Identification of ‘pathogenic’ IgA in human serum samples

Immunoglobulin A Nephropathy (IgAN) is the most common form of primary glomerulonephritis worldwide. The disease is defined by the deposition of IgA1, but not IgA2, in the glomeruli of patients, with subsequent mesangial cell proliferation and tubulointerstitial fibrosis, which can lead to end stage renal disease (ESRD). Although raised levels of IgA1 with decreased galactosylation at the hinge region is seen in IgAN patients, this is not a unique feature that can be considered the cause of the disease. Indeed, the pathogenesis of IgAN is not fully understood and is complicated by evidence of glomerular deposition of IgA in people with no history of renal disease. It is thought that the deposited IgA1 originates from serum and is recognised by autoantibodies, mainly of IgG and IgA type, forming circulating immune complexes. These deposit in the glomeruli and trigger mesangial cells to proliferate and release cytokines. To date, the composition of these immune complexes has not been clearly identified. This thesis aimed to separate serum IgA1 into subgroups, to characterise the subgroups and to identify pathogenic features of the immune complexes in the subgroups. Serum IgA1 was separated into subgroups by affinity chromatography using the lectin jacalin. The structural and functional characteristics of the IgA containing immune complexes in each subgroup was determined using ELISA and western blot methods and a novel use of IgA1 protease isolated from Streptococcus oralis. Unexpectedly, it was observed that the pathogenicity of the immune complexes is not driven by amounts of IgA1 molecules with different molecular features or by the presence of IgG autoantibodies. Proteomic data by mass spectrometry analysis revealed a fundamental contribution of complement proteins C7, C8, and C9 and serum proteins; fibulin, pigment epithelium-derived factor and monocyte differentiation antigen CD14 in the pathogenicity leading to the mesangial activation in IgAN.