Background Decreased number or altered function of regulatory T cells (Tregs) has been reported in many inflammatory rheumatic diseases, and Tregs are considered promising therapeutic targets for autoimmune diseases. It is thus of high importance to delineate the pathways controlling Tregs biology and the onset of autoimmunity. Fra2 is a transcription factor belonging to the Fos family proteins which takes part in the AP-1 transcription complex. The role of Fra2 in Tregs is so far unknown.
Objectives To characterize the potential role of Fra2 in controlling Tregs and autoimmunity in Fra2 transgenic mice.
Methods Fra2 transgenic mice were generated, in which the Fra2 transcription factor is ubiquitously overexpressed under the control of an MHCI promotor. T lymphocyte populations were analyzed by flow cytometry, and pathological manifestations in multiple organs by histology. Bone marrow cells were transferred into lethally irradiated recipients to create Fra2-wt chimeric mice.
Results At 3 weeks of age, Fra2 mice showed a striking decrease of the Treg population (CD4+CD25+FoxP3+ in wt and Fra2 littermates spleens: 11.35 and 5.75% of CD4+, respectively, p=0.0005, Mann-Whitney test, n=6). The strong decrease in Tregs was stable over time and also observed in CD4+ single positive thymocytes, indicating that Fra2 mice have a defect in natural Treg development. Interestingly, from 7 weeks on, we could also detect the appearance of activated T cells (CD4+ and CD8+, with CD62LhighCD44highCD127low profile) which represented up to 60% of total T cells by 16 weeks of age. While the phenotype of young mice was limited to decreased Tregs, macroscopic and histological observations in 16 week-old mice showed the presence of an extensive multi-organ autoimmune phenotype (see figure). Perivascular inflammatory infiltrates, containing T cells, B cells and numerous granulocytes were observed in the lung and liver. Extensive inflammation and fibrosis were observed in the thymus. Spleens were also enlarged with pronounced extramedullary haematopoiesis. Fra2 mice also developed dermatitis on eyelids and back-skin, with an increase in dermal and epidermal thickness. Finally, duodenum was enlarged due to acute inflammation and reactive hyperplasia.
To understand whether the effect of Fra2 is T cell intrinsic or extrinsic, we also performed bone marrow transfer experiments in which irradiated wt recipient mice received Fra2 transgenic bone marrow. Interestingly, chimera mice displayed a decreased percentage of Tregs in blood, spleen, but also thymus, confirming an intrinsic role of Fra2 in natural Treg development.
Conclusions These data suggest that Fra2 overexpression inhibits natural Treg development, resulting in 1: diminished Tregs, 2: activation of effector T cells and 3: development of inflammation in multiple organs. This is the first evidence for a role of Fra2 in controlling Treg development and autoimmunity. This murine model also provides a unique opportunity to delineate the function of the Fra2 pathway in T cells and Tregs and its impact on autoimmunity.
Disclosure of Interest F. Renoux: None declared, M. Stellato: None declared, E. Pachera: None declared, D. Impellizzieri: None declared, C. Dees: None declared, J. Distler: None declared, G. Kania: None declared, O. Boyman: None declared, O. Distler Grant/research support from: Bayer, Sanofi, Ergonex, Boehringer Ingelheim, Actelion, Pfizer, Consultant for: 4 D Science, Actelion, Active Biotec, Bayer, BiogenIdec, BMS, Boehringer Ingelheim, EpiPharm, Ergonex, espeRare foundation, Genentech/Roche, GSK, Inventiva, Lilly, medac, MedImmune, Pharmacyclics, Pfizer, Serodapharm, Sinoxa