Gardiquimod is an agonist of human toll-like receptor 7 (TLR7) [1][2].

Toll-like receptors (TLRs) are a family of highly conserved pattern recognition receptors involved in innate immune responses to pathogen infection. TLR7 is highly expressed by plasmacytoid dendritic cells, and is also found on other leukocyte subpopulations. TLR7 and TLR8 recognize single-stranded RNA from viruses [4].

Gardiquimod is an agonist of human toll-like receptor 7 (TLR7), but not human TLR8, although it activates both porcine TLR7 and TLR8. In transfected 293T cells, Gardiquimod preferentially activated porcine TLR8 (pTLR8) [1]. Gardiquimod is a member of imidazoquinoline family. When codelivered intranasally with plant-derived Norwalk virus virus-like particles (NV VLPs), gardiquimod (GARD) induced NV VLP-specific serum IgG and IgG isotype responses and mucosal IgA responses in the gastrointestinal, respiratory, and reproductive tracts [3]. In macrophages and activated peripheral blood mononuclear cells (PBMCs) infected with X4, R5 or dual-tropic (R5/X4) strains of HIV-1, Gardiquimod inhibited HIV-1 reverse transcriptase and significantly reduced infection. Gardiquimod also induced interferon-alpha (IFN-a) transcription and sustained IFN-a protein secretion [4]. In BxPC-3 cells, Gardiquimod inhibited cell proliferation and migration, and induced apoptosis [5].

In C57BL/6 mice, Gardiquimod enhanced the ability of Treg cell to suppress the proliferation of Tresp cells. The CD4+CD25+Treg cells from mice that had received gardiquimod exhibited an increased inhibitory effect on anti-CD3/anti-CD28 mAb-coated bead-stimulated Tresp cells [2].

References:
[1]. Zhu J, Lai K, Brownile R, et al. Porcine TLR8 and TLR7 are both activated by a selective TLR7 ligand, imiquimod. Mol Immunol. 2008 Jun;45(11):3238-43.
[2]. Forward NA1, Furlong SJ, Yang Y, et al. Signaling through TLR7 enhances the immunosuppressive activity of murine CD4+CD25+ T regulatory cells. J Leukoc Biol. 2010 Jan;87(1):117-25.
[3]. Velasquez LS, Hjelm BE, Arntzen CJ, et al. An intranasally delivered Toll-like receptor 7 agonist elicits robust systemic and mucosal responses to Norwalk virus-like particles. Clin Vaccine Immunol. 2010 Dec;17(12):1850-8.
[4]. 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 Jun;29(6):907-18.
[5]. Zou BB, Wang F, Li L, et al. Activation of Toll-like receptor 7 inhibits the proliferation and migration, and induces the apoptosis of pancreatic cancer cells. Mol Med Rep. 2015 Oct;12(4):6079-85.