Pattern recognition receptors in microbial keratitis

Abstract

Microbial keratitis is a significant cause of global visual impairment and blindness. Corneal infection can be caused by a wide variety of pathogens, each of which exhibits a range of mechanisms by which the immune system is activated. The complexity of the immune response to corneal infection is only now beginning to be elucidated. Crucial to the cornea's defences are the pattern-recognition receptors: Toll-like and Nod-like receptors and the subsequent activation of inflammatory pathways. These inflammatory pathways include the inflammasome and can lead to significant tissue destruction and corneal damage, with the potential for resultant blindness. Understanding the immune mechanisms behind this tissue destruction may enable improved identification of therapeutic targets to aid development of more specific therapies for reducing corneal damage in infectious keratitis. This review summarises current knowledge of pattern-recognition receptors and their downstream pathways in response to the major keratitis-causing organisms and alludes to potential therapeutic approaches that could alleviate corneal blindness.

Figure 1

Assembly and activation of the inflammasome. NLRP3 and NLRC4 are the two inflammasomes described to date that assemble at the corneal surface in response to infectious agents. TLR4 stimulation induces Nlrp3 gene expression and NLRP3 transcription; in contrast, NLRC4 is constitutively expressed in the cell. Both inflammasomes share common structural domains (nucleotide-binding domain and LRR) and the ability to recruit the adaptor molecule ASC, which facilitates the association of pro-caspase-1. Different mechanisms of activation have been described for each inflammasome: extracellular ATP activates NLRP3 via P2RX7 receptor stimulation that provokes a decrease of the intracellular K⁺ levels. Alteration of Cl⁻ or Ca²⁺ fluxes or production of reactive oxygen species by mitochondria (a consequence of cell damage), are also able to stimulate the NLRP3 inflammasome. Furthermore, phagocytosis of damage-associated molecular pattern molecules promotes lysosome destabilization and the release of cathepsin B protein into the cytosol, activating the canonical inflammasome.^, NLRC4 is activated via internalisation of flagellin and components of the Pseudomonas bacterial virulence factor type 3 secretion system. NLRC4 needs a protein known as Naip for its stimulation: Naip5-6 proteins are responsible for binding the internalised flagellin and Naip2 binds the components of the type 3 secretion system.^{, ,} These inflammasomes lead to the activation of Caspase-1 protease. Caspase-1 protease is required to cleave the pro-interleukins necessary to generate the mature forms of IL-18 and IL-1β. The cysteine protease Caspase-11 can be also activated by NLRC4 and promote the maturation of these pro-interleukins.

Figure 2

Immune recognition of Pseudomonas aeruginosa molecular patterns. LPS-binding protein binds the LPS from the P. aeruginosa cell wall and the complex is recognised by the CD14 receptor. CD14 transfers the LPS to the MD-2 molecule that is associated with TLR4. MD-2 undergoes changes in its conformation and triggers the stimulation of TLR4. TLR4 dimerises and recruits TIR-domain-containing adaptor proteins (TRIF and TIRAP) to start the signal transduction. TIRAP requires MyD88 protein for its activation, which also recruits IL-1 receptor-associated kinases IRAK4 and IRAK1. Phosphorylation of IRAK1 by IRAK4 activates the E3 ubiquitin protein ligase TNFR-associated factor 6 (TRAF6). This activates TGF-β associated kinase (TAK)1 which results in transcription of pro-inflammatory cytokines via the NF-kB and mitogen-activated (MAP) kinase/JNK pathways. TLR3 can also signal via TRIF. Activation of TLR3/4 recruits TRIF via TRAM which signals through RIP1 and TRAF6 to the NF-kB and JNK pathways. TRIF also activates TRAF3, which through TBK1 and IKK phosphorylates IRF3 and stimulates the production of type I interferons.^{, ,} ssDNA containing unmethylated CpG motifs is sensed by TLR9. In contrast to the other TLRs mentioned above, TLR9 is localised in the endosome. It also activates the MyD88-dependent pathway.^,