Background Nuclear factor (NF)-κB signalling plays an important role in the regulation of immune responses. We have previously demonstrated that CD40-mediated noncanonical NF-κB signalling in dendritic cells (DC) induces the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). Noncanonical NF-κB signalling is also important for AIRE expression in the thymus, which is essential for establishment of central tolerance. In the periphery, extrathymic AIRE-expressing antigen presenting cells (including DC) can induce or maintain peripheral tolerance. However, stimuli that induce extrathymic AIRE expression in DC are hitherto unknown.

Materials and methods AIRE expressing cells were characterised by confocal microscopy in tonsil tissue and inflamed tissues from patients with rheumatoid arthritis (RA) and primary Sjögren’s Syndrome (pSS). Monocyte-derived DC (moDC), peripheral blood conventional DC (cDC) and plasmacytoid DC (pDC) were stimulated by anti-CD40 to induce NF-κB activation. Gene transcription and protein expression levels of AIRE, IDO and noncanonical NF-κB signalling components were detected by real time-PCR, Western blot and confocal microscopy. Expression of NF-κB-inducing kinase (NIK), the main activating kinase of the noncanonical pathway, was modulated by siRNA-mediated silencing.

Results First, we identified AIRE expressing cells in inflamed tonsil tissue, RA synovial tissue and pSS glandular tissue and could demonstrate that these cells have DC characteristics, including MHCII, CD40 and CD11c expression. Next, we demonstrated that CD40-stimulation of moDC and pDC, but not cDC was accompanied by upregulation of AIRE expression. This upregulation was crucially dependent on noncanonical NF-κB signalling, since AIRE expression was abrogated by NIK siRNA-mediated silencing and was not affected by either inhibition or induction of canonical NF-κB signalling. Furthermore, preliminary data show that CD40-stimulated moDC from APECED (AIRE deficient) patients exhibited increased T cell proliferation in a mixed lymphocyte reaction compared to healthy individuals.

Conclusions Collectively, our data demonstrate that AIRE expression in DC is critically dependent on CD40-mediated noncanonical NF-κB signalling. These findings suggest a role for peripheral AIRE expression in controlling T cell proliferation during adaptive immune responses, including the inflamed target tissues of patients with RA and pSS. These findings could potentially be exploited as a novel approach to boost peripheral tolerance and induce immune regulation.