Nucleic acid (NA) sensors are part of the innate immune system. they can recognize multiple forms of intracellular self and nonself RNA and DNA macromolecules. Besides, the activation of NA sensors initiates signal transduction pathways. In humans, there exist four endosomal TLRs: TLR3, 7, 8, and 9.
The endosomal TLRs detect viral and endogenous double-stranded RNA (dsRNA; TLR3), single-stranded RNA (ssRNA; TLR7/8), and unmethylated CpG sequences in DNA (TLR9). Whatmore, TLR7 and TLR8 have the sequence and function. Both of them can mount an immune response by detecting and responding to degraded ssRNA products from bacteria and viruses.

In this article, we will introduce a potent TLR8 antagonist, CU-115 (IC₅₀=1.04 µM).

In endosomal and non-endosomal TLR specificity studies, Human embryonic kidney (HEK) 293 cells expresses the human toll-like receptor (hTLR) gene. Additionally, it also expresses inducible secreted embryonic alkaline phosphatase (SEAP) reporter gene. After 16-hours’ incubation, CU-115 displays activity for TLR7 and TLR8 at low concentrations (0.5 μM).

CU-115 does not modulate the NF-kB inhibition induced by Pam2CSK4, Pam3CSK4, Poly(I: C), LPS, R848, and Flic in HEK-293 TLR1/2, TLR2/6, TLR3, and TLR4 cells. However, CU-115 inhibits TLR9 signaling at 1, 5, and 20 µM and ~10-25% inhibition.

in a luciferase reporter assay, after an introduction of ssRNA nucleic acid ligands 3p-hpRNA or G3-YSD. CU-115 (5-20 µM) inhibits increases in type I IFN transcriptional activity.
CU-115 is nontoxic at low concentrations at 0.5-20 µM. Additionally, it toxic at 100 μM in Hek293 TLR7 and TLR8 cells. Besides, CU-115 also is nontoxic at low concentrations (0.5 and 20 μM) and displays partial toxicity at 100 μM in THP Dual cells.
The enzyme-linked immunosorbent assay (ELISA) is performed to measure the upregulation/inhibition of TNF-α in human THP-1 cells (hTHP-1). CU-115 (5-20 µM) abolishes the R848-induced TNF-α production activated in hTHP1. It also represses the expression of IL-1β in hTHP-1 cells. These results suggest that CU-115 suppresses TLR8 and TLR7 signaling pathways.

In conclusion,  as a potent TLR8 antagonist,  CU-115 shows selective for TLR8 over TLR7 (IC₅₀=>50 µM).  And the compound decreases TNF-α and IL-1β production activated by R-848 in THP-1 cells.

[1]. Rosaura Padilla-Salinas, et al. J Med Chem. 2019 Nov 27;62(22):10221-10244.