family cytokines, would promote renal injury and systemic inflammatory responses in experimental RPGN. FL enhanced pDCs and Tregs in steady state conditions. However, when given during experimental RPGN, DCs developed a conventional phenotype within secondary lymphoid organs, effector T cell responses were enhanced (predominantly Th17) and local immune responses were increased, with increased CD11c+ DCs and neutrophils being recruited to glomeruli. iTregs, from in vitro cultures of naïve CD4+ T cells with all-trans retinoic acid and transforming growth factor beta, had a regulatory phenotype in vitro, which was further enhanced by the administration of a monoclonal antibody to IL-12p40 (a subunit of both IFN gamma and IL-23) during iTreg culture. Only iTregs cultured with anti-IL-12p40 suppressed dermal delayed type hypersensitivity and modulated proinflammatory cytokine production by stimulated splenocytes, displaying a more ‘stable’ phenotype, as a greater proportion of the transferred cells remained Foxp3+ when retrieved from the spleen. In experimental RPGN, neither iTregs nor iTregs+anti-IL-12p40 protected mice from GN. Both types of iTregs homed poorly to the kidney, with reduced Foxp3 expression, indicating they had an unstable phenotype. IL-17RA-/- mice were protected from experimental RPGN. IL-17RA expression on leukocytes was required for glomerular segmental necrosis and neutrophil recruitment to glomeruli. IL-17RA expression on leukocytes and renal tissues cells were important in generating systemic Th1 effector responses and humoral immunity. Stromal cell expression of IL-17RA was important for IgG2b class switching and for the early expression of CXCR5 on B cells. Compared to controls, sensitised IL-17RA-/- mice had reduced antigen-specific humoral immune responses, abnormal germinal centre formation within the spleen, fewer T follicular helper cells and fewer B cells expressing CXCR5. In conclusion, blockade of IL-17RA is promising as a novel therapy for RPGN, but further exploration of the potential impact this may have on humoral immunity is required. Utilising iTregs as a cellular therapy will require cells with a stable regulatory phenotype when transferred into an inflammatory environment.