Chlamydia pneumoniae is an obligate intracellular human pathogen causing diseases such as pneumonia, bronchitis, and pharyngitis. Because of its intracellular replication, cell-mediated immune responses are needed to mediate successful defenses of the host. Because dendritic cells play a central role in linking innate immunity and Ag-specific cell-mediated immune responses we asked whether dendritic cells are activated upon contact with C. pneumoniae and whether known Toll like receptors (TLR) are involved in this process. Here we show that C. pneumoniae was taken up by bone marrow-derived murine dendritic cells. Ingested C. pneumoniae appeared to be unable to develop mature inclusion inside dendritic cells. Furthermore, upon contact with C. pneumoniae dendritic cells were potently stimulated because NF-κB was activated and translocated to the nucleus, cytokines like IL-12p40 and TNF-α were secreted, and expression of MHC class II molecules, CD40, CD80, and CD86 was up-regulated. Importantly, secretion of cytokines as well as translocation of NF-κB were dependent on the presence of TLR2 and independent from TLR4 with the exception of IL-12p40 secretion, which was attenuated in the absence of either a functional TLR2 or 4. In conclusion, we show here that recognition of the Gram-negative bacterium C. pneumoniae depends largely on TLR2 and only to a minor extent on TLR4.

Chlamydia pneumoniae stimulates potently maturation of and cytokine secretion by bone marrow‐derived dendritic cells (BMDDC). BMDDC responses depend mainly on Toll‐like receptor (TLR)2 and to a minor extent on TLR4. We demonstrate here using C. pneumoniae in an infectious model with the replication‐permissive epithelial cell line HEp2 that HSP60 is produced in substantial amounts in chlamydial inclusions during infection. Electron microscopy of chlamydial inclusions revealed that HSP60 was mainly associated with reticulate bodies, but was also located in between the different chlamydial developmental forms. Supernatants of permissive HEp2 cells infected with C. pneumoniae contained soluble chlamydial HSP60 as demonstrated by Western blotting and were able to stimulate BMDDC of wild‐type mice. The stimulatory capacity of culture supernatants correlated with the presence of chlamydial HSP60. In contrast, BMDDC from TLR4‐mutant mice crossed to TLR2‐deficient mice were not stimulated by the culture supernatant, indicating that chlamydial HSP60 but not cytokines, possibly secreted by infected HEp2 cells, are responsible for the observed stimulation of BMDDC. Purified recombinant HSP60 from C. pneumoniae stimulated BMDDC in a TLR2‐ and TLR4‐dependent fashion similar to the whole microorganism. In summary, these data suggest chlamydial HSP60 as an important mediator of inflammatory responses during infection with C. pneumoniae.

No abstract