Internalization of FimH⁺ Escherichia coli by the human mast cell line (HMC-1 5C6) involves protein kinase C

Rodent mast cells (MC) play critical roles in host defense against bacterial infection. However, bacteria-mediated signaling mechanisms in MC have not been studied. In addition, the response of human MC to bacteria is not fully investigated. This study examined the interaction between human MC and type 1 fimbriated Escherichia coli and the mechanisms involved using the human MC line HMC-1 5C6 and human cord blood-derived MC. These MC internalized significant numbers of FimH⁺ E. coli, but not its isogenic FimH^- mutant. In HMC-1 cells, bacterial internalization was stimulated by protein kinase C (PKC) activation [short-term phorbol myristate acetate (PMA) treatment] and dramatically decreased by PKC inhibitors or PKC depletion (long-term PMA treatment). Moreover, bacterial internalization was accompanied by significant expression of PKCβ₁ and δ. Fluorescence microscopy demonstrated accumulation of PKCβ₁ on internalized bacteria. These data indicate that human MC has the capacity to internalize bacteria and PKC may be a critical intracellular mediator of this function.