Protein D expression promotes the adherence and internalization of non-typeable Haemophilus influenzae into human monocytic cells

Protein D, having a glycerol-3-phosphodiester phosphodiesterase activity, i s found at the surface of all Haemophilus influenzae strains and is a possi ble virulence factor. In the present study, the involvement of protein D in the entry of NTHi into human monocytic cells is reported. Primary monocyte s and the monocytic cell lines U-937 and THP-1 were infected with NTHi stra in 772 and the mutant 772 Delta hpd1 (lacking the gene for protein D). NTHi 772 adhered to and entered monocytic cells up to four-fold more efficientl y compared to 772 Delta hpd1. When an Escherichia coli transformant express ing protein D was incubated with monocytic cells, the number of intracellul ar bacteria increased 1.6-fold compared to protein D-deficient controls. An y correlation between internalization and phosphorylcholine expression was not detected. In conclusion, our data suggest that surface-expressed protei n D promotes the adherence of NTHi to human monocytes leading to a higher n umber of internalized bacteria. (C) 2001 Academic Press.