Signal thresholds and modular synergy during expression of costimulatory molecules in B lymphocytes

We analyzed intracellular pathways modulating surface densities of CD80 and CD86 in B cells activated through ligation of the Ag receptor, and the adh esion molecule CD54. Whereas B cell Ag receptor (BCR) cross-linking alone s timulated increased expression of CD86, up-regulation of CD80 required dual stimulation with anti-IgM and anti-CD54. The principal downstream componen t contributed by BCR signaling, toward both CD80 and CD86 induction, was th e elevated concentration of free cytoplasmic Ca2+, recruited by way of capa citative influx. This alone was sufficient to generate an increase in CD86 levels. However, CD80 enhancement required the concerted action of both int racellular Ca2+ concentration and CD54-initiated pathways. The nexus betwee n anti-IgM and anti-CD54 stimulation, in the context of CD80 regulation, wa s identified to involve a self-propagating process of sequential synergy. T he first step involved amplified accumulation of intracellular cAMP, as a r esult of cross-talk between BCR-mobilized Ca2+ and CD54-derived signals. Th is then facilitated a second synergistic interaction between Ca2+ and cAMP, culminating in CD80 expression. Our findings of distinct signal transducer requirements, with the added consequences of cross-talk, offers an explana tion for variable modulation of costimulatory molecule expression in respon se to diverse physiological stimuli. Importantly, these results also reveal how concentration threshold barriers for recruitment of individual second messengers can be overcome by constructive convergence of signaling modules .