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Editorial
Toll-like Receptor Agonists Induce Anti-Viral Responses that Inhibit HIV-1 Replication
Alexandra L. Howell*
Department of Veterans Affairs, Office of Research and Development, White River Junction, VT, and Department of Medicine, NH, USA

Innate immune responses represent the first line of defense against invading microbes including viruses, bacteria and fungi. These responses are mediated by several immune cell populations following activation of specific receptors called "pattern recognition receptors" (PRR) [1-3]. These PRR bind distinct "pathogen-associated molecular patterns" (PAMPs) present on microbes, triggering a rapid signaling cascade in the cell that leads to the production of inflammatory mediators and anti-viral molecules [4,5]. Each PRR binds a specific ligand such as double- or single-stranded RNA or DNA, or bacterial lipopolysaccharide, allowing specificity of the response according to the type of invading pathogen. Some of the best studied PRR are the toll-like receptors (TLR) that are present on myeloid and lymphoid cells, and are located either on the cell membrane or within endosomal vesicles [3,4,6]. TLR, of which there are more than a dozen well-characterized receptors, become activated once bound to ligand. Activation of the TLR leads to downstream signaling events designed to rapidly induce expression of inflammatory mediators and innate immune molecules designed to thwart an infection [7-10]. In addition, TLR activation leads to the up-regulation of surface receptors that promote interactions among immune cells leading to induction of acquired immune responses [11-13]. These responses likely enhance the adaptive immune responses mediated by the T and B cell compartments of the immune system, thus ensuring a long-lasting, durable, and memory immune response to the pathogen.
The use of agonists to endosomal TLR have been studied for the potential to mediate anti-viral responses in general, and anti-human immunodeficiency virus type 1 (HIV-1) responses in particular. Endosomal TLR respond primarily to viral nucleic acids at the time of initial infection [14]. Activation of these TLR induces expression of type I interferons and interferonstimulated genes (ISG) along with additional anti-viral factors [4,5]. Some of these factors include interferon (IFN) -alpha, IFNbeta, IFN-gamma, as well as the interferon-induced proteins such as myxovirus resistance gene (MxA) [15], 2'-5' oligoadenylate synthase (OAS) [16], double-stranded RNA-dependent protein kinase R (PKR) [17,18], ribonuclease L (RNAL) [16] and its endogenously-expressed and HIV-induced suppressor protein termed ribonuclease L inhibitor (RLI) [19]. Two additional HIV-1-specific restriction factors that impart potent HIV inhibitory activity include apolipoprotein B mRNA-editing catalytic 3G (Apobec3G) [20], and SAM-domain and HD-domain containing protein 1 (SAMHD1) [21]. All of these proteins have been shown to be critically important to the antiviral innate immune response.
We studied the anti-HIV-1 activity of several TLR agonists and have found that these compounds can potently inhibit HIV-1 replication in primary cultures of peripheral blood mononuclear cells (PBMC). Our initial studies focused on a TLR7 agonist, gardiquimod, that was shown to effectively inhibit HIV-1 replication even when added to PBMC 48 hours after HIV-1 infection [7]. We determined that gardiquimod, an imidiazoquinoline compound, functioned to inhibit HIV-1 replication by two distinct mechanisms. This compound activated TLR7 and induced high levels of IFN-alpha in PBMC. By blocking the MyD88 adaptor protein with a peptide inhibitor, we showed that IFN-alpha production was blocked, and some, but not all, of the anti-HIV activity was reversed. However, because of its molecular structure, gardiquimod also functioned as a reverse transcriptase inhibitor, blocking HIV-1 reverse transcriptase in newly infected cells, and effectively stopping the spread of infection. Our current studies focus on defining the intracellular signaling pathways induced by agonists to other endosomal TLR,including TLR3, TLR8 and TLR9. It is likely that these agonists,similar to gardiquimod, can function to inhibit HIV-1 replication by both TLR-mediated and non-TLR-mediated mechanisms. For example, because these agonists mimic both single- and doublestranded nucleic acids, it is likely that they can activate non-TLR nucleic acid sensors in the cytoplasm, and activate anti-viral mechanisms by bypassing TLR activation. Such agonists would have greater utility in therapeutic applications if they could prevent the inflammatory cytokine and chemokine responses that also recruit additional immune cells to the site of infection.
Moreover, because TLR agonists have been shown to augment vaccine responses to HIV-1 peptides in both mice [22,23] and non-human primates [24,25], it is possible that these compounds could have a combined therapeutic use to enhance T and B cell responses to HIV-1 vaccines, while at the same time, induce protective innate immune responses in HIV-exposed individuals. If designed appropriately, TLR agonists, or other compounds that would be capable of triggering anti-viral immune responses,could function to induce anti-viral responses while avoiding the induction of potentially harmful inflammatory responses.
In sum, the use of TLR agonists and other small molecules that mimic viral genomes, may hold great promise as novel anti-viral therapeutics to inhibit HIV-1 infection and replication. Whether these compounds could be developed as microbicides to prevent HIV transmission across mucosal tissues at the initial point of entry, or as post-exposure therapeutic agents, is not clear at this point. However, harnessing innate immune responses against viral infection is one area that deserves to be developed further in the fight against HIV-1 infection.
ACKNOWLEDGEMENTS
The author gratefully acknowledges the assistance of Maarten Buitendijk, graduate student in the Program of Experimental and Molecular Medicine, Geisel School of Medicine at Dartmouth, and Susan K. Eszterhas, Ph. D., Assistant Professor of Medicine, Department of Microbiology/Immunology, Geisel School of Medicine at Dartmouth, for their insightful comments on this review article.

References
  1. Pichlmair A, Reis e Sousa C. Innate recognition of viruses. Immunity.2007; 27: 370-383.
  2. Szatmary Z. Molecular biology of toll-like receptors. Gen Physiol Biophys. 2012; 31: 357-366.
  3. Hua Z, Hou B. TLR signaling in B-cell development and activation. Cell Mol Immunol. 2013; 10: 103-106.
  4. Qian C, Cao X. Regulation of Toll-like receptor signaling pathways in innate immune responses. Ann N Y Acad Sci. 2013; 1283: 67-74.
  5. Kipanyula MJ, Seke Etet PF, Vecchio L, Farahna M, Nukenine EN,Nwabo Kamdje AH. Signaling pathways bridging microbial-triggered inflammation and cancer. Cell Signal. 2013; 25: 403-416.
  6. Bao M, Liu YJ. Regulation of TLR7/9 signaling in plasmacytoid dendritic cells. Protein Cell. 2013; 4: 40-52.
  7. Buitendijk M, Eszterhas SK, Howell AL. Gardiquimod: a Toll-like receptor-7 agonist that inhibits HIV type 1 infection of human macrophages and activated T cells. AIDS Res Hum Retroviruses. 2013; 29: 907-918.
  8. Caramalho I, Lopes-Carvalho T, Ostler D, Zelenay S, Haury M,Demengeot J. Regulatory T cells selectively express toll-like receptors and are activated by lipopolysaccharide. J Exp Med. 2003; 197: 403-411.
  9. Hamilton MJ, Antignano F, von Rossum A, Boucher JL, Bennewith KL, Krystal G. TLR agonists that induce IFN-beta abrogate resident macrophage suppression of T cells. J Immunol. 2010; 185: 4545-4553.
  10. Ishii KJ, Coban C, Kato H, Takahashi K, Torii Y, Takeshita F, et al. A Toll-like receptor-independent antiviral response induced by doublestranded B-form DNA. Nat Immunol. 2006; 7: 40-48.
  11. Fujita Y, Taguchi H. Overview and outlook of Toll-like receptor ligandantigen conjugate vaccines. Ther Deliv. 2012; 3: 749-760.
  12. Gujer C, Sundling C, Seder RA, Karlsson Hedestam GB, Loré K. Human and rhesus plasmacytoid dendritic cell and B-cell responses to Tolllike receptor stimulation. Immunology. 2011; 134: 257-269.
  13. Kaisho T, Akira S. Toll-like receptors as adjuvant receptors. Biochim Biophys Acta. 2002; 1589: 1-13.
  14. Iwasaki A. Innate immune recognition of HIV-1. Immunity. 2012; 37:389-398.
  15. Haller O, Staeheli P, Kochs G. Protective role of interferon-induced Mx GTPases against influenza viruses. Rev Sci Tech. 2009; 28: 219-231.
  16. Chakrabarti A, Jha BK, Silverman RH. New insights into the role of RNase L in innate immunity. J Interferon Cytokine Res. 2011; 31: 49-57.
  17. Jha BK, Polyakova I, Kessler P, Dong B, Dickerman B, Sen GC, et al.Inhibition of RNase L and RNA-dependent protein kinase (PKR) by sunitinib impairs antiviral innate immunity. J Biol Chem. 2011; 286:26319-26326.
  18. Toroney R, Hull CM, Sokoloski JE, Bevilacqua PC. Mechanistic characterization of the 5'-triphosphate-dependent activation of PKR: lack of 5'-end nucleobase specificity, evidence for a distinct triphosphate binding site, and a critical role for the dsRBD. RNA. 2012;18: 1862-1874.
  19. Tian Y, Han X, Tian DL. The biological regulation of ABCE1. IUBMB Life. 2012; 64: 795-800.
  20. Lehner T, Wang Y, Whittall T, Seidl T. Innate immunity and HIV-1 infection. Adv Dent Res. 2011; 23: 19-22.
  21. Ayinde D, Casartelli N, Schwartz O. Restricting HIV the SAMHD1 way:through nucleotide starvation. Nat Rev Microbiol. 2012; 10: 675-680.
  22. Buffa V, Klein K, Fischetti L, Shattock RJ. Evaluation of TLR agonists as potential mucosal adjuvants for HIV gp140 and tetanus toxoid in mice. PLoS One. 2012; 7: e50529.
  23. San Román B, De Andrés X, Muñoz PM, Obregón P, Asensio AC, Garrido V, et al. The extradomain A of fibronectin (EDA) combined with poly(I:C) enhances the immune response to HIV-1 p24 protein and the protection against recombinant Listeria monocytogenes-Gag infection in the mouse model. Vaccine. 2012; 30: 2564-2569.
  24. Wille-Reece U, Flynn BJ, Loré K, Koup RA, Kedl RM, Mattapallil JJ, et al. HIV Gag protein conjugated to a Toll-like receptor 7/8 agonist improves the magnitude and quality of Th1 and CD8+ T cell responses in nonhuman primates. Proc Natl Acad Sci U S A. 2005; 102: 15190-15194.
  25. Wille-Reece U, Wu CY, Flynn BJ, Kedl RM, Seder RA. Immunization with HIV-1 Gag protein conjugated to a TLR7/8 agonist results in the generation of HIV-1 Gag-specific Th1 and CD8+ T cell responses. J Immunol. 2005; 174: 7676-7683.

Cite this article: Howell AL (2013) Toll-like Receptor Agonists Induce Anti-Viral Responses that Inhibit HIV-1 Replication. J Immunol Clin Res 1: 1007.
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