| Abstract: | As an important intracellular second-messenger, the concentration of calcium in the cytosol Ca+2]i influences diverse cellular activities. To investigate the calcium requirement for distinct phases of B cell activation, we studied the effect of altering the quantity of extracellular calcium on the induction of increased B cell MHC class II molecule (Ia) expression and DNA synthesis by different B cell mitogens. During short-term cultures (less than 24 hr), the induction of class II molecules by anti-immunoglobulin (anti-Ig) and calcium ionophore were dependent on the presence of extracellular calcium, whereas activation induced by B cell stimulation factor-1 (BSF-1) was minimally dependent on extracellular calcium, and that induced by LPS was independent of it. During longer-term cultures (i.e., greater than 24 hr), the heightened class II molecule expression that was induced by all of the B cell mitogens used was significantly compromised by depletion of extracellular free calcium. Although the anti-Ig-stimulated increase in expression of Ia could be restored by the addition of calcium to the medium at 12 hr, it could not be restored when the addition of calcium was delayed to 24 hr after the onset of culture. This was in marked contrast to the finding that BSF-1-stimulated B cell responses which were suppressed after 24 hr of culture in the presence of EGTA could be restored by the addition of calcium. Activation of B cells along the pathway leading to DNA synthesis demonstrated a requirement for extracellular calcium which was greater than that required for induction of MHC class II molecule expression. Thus, LPS-stimulated size increases of B cells after 12 hr of culture was dependent on extracellular calcium while its induction of MHC class II molecule expression was independent of extracellular calcium at this early time point. These observations indicate that the extracellular calcium requirement for B cell activation is dependent both on the activation pathway utilized by the mitogenic signal and on the duration of cell activation. Furthermore, they demonstrate that B cell stimuli that can initiate B cell activation in the relative absence of extracellular calcium may require extracellular calcium for maintenance of this activational state. |
