Collagen type VI is the antigen recognized by the ER-TR7 antibodySince its first description published in this journal in 1984 [1], the monoclonal antibody ER-TR7 has been used in a great number of immunological and nonimmunological studies. Even so, its antigen is still unknown. The ER-TR7 (Erasmus University Rotterdam-Thymic Reticulum) antibody was originally produced in a screen aimed at identifying monoclonal antibodies reactive against nonlymphoid stromal cells of the mouse thymus [1]. Therefore, its staining pattern in secondary lymphoid organs was characterized in particular detail. Staining of mouse spleen with ER-TR7 revealed a system of extracellular reticular fibrils that delineates the microanatomical architecture of the organ and allows a clear distinction of the red and white pulp and their interconnecting marginal zone. Other structures that contain extracellular matrix (ECM) components, such as the capsule, trabeculae, central arteriole, and other vascular walls were also strongly labeled by ER-TR7. Similarly, a fine reticular ER-TR7 staining delineated the T and B cell microenvironments within lymph nodes [2]. In the same lymphoid organs, ER-TR7 also marked the stromal cells known as Fibroblastic Reticular Cells (FRCs), which
The widely expressed microfibril-forming collagen VI is subject to proteolytic cleavage and it has been proposed that the cleaved off C-terminal Kunitz domain (C5) of the α3 chain is an adipokine important for tumor progression and fibrosis. Under the name “endotrophin” the C5 fragment has also been shown to be a potent biomarker for fibro-inflammatory diseases. However, the biochemical mechanisms behind endotrophin activity have not been investigated. In earlier studies, the anthrax toxin receptor 1 was found to bind to C5, but this potential interaction has not been further studied. Given the proposed physiological role of endotrophin we aimed to determine how the endotrophin signal is transmitted to the recipient cells. Surprisingly, we could not detect any interaction between endotrophin and anthrax toxin receptor 1 or its close relative, anthrax toxin receptor 2. Moreover, we could not detect binding of fully assembled collagen VI to either anthrax toxin receptor. We also performed similar experiments with the collagen VI surface receptor NG2 (CSPG4). We could confirm that NG2 is a collagen VI receptor that binds to assembled collagen VI, but not to the cleaved C5/endotrophin. A cellular receptor for C5/endotrophin therefore still remains elusive.
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