Background: It has been suggested that defective handling of apoptotic cells by macrophages plays a key role in the development of systemic lupus erythematosus (SLE). The relative contribution of intrinsic defects and serum factors remains controversial.

Objective: To compare monocytes from SLE patients, patients with rheumatoid arthritis, and healthy controls for their ability to differentiate in vitro into macrophages and to bind/engulf apoptotic cells.

Methods: Peripheral blood derived monocytes from healthy donors or from patients with SLE or rheumatoid arthritis were allowed to differentiate into macrophages. The in vitro uptake of apoptotic cells by macrophages was evaluated by a flow cytometry assay that allowed discrimination between binding and internalisation.

Results: Monocytes from SLE and rheumatoid patients showed a striking defect in adherence to plastic compared with healthy donors. Absence or heat inactivation of serum resulted in a reduction in the binding and engulfment of apoptotic cells by macrophages. Macrophages from rheumatoid and SLE patients had similar percentages of apoptotic cells bound to their surface compared with normal controls. However, macrophages from SLE patients showed a significant defect in the internalisation of apoptotic cells compared with those from healthy controls, even in the presence of normal human serum.

Conclusions: Monocytes from patients with SLE and rheumatoid arthritis have a similar defect in their capacity to adhere to plastic. However, only macrophages from SLE patients showed an impaired ability to engulf apoptotic cells, which indicates that an intrinsic cellular defect may be responsible for this phenomenon.