Background Innate immune cells in particular monocytes (Mo) play a role in the pathogenesis of SLE. Depending on the stimulus they encounter Mo can acquire either a pro (M1) or anti (M2a) inflammatory phenotype. B Lymphocyte Stimulator (BLyS) has been shown to promote Mo survival in health however its effect on Mo subpopulation differentiation in health or SLE, a disease characterised by high levels of circulating BLyS, has not been studied.
Objectives To investigate the effect of BLyS on Mo subpopulation differentiation.
Methods 25 Patients and controls were recruited. CD14+ Mo subpopulations were analysed by Flow Cytometry using the following markers: M1 (pro-inflammatory) subset = CD14+CD86+HLA-DR+ CD206-CD 163-; M2a (anti-inflammatory) subset = CD14+CD 206+CD163+CD86-HLA-DR-. Serum levels of cytokines associated with M1 Mo (CXCL10) and M2a Mo (CCL17) were assessed by ELISA. Clinical data and disease activity were recorded. Differences between groups were analysed by the Mann-Whitney U test.
Results Basally, SLE patients had higher levels of pro-inflammatory M1 Mo (25.2% v 16.3%) and significantly lower levels of anti-inflammatory M2a Mo in comparison to healthy controls (1.8% v 3.4%, p<0.01).
Following stimulation with BLyS a significant increase in both the M1 and M2a Mo subpopulations was observed in healthy controls (M1:16.3% v 25.4%, p<0.01; M2a 3.4% v 6%, p<0.01). Similarly SLE patients had enhanced M1 and M2a subpopulations following BLyS stimulation [(M1: 25.2% v 40.4%, p<0.001), (M2a: 1.8% v 6.5%, p<0.05)] such that SLE patients demonstrated significantly higher levels of M1 Mo in comparison to healthy controls following BLyS stimulation (40.4% v 25.4%, p<0.001). Although no absolute difference was observed between patients and controls in the M2a subpopulation following stimulation, when the fold change from baseline was calculated SLE patients had a significant 2 fold increase from baseline in response to BLyS stimulation suggesting BLyS promotes a mixed inflammatory response in SLE.
When SLE patients were analysed as per their clinical phenotype a significant difference in % expression of M1 monocytes was demonstrated in SLE patients with evidence of immunological activity (dsDNA +ve) at the time of study visit in response to BLyS stimulation in comparison to those who were dsDNA–ve (63.5% vs 28.2%, p<0.001). BLyS stimulation also appeared to enhance expression of M2a Mo in dsDNA+ve patients, albeit in a non-significant manner (10.3% v 5.9%).
Determination of M1 and M2a subset associated cytokines by ELISA confirmed significantly enhanced levels of both subset markers in the serum of SLE patients. Significantly higher levels of CXCL10 were observed in those patients with active disease (493.5pg/ml v 94.2pg/ml, p=0.0045). Regarding clinical involvement CXCL10 levels were higher in those with CNS involvement (649.7pg/ml v 151.7pg/ml, p=0.02) as well as those with evidence of immunological activity. Interestingly CCL17 levels were also higher in patients with active disease (211.7pg/ml v 108.2pg/ml, p<0.0001) with elevated levels also observed in those with both renal involvement (146.8pg/ml v 113.4pg/ml, p=0.046) and serositis (166.8pg/ml v 108.8pg/ml, p=0.039).
Conclusions SLE patient Mo demonstrated enhanced pro and anti-inflammatory responses to BLyS stimulation. Enhanced levels of M1 and M2a Mo associated cytokines, which correspond to clinical phenotypes, were also demonstrated in SLE patients.
Disclosure of Interest None declared