An insight on antibody-mediated immune-metabolism in inflammation and homeostasis: the role of Fc gamma receptors in regulation of indolemaine 2,3 dioxygenase activity in dendritic cells (IRM9P.460)

Abstract

Indoleamine2, 3-dioxygenase (IDO) is a key immune-modulatory enzyme in Tryptophan catabolism that is highly expressed by Dendritic cells (DCs). Notably, DCs are the main mediators of Fc-dependent anti-inflammatory effects of Intravenous Immunoglobulins (IVIgs) as well as Immune Complexes (ICs)-induced inflammation. Hence, we aimed to study the role of FcγRs in regulation of IDO activity in human DCs from healthy individuals, which remains elusive compared to the well-established role of TLRs. Plasma-pooled human IgG showed no effect on IDO activity in un-stimulated monocyte-derived DCs (Mo-DCs). However, on TLR4 stimulation, IgG down-regulates LPS-induced IDO activity. This counter-regulation was enhanced further, by blocking FcγRIIA, the highest expressed FcγR on Mo-DCs. Interestingly, this effect was elicited by both soluble and plate-bound IgG, resembling IVIgs and ICs, respectively, but with higher potency with soluble IgG. Strikingly, mouse IgG demonstrated a similar effect to human IgG. Herein, we propose a threshold model for IDO regulation through a crosstalk between TLR and FcγRs in homeostatic and inflammatory conditions. IgG was only able to down-regulates TLR-4-induced IDO activity in human DCs, with no effect in the steady state. Furthermore, data suggest that recognition of conserved proteins or glycan moieties of IgG by gamma associated receptors modulate IDO activity. Such insight has potential therapeutic implications for antibody-based immunotherapy.