Studies of immunoglobulin receptors and human proximal tubular cells

Natto, Manal Jamal (2008) Studies of immunoglobulin receptors and human proximal tubular cells. PhD thesis, University of Glasgow.

Abstract

Glomerulonephritis (GN) is a major cause of progressive chronic renal failure (CRF) and accounts for less than 20% in the UK of patients requiring dialysis and transplantation. There are many forms of GN, of which IgA nephropathy (IgAN) is the most common, affecting up to 2% of the adult population. GN is characterised by irreversible and progressive glomerular and tubulointerstitial fibrosis, reduction in glomerular filtration rate (GFR), and retention of uremic toxins. The rate of progression of GN is increased by various clinical factors including hypertension, and the quantity and specificity of proteinuria. Although albuminuria is most widely studied, higher molecular weight proteins such as immunoglobulins (Ig) are more associated with progression of renal disease. In this thesis I have explored the hypothesis that an interaction between tubular cells and filtered Ig/immuno complex (Ig/IC) may involve specific cellular receptors, and thus provide a novel mechanism for the initiation of tubular injury and fibrosis.
Deposition of Ig and IC is seen in renal diseases such as membranous glomerulonephritis, and IgAN. Ig/IC are regarded as responsible for the initiation of glomerular injury in these diseases. However, whether Ig/IC contribute directly to tubular injury is unknown. We previously identified a novel IgA receptor, the Fcα/μ receptor (Fcα/μR), in mesangial cells which may contribute to IgAN. This receptor binds both IgA and IgM but not IgG. I hypothesised that Ig and IC, filtered at the glomerulus, may act on Ig receptors in human proximal tubular epithelial cells (PTEC) and may play a direct pathophysiological role in the development of interstitial fibrosis. Specifically, that filtered IgA may bind to the Fcα/μR in IgAN. The activation of Ig receptors may stimulate tubular cells to alter cell proliferation and extracellular matrix formation (e.g fibronectin (FN)), which are pathophysiological characteristics of tubulointerstitial fibrosis in GN.
I studied primary and immortalised human PTEC using qualitative RT-PCR and quantitative real-time PCR (qRT-PCR) for expression of candidate Ig receptors. I investigated the expression of Ig receptors and their regulation by cytokines (e.g. IFN-γ, TGF-β1 and IL-1α) known to be implicated in GN. The receptors studied include the polymeric immunoglobulin receptor (pIgR), the neonatal receptor (FcRn), the Fcα/μR, the classical IgG receptors (FcγR1, γIIa, γIIb, γIII and the related FcR γ-chain) and the classical IgA receptor (FcαR). None of the classical IgG receptors were expressed by PTEC, nor was the classical IgA receptor. However, the FcRn, pIgR and the Fcα/μR were expressed.
Gene expression of these receptors was investigated under cytokine regulation. IFN-γ, IL-1α and TGF-β1 had no effect on FcRn gene expression. The expression of the pIgR gene was up-regulated by IFN-γ and the Fcα/μR transcript level was up-regulated by IL-1α and IFN-γ but down-regulated by TGF-β1. Expression of Fcα/µR protein by tubular cells was confirmed by western blot analysis by using specific Fcα/µR monoclonal antibody.
Immunohistochemistry confirmed the expression of Fcα/µR by PTEC in both normal and diseased kidney and that IgG, IgA and IgM binding varies in parallel with gene expression of related Fc receptors. This binding was associated with increased FN production (assessed by ELISA) and reduced the proliferation of PTEC, demonstrated by [3H]-thymidine uptake assay.
I screened the ability of PTEC to release cytokines under normal conditions and confirmed this by ProteoPlex cytokine array. Among these cytokines there was no significant effect of Ig/IC on IL-6, IL-8 and GM-CSF released by PTEC, and this was confirmed by specific ELISA for each individual cytokine. The phosphotyrosine/phospho-ERK signalling pathway was unaffected by Ig/IC; however, IgM was able to activate the phospho-ERK pathway. These results suggest that there might be alternative signalling pathways activated by Ig/IC in PTEC.
I then studied the effect of calcineurin inhibitor immunosuppressive drugs as a model of tubular toxicity, and statins as immunomodulatory drugs, reputed to protect renal function and known to have immunomodulatory effects. I hypothesised that immunosuppressant agents/statins either alone or in combination with LPS might affect Ig receptors expression, cytokine release, FN production and proliferation by PTEC.
My results inferred that immunosuppressive agents alone have a non-inflammatory response on Ig receptor expression. However, in combination with LPS, the study drugs showed a slight inhibitory effect on the expression of the pIgR on PTEC at the highest concentration only. There was no significant effect of immunosuppressant agents alone or in combination with LPS on IL-6, IL8 and GM-CSF released by PTEC. Cyclosporine, FK506 and sirolimus were shown to inhibit both the production of FN and the growth of PTEC at high doses, thereby exerting an antiproliferative effect.
Both simvastatin and fluvastatin inhibited LPS induced production of IL-6, IL-8 and GM-CSF, FN and proliferation in a dose-dependent manner. 1 micromolar simvastatin/fluvastatin was associated with a 35% reduction in unstimulated FN production, a 51% reduction in proliferation, and reduced production of IL-6 (58%), IL-8 (65%) and GM-CSF (57%). The expression of Ig receptors was increased in a dose-dependent manner in PTEC treated with statins alone or in combination with LPS.
In conclusion, the expression of tubular FcRn, pIgR and Fcα/µR by PTEC is regulated by proinflammatory cytokines. The binding of IgA and IgM, possibly through the Fcα/µR, by PTEC may contribute to immune-mediated nephropathy. The FN production and the proliferation suggest that Ig/IC binding may contribute to the pathophysiology of IC-mediated renal disease. The ability of immunosuppressant agents to reduce FN production, proliferation, and pIgR expression by PTEC may offer potential new strategies for the prevention and treatment of nephrotoxicity, proteinuria and chronic allograft nephropathy (CAN). The inhibitory effect of statins alone or in combination with LPS on FN production, proliferation and cytokines released suggest that statin therapy may have potential benefit in interstitial nephritis and fibrosis. On other hand, statins alone or in combination with LPS increased the expression of Ig receptors by PTEC.
This thesis shows that the novel human Fcα/µR in PTEC may play an important role in the immune mechanisms involved in the tubular response during renal injury associated with tubular Ig/IC deposits in renal disease.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Proximal Tubular Cells, Immunoglobulin, IgA nephropathy, Receptors, FCαμ receptor, Statins, Cyclosporine, FK506, Sirolimus
• Subjects: R Medicine > R Medicine (General); R Medicine > RS Pharmacy and materia medica; Q Science > Q Science (General)
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
• Supervisor's Name: Jardine, Professor Alan and MacFarlane, Dr Niall
• Date of Award: 2008
• Depositing User: Miss Manal Natto
• Unique ID: glathesis:2008-303
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 24 Jun 2008
• Last Modified: 10 Dec 2012 13:17
• URI: https://theses.gla.ac.uk/id/eprint/303