Regional Distribution in Human of a Novel Aortic Collagen-Associated Microfibrillar Protein☆

We have reported two hypothetical proteins of human aorta, based on sequences cloned from a cDNA library constructed from mRNAs purified from the adventitia. These sequences have immunoglobulin-κ (IgK)-like domains, and we have shown that microfibrils of the aortic adventia are immunoreactive with antibodies against IgK. The present study was performed to characterize more specifically the regional distribution in human of one of these proteins in particular and the distribution of matrix proteins with IgK-like motifs in general. An antibody was raised in rabbit against a synthetic peptide based on a unique sequence in one of the hypothetical proteins (NPSNRVTPQKNFP), which has not been reported in the sequence of any other known protein. This antibody and a rabbit anti-human IgK antibody were used as first antibodies in the staining reactions. A monoclonal mouse anti-smooth muscle cell α-actin antibody was also used. Immunoreactivity with the sequence-specific antibody was limited to the aorta and large vessels. Adventitial microfibrillar staining was more conspicuous in the abdominal aorta than in the thoracic aorta and in the internal iliac than in the external iliac artery. The immunoreactive protein was associated with fibroblasts and not smooth muscle cells. Immunoreactivity coated the collagen fibers diffusely, while elastin fibers were not stained. Further studies using antibodies against IgK demonstrated immunoreactivity of collagen and fibroblasts in a variety of tissues: spleen, ovary, testicle, cervix, prostate, skin, and breast (but not brain). Immunoglobulin motifs may be a feature of matrix proteins produced by fibroblasts in many tissues, but the first motif that we identified from a cDNA library of aortic adventitia appears to be specific to aorta and large vessels.