Table 1.

List of discussed AhR ligands.

Compound	Class	Origin	Immune Mechanism in Asthma	Ref
I3S 1	Tryptophan metabolite	Uremic toxin from microbiota and host metabolism	Suppression of Th2 differentiation	[14]
Kynurenine	Tryptophan metabolite	Host metabolism	Induction of Treg generation	[15]
FICZ 2	Tryptophan derivative	Oxidation product of tryptophan (skin and microbiota metabolism)	Endogenous AhR agonist;Inhibition of Th2 differentiation; Th17 differentiation;Mast cell degranulation	[16,17,18]
Lipoxin A4	Arachidonic acid derivative	Host metabolism (immune cells)	Global reduction of inflammation	[19,20]
TCDD 3	Chlorinated dioxin	Released from industrial process	Depending on the model decreased Th17 cytokines and neutrophils with Treg induction or increased mucus production and profibrotic molecules	[21,22,23,24,25]
	Benzo(a)pyrene (B(a)P)	Pollutant	Mucus secretion, epithelial, macrophage, T cell activation: synergistic effect with allergen co-exposure	[26,27]
	Indeno[1,2,3-cd]pyrene	Pollutant	Dendritic cell activation: synergistic effect with allergen co-exposure	[28,29]
	4-nonylphenol	Pollutant
Complex PAH 4 mix	Diesel exhaust particles (DEP)	Pollutant	Mucus secretion, epithelial, ILC2, T cell and dendritic cell activation: synergistic effect with allergen co-exposure	[30,31,32,33,34,35]
	Adsorbed on particle matters < 2.5 µm (PM2.5)	Pollutant
	Adsorbed on ultrafine particles (UFP)	Pollutant	Dendritic cell, macrophage activation	[34,36]
VAF347 5		Synthetic	Blockade of B and DC function	[37,38]
Flavonoïds	Polyphenolic metabolites	Vegetables (tea, garlic, berries, broccoli)	Immune mechanism unknown - used in traditional medicine as anti-asthmatic agents	[39,40,41]
Khellin & visnagin	Furanochromone derivative	Fruit of Ammi visnaga
Curcumin	Diferuloyl methane	Curcuma longa
CH223191 6		Synthetic	AhR antagonist	[23,26,42]
α-naphtoflavone	Synthetic flavonoid	Synthetic	AhR antagonist	[43]
Tapinarof 7		Natural bacterial product	AhR agonist	[44,45]

¹ Indoxyl 3-sulfate; ² 6-formylindolo[3,2-b]carbazole; ³ 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin. ⁴ polycyclic aromatic hydrocarbons; ⁵ [4-(3-chloro-phenyl)-pyrimidin-2-yl]-[4-trifluoromethyl-phenyl]-amine; ⁶ 1-Methyl-N-[2-methyl-4-[2-(2-methylphenyl)diazenyl]phenyl-1H-pyrazole-5-carboxamide; ⁷ GSK2894512.