Does the TLR3 rs3775291 variant reduce TLR3 signaling and increase susceptibility to viral infections?

Toll-like receptor 3 (TLR3) plays a critical role in the innate immune response by recognizing double-stranded RNA (dsRNA), a hallmark of viral replication. The rs3775291 variant (c.1234C>T; p.Leu412Phe) is a well-characterized polymorphism that significantly alters this defensive mechanism.

Mechanistic impact on TLR3 signaling

The rs3775291 variant results in a leucine-to-phenylalanine substitution at position 412 within the leucine-rich repeat (LRR) domain. This substitution is functionally significant as it:

• Disrupts receptor localization: The variant impairs the proper endosomal trafficking and localization of the receptor, which is essential for encountering viral dsRNA.
• Impairs ligand binding: The structural change reduces the binding affinity for dsRNA, leading to defective activation of downstream signaling pathways, specifically NF-κB and IRF3.
• Reduces cytokine production: Functional assays show that cells carrying the T allele exhibit significantly diminished production of Type I interferons (particularly IFN-β) and proinflammatory cytokines like TNF-α. It has also been linked to impaired macroautophagy, further weakening the cellular response to intracellular pathogens.

Clinical evidence and viral susceptibility

The functional impairment of TLR3 translates into increased clinical vulnerability to a broad spectrum of viral pathogens:

• COVID-19 and Sepsis: Research indicates a strong association with severe COVID-19 outcomes; one cohort study reported an odds ratio (OR) of 2.261 for susceptibility, while septic patients with the T/T genotype showed a four-fold increase in mortality.
• Neurotropic Viruses: The variant is linked to a higher frequency of West Nile Virus (WNV) complications, with the C/T genotype found in 65% of hospitalized patients versus 27.3% of controls. It is also associated with increased risk for herpes simplex encephalitis and cytomegalovirus (HCMV) disease.
• Population Variance: A 2023 meta-analysis of over 7,000 participants confirmed an overall increased risk of infection (OR 1.16). However, effects can vary by geography, with the variant showing higher risk associations in Asian and American populations compared to some European cohorts.

Bottom line

The TLR3 rs3775291 variant is a high-impact polymorphism that reduces TLR3 signaling through impaired receptor localization and ligand binding, leading to deficient interferon responses and a significantly increased risk of severe viral infections, including COVID-19 and West Nile Virus.