Insights into the European rabbit (Oryctolagus cuniculus) innate immune system: genetic diversity of the toll-like receptor 3 (TLR3) in wild populations and domestic breeds.

Background: Toll-like receptors (TLRs) belong to the innate immune system and are a major class of pattern recognition receptors representing the first line of the innate immune response. The TLR molecule is structurally composed by an ectodomain that contains leucine rich repeats (LRRs) that interact with pathogen associated molecular patterns (PAMPs), a transmembrane domain and a conserved cytoplasmic domain designated TIR (Toll-IL1 receptor) that is responsible for the intracellular signaling. TLR3 has been associated with the direct recognition of double-stranded viral RNA resulting from viral replication, while TLR7 and TLR8 target single-stranded viral RNA. In the European rabbit (Oryctolagus cuniculus), TLR7 and TLR8 were reported to be absent and pseudogenised, respectively, making TLR3 the only available TLR for the recognition of viral RNA. Thus, the levels of diversity of TLR3 were evaluated in the European rabbit by analysing different genetic backgrounds and exposure to pathogen pressures.

Results: We detected 41 single nucleotide polymorphisms (SNPs) in the coding sequence of TLR3. The highest diversity was observed in the wild populations of Iberian Peninsula, between 22-33 polymorphic positions. In the French population, 18 SNPs were observed and only 4 polymorphic positions were detected in the domestic breeds. 14 non-synonymous substitutions were observed, most of them in the LRR molecules. The remaining were scattered across the transmembrane and TIR domains.

Conclusion: The study of TLR3 in European rabbit populations might be relevant to understand the interplay between RNA viruses and innate immunity. Wild rabbit populations presented more diversity than domestic breeds and other mammals previously studied. This might be linked to the absence of population bottlenecks during their evolution and to the almost inexistence of man-mediated selection. The observed variability might have also been potentiated by the contact of the wild populations with various pathogens. The study of these patterns of variability might reveal scenarios of host-pathogen interaction and identify TLR3 polymorphisms' that arose due to viral pathogens affecting wild populations.