Costimulation of fibronectin receptor promotes Fc gamma R-mediated rescue of IL-3-dependent bone marrow-derived cells from apoptosis.

The IL-3-dependent murine bone marrow-derived cell line FDC-P2/185-4 (185-4) undergoes apoptosis when IL-3 is withdrawn from the culture medium. However, a high concentration of aggregated mouse IgG prevents apoptosis of 185-4 cells by an autocrine mechanism, producing IL-3. An analysis of flow cytometry revealed that 185-4 cells expressed Fc gamma RIII on their surface and that these effects of IgG are inhibited by anti-Fc gamma RIII mAb. These results indicated that the effect of IgG is mediated by low affinity Fc gamma RIII. In contrast, a low concentration of mouse IgG cannot prevent the apoptosis of 185-4 cells, but in the presence of fibronectin (FN), cell survival is prolonged. It is generally accepted that the interaction of cells with FN is mediated by several integrins such as alpha 5 beta 1 (VLA-5) or alpha 4 beta 1 (VLA-4), and analysis of flow cytometry showed that 185-4 cells express these integrins on their surface. Furthermore, these effects of FN are blocked specifically by RGD peptide or anti-VLA-4 mAb. These findings indicated that FN induces the costimulatory signal through integrin receptor and enhances the proliferative effect through Fc gamma RIII by a low concentration of IgG. The findings presented here suggested that the engagement of FN-integrin receptors on 185-4 cells increases the sensitivity of the cells for cellular activation of IgG. Since inflammatory cells in the microenvironment are surrounded by extracellular matrix proteins, it is possible that adhesion molecules play important roles in inflammatory states such as autoimmune diseases caused by increased levels of IgG.