Pattern recognition receptors in companion and farm animals - the key to unlocking the door to animal disease?

Abstract

The innate immune system is essential for host defence and is responsible for early detection of potentially pathogenic microorganisms. Upon recognition of microbes by innate immune cells, such as macrophages and dendritic cells, diverse signalling pathways are activated that combine to define inflammatory responses that direct sterilisation of the threat and/or orchestrate development of the adaptive immune response. Innate immune signalling must be carefully controlled and regulation comes in part from interactions between activating and inhibiting signalling receptors. In recent years, an increasing number of pattern recognition receptors (PRRs), including C-type lectin receptors and Toll-like receptors (TLRs), has been described that participate in innate recognition of microbes, especially through the so called pathogen-associated molecular patterns (PAMPs). Recent studies demonstrate strong interactions between signalling through these receptors. Whereas useful models to study these receptors in great detail in the murine and human system are now emerging, relatively little is known regarding these receptors in companion and farm animals. In this review, current knowledge regarding these receptors in species of veterinary relevance is summarised.

Fig. 1

SMART representation of bovine C-type lectin receptors. Each receptor has several extracellular C-type lectin binding domains, followed by a neck region and the transmembrane region. The function of the neck region is not yet fully understood.

Fig. 2

Radial tree of published bovine C-type lectin receptor sequences with their human and murine counterparts. Each major family is shown in a circle. Note that for DEC205 and dectin-2, the bovine molecule seems to be more closely related to the murine than the human counterpart.

Fig. 3

Diagram illustrating the current knowledge regarding TLRs, their known agonists and the signalling pathways activated by the interaction of the receptor with an agonist.

Fig. 4

Radial tree of companion and farm animal TLRs, compared to their human orthologues. Each TLR group is shown in a circle, with the TLR 1/6/10 family highlighted. Note the branch structure within a TLR group can be variable (e.g., TLR9), which may have implications for differences in pathogen recognition between TLRs from different species. Some canine TLR sequences were omitted from the tree due to concerns over the validity of the sequences (see text for further explanation).