ABSTRACT
Effective clearance of apoptotic cells by phagocytes prevents the release of intracellular alarmins and manifestation of autoimmunity. This prompt efferocytosis is complemented by intracellular proteolytic degradation that occurs within the apoptotic cells and in the efferosome of the phagocytes. Although the role of extracellular proteases in apoptotic cells clearance is unknown, the strong association of congenital C1s deficiency with Systemic Lupus Erythematosus highlights the protective nature that this extracellular protease has against autoimmunity. The archetypical role of serine protease C1s as the catalytic arm of C1 complex (C1qC1r2C1s2) involve in the propagation of the classical complement pathway could not provide the biological basis for this association. However, a recent observation of the ability of C1 complex to cleave a spectrum of intracellular cryptic targets exposed during apoptosis provides a valuable insight to the underlying protective mechanism. High-mobility group box 1 (HMGB1), an intracellular alarmin that is capable of inducing the formation of antinuclear autoantibodies and causes lupus-like conditions in mice, is identified as a novel potential target by bioinformatics analysis. This is verified experimentally with C1s, both in its purified and physiological form as C1 complex, cleaving HMGB1 into defined fragments of 19 and 12 kDa. This cleavage diminishes HMGB1 ability to enhance lipopolysaccharide mediated pro-inflammatory cytokines production from monocytes, macrophages and dendritic cells. Further mass spectrometric analysis of the C1 complex treated apoptotic cellular proteins demonstrated additional C1s substrates and revealed the complementary role of C1s in apoptotic cells clearance through the proteolytic cleavage of intracellular alarmins and autoantigens. C1 complex may have evolved as, besides the bacteriolytic arm of antibodies in which it activates the complement cascade, a tissue renewal mechanism that reduces the immunogenicity of apoptotic tissue debris and decreases the likelihood of autoimmunity.
ABSTRACT
Childhood-onset systemic lupus erythematosus (cSLE) is a multisystem autoimmune disease characterized by immune dysregulation affecting patients less than 18 years old. One-fifth of SLE cases are diagnosed during childhood. cSLE presents differently from adults and has a more severe and aggressive course. We describe the clinical and antibody profiles in our cSLE Singapore cohort. All cSLE patients who satisfied the 1997 American College of Rheumatology diagnostic criteria were captured in our lupus registry from January 2009 to January 2014. Data including demographic, cumulative clinical, serologic data, and damage indices were collected. Adjusted mean SLEDAI-2K (AMS) was used to summarize disease activity over multiple visits. Cluster analysis using non-hierarchical K-means procedure was performed on eight selected antibodies. The 64 patients (female:male ratio 5:1; Chinese 45.3%, Malay 28.1%, Indian 9.4%, and other races 17.2%) had a mean onset age of 11.5 years (range 2.1-16.7) and mean age at diagnosis was 11.9 years (range 2.6-18.0). Our study demonstrated differences in clinical manifestations for which hematologic involvement was the most common manifestation with less renal disease and uncommon neurologic manifestation as compared to other cSLE cohorts reported in our region. Antibody clusters were identified in our cohort but their clinical association/discrimination and outcome prediction required further validation study. Outcomes of our cohort in regard to disease activity after therapy and organ damages were comparable if not better to other cSLE cohorts elsewhere. Steroid-related damage, including symptomatic multifocal avascular necrosis and cataract, were not uncommon locally. Infection remains the major cause of death for the continent. Nevertheless, the five year survival rate of our cohort (98.4%) was high.
