Can MMTV exploit TLR4?

(C3) The Oral Epithelial Cell and First Encounters with HIV-1

The oral epithelium is the site of first exposure of HIV-1 to host tissues during oral sex with an infected partner or through breast-feeding by an infected mother. Although the oral epithelium is distinguishable by its apparent resistance, the mucosal surfaces represent a primary target of HIV-1. After oral exposure and swallowing, infection is detected prominently in the gastrointestinal tract, which becomes depleted of CD4+ T-cells. The oral cavity and palatine tonsils appear to resist infection and transfer to susceptible lymphoid cells in the lamina propria by local anti-HIV-1 mechanisms. In some cases, expression of these antiviral mechanisms increases after exposure to HIV-1. During primary exposure and before seroconversion, based on limited in vitro and primate data, a window of opportunity for capture of HIV-1 by the oral epithelium may exist. After seroconversion, the risk of infectious HIV-1 appearing in saliva is negligible. This report considers evidence that oral epithelium has the potential both to enable and to resist infection by HIV-1.

Role of novel retroviruses in chronic liver disease: assessing the link of human betaretrovirus with primary biliary cirrhosis

A human betaretrovirus resembling mouse mammary tumor virus has been characterized in patients with primary biliary cirrhosis. The agent triggers a disease-specific phenotype in vitro with aberrant cell-surface expression of mitochondrial antigens. The presentation of a usually sequestered self-protein is thought to lead to the loss of tolerance and the production of anti-mitochondrial antibodies associated with the disease. Similar observations have been made in mouse models, where mouse mammary tumor virus infection has been linked with the development of cholangitis and production of anti-mitochondrial antibodies. The use of combination antiretroviral therapy has been shown to impact on histological and biochemical disease in mouse models of autoimmune biliary disease and in clinical trials of patients with primary biliary cirrhosis. However, the HIV protease inhibitors are not well tolerated in patients with primary biliary cirrhosis, and more efficacious regimens will be required to clearly link reduction of viral load with improvement of cholangitis.

IFN-α and lipopolysaccharide upregulate APOBEC3 mRNA through different signaling pathways

APOBEC3 (A3) proteins are virus-restriction factors that provide intrinsic immunity against infections by viruses like HIV-1 and mouse mammary tumor virus. A3 proteins are inducible by inflammatory stimuli, such as LPS and IFN-α, via mechanisms that are not fully defined. Using genetic and pharmacological studies on C57BL/6 mice and cells, we show that IFN-α and LPS induce A3 via different pathways, independently of each other. IFN-α positively regulates mouse APOBEC3 (mA3) mRNA expression through IFN-αR/PKC/STAT1 and negatively regulates mA3 mRNA expression via IFN-αR/MAPKs-signaling pathways. Interestingly, LPS shows some variation in its regulatory behavior. Although LPS-mediated positive regulation of mA3 mRNA occurs through TLR4/TRIF/IRF3/PKC, it negatively modulates mA3 mRNA via TLR4/MyD88/MAPK-signaling pathways. Additional studies on human peripheral blood mononuclear cells reveal that PKC differentially regulates IFN-α and LPS induction of human A3A, A3F, and A3G mRNA expression. In summary, we identified important signaling targets downstream of IFN-αR and TLR4 that mediate A3 mRNA induction by both LPS and IFN-α. Our results provide new insights into the signaling targets that could be manipulated to enhance the intracellular store of A3 and potentially enhance A3 antiviral function in the host.

Increases in IgA(+) B cells in Peyer's patches during milk-borne mouse mammary tumor virus infection are influenced by Toll-like receptor 4 and are completely dependent on the superantigen response

Mouse mammary tumor virus (MMTV) is a milk-borne betaretrovirus that has developed strategies to exploit and subvert the host immune system. Although mammary glands are the final target of infection, Peyer's patches (PP) are the entry site of the virus. Herein, we show that the infection induces increases in the number of PP IgA(+) B cells and higher expression of the α circular transcript, which is a specific marker of the switch to IgA. In addition, IgA(+) B-cell increases correlated with higher levels of cytokines related to IgA class switching, such as interleukin (IL)-5 and IL-6. Of interest, the increases in IgA(+) B cells were lower in Toll-like receptor 4-deficient mice and were completely dependent on the presence of superantigen-reactive T cells. Our results point to a novel mechanism involved in MMTV infection and suggest that IgA(+) B cells may play an important role in carrying the virus to the mammary glands.

Sensitization to TLR7 agonist in IFN-beta-preactivated dendritic cells

TLRs interact with a growing list of pathogen-derived products and these interactions drive the activation of innate and adaptive immune responses. Dendritic cells (DC) play a key role in these events expressing a heterogeneous repertoire of TLRs. We have previously demonstrated the production of type I IFNs in DC following bacterial infections and TLR triggering. In this study, we sought to characterize the transcriptome specifically induced in human DC by IFN-beta production stimulated upon LPS treatment. To this aim, by using cDNA microarrays, we compared the transcriptome of DC following LPS treatment in the absence or presence of neutralizing anti-type I IFN Abs. Interestingly, we found that the expression of TLR7 was induced during LPS-induced maturation of DC in a type I IFN-dependent manner. The induction of TLR7 in maturing DC was mainly a consequence of the transcriptional activity of IRF-1, whose binding site was located within TLR7 promoter. Moreover, we also demonstrated that "priming" of immature DC, that usually express TLR8 but not TLR7, with exogenous IFN-beta induced a functionally active TLR7. In fact, treatment with the TLR7-specific ligand 3M-001 up-regulated the expression of CD83, CD86, and CD38 in IFN-beta-primed DC but not in immature DC. Therefore, a robust enhancement in proinflammatory as well as regulatory cytokines was observed. These data suggest that TLR4-mediated type I IFN release activates specific transcription programs in DC amplifying the expression of pathogen sensors to correctly and combinatorially respond to a bacterial as well as viral infection.

The Toll-like receptors: analysis by forward genetic methods

Many genes, and conceivably most genes, are constitutively expressed yet have conditional functions. Their products are utilized only under special circumstances, and enforce homeostatic regulation. Mutations do not disclose the function of such genes unless the proper conditions are applied. The genes that encode the Toll-like receptors (TLRs) fall into this category. The TLRs represent the principal sensors of infection in mammals. Absent infection, mammals have little need for the TLRs; they are essential only when microbes gain access to the interior milieu of the host. The function of the TLRs in mammals was first disclosed by a spontaneous mutation in a locus called Lps, when it was shown by positional cloning to be identical to Tlr4. Random germline mutagenesis has since permitted an estimate of the total number of proteins required for TLR signaling to the level of tumor necrosis factor (TNF) synthesis and activity, and has also shown that these sensors are extremely broad in their ability to detect microbes. Ultimately, the TLRs are responsible for most infection-related phenomena, both good and bad. These include the development of fever, shock, and tissue injury, but also the activation of innate and adaptive effector mechanisms that lead to the elimination of microbes.

A type I interferon autocrine-paracrine loop is involved in Toll-like receptor-induced interleukin-12p70 secretion by dendritic cells

Dendritic cells (DC) produce interleukin-12 (IL-12) in response to Toll-like receptor (TLR) activation. Two major TLR signaling pathways participate in the response to pathogens: the nuclear factor-kappaB (NF-kappaB)-dependent pathway leading to inflammatory cytokine secretion including IL-12 and the interferon (IFN)-dependent pathway inducing type I IFN and IFN-regulated genes. Here we show that the two pathways cooperate and are likely both necessary for inducing an optimal response to pathogens. R-848/Resiquimod (TLR7 ligand in the mouse and TLR7/8 ligand in human) synergized with poly(I:C) (TLR3 ligand) or lipopolysaccharide (LPS; TLR4 ligand) in inducing high levels of bioactive IL-12p70 secretion and IFN-beta mRNA accumulation by mouse bone marrow-derived DC (BM-DC). Strikingly, IL-12p70 but not IL-12p40 secretion was strongly reduced in BM-DC from STAT1(-/-) and IFNAR(-/-) mice. STAT1 tyrosine-phosphorylation, IL-12p35, and IFN-beta mRNA accumulation were strongly inhibited in IFNAR(-/-) BM-DC activated with the TLR ligand combinations. Similar observation were obtained in human TLR8-expressing monocyte-derived DC (moDC) using neutralizing anti-IFNAR2 antibodies, although results also pointed to a possible involvement of IFN-lambda1 (also known as IL-29). This suggests that TLR engagement on DC induces endogenous IFNs that further synergize with the NF-kappaB pathway for optimal IL-12p70 secretion. Moreover, analysis of interferon regulatory factors (IRF) regulation in moDC suggests a role for IRF7/8 in mediating IRF3-independent type I IFN and possibly IL-12p35 synthesis in response to TLR7/8.