Identification of novel IgA receptors expressed by human mesangial cells and proximal tubule epithelial cells

Abstract

Immunoglobulin A Nephropathy (IgAN) is the most common cause of primary glomerulonephritis worldwide. The majority of patients progress to kidney failure within 10 to 20 years from the time of diagnosis. Glomerular injury in IgAN is initiated by the binding of immune deposits containing galactose deficient IgA1 (gd-IgA1) to human mesangial cells (HMC), which induces inflammation, mesangial cell proliferation, and mesangial matrix production. Two of the most powerful predictors of future kidney function decline in IgAN are the extent of proteinuria and tubulointerstitial fibrosis. This supports the importance of the role of tubular factors together with the mesangial response to IgA deposition in determining the severity of IgAN. The aim of this thesis was to explore whether IgA1 exerts a direct effect on HMC and proximal tubule epithelial cells (PTEC) activation and generation of pro-inflammatory and pro-fibrotic cytokines and to identify putative cell surface receptors for IgA1. In this thesis, I show that human IgA1, and especially galactose deficient polymeric IgA, stimulates the release of pro-inflammatory and pro-fibrotic cytokines from cultured HMC and PTEC. I show that IgA1 isolated from IgAN patients’ serum binds with greater avidity to HMC and PTEC compared to IgA1 from healthy subjects. I developed a novel method to isolate cell surface IgA1 receptors from HMC and PTEC. The isolated IgA/receptor complexes were analysed by mass spectrometry, and candidate receptors were selected following bioinformatic analyses. Expression of candidate receptors on PTEC was confirmed using confocal microscopy, and expression of candidate receptors for both HMC and PTEC confirmed in human kidney biopsy samples. Functional activity of PTEC candidate IgA receptors was assessed in siRNA knockdown experiments. A number of novel putative receptors for IgA1 have been identified, and future work should examine their contribution to HMC and PTEC activation in IgAN.