Background Increased numbers of apoptotic leukocytes¹ and impaired clearance of apoptotic cells² have been demonstrated in SLE. Patients with C1q deficiency almost invariably develop SLE, probably because C1q is important in the clearance of apoptotic cells by monocyte/macrophages.³ Calreticulin has been shown to bind to the early complement component C1q, and interfere with complement-mediated immune processes. Patients with systemic lupus erythematosus (SLE) have increased serum levels of calreticulin.⁴ C-reactive protein (CRP) facilitates the clearance of apoptotic cells in the presence of C1q,⁵ but CRP levels are inappropriately low in SLE.

Objectives To measure the expression of calreticulin on the surface of peripheral blood monocytes and neutrophils in SLE patients compared to normal and rheumatoid arthritis (RA) subjects, and correlate these with disease activity.

Methods Peripheral blood was sampled from 28 SLE, 17 normal, and 19 RA subjects. Calreticulin expression on monocytes and neutrophils was measured by immunofluorescence labelling and flow cytometry, and the density of surface expression of calreticulin expressed as mean channel fluorescence (MCF) values.

Results Calreticulin was expressed on the surface of over 90% of monocyes and neutrophils in all three groups. No differences in the percentage of cells expressing calreticulin were seen when the three groups were compared. The density of surface expression of calreticulin on monocytes was significantly higher in SLE (median MCF = 4.45) compared with normal subjects (median MCF = 3.25, p = 0.014). Density of surface expression of calreticulin on neutrophils was also higher in SLE (median MCF = 19.6) compared with normal subjects (median MCF = 14.50, p = 0.002). Calreticulin expression did not correlate with SLAM or BILAG scores. There were trends towards higher surface calreticulin expression on monocytes and neutrophils in SLE compared with RA, but these did not reach statistical significance.

Conclusion Calreticulin expression is increased on monocyte and neutrophil cell surface membranes in SLE. The combination of increased monocyte and neutrophil surface calreticulin expression (which can interfere with C1q-mediated clearance), and inappropriately low levels of serum CRP and/or C1q (which facilitate this clearance), may contribute to the increased circulating apoptotic leukocytes seen in SLE.

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