Lipoteichoic acid upregulates plasminogen activator inhibitor-1 expression in parapneumonic effusions : LTA upregulates PAI-1 expression in PPE

The Toll-like receptor (TLR) family plays an instructive role in innate immune responses against microbial pathogens, as well as the subsequent induction of adaptive immune responses. TLRs recognize specific molecular patterns found in a broad range of microbial pathogens such as bacteria and viruses, triggering inflammatory and antiviral responses and dendritic cell maturation, which result in the eradication of invading pathogens. Individual TLRs interact with different combinations of adapter proteins and activate various transcription factors such as nuclear factor (NF)-kappaB, activating protein-1 and interferon regulatory factors, driving a specific immune response. This review outlines the recent advances in our understanding of TLR-signaling pathways and their roles in immune responses. Further, we also discuss a new concept of TLR-independent mechanisms for recognition of microbial pathogens.

Lipoteichoic acid (LTA) derived from Streptococcus pneumoniae, purified employing a chloroform/methanol protocol, and from Staphylococcus aureus, prepared by the recently described butanol extraction procedure, was investigated regarding its interaction with lipopolysaccharide (LPS)-binding protein (LBP), CD14, Toll-like receptors (TLRs)-2 and -4, and MD-2. LTA from both organisms induced cytokine synthesis in human mononuclear phagocytes. Activation was LBP- and CD14-dependent, and formation of complexes of LTA with LBP and soluble CD14 as well as catalytic transfer of LTA to CD14 by LBP was verified by PhastGel(TM) native gel electrophoresis. Human embryonic kidney (HEK) 293/CD14 cells and Chinese hamster ovary (CHO) cells were responsive to LTA only after transfection with TLR-2. Additional transfection with MD-2 did not affect stimulation of these cells by LTA. Our data suggest that innate immune recognition of LTA via LBP, CD14, and TLR-2 represents an important mechanism in the pathogenesis of systemic complications in the course of infectious diseases brought about by the clinically most important Gram-positive pathogens. However, the involvement of TLR-4 and MD-2 in this process was ruled out.

LPS elicits several immediate proinflammatoy responses in peripheral blood leukocytes via a recently described pathway including CD14, Toll-like receptors (TLR), serine-threonine kinases, and NF-kappaB transcription factor. However, the functional responses of intestinal epithelial cells (IEC) to stimulation with LPS are unknown. Expression of mRNA and protein for CD14 and TLRs were assessed by RT-PCR, immunoblotting, and immunohistochemistry in mouse and human IEC lines. LPS-induced activation of signaling pathways (p42/p44 mitogen-activated protein kinase (MAPK), c-Jun NH2-terminal kinase (JNK), p38, p65, NF-kappaB) were assessed by immunoblotting and gel shifts. CD14 mRNA and protein expression were not detectable in IEC. However, human TLR2, TLR3, and TLR4 mRNA were present in IEC. TLR4 protein was expressed in all cell lines; however, TLR2 protein was absent in HT29 cells. Immunofluorescent staining of T84 cells demonstrated the cell-surface presence of the TLRs. LPS-stimulation of IEC resulted in activation (>1.5-fold) of the three members of the MAPK family. In contrast, LPS did not significantly induce activation of JNK and p38 in CMT93 cells, p38 in T84 cells and MAPK and JNK in HT29 cells. Downstream, LPS activated NF-kappaB in IEC in a time-, dose-, and serum-dependent manner. IEC express TLRs that appear to mediate LPS stimulation of specific intracellular signal transduction pathways in IEC. Thus, IEC may play a frontline role in monitoring lumenal bacteria.