Glucocorticoids Synergistically Enhance NontypeableHaemophilus influenzae-induced Toll-like Receptor 2 Expression via a Negative Cross-talk with p38 MAP Kinase

Abstract

The recognition of invading microbes followed by the induction of effective innate immune response is crucial for host survival. Human surface epithelial cells are situated at host-environment boundaries and thus act as the first line of host defense against invading microbes. They recognize the microbial ligands via Toll-like receptors (TLRs) expressed on the surface of epithelial cells. TLR2 has gained importance as a major receptor for a variety of microbial ligands. In contrast to its high expression in lymphoid tissues, TLR2 is expressed at low level in epithelial cells. Thus, it remains unclear whether the low amount of TLR2 expressed in epithelial cells is sufficient for mediating bacteria-induced host defense and immune response and whether TLR2 expression can be upregulated by bacteria during infection. Here, we show that TLR2, although expressed at very low level in unstimulated human epithelial cells, is greatly up-regulated by nontypeable Hemophilus influenzae (NTHi), an important human bacterial pathogen causing otitis media and chronic obstructive pulmonary diseases. Activation of an IKK␤-IB␣-dependent NF-B pathway is required for TLR2 induction, whereas inhibition of the MKK3/6-p38␣/␤ pathway leads to enhancement of NTHi-induced TLR2 up-regulation. Surprisingly, glucocorticoids, well known potent anti-inflammatory agents, synergistically enhance NTHi-induced TLR2 up-regulation likely via a negative cross-talk with the p38 MAP kinase pathway. These studies may bring new insights into the role of bacteria and glucocorticoids in regulating host defense and immune response and lead to novel therapeutic strategies for modulating innate immune and inflammatory responses for otitis media and chronic obstructive pulmonary diseases.

NLRP2, an Inhibitor of the NF-κB Pathway, Is Transcriptionally Activated by NF-κB and Exhibits a Nonfunctional Allelic Variant

“…Because NLRP2 inhibits NF-B, upregulation of NLRP2 may be part of a negative regulatory loop induced via inflammatory stimuli. A similar NF-B-dependent mechanism has been described for NOD2 (14) and TLR2, which activates the NF-B signaling pathway following recognition of microbial components (31), and for the NF-B inhibitor I B␣ after stimulation with TNF-␣ or transfection with the p65 subunit of NF-B (21), which has been proposed as an inducible autoregulatory pathway.…”

Synergistic activation of NF-κB by nontypeable Haemophilus influenzae and tumor necrosis factor α

“…The expression plasmids I B␣(S32͞36A), NIK(K429͞430A), MEKK1(K432M), fp38␣(AF), fp38␤2(AF), MKK3␤(A), MKK6␤(A), IKK-␤(K49A), and IKK-␥ were described (7,(14)(15)(16)(17). The reporter construct NF-B luc was generated as described (7).…”

“…Abs against phospho-I B␣(Ser-32), I B␣, phospho-p38, p38, phospho-MKK3͞6(Ser-189͞207), and MKK3 were purchased from Cell Signaling Technology (Beverly, MA). Phosphorylation of I B␣, p38, and MKK3͞6 was detected as described and by following the manufacturer's instructions (7,(14)(15)(16)(17).…”

The contribution of PARs to inflammation and immunity to fungi

“…33 p38MAPK has a central role in the activation of homeostatic cyclooxygenase-2 in the airways 11 and in the inhibition of NF-B activation by salicylates. 43 Thus, PAR 2 agonists will share with salicylates a common mechanism of anti-inflammatory action and, at the same time, will rescue the host from toxicity associated with glucocorticoids, which are known to antagonize both p38MAPK 44 and PAR 2 . 19 The ability of PARs and TLRs to have cis-and trans-interactions with other receptors as well as the redundancy in their signaling pathways 6 precludes a definite mechanistic view of events regulating the inflammatory response at the sites of infection.…”