Chlamydia pneumoniae entry into epithelial cells by clathrin-independent endocytosis

A gram-negative obligate intracellular bacterium, Chlamydia pneumoniae, is a common respiratory pathogen. Here, we examined the invasion and attachment of C. pneumoniae K6 into nonphagocytic HL epithelial cell line by manipulating host plasma membranes by using cholesterol-depleting methyl-beta-cyclodextrin (MβCD) and cholesterol-loading MβCD complexed cholesterol (chol-MβCD). The invasion was attenuated by MβCD-treatment while chol-MβCD augmented the attachment and invasion. In addition, the invasion was inhibited by cholesterol sequestering reagents, nystatin and filipin. Furthermore, exposure of host cells to sphingomyelinase inhibited the invasion. RNA interference was used to assay the role of clathrin and human scavenger receptor B, type I (SR-BI) in the entry of C. pneumoniae into A549 lung epithelial adenocarcinoma cells. In contrast to Chlamydia trachomatis L2, the entry of C. pneumoniae was found to be independent of clathrin. In addition, the entry was found to be SR-BI-independent, but interestingly, the chlamydial growth was attenuated in the SR-BI-silenced cells. These findings suggest that the attachment and invasion of C. pneumoniae into nonphagocytic epithelial cells is dependent on the formation of cholesterol- and sphingomyelin-rich plasma membrane microdomains, and the entry is a clathrin-independent process. In addition, our data indicate that SR-BI supports the growth of C. pneumoniae in epithelial cells.