Continual signaling is responsible for constitutive ERK phosphorylation in B-1a cells.

B-1a cells constitutively express phosphorylated, activated ERK, but the origin of pERK in B-1 cells has not been determined. To address this issue, we examined specific mediators of intracellular signaling in unmanipulated B-1a cells. We found that constitutive pERK was rapidly lost from B-1a cells following addition of metabolic inhibitors that block src kinase, Syk, PI-3K, and PLC function. We examined Syk and PLC in more detail and found rapid accumulation of phosphorylated forms of these molecules in B-1a cells, but not B-2 cells, when phosphatase activity was inhibited, and this change occurred in the majority of B-1a cells. Further, we showed that inhibition of src kinase activity eliminated "downstream" pSyk and pPLC accumulation in phosphatase-inhibited B-1a cells, indicating a pathway connection. CD86 expression is greater on B-1 than B-2 cells and plays a role in antigen presentation by B-1 cells to T cells. We found that when Syk or PI-3K was inhibited, CD86 expression was diminished in a reversible fashion. All together, these results indicate that continual activation of intracellular signaling leads to constitutive activation of ERK in B-1 cells, with attendant consequences for co-stimulatory molecule expression.