Surface complement C3 fragments and cellular binding of microparticles in patients with SLE
Line Kjær Winberg1, Claus Henrik Nielsen1, Søren Jacobsen1
1Institute for Inflammation Research and Copenhagen Lupus and Vasculitis Clinic, Center for Rheumatology and Spine Diseases, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
Objectives:
To examine microparticles (MPs) from patients with SLE and healthy controls (HCs) by determining the cellular origin of the MPs, quantifying attached fragments of complement component 3 (C3) and assessing the ability of MPs to bind to circulating phagocytes and erythrocytes. These features may be relevant for clearance of MPs in SLE pathogenesis.
Methods:
Attached C3 fragments (C3b, iC3b, C3d), membrane integrity and cell surface markers of MPs from 18 patients with SLE and 11 HCs were measured by adding specific antibodies, 7-aminoactinomycin D (7AAD) and annexin V. MPs from all subjects were labelled with carboxyfluorescein diacetate succinimidyl ester and allowed to bind to autologous phagocytes and erythrocytes in the presence of autologous serum, and the binding to individual cell populations was assessed by flow cytometry.
Results:
The proportion of MPs bearing C3 fragments was higher in patients with SLE than in HCs (p=0.026), but the amount of opsonising C3b/iC3b molecules was lower (p=0.004). The C3b/iC3b level correlated with the concentration of circulating C3 (rs=0.53, p=0.036). Phagocytes and erythrocytes from patients and HCs bound autologous MPs, and granulocytes from patients bound 13% more MPs than those from HCs (p=0.043). The presence of erythrocytes inhibited the MP binding to granulocytes by approximately 50%.
Conclusions:
Our demonstration of altered composition of C3 fragments on MPs from patients with SLE, including decreased numbers of opsonising C3 fragments, and competitive binding of MPs to circulating phagocytes and erythrocytes corroborates the hypothesis of defective clearance of apoptotic material in SLE, and indicates that differences in both MP opsonisation and binding of MPs to cells are important in the pathogenesis of SLE.
Insights
Microparticles (MPs) in systemic lupus erythematosus (SLE) patients show altered complement component 3 (C3) fragment composition and increased binding to phagocytes. This suggests defective clearance of apoptotic material contributes to SLE pathogenesis.
Area of Science:
- Immunology
- Rheumatology
- Cell Biology
Background:
- Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by impaired clearance of apoptotic material.
- Microparticles (MPs) are small vesicles released from cell membranes, implicated in various pathological processes.
- Complement component 3 (C3) plays a crucial role in immune responses and clearance mechanisms.
Purpose of the Study:
- To investigate the characteristics of MPs in SLE patients compared to healthy controls (HCs).
- To determine the cellular origin, C3 fragment deposition, and binding capacity of MPs to phagocytes and erythrocytes.
- To explore the relevance of these MP features in the pathogenesis and clearance of apoptotic material in SLE.
Main Methods:
- Analysis of C3 fragment deposition (C3b, iC3b, C3d) on MPs using specific antibodies.
- Assessment of MP membrane integrity and cell surface markers via 7-aminoactinomycin D (7AAD) and annexin V staining.
- Flow cytometry to evaluate the binding of fluorescently labeled MPs to autologous phagocytes and erythrocytes in the presence of serum.
Main Results:
- MPs from SLE patients exhibited a higher proportion of C3 fragments but lower levels of opsonising C3b/iC3b compared to HCs.
- C3b/iC3b levels on MPs correlated with circulating C3 concentrations in SLE patients.
- Granulocytes from SLE patients bound significantly more autologous MPs than those from HCs, with erythrocytes inhibiting this binding.
Conclusions:
- Altered C3 fragment composition and increased MP binding to phagocytes in SLE patients suggest defective clearance of apoptotic material.
- Both MP opsonisation and cellular binding dynamics are implicated as critical factors in the pathogenesis of SLE.
- These findings highlight potential mechanisms contributing to disease progression and offer targets for therapeutic interventions.
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