Systemic Activation of TLR3-Dependent TRIF Signaling Confers Host Defense against Gram-Negative Bacteria in the Intestine

Recognition of Gram-negative bacteria by toll-like receptor (TLR)4 induces MyD88 and TRIF mediated responses. We have shown that TRIF-dependent responses play an important role in intestinal defense against Gram-negative enteropathogens. In the current study, we examined underlying mechanisms of how systemic TRIF activation enhances intestinal immune defense against Gram-negative bacteria. First we confirmed that the protective effect of poly I:C against enteric infection of mice with Yersinia enterocolitica was dependent on TLR3-mediated TRIF signaling by using TLR3-deficient mice. This protection was unique in TRIF-dependent TLR signaling because systemic stimulation of mice with agonists for TLR2 (Pam3CSK4) or TLR5 (flagellin) did not reduce mortality on Y. enterocolitica infection. Systemic administration of poly I:C mobilized CD11c+, F4/80+, and Gr-1(hi) cells from lamina propria and activated NK cells in the mesenteric lymph nodes (MLN) within 24 h. This innate immune cell rearrangement was type I IFN dependent and mediated through upregulation of TLR4 followed by CCR7 expression in these innate immune cells found in the intestinal mucosa. Poly I:C induced IFN-γ expression by NK cells in the MLN, which was mediated through type I IFNs and IL-12p40 from antigen presenting cells and consequent activation of STAT1 and STAT4 in NK cells. This formation of innate immunity significantly contributed to the elimination of bacteria in the MLN. Our results demonstrated an innate immune network in the intestine that can be established by systemic stimulation of TRIF, which provides a strong host defense against Gram-negative pathogens. The mechanism underlying TRIF-mediated protective immunity may be useful to develop novel therapies for enteric bacterial infection

TIR Domain-Containing Adapter-Inducing Beta Interferon (TRIF) Mediates Immunological Memory against Bacterial Pathogens

Induction of adaptive immunity leads to the establishment of immunological memory; however, how innate immunity regulates memory T cell function remains obscure. Here we show a previously undefined mechanism in which innate and adaptive immunity are linked by TIR domain-containing adapter-inducing beta interferon (TRIF) during establishment and reactivation of memory T cells against Gram-negative enteropathogens. Absence of TRIF in macrophages (Mϕs) but not dendritic cells led to a predominant generation of CD4(+) central memory T cells that express IL-17 during enteric bacterial infection in mice. TRIF-dependent type I interferon (IFN) signaling in T cells was essential to Th1 lineage differentiation and reactivation of memory T cells. TRIF activated memory T cells to facilitate local neutrophil influx and enhance bacterial elimination. These results highlight the importance of TRIF as a mediator of the innate and adaptive immune interactions in achieving the protective properties of memory immunity against Gram-negative bacteria and suggest TRIF as a potential therapeutic target

Human Intestinal Epithelial Cells Express Interleukin-10 through Toll-Like Receptor 4-Mediated Epithelial-Macrophage Crosstalk

In the intestine, interaction between epithelial cells and macrophages creates a unique immunoregulatory microenvironment necessary to maintain local immune and tissue homeostasis. Human intestinal epithelial cells (IECs) have been shown to express IL-10, which keeps epithelial integrity. We have demonstrated that bacterial signaling through TLR4 induces 15-Deoxy-Delta-12,14-prostaglandin J2 (15d-PGJ2) synthesis in intestinal macrophages by Cox-2 expression. Here we show that TLR4 signaling generates crosstalk between IECs and macrophages that enhances IL-10 expression in IECs. Direct stimulation of TLR4 leads to the expression of IL-10 in IECs, while the presence of macrophages in a transwell system induces another peak of IL-10 in IECs at a later time point. The second peak of the IL-10 expression is two times greater than the first peak. This late induction of IL-10 depends on a nuclear receptor peroxisome proliferator-activated receptor (PPAR)γ that is accumulated in IECs by TLR4-mediated inhibition of the ubiquitin-proteasomal pathway. TLR4 signaling in macrophages in turn synthesizes 15d-PGJ2 through p38 and ERK activation and Cox-2 induction, which activates PPARγ in IECs. These results suggest that TLR4 signaling maintains IL-10 production in IECs by generating epithelial-macrophage crosstalk, which is an important mechanism in maintenance of intestinal homeostasis mediated through host-bacterial interactions