Pathogenic species of the genus Yersinia evade the bactericidal functions of phagocytes. This evasion is mediated through their virulence effecters, Yops, which act within target cells, in this study we investigated the effect of Yersinia pseudotuberculosis on Ca2+ signaling in polymorphonuclear neutrophils, The intracellular free calcium concentration in single adherent human neutrophils was monitored during bacterial infection and, in parallel, the encounter between the bacteria and cells was observed, When a plasmid-cured strain was used for infection, adherence of a single bacterium to the cellular surface induced a beta(1) integrin-dependent transient increase in the intracellular concentration of free calcium. This was, however, not seen with Yop-expressing wild-type bacteria, which adhered to the fell surface without generating any Ca2+ signal. Importantly, the overall Ca2+ homeostasis was not affected by the wild-type strain; the Ca signal mediated by the C-protein coupled formyl- methionyl-leucyl-phenylalanine receptor was still functioning, Hence, the blocking effect was restricted to certain receptors and their signaling pathways. The use of different Yop mutant strains revealed that the protein tyrosine phosphatase YopH was responsible for the inhibition. This virulence determinant has previously been implicated in very rapid Yersinia-mediated effects on target cells as the key effector in the blockage of phagocytic uptake. The present finding, that Y. pseudotuberculosis, via YopH, specifically inhibits a self-induced immediate-early Ca2+ signal in neutrophils, offers more-detailed information concerning the effectiveness of this virulence effector and implies an effect on Ca2+-dependent, downstream signals.
