Soluble erythropoietin receptor levels associate with inflammatory mediators but not with disease activity or cumulative organ damage in patients with systemic lupus erythematosus
The erythropoietin receptor (EpoR) stimulates erythrocyte proliferation after erythropoietin binding. EpoR belongs to the cytokine receptor superfamily and can be found on macrophages and endothelial cells. As there are no data on the role of EpoR systemic autoimmune diseases, we investigated the role of soluble EpoR (sEpoR) in patients with systemic lupus erythematosus (SLE). In a cross-sectional study we recorded clinical characteristics, disease activity (SLEDAI-2K) and organ damage (SDI). sEpoR, autoantibodies and cytokines were measured by enzyme-linked immunosorbent assay (ELISA) in SLE patients (n=100) and compared with a rheumatoid arthritis (RA) cohort (n=57) and a cohort with noninflammatory back pain (NIBP; n=89). Data were analysed with non-parametric techniques. We found no significant difference in sEpoR levels across the SLE, RA and NIBP groups and sEpoR levels were similar in patients with (6% of SLE and 31% of RA) or without anaemia. sEpoR levels were unrelated to haemoglobin levels, SLEDAI-2K or SDI scores, but in both cohorts correlated with levels for C-reactive protein (CRP), interleukin-6 (IL-6), tumour necrosis factor (TNF) and IL-1 (all P<0.001). sEpoR levels are not involved in anaemia or erythropoietin resistance in SLE and RA patients, but closely mirror the underlying inflammatory process. This suggests that increased shedding of sEpoR during inflammation occurs at other sites than bone marrow.