Defective phagocytosis in systemic lupus erythematosus

Reduced expression of C1q-mRNA in monocytes from patients with systemic lupus erythematosus

Inherited C1q deficiency is associated strongly with the development of systemic lupus erythematosus (SLE). The aim of our study was to evaluate the ability of monocytes from SLE patients without inherited C1q deficiency to up-regulate C1q-mRNA upon stimulation. Furthermore, we wanted to elucidate the physiological stimulus for up-regulation of C1q-mRNA. Peripheral blood mononuclear cell (PBMC)-derived monocytes from 10 SLE patients, 10 patients with rheumatoid arthritis (RA) and 10 healthy controls (HC) were stimulated with dexamethasone (DXM), interferon-gamma or both. Additionally, purified monocytes from HC were stimulated with interleukin (IL)-10. C1q-mRNA expression was measured by quantitative reverse transcription-polymerase chain reaction (RT-PCR). C1q protein was detected using the standard alkaline phosphatase/anti-alkaline phosphatase (APAAP) technique. SLE monocytes were significantly less able to up-regulate C1q-mRNA when compared to RA or HC. IL-10 was identified as an important stimulus for C1q synthesis. In SLE patients there is a significant functional impairment of monocytes to synthesize C1q upon stimulation. As C1q is linked to the process of recognition and removal of apoptotic cells, this relative C1q deficiency is likely to contribute to the reduced phagocytosis of apoptotic material observed in SLE and thereby might be a central pathogenetic factor.

Environment-dependent down-modulation of CD45 cell surface expression on polymorphonuclear cells

CD45, a transmembrane protein tyrosine phosphatase, regulates important cell functions. Its involvement in clearing tissues from dead or dying cells by polymorphonuclear cells (PMNs) is unclear. Therefore, in this study the influence of exogenous factors (e.g., temperature and phagocytosis) on the CD45 expression on PMNs (in patients with lupus erythematosus (LE) vs. controls) was analysed by flow cytometry. The data showed a significant down-regulation of CD45 at 4 degrees C in controls, but not on LE-PMNs. In the presence of targets, CD45 was significantly decreased (P<0.00002). Substrate-dependent down-regulation was greater in the presence of nuclei than in the presence of dead cells(68.98%+/-2.28% vs. 47.89%+/-2.75%; P<0.002), and on LE-PMNs than on control-PMNs (45.93%+/-4.92% vs. 53.65%+/-2.2%). Target pretreatment with anti-dsDNA antibody-containing serum resulted in a lower CD45 expression compared to treatment with normal serum. This effect was more pronounced on PMNs of LE patients than on those of controls. Phagocytosis was inversely correlated with CD45 expression (P<0.0007). These data suggest that phagocytic clearance of cellular debris by PMNs may modulate the function of the effector cells, and that anti-dsDNA antibodies apparently enhance the phagocytic clearance.

Disruption of the cytoskeleton after apoptosis induction with autoantibodies

F-actin cleavage was studied in PBMC after treatment with anti-dsDNA antibodies. Significant changes in F-actin disruption detected by decrease of FITC-phalloidin staining occurred after apoptosis induction with anti-dsDNA antibodies (p < 0.006). Despite of similar F-actin disruption, the switch of phosphatidylserine (PS) to the outer leaflet of the cell membrane as detected by annexin V binding was lower after anti-dsDNA antibody than without antibody treatment (58.4 +/- 11.0% vs. 81.9 +/- 7.7%). F-actin disruption was accompanied by activation of caspase 3 within the cytoplasm (r = -0.92599; p < 8.87446 x 10-(10)) under both conditions with and without autoantibodies. These findings indicate that anti-dsDNA antibody-induced apoptosis is more marked within the cell than upon the cell surface. The diminished externalization of PS might result in a decreased phagocytosis. Thereby, the reduced clearance of apoptotic cells could induce autoantibody production possibly against epitopes which arise due to the apoptotic disruption of cells.

Time between onset of apoptosis and release of nucleosomes from apoptotic cells: putative implications for systemic lupus erythematosus

OBJECTIVE

To investigate the kinetics of nucleosome leakage from apoptotic cells in an in vitro system and extrapolate the results to autoimmune disease, in particular systemic lupus erythematosus.

METHODS

A sensitive nucleosome enzyme linked immunosorbent assay (ELISA) was developed, using a monoclonal antibody (mAb) against histone 3 and an mAb against nucleosomes. Nucleosome release during apoptotic cell death was studied in Jurkat cells. AnnexinV binding (early apoptosis) and propidium iodide positivity (late apoptosis) of the cells were compared with nucleosome release at different times after apoptosis induction.

RESULTS

Nucleosomes appeared in culture supernatant of Jurkat cells 24 to 48 hours after apoptosis induction, when the cells had been late apoptotic for more than 12 hours.

CONCLUSION

Nucleosomes are released from late apoptotic Jurkat cells, with a 12 hour delay from the appearance of AnnexinV binding cells. This result suggests that in vivo scavenger mechanisms have 12 hours to remove apoptotic material from the circulation.