Radioimmunoassay for human type VI collagen and its application to tissue and body fluids.

A liquid phase radioimmunoassay (RIA) was developed for pepsin-solubilized human type VI collagen, allowing quantitative analysis of this protein down to a concentration of 3 ng/ml. No cross-reactivity was observed with human collagens type I, III, IV (triple helical portion and 7-S domain), and V, nor with laminin fragment Pl and plasma fibronectin. Significant amounts of closely related antigenic material were detected in serum, bile, ascites, and mesenchymal cell culture media. Type VI collagen could be completely solubilized from several tissues by a repeated pepsin digest, and its content as determined by RIA was found to be less than 0.1% of total collagen (55-70 micrograms/g protein). In fibrotic liver tissue type VI collagen was elevated up to 10-fold (620 micrograms/g protein) when compared to normal liver. Sera of patients with fibrotic liver disease, however, revealed antigen levels usually below the narrow normal range of 22 +/- 7.8 ng/ml (mean +/- 2.5 SD). We conclude that, although type VI collagen represents a minor fraction of the interstitial collagens, its comparatively high serum levels point to a considerable turnover in the normal individual. Our data suggest that in fibrosis as exemplified in fibrotic liver disease, the metabolism of this collagen is down-regulated, while at the same time, it accumulates in the interstitial matrix.