TABLE 1.

Putative location and ligands of toll-like receptors in the human cornea, based on in vivo animal and in vitro human cell culture studies.

TLRα	Putative Location in Cornea	Mice or Human?	Putative Location in Cell	Ligands	Activation of NF-kB or TRIF (IFN-3) pathways?	References
1	Epithelium Stroma	Both	Surface	LTA and PGN (Gram-positives) LPS (Gram-negatives)	NF-κB	(Kumar et al., 2004)
2δ	Epithelium Stroma Endothelium	Both	Surface	Fungal zymosan and β-glycan (fungi) LTA and PGN (Gram-positives) LPS (Gram-negatives) Phospholipomannan Envelope glycopeptides (viruses)	NF-κB	(Adhikary et al., 2008; Choi et al., 2011;Ebihara et al., 2007; Jin et al., 2009;Johnson et al., 2005; Kot et al., 2019; Kumar et al., 2004, 2006a; Tullos et al., 2013; Zhao and Wu, 2008)
3	Epithelium Stroma	Both	Endosomal	dsRNA	TRIF (IFN-3)	(Kumar et al., 2006b; Liu et al., 2008; Ueta et al., 2005)
4	Epithelium Stroma Endothelium	Both	Surface	LPS (Gram-negatives) Glycoinositol phospholipids (protozoa) Envelope glycopeptides (viruses)	TRIF (IFN-3) and NF-kB	(Choi et al., 2011; Jin et al., 2009; Johnson et al., 2005; Kot et al., 2019; Kumagai et al., 2005; Kumar et al., 2006a; Zhao and Wu, 2008)
5	Epithelium Stroma	Both	Surface	Flagellin	NF-κB	(Hayashi et al., 2001; Zhang et al., 2003)
6	Epithelium Stroma	Both	Surface	LTA and PGN (Gram-positives) LPS (Gram-negatives)	NF-κB	(Redfern et al., 2011)
7	Epithelium Stroma	Both	Endosomal	Viral ssRNA	NF-κB	(Chang et al., 2006; Redfern et al., 2011)
8	No known corneal presence	Both	Surface	Viral ssRNA	NF-κB	(Heil et al., 2004; Redfern et al., 2011)
9	Epithelium Stroma Endothelium	Both	Endosomal	Bacterial CpG DNA motifs Viral CpG DNA motifs	NF-κB	(Ebihara et al., 2007; Inoue, 2014; Johnson et al., 2005;Takeda et al., 2011)
10	Epithelium Stroma	Human	Surface	Unknown†	NF-κB	(Hasan et al., 2005; Redfern et al., 2011)

Key–dsRNA: double stranded ribonucleic acid; IFN: interferon; LPS: lipopolysaccharide; LTA: lipoteichoic acid; NF-κB: nuclear factor κB; PGN: peptidoglycans; ssRNA: single-stranded ribonucleic acid; TRIF: TIR-domain-containing adapter-inducing interferon-β

^αIn total, 13 TLRs have been discovered among mice and humans; only those found in humans are listed in this table.

^†TLR10 homodimerizes with TLR1 and TLR2, su estin they may have similar li ands (Hasan et al., 2005).

^δTLR2 dimerizes with TLR1 to recognize tri-acylated lipoproteins; TLR2 dimerizes with TLR6 to recognize di-acylated lipoproteins (Pearlman et al., 2008).