Lipopolysaccharide‐primed heterotolerant dendritic cells suppress experimental autoimmune uveoretinitis by multiple mechanisms

Summary Exposure of bone‐marrow‐derived dendritic cells ( BMDC ) to high‐dose ultrapure lipopolysaccharide for 24 hr ( LPS ‐primed BMDC ) enhances their potency in preventing inter‐photoreceptor retinoid binding protein: complete Freund's adjuvant‐induced experimental autoimmune uveoretinitis (EAU). LPS ‐primed BMDC are refractory to further exposure to LPS (= endotoxin tolerance), evidenced here by decreased phosphorylation of TANK ‐binding kinase 1, interferon regulatory factor 3 ( IRF 3), c‐Jun N‐terminal kinase and p38 mitogen‐activated protein kinase as well as impaired nuclear translocation of nuclear factor κ B ( NF ‐ κ B) and IRF 3, resulting in reduced tumour necrosis factor‐ α ( TNF ‐ α ), interleukin‐6 ( IL ‐6), IL ‐12 and interferon‐ β secretion. LPS ‐primed BMDC also show reduced surface expression of Toll‐like receptor‐4 and up‐regulation of CD 14, followed by increased apoptosis, mediated via nuclear factor of activated T cells ( NFAT c)‐2 signalling. LPS ‐primed BMDC are not only homotolerant to LPS but are heterotolerant to alternative pathogen‐associated molecular pattern ligands, such as mycobacterial protein extract ( Mycobacterium tuberculosis ). Specifically, while M. tuberculosis protein extract induces secretion of IL ‐1 β , TNF ‐ α and IL ‐6 in unprimed BMDC , LPS ‐primed BMDC fail to secrete these cytokines in response to M. tuberculosis . We propose that LPS priming of BMDC , by exposure to high doses of LPS for 24 hr, stabilizes their tolerogenicity rather than promoting immunogenicity, and does so by multiple mechanisms, namely (i) generation of tolerogenic apoptotic BMDC through CD 14: NFAT c signalling; (ii) reduction of NF ‐ κ B and IRF 3 signalling and downstream pro‐inflammatory cytokine production; and (iii) blockade of inflammasome activation.