Proteomic Profiling of Complement Components in Glomerular Disease

The complement system plays a central role in glomerular disease development and resolution. Currently, renal biopsies assess the presence of complement through limited immunofluorescence stains (C3 and C1q). With >50 total proteins and fragments involved in the complement cascade, this method offers a severely limited view into the mechanisms of tissue injury orchestrated by complement activation. A more comprehensive evaluation of complement components will advance the understanding of complement involvement in glomerular diseases by allowing for multiplex detection of complement cascade proteins and activation products, which can be achieved by mass spectrometry (MS). Data-independent acquisition MS was performed following extraction of proteins from tissue lysates, microdissected glomeruli, or protein G immunoprecipitates from residual kidney biopsy tissue. Cohorts included patients with lupus nephritis, membranous nephropathy and membranous lupus nephritis, diabetic glomerulosclerosis, C3 glomerulonephritis, and control biopsies. Abundances of complement components by MS correlated with immunofluorescence intensity of C1q on kidney biopsies. Increased abundances of complement classical, lectin, final common pathway, and regulatory proteins correlated with disease activity in lupus nephritis. Complement protein abundances of final common pathway components were heterogeneous between patients with the same disease state, including diabetic glomerulosclerosis and various forms of proliferative glomerulonephritis. Finally, complement proteins and their activation products can be mapped to determine which components are impacted among individuals and between disease states. In conclusion, complement proteins and some of their split products can be reliably measured by MS of kidney biopsies, which can enhance our understanding of complement-mediated tissue injury and heterogeneity in glomerular diseases.