Background/Purpose: Innate immune cells with a myeloid phenotype and suppressor activity towards T cells have been recently described in cancer patients. Similar myeloid-derived suppressor cells (MDSCs) have been also found in transplant recipients. While such suppressor cells can weaken anti-tumor T cell responses with detrimental consequences in cancer, they have the potential to suppress transplant rejection or autoimmunity. If MDSCs were present in patients with autoimmune diseases such as rheumatoid arthritis (RA), their suppressor activity could be exploited to curtail the expansion of autoreactive T cells. The goal of the present study was to identify potential MDSCs in mice with cartilage proteoglycan-induced arthritis (PGIA), an autoimmune model of RA.
Methods: Various organs of naïve BALB/c mice and BALB/c mice with PGIA were screened for the expression of MDSC-related genes and phenotypic markers using RT-PCR, Western blot, and flow cytometry. MDSCs (which were found predominantly in the synovial fluid and spleen of arthritic mice) were then isolated and tested for suppressor activity towards proteoglycan (PG)-specific T cells using PG-loaded bone marrow-derived dendritic cells (DCs) and T cells from naïve PG-specific T cell receptor transgenic (PG-TCR-Tg) mice. The effect of MDSCs on DC maturation (expression of MHC-II and the co-stimulatory molecule CD86) was also examined. The mechanisms of MDSC-mediated suppression were investigated using inhibitors of MDSC-produced effector molecules such as arginase-1 (arg-1), nitric oxide (NO), and reactive oxygen species (ROS).
Results: We identified cells expressing a myeloid phenotype (CD11b+Gr-1+) and co-expressing MDSC-specific molecules including arg-1 and inducible NO synthase (iNOS) in the synovial fluid (SF) of arthritic joints and spleens of mice with PGIA. Such MDSC-like cells were detectable at much lower numbers in the blood and bone marrow of the same animals, and were virtually absent in naïve mice. Upon co-culture with PG-TCR-Tg T cells in the presence of PG-loaded DCs, SF MDSCs profoundly inhibited the proliferation of T cells, thereby confirming their suppressor activity. Phenotypically similar CD11b+ cells isolated from the spleens of arthritic mice were much less potent suppressors of T cell proliferation. Intriguingly, SF MDSCs also significantly inhibited the maturation of DCs through down-regulation of MHC-II and CD86 expression. SF-MDSCs did not suppress the PG/DC-independent proliferation of anti-CD3/CD28-stimulated T cells, suggesting that they exhibited suppressor activity, at least in part, via inhibition of DC activation and antigen presentation. Experiments with inhibitors of arg-1, iNOS, and ROS reveled that the primary mechanisms of suppression of both DC maturation and T cell proliferation involved NO and ROS production by SF MDSCs.
Conclusions: Our study is the first to identify MDSCs in the SF of arthritic joints and spleen in an animal model of RA. As potent suppressors of DC maturation and antigen-specific T cell proliferation, MDSCs have a potential to down-regulate autoimmunity, and by doing so, prevent further inflammatory attacks on the joints in RA.
Disclosure: J. Kurko, None; C. Egelston, None; T. Besenyei, None; B. Tryniszewska, None; T. Kobezda, None; T. A. Rauch, None; T. T. Glant, None; K. Mikecz, None.