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Magdalena Massalska,1 Marzena Ciechomska,1 Ewa Kuca-Warnawin,1 Tomasz Burakowski,1 Anna Kornatka,1 Anna Radzikowska,1 Dariusz Pawlak,2 Barbara Muz,3 Adrianna Loniewska-Lwowska,4 Andrzej Palucha,5 Pawel Maldyk,6,7 Wlodzimierz Maslinski1 1Department of Pathophysiology and Immunology, National Institute of Geriatrics, Rheumatology, and Rehabilitation (NIGRiR), Warsaw, 02-637, Poland; 2Department of Pharmacodynamics, Medical University of Bialystok, Bialystok, 15-222, Poland; 3Department of Radiation Oncology, Cancer Biology Division, Washington University School of Medicine, St. Louis, MO, 63108, USA; 4Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw, 02-106, Poland; 5Genomed S.A. Ponczowa 12, Warsaw, 02-971, Poland; 6Department of Rheumoorthopaedic Surgery, National Institute of Geriatrics, Rheumatology, and Rehabilitation (NIGRiR), Warsaw, 02-637, Poland; 7Clinical Department of Orthopaedic and Traumatology of Locomotor System, Enfant-Jesus Clinical Hospital, Warsaw, 02-005, PolandCorrespondence: Magdalena Massalska, Department of Pathophysiology and Immunology, National Institute of Geriatrics, Rheumatology, and Rehabilitation, Spartanska 1, Warsaw, 02-637, Poland, Tel/Fax +48 22 670 94 94, Email magdalena.massalska@spartanska.plBackground: Rheumatoid arthritis (RA) is a chronic autoimmune disease with systemic inflammation finally resulting in damaged joints. One of the RA development models suggests bone marrow (BM) as a place of inflammation development further leading to disease progression. We aimed to investigate the potential of CTLA-4-Fc molecule in inducing tolerogenic milieu in BM measured as indoleamine 2,3-dioxygenase (IDO) expression, CD4+Foxp3+ Treg induction, and T cell activation control. The expression of IDO-pathway genes was also examined in monocytes to estimate the tolerogenic potential in the periphery.Methods: Bone marrow mononuclear cells (BMMC) were stimulated by pro-inflammatory cytokines and CTLA-4-Fc. Next IDO expression, CD4+CD69+ and CD4+Foxp3+ percentage were estimated by PCR and FACS staining, respectively. Enzymatic activity of IDO was confirmed by HPLC in BM plasma and blood plasma. Genes expressed in IDO-pathway were analyzed by NGS in peripheral monocytes isolated from RA patients and healthy controls.Results: We found that CTLA-4-Fc and IFN-γ stimulation results in IDO production by BMMC. CTLA-4-Fc induced tryptophan catabolism can inhibit mitogen-induced CD4+ T cells activation without influencing CD8+ cells, but did not control CD25 nor Foxp3 expression in BM cells. Significantly higher expression of selected IDO-pathway genes was detected on peripheral monocytes isolated from RA as compared to healthy controls.Conclusion: This study sheds light on some immunosuppression aspects present or induced in BM. The potential of IDO-mediated pathways were confirmed in the periphery, what may represent the promising candidates for therapeutic strategies in RA.Keywords: rheumatoid arthritis, bone marrow, CTLA-4-Fc, indoleamine 2,3- dioxygenase, CD4+Foxp3+, monocytes