Increased rate of apoptosis and diminished phagocytic ability of human neutrophils infected with Afa/Dr diffusely adhering Escherichia coli strains

The proinflammatory effect of Afa/Dr diffusely adhering Escherichia coli (Afa/Dr DAEC) strains have been recently demonstrated in vitro by showing that polymorphonuclear leukocyte (PMN) transepithelial migration is induced after bacterial colonization of apical intestinal monolayers. The effect of Afa/Dr DAEC-PMN interaction on PMN behavior has been not investigated. Because of the putative virulence mechanism of PMN apoptosis during infectious diseases and taking into account the high level of expression of the decay-accelerating factor (DAF, or CD55), the receptor of Afa/Dr DAEC on PMNs, we sought to determine whether infection of PMNs by Afa/Dr DAEC strains could promote cell apoptosis. We looked at the behavior of PMNs incubated with Afa/Dr DAEC strains once they had transmigrated across polarized monolayers of intestinal (T84) cells. Infection of PMNs by Afa/Dr DAEC strains induced PMN apoptosis characterized by morphological nuclear changes, DNA fragmentation, caspase activation, and a high level of annexin V expression. However, transmigrated and nontransmigrated PMNs incubated with Afa/Dr DAEC strains showed similar elevated global caspase activities. PMN apoptosis depended on their agglutination, induced by Afa/Dr DAEC, and was still observed after preincubation of PMNs with anti-CD55 and/or anti-CD66 antibodies. Low levels of phagocytosis of Afa/Dr DAEC strains were observed both in nontransmigrated and in transmigrated PMNs compared to that observed with the control E. coli DH5alpha strain. Taken together, these data strongly suggest that interaction of Afa/Dr DAEC with PMNs may increase the bacterial virulence both by inducing PMN apoptosis through an agglutination process and by diminishing their phagocytic capacity.