Psoriatic Arthritis (PsA) is a chronic inflammatory arthropathy commonly associated with psoriasis (PsO). Several evidences suggest a critical role of the immune cells, especially T cells, in the pathogenesis of PsA. Regulatory T cells (Tregs), a subset of FOXP3+ CD4 T cells endowed with multiple immunosuppressive functions, play key roles in preventing unwanted immune responses. Reduced frequencies of circulating Tregs was revealed in a variety of autoimmune disorders including PsA. Tregs suppress target cells by transferring cyclic adenosine monophosphate (cAMP), an intracellular second messenger, through gap junctions; notably, cAMP intracellular levels are especially high in Tregs compared to conventional T cells (Tconvs) mostly because of a constitutively low expression of phosphodiesterases (PDEs). Treg suppression results in impaired calcium influx in target cells, and defects in this process was reported in autoimmunity. An increased expression of type-I interferon (IFN)-induced gene signature has been reported in PsA. Type I IFN inhibits human Treg suppressive function by repressing their intracellular cAMP levels in a PDE4-dependent fashion.
The aims of this project are to assess whether Tregs show lower frequency, altered phenotype, and less cAMP content, while Tconvs display defective suppression of calcium influx, ex vivo from PsA patients compared to healthy controls; to evaluate whether the expression of IFN-stimulated genes is increased in PsA patient and correlates to the extent of Treg functional defects, to disease activity and severity and to test whether, in vitro, treatment with PDE inhibitors can rescue normal cAMP levels and suppressive functions in Tregs from PsA patients.
The results of the study will increase the knowledge about the role of Tregs in the pathogenesis of PsA and shed light on the link between the psoriatic disease and the metabolic syndrome.
Previous data in the literature indicate that the frequency of circulating Tregs, identified as CD4+ CD25high CD127low or FOXP3+, is lower in PsA patients compared to healthy controls: with our extended flow cytometry analysis, including major markers of Treg activity and stability (CD45RA, Helios and Ki67), we expect to generate a more comprehensive scenario of the complexity of Treg subpopulations in PsA.
The set up of flow-cytometry-based assays, for the ex vivo analysis of cAMP levels in Tregs and calcium influx in Tconvs, will provide novel experimental tools for estimating Treg suppressive function directly in vivo, without the need of cell isolation from small blood samples.
We expect to find a positive association between IFN signature activation, cAMP reduction in Tregs, and higher calcium flux in Tconvs, in PsA patients: analyzing the correlation with clinical parameters, and especially with indexes of metabolic syndrome, will support the notion that Treg dysfunctions may contribute to the loss of immune control over inflammation at several levels.
The experiments aimed at testing whether PDE inhibitors rescues Treg functions in vitro will elucidate whether the Treg paralysis, induced by the prolonged exposure to type-I IFNs in vivo, is therapeutically reversible, in line with previously published results on Tregs treated with IFN in vitro.