Background and Objectives Our recent experiments have suggested that similarly to SLE, patients with primary Sjögren’s syndrome (SS) manifest significantly increased phagocytosis of necrotic cell debris (secondary necrotic cell material, SNEC). This pheno-menon has been attributed to serological aberrations of these patients, as indicated by the capacity of patients’ sera to promote the uptake of SNEC by healthy phagocytes. In this study, we comparatively investigated the role of serum DNAse-I activity and IgG immunoglobulins from SS, SLE and RA patients in the promotion of SNEC-phagocytosis by healthy monocytes.
Materials and Methods The activity of DNase-I was assessed by single radial enzyme-diffusion assay (SRED) in the serum of patients with SS (n = 60), SLE (n = 22) and RA (n = 14) and healthy donors (HBD, n = 52). Total IgG immunoglobulins were isolated by negative selection from the serum of patients and controls using Melon Gel Resin columns. SNEC were prepared by heat-induced necrosis of normal lymphocytes and labelling with propidium iodide. The influence of serum components on SNEC-phagocytosis was assessed by flow cytometry in admixture experiments using normal phagocytes and SNEC pre-incubated with whole sera or purified serum IgG from patients or HBD.
Results Serum DNase-I activity in patients with SS and SLE was found significantly reduced compared to HBD and RA patients (p < 0.0001) and correlated inversely with the ability of these sera to promote SNEC-phagocytosis by healthy monocytes (p = 0.0003). The capacity of HBD sera to promote SNEC-phagocytosis by normal monocytes was significantly increased (by 90%) following the addition of the DNase-I-specific inhibitor G-actin (800 µg/ml), supporting the important physiological role of DNA degradation by serum DNase-I in the prevention of SNEC-phagocytosis. SNEC opsonised with IgG isolated from autoimmune patients or from HBD were found to be similarly ingested by normal monocytes. However, in the presence of normal serum, the opsonisation of SNEC with IgG isolated from SS or SLE sera was found to induce significantly increased SNEC-phagocytosis, compared to that observed with SNEC opsonised with IgG isolated from HBD sera (p = 0.001).
Conclusions Our results indicate that, in a manner similar to SLE, SS patients are characterised by deficient serum DNase-I activity. Such reduced serum capacity for degradation of nucleic acids, in conjunction with the opsonisation of SNEC by serum autoantibodies appears to lead to increased exposure of the immune system of these patients to necrotic cell debris, to enhanced SNEC-phagocytosis and consequently to the inflammatory responses that characterise the disorder.