AB0108 Increased Expression of GM-CSF Secreting Peripheral B Cells in Patients with Rheumatoid Arthritis

Background B cells secreting GM-CSF have been recently identified to participate in innate immunity against bacteria. GM-CSF is known also to play a key role in rheumatoid arthritis (RA) pathogenesis and is currently a target of investigational therapies. The existence of a B cell population secreting GM-CSF and its potential role in RA pathogenesis has not been studied so far. Objectives To determine the expression of GM-CSF secreting peripheral B cells in patients with RA. Methods 20 treatment-naïve patients with active RA (females/males=17/3, mean age =56±11.6 years, median disease duration =1.5 years, RF and/or anti-CCP+=50%), 10 disease controls (psoriatic arthritis n=4, ANCA-associated vasculitides n=2, osteoarthritis n=2, Sjogren's syndrome n=1, giant cell aortitis n=1) and 9 healthy controls were included in the study. Peripheral blood mononuclear cells (PBMC) isolated from heparinized whole blood after density gradient centrifugation were stimulated overnight with Phorbol Myristate Acetate (PMA) and ionomycin in the presence of Brefeldin. Cells were then stained with antibodies directed against CD19 and intracellular cytokine GM-CSF. Positive cells were quantified by flow cytometry and compared between the different groups. Results The % of B cells (CD19+) did not differ between the three groups (7.14±4.01 vs. 6.63±3.52 vs. 8.54±1.99, p=NS). Nevertheless, an expanded population of CD19GM-CSF + cells was detected in RA patients (4.44±2.64%) compared to disease (0.89±1.44%, p=0.0004) and healthy (0.18±00.24%, p=0.00002) controls. There was no difference in GM-CSF expression between disease and healthy controls. There was no difference in CD19-GM-CSF expression between seropositive (RF and/or anti-CCP+, 4.41±2.68%) and seronegative (4.46±2.8%, p=0.86) RA patients. No correlation was observed between the % of GM-CSF+ B cells and CRP levels (p=0.86, Spearman correlation). Conclusions Our study shows for the first time an expanded population of peripheral GM-CSF secreting B cells in untreated RA patients with active disease, not present in patients with other inflammatory or non-inflammatory rheumatic diseases as well as in healthy controls. More studies are needed in order to determine the functional role of these cells in RA pathogenesis. References Cornish AL et al. G-CSF and GM-CSF as therapeutic targets in rheumatoid arthritis. Nat Rev Rheumatol 2009; 5:554-9 Rauch P et al. Innate Response Activator B Cells Protect Against Microbial Sepsis. Science