Objective. To investigate potential mechanisms by which CD40L‐mediated signals may be involved in the pathogenesis of lupus glomerulonephritis (GN).

Methods. Renal in situ CD40L and CD40 expression was examined in patient biopsy specimens. Immuno‐histochemical studies were performed on frozen sections utilizing anti‐CD40L monoclonal antibody (MAb), anti‐CD40 MAb, or control MAb. As controls, we analyzed normal kidney specimens and specimens obtained from patients with IgA nephropathy, focal segmental glomerulosclerosis, minimal change disease, idiopathic membranous GN, and antineutrophil cytoplasmic antibody‐positive pauci‐immune GN. Staining distribution was noted and staining intensity scored on a semiquantitative scale of 0 (no staining) to 3+ (intense staining).

Results. In normal kidney, CD40 was expressed on parietal epithelial cells, mesangial cells, endothelial cells, and distal tubules but not proximal tubules. Glomerular and tubular CD40 expression was markedly up‐regulated in class III and class IV lupus GN, where there was intense staining of crescents, proximal and distal tubules, and interstitial mononuclear cells. In contrast, CD40 expression in class V lupus GN was similar to that in normal kidney. Interstitial mono‐nuclear cells expressing CD40L were present in class IV lupus GN. However, these findings were not unique to lupus GN: up‐regulation of CD40 and CD40L expression was similarly observed in other inflammatory renal diseases.

Conclusion. This study shows that CD40 is expressed on a variety of renal parenchymal and non‐parenchymal cells in normal kidney. Renal CD40 expression is up‐regulated in class III and class IV lupus nephritis, as well as in other inflammatory renal diseases, and is associated with the presence of CD40L+ mononuclear cells.