Hemonectin Mediates Adhesion of Engrafted Murine Progenitors to a Clonal Bone Marrow Stromal Cell Line From S1/S1d Mice

Mutant Sl/SId mice exhibit decreased marrow hematopoie-sis. The defect is known to reside in the marrow microenviron-ment of these animals, which is reproduced in vitro by primary marrow explants as well as by cloned marrow stromal cell lines. Bone marrow progenitor cells are incapable of adhering to primary Sl/SId stromal cells or cloned stromal cell lines derived from them to form cobblestone-islands and proliferate. The role of hemonectin, a marrow-specific adhesion protein in the defective hematopoiesis of the Sl/SId mice, was studied. Indirect immunoperoxidase staining of marrow in situ from Sl/SId mice showed little specific staining while specific staining was seen in a pericellular distribution in marrow from +/+ mice. Hemonectin expression in several cloned stromal cell lines from Sl/SId mice was compared by immunoblotting with that in cloned stromal cell lines from normal +/+ littermates. Cell line Sld3, which has the least hematopoiesis supportive capacity in vitro, showed no detectable hemonectin by immunoblotting, while SCI and Sld2 showed detectable but greatly reduced amounts compared with normal +/+ 2.4, GBI/6, and D2XRII. Confluent cultures incubated with purified hemonectin and engrafted with enriched progenitors showed a significant increase in the cumulative number of cobblestone-islands and day 14 spleen colony-forming units (CFU-s) forming progenitors (39.15 ± 3.6/dish; 16.3 ± 3.1/dish, respectively), compared with untreated Sld3 cultures (cobblestone-islands 8.1 ± 3.6/dish; CFU-s forming progenitors 8.8 ± 0.05/dish). Hemonectin-mediated progenitor cell binding to the Sld3 stromal cells was specifically inhibited by antihemonectin but not by preimmune serum. These data support the role of hemonectin in early progenitor-stromal cell interactions.