Sensitive immunoenzymatic assay for urinary immunoglobulin subclasses of different pH: its significance in diabetic patients.

In search of a marker for monitoring the progression of diabetic nephropathy from the stage of charge- to that of size-selectivity loss, attention has been focused on the evaluation of immunoglobulin G (IgG) subclasses in the urine. We have developed a new sensitive enzyme immunoassay for the quantitation of urinary IgG1 and IgG4. Mouse monoclonal antibodies specific for each subclass were bound to microtitre wells precoated with rabbit anti-mouse immunoglobulin antibody. IgG1 and IgG4 of standard preparations (or of samples to be tested) were revealed using peroxidase-conjugated rabbit anti-human IgG. The procedure was carried out at 4 degrees C. This method can detect about 2 ng/ml of IgG4 and 20 ng/ml of IgG1. The monoclonal antibodies used were shown to be highly subclass-specific. IgG1 and IgG4 have a similar molecular weight but a different pH (about 9 and 4.6 respectively); a change in their ratio in the urine of diabetic patients may indicate a progressive deterioration of kidney function at the stage of incipient diabetic nephropathy.