Deposition of chromatin‐IgG complexes in skin of nephritic MRL‐lpr/lpr mice is associated with increased local matrix metalloprotease activities

Abstract

Chromatin-IgG complexes appear as electron dense structures (EDS) in glomerular basement membranes in lupus nephritis. Here, we present results of comparative analyses of the composition of EDS in murine lupus dermatitis and nephritis. One focus was to perform an analytical approach to understand why such complex structures bind skin basement membrane components. Transcription of skin membrane-encoding genes was analysed to see if expression of such genes was increased, eventually indicating that binding capacity of immune complexes increased when dermatitis developed. Variations in matrix metalloprotease 2 (MMP2), MMP9 and Dnase1 mRNA levels and enzymatic activities were correlated with circulatory chromatin-IgG complexes and deposition in skin. We also examined if glomerular deposits of EDS predicted similar deposits in skin of (NZB x NZW)F1 or MRL-lpr/lpr mice, as we observed chromatin-IgG complexes in capillary lumina in skin and glomeruli in both strains. EDS consisting of chromatin fragments and IgG were found sub-epidermally in skin with LE-like lesions of end-stage nephritic MRL-lpr/lpr mice. Dermal MMP-encoding genes were up-regulated during disease progression, and gelatinolytic activity was increased in affected skin. Dnase1 mRNA level and total nuclease activity remained stable in skin during the disease, in contrast to progressive loss of renal Dnase1 mRNA and total renal nuclease activity during development of nephritis. Loss of renal Dnase1 may explain release of chromatin fragments, while increased MMP activity may disrupt membranes making them accessible for chromatin fragment-IgG complexes. Circulatory chromatin-IgG complexes, and up-regulated intradermal MMP activity may be crucial for deposition of immune complexes in skin of lupus-prone mice.