What is the significance of HLA-DR antigen expression in the extraglomerular mesangium in glomerulonephritis?

The HLA-DR antigen is a HLA class II molecule involved in the presentation of antigenic peptides to the T cell receptor, thus regulating the immune response. Renal expression of the HLA-DR antigen may indicate specific sites of immunologically-mediated kidney injury in glomerulonephritis (GN). The aim of our study was to assess the presence of the HLA-DR antigen along the nephron including the extraglomerular mesangium in GN.

A cross-sectional study of 22 patients with glomerulonephritis, mean age: 46.59±10.77 years, 14 male and 8 female, was conducted. Conventional stains, as well as immunohistochemistry for the HLA-DR Antigen Alpha-Chain were employed on kidney biopsies. Immunohistochemistry was assessed using a semi-quantitative score: 0-absent, 1-mild, 2-moderate, 3-intense. Statistical analysis was performed using SPSS17.

Four patients presented Focal and Segmental Glomerulosclerosis (FSGS), 5 patients: membranoproliferative GN, 7 patients: membranous nephropathy, 3 patients: mesangial proliferative GN, 2 patients: minimal change disease (MCD), and 1 patient: crescentic GN. Regarding the percentage of cases with HLA-DR positive cells along the nephron out of 22 patients: glomerular endothelial cells were 100% positive, intraglomerular mesangium cells were 81.8% positive, podocytes were 36.4% positive, extraglomerular mesangium cells were 31.8% positive, proximal tubule cells were 95.5% positive, distal tubule cells were 68.2% positive, interstitial capillaries were 77.3% positive, and cells of interstitial infiltrates were 27.3% positive. The percentage of cases staining positively for the HLA-DR antigen in the extraglomerular mesangium was 25% in FSGS, 60% in membranoproliferative GN, 0% in membranous nephropathy, 33.3% in mesangial proliferative GN, 100% in minimal change disease and 0% in crescentic GN.

A prominent HLA-DR antigen distribution was found on glomerular endothelial cells, intraglomerular mesangium cells and proximal and distal tubular cells. Extraglomerular mesangium cells and podocytes stained variably for the HLA-DR antigen, as did the cells of the interstitial infiltrates. The extraglomerular mesangium which serves as a portal of entry into the intraglomerular mesangium is endowed with antigen-presenting capabilities and is a region where induction of immune reactions could take place.