Higher urinary IgM excretion in type 2 diabetic nephropathy compared to type 1 diabetic nephropathy

Proteinuria, due to impairment of the charge- and/or size selectivity of the glomerular capillary wall (GCW) is the earliest clinical evidence of diabetic nephropathy (DN). To study the pathophysiological differences between patients with DN in type 1 diabetes mellitus (type 1 DN) and type 2 diabetes mellitus (type 2 DN), we compared the patterns of urinary proteins of different size and charge in the two entities of diabetic kidney disease.

Urine concentrations of albumin, IgG2, IgG4 and IgM were assessed in 22 (15 males and 7 females) patients with type 1 DN, and in 20 (18 males and 2 females) patients with type 2 DN. Comparisons with one control group of 13 (12 males and one female) patients with nephrosclerosis due to systemic hypertension and a second control group of 16 (14 males and 2 females) healthy controls were made.

The urine excretion of IgG2 and IgM and the ratio of IgG2 to IgG4 (IgG2/IgG4), were significantly higher in type 2 DN compared to type 1 DN (P < 0.01). Patients with type 2 DN and patients with nephrosclerosis had significantly higher urine excretion of IgG and IgM compared to the age-matched healthy subjects (P < 0.001). The IgG2/IgG4 ratio was higher in type 2 DN compared to nephrosclerosis and healthy controls (P < 0.01).

The increased urine excretion of IgG and IgM that accompanies albuminuria in type 2 DN suggests that the dominant pathophysiological mechanism of proteinuria in type 2 DN might be an alteration of the size selective properties of the glomerular capillary wall, including the occurrence of non-discriminatory "shunt pathways." The charge selective properties of the glomerular capillary wall seem to be intact in type 2 DN, as indicated by the high IgG2/IgG4 ratio. The mechanisms of proteinuria in type 1 DN seem to be merely a consequence of an impaired charge selectivity of the glomerular capillary wall.