IMMU-KYN-PARK: exploring indoleamine 2,3-dioxygenase 1 in peripheral immune system in Parkinson’s Disease

Parkinson’s Disease (PD) is a neurodegenerative disease where central and peripheral immune dysfunctions have been pointed out as a critical component of susceptibility and progression of this disease. Dendritic cells (DCs) and monocytes are key players in promoting immune response regulation and can induce the immunomodulatory enzyme indoleamine 2,3-dioxygenase 1 (IDO1) under proinflammatory environments. IDO1 is a rate-limiting enzyme of the kynurenine pathway (KP) that converts tryptophan (Trp) into kynurenine (KYN) as the first step of the pathway. The immune functions of the pathway, which are extensively described in cancer, are unexplored in neurodegeneration. The dysfunctions of KP that have been described in PD are associated with neurotoxic functions. This thesis focuses on IDO1+DCs and IDO1+monocytes in PD aiming to explore the immune role of IDO1 in PD. We also highlight the importance of exploring the role of dopaminergic therapeutics in the immune system including IDO1 modulation. We first induced IDO1 in myeloid cells with different pro-inflammatory stimuli. The myeloid stimulus lipopolysaccharide (LPS), as well as the lymphoid stimulus anti-CD3/CD28, were able to induce IDO1 in DCs and monocytes (and not in B, T, and NK cells) supporting an important immune cellto-cell interaction. This knowledge allowed us to optimize an in vitro model that enabled us to induce and manipulate pharmacologically IDO1+cells. Before characterizing in vitro, the circulating myeloid cells of PD patients, including their ability to induce IDO1, we immunophenotyped these unstimulated cells of patients. DCs and monocytes of PD patients displayed inflammatory features including increased levels of pro-inflammatory subpopulations [plasmacytoid DCs and double negative (DN) classical DCs (cDCs) expressing CD16)] and higher frequencies of monocytes (total, classical, and NC), expressing the adhesion molecule CD62L. Importantly, these patients also showed increased expression of IL-10, which is known to induce T regulatory (Tregs) cells. This is another immunoregulatory mechanism. Other pro-inflammatory cytokines were increased (IFNs, IL-6, IL-12p70, IL-17A, and IL-33) indicating immune dysfunction. However, IDO1+DCs and IDO1+monocytes from PD patients remained at low levels. This is suggestive that the inflammatory profile of our mild-stage PD patients was not sufficient to significantly induce this enzyme. In vitro characterization of monocytes and DCs showed that LPS-stimulated monocytes from PD patients had a higher reduction of the relative frequency (RF) of non-classical monocytes (NC-Mo) and a higher IDO1-induction when compared with healthy subjects (HS). Importantly, DA seems to accentuate this NC-Mo reduction in PD patients. Moreover, DA decreased IDO1-induction in DCs and monocytes through βeta2 adrenergic receptors (β2AR). The effect of DA on cytokines and IDO1 suggests an anti-inflammatory profile of DA by attenuating the effect of LPS stimulation. Salbutamol, which is a β2AR agonist, seems to have a similar effect to DA, but milder. Salbutamol showed a significantly lower impact on PD patients’ monocytes when compared with HS. Overall, our data suggest that β2AR signaling is impaired in IDO1+monocytes from PD patients. The immunomodulatory effects of DA may also be mediated by other adrenoceptors including αAR, β1AR, or β3AR. This thesis highlighted IDO1+monocytes as diagnostics and therapeutic avenues in PD. We also argue that further understanding the role of catecholamines as immunomodulators is of paramount importance to optimize current therapeutic strategies in PD.