EXPANSION OF THE NONADHERENT MYELOID CELL-POPULATION BY MONOCLONAL-ANTIBODIES AGAINST TENASCIN-C IN MURINE LONG-TERM BONE-MARROW CULTURES

Tenascin-C, a predominantly mesenchymal extracellular matrix protein, has a restricted distribution in adult tissues. It has previously been shown that this protein is expressed in the bone marrow. In this pape r we show that murine myeloid and lymphoid long-term bone marrow cultu res differ in their expression of tenascin-C splice variants. In the a dherent stromal layer of myeloid cultures, the 260-kDa polypeptide enc oded by the 8-kb mRNA was the major splice variant, whereas in the str omal layer of lymphoid cultures both the shorter 210-kDa polypeptide e ncoded by the 6-kb mRNA and the 260-kDa polypeptide were abundantly ex pressed. However, in both culture systems the larger 260-kDa tenascin- C polypeptide was the major isoform secreted in the culture supernatan t. This finding is in agreement with previous reports indicating that the smaller 210-kDa isoform is preferentially deposited in the stroma, whereas the alternatively spliced segment in the 260-kDa tenascin-C m ay contain antiadhesive domains. Glucocorticoids in myeloid long-term bone marrow cultures and in the MC3T3-G2/PA6 cell line downregulated t he expression of tenascin-C. In the present study we observed that thi s was due primarily to downregulation of the 8-kb major splice variant of the tenascin-C mRNA. We also studied the possible role of tenascin -C in the bone marrow by using antibodies against tenascin-C in long-t erm bone marrow cultures. We found that three monoclonal antibodies ag ainst the carboxyterminal type III fibronectin repeats of tenascin-C ( TNCfn 7-8) increased the number of the nonadherent myeloid cells in my eloid long-term bone marrow cultures. It has recently been suggested t hat the TNCfn 6-8 domain of tenascin-C binds to the alpha 8 beta 1 int egrin. Using Northern blotting, we found that the integrin alpha 8 sub unit was expressed in adherent cells in bone marrow cultures, raising the possibility that tenascin-C acts in bone marrow cultures by bindin g to the alpha 8 beta 1 integrin.