Chlamydial Adhesion and Adhesins

This chapter focuses on recent advances in understanding the attachment of chlamydial elementary body (EB) to target cells. The early hints of a role for OmcB in adhesion to mammalian cells and the subsequent identification of OmcB as an EB surface protein that binds heparin were important first steps in the understanding of chlamydial adhesion. Enzyme-linked immunosorbent assay and flow cytometric analyses showed that preincubation with recombinant C. pneumoniae OmcB dramatically decreased the incidence of EB attachment to epithelial and endothelial cells. These data strongly argue that OmcB acts as an adhesin for association to human cells and is of primary importance for infection by several chlamydial species. Regardless of the structure of the glycosaminoglycan (GAG) recognized by OmcB, it has been proposed that the attachment of chlamydiae to human cells via OmcB-GAG interactions is a first step towards successful internalization. Blocking this initial interaction between an EB and the target cell by the addition of excess soluble HS, recombinant OmcB, or anti-OmcB antibody never inhibits the infection by more than 90%. This residual infectivity points to the presence of additional adhesin-receptor interactions, and the recent identification of the polymorphic membrane protein (Pmp) family as a new group of chlamydial adhesins supports this concept. Adhesion studies suggest that perhaps all Pmp proteins act as adhesins. Given that Pmp proteins act as adhesins, the data may suggest that there is considerable pressure to diversify their expression to adapt to new niches.