Supported by BayImmuNet we have generated a series of monoclonal antibodies against human IL-3. Two of the antibodies are able to detect IL-3 with high sensitivity in human plasma or serum, while commercially available ELISAs show a high background. Using this newly developed ELISA we found that a considerable percentage of patients with rheumatoid arthritis express elevated plasma IL-3 levels while only 2% of healthy controls show elevated IL-3. Unlike IL-3 no differences were seen for IL-6 and TNF between patients with rheumatoid arthritis and other types of joint disease. The human data are consistent with data in mice and rhesus monkeys. Using the model of collagen induced arthritis in DBA/1 mice, we found that blockade of IL-3 significantly reduces the development of arthritis and the humoral immune response against collagen, while injection of recombinant IL-3 exacerbates arthritis and increases titers of anti-collagen antibodies. Other groups have found that injection of IL-3 in rhesus monkeys induces development of polyarthritis, most likely in a HLA-dependent manner. IL-3 is mainly produced by T cells in mice and humans.
To better understand the role of IL-3 in patients with rheumatoid arthritis we need to find out which cells are responsible for IL-3 production in patients with rheumatoid arthritis, how IL-3 expression is regulated (especially in T cells) and how cells involved in the pathogenesis of rheumatoid arthritis respond to IL-3. We also aim at establishing IL-3 as diagnostic and predictive biomarker for rheumatoid arthritis.
We will analyze which cells express IL-3 in patients with rheumatoid arthritis and quantify the frequency of IL-3 expressing cells in patients and healthy controls. This analysis is possible with one of our antibodies that nicely works for intracellular staining of IL-3.
As IL-3 is mainly produced by T cells, we will analyze how IL-3 expression is regulated in T cells and investigate which cytokines and conditions of stimulation induce or inhibit expression of IL-3. In addition, we will study, which other cytokines are co-expressed with IL-3 and analyze the IL-3 expression in classically differentiated TH-1, TH-2, TH-17 and Treg cells.
To better understand functional consequences of elevated IL-3 in patients, we will analyze how cells involved in the pathogenesis of rheumatoid arthritis, especially B and T cells, respond to IL-3. So far, there are very few data on B cells and no data on T cells. We found that the IL-3 receptor is markedly upregulated on activated T cells, suggesting that IL-3 has an impact on T cells. To this end, we will also analyze binding of IL-3 to various cell types using FITC-labelled IL-3 and one of our antibodies that recognizes receptor bound IL-3.
We also plan to further evaluate IL-3 as diagnostic and predictive biomarker for rheumatoid arthritis. Currently we have clinical data on about 200 patients from Regensburg that show increased expression of IL-3 in patients with rheumatoid arthritis compared to controls and other rheumatic diseases. We need to confirm these results in a second independent cohort of patients and include more controls with psoriasis arthritis, ankylosing spondylitis and osteoarthritis. We will also investigate if mutations in the IL-3 gene account for increased IL-3 expression levels in patients.
Overall these investigations will improve our understanding of the role of IL-3 in rheumatoid arthritis and support the development of therapeutic approaches to block IL-3 in autoimmune diseases.