MONOCLONAL-ANTIBODY CROSS-LINKING OF THE ALPHA-4 OR BETA-1 INTEGRIN INHIBITS COMMITTED CLONOGENIC HEMATOPOIETIC PROGENITOR PROLIFERATION

Adhesion receptors can serve as primary signal transduction molecules that convey information into cells that can affect cell proliferation and differentiation. Since hematopoietic progenitors adhere to marrow stroma and fibronectin via the alpha 4 beta 1 integrin and CD44, we ex amined the role of these receptors in the transfer of proliferation-re gulatory signals to progenitors. Actively proliferating colony-forming cells (CFCs) present in cultured CD34(+) cells were incubated with mo use monoclonal antibodies against the alpha 4, beta 1, or CD44 recepto rs and crosslinking was performed with a secondary goat-anti-mouse ant ibody. The effect on CFC proliferation was examined with a H-3 thymidi ne suicide assay. Compared with controls (39 to 51% kill), crosslinkin g the alpha 4 or beta 1 integrins significantly reduced CFC proliferat ion (12 to 26% kill, p = 0.01), indicating that proliferation-inhibito ry signals are transmitted through the VLA-4 integrin. Cytochalasin D, a compound that prevents actin polymerization, prevented not only alp ha 4 receptor capping, but also the inhibition of CFC proliferation ob served following alpha 4 crosslinking. However, crosslinking of the CD 44 receptor with the antibodies Hermes-3 and 50B4, which inhibit adhes ion of CFC to fibronectin, failed to cap the CD44 receptor in the majo rity of CD34(+) cells. Furthermore, crosslinking of the CD44 receptor with these antibodies also failed to inhibit proliferation of CFCs. Th ese studies demonstrate that adhesion receptor crosslinking of the alp ha 4 beta 1 integrin, together with subsequent changes in F-actin poly merization, negatively regulates hematopoietic progenitor proliferatio n in a manner independent of the shape change associated with adhesion .