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.

The Toll/IL-1 receptor (TIR) domains are crucial signaling modules during innate immune responses involving the Toll-like receptors (TLRs) and IL-1 receptor (IL-1R). Myeloid differential factor 88 (MyD88) is a central TIR domain-containing adapter molecule responsible for nearly all TLR-mediated signaling and is targeted by a TIR domain-containing protein C (TcpC) from virulent uropathogenic Escherichia coli, a common human pathogen. The mechanism of such molecular antagonism has remained elusive. We present the crystal structure of the MyD88 TIR domain with distinct loop conformations that underscore the functional specialization of the adapter, receptor, and microbial TIR domains. Our structural analyses shed light on the genetic mutations at these loops as well as the Poc site. We demonstrate that TcpC directly associates with MyD88 and TLR4 through its predicted DD and BB loops to impair the TLR-induced cytokine induction. Furthermore, NMR titration experiments identify the unique CD, DE, and EE loops from MyD88 at the TcpC-interacting surface, suggesting that TcpC specifically engages these MyD88 structural elements for immune suppression. These findings thus provide a molecular basis for the subversion of TLR signaling by the uropathogenic E. coli virulence factor TcpC and furnish a framework for the design of novel therapeutic agents that modulate immune activation.innate immune cells | bacterial pathogens

The relevance of TLR2 and TLR4 for recognizing Chlamydia pneumoniae in vivo during pulmonary infection and to survive the infection was explored. We found that early immune responses triggered by C. pneumoniae partially depended on TLR2, but not on TLR4. The chemokines MIP-2 and MIP-1a were not induced, while IL-12p40 levels were higher in TLR2 -/-mice compared to wild-type mice. Secretion of TNF, keratinocytederived chemokine and monocyte chemoattractant protein-1 was attenuated in TLR2 -/-mice, while IFN-c was increased as in wild-type mice. The pulmonary cyto-and chemokine response of TLR2 -/-ÂTLR4 d/d was similar to TLR2 -/-mice. TLR2 -/-and TLR2 -/-ÂTLR4 d/d mice also attracted fewer polymorphonuclear neutrophils into the lung, while TLR4 d/d mice recruited them. Attenuated recruitment of polymorphonuclear neutrophils correlated with reduced weight loss in TLR2 -/-and TLR2 -/-ÂTLR4 d/d mice and a lower chlamydial burden 3 days post infection. At 9 days post infection, TLR2 -/-and TLR2 -/-ÂTLR4 d/d mice produced cyto-and chemokines as efficiently as wild-type mice, indicating that the involvement of TLR in inflammation varies over time. All TLR2 -/-ÂTLR4 d/d mice succumbed to the infection, while about 50% of TLR2 -/-mice died. Taken together, the function of TLR2 and TLR4 is required to survive pulmonary infection with C. pneumoniae.