Systemic lupus erythematosus (SLE) is a multi-factorial chronic autoimmune disorder characterized by dysfunction of T and B lymphocytes and affects various vital organ systems; 70 to 90% of SLE patients are female 1. The etiology of the disease is still unclear, although environmental, host genetic and hormonal factors have been proposed to play major roles in pathogenesis of SLE 2. There are limited studies on Indian SLE patients. Although the prevalence of SLE in India is rare (3/100,000) 3, the survival rates of these patients (70% at 5 years, 50% at 10 years) are very low compared to Western cohorts 45. Several factors are thought to be responsible for poor survival rates, the most important being delayed diagnosis 6. Several serological biomarkers such as complement components C3 and C4, and anti-dsDNA have been identified, which are not always consistent 78.

The innate immune system plays an important role in the pathogenesis of SLE 9. Mannose binding lectin (MBL) is an essential component of innate immunity that recognizes carbohydrate residues on the surface of micro-organisms, namely, bacteria, viruses, fungi, and protozoa, and activates the complement system through MBL-associated serine proteases 10. Upon activation, early complement factor C3 is cleaved to C3a and C3b. C3a induces inflammation and recruits phagocytes 11. C3b enhances opsonisation by binding to both phagocytes and pathogens 12. In addition, C3b binds to other proteins on the surface of pathogens, forming membrane attack complexes which clear foreign pathogens 13.

The role of MBL in the pathogenesis of SLE has so far not been clearly defined. Higher plasma MBL levels in SLE patients have been reported in various populations 141516. MBL appears to have a dual mode of action: increased MBL leads to enhanced complement activation and tissue damage, while its deficiency has been linked with defective clearance of apoptotic cells that provide a stimulus for autoantibody formation 17. Also, MBL deficiency is linked to susceptibility to secondary infections, which is presumed to be a contributory factor in the development of SLE 18.

Variants of the MBL2 gene associated with lower plasma MBL levels have been associated with susceptibility to SLE 1920212223 and the development of nephritis 1920. Recently, we have observed predisposition of patients with low MBL-producing genotype (LXA/LYB, LYB/LYB and LXB/LXB) to develop SLE and autoimmune hemolytic anemia (AIHA) 24. Literature is limited on the association of MBL with disease activity 1425. The current study investigates the association between plasma MBL levels and SLE disease activity in a cohort of female patients from Eastern India.

MBL is a key molecule of the innate immune system that opsonises microbes through carbohydrate moieties and eliminates them through complement activation. Although the precise role of MBL in SLE pathogenesis is largely unknown, MBL plays a dual role in autoimmune disorders. Low levels of MBL impair the clearance of apoptotic bodies leading to production of antibody against self antigens, while increased MBL enhance activation of the complement system leading to the tissue damage observed in severe disease in SLE 18. The present study demonstrates plasma MBL as a potential biomarker of SLE disease activity. Genetic aspects of MBL in SLE have been well investigated 192930. The variability in levels of serum MBL in lupus patients could be attributed to variants in the MBL 2 gene 3132. However, the reason for high MBL levels in SLE compared to healthy controls is not known. Some attribute it to an acute phase reactant consequent to ongoing inflammation 33. However, literature is limited on the association of plasma MBL and disease activity. A study on Taiwanese pediatric SLE patients showed a positive correlation. In addition, plasma MBL also correlated positively with anti-dsDNA but not with complement factors (C3 and C4) 14. A Japanese study of 147 adult patients did not conclusively demonstrate an association between MBL levels and disease activity, although serum MBL concentration did show significant association with serum C3 or CH₅₀ levels 25. Our study of 93 patients with lupus and 67 controls showed elevated plasma MBL in SLE compared to healthy controls corroborating the observations made in previous studies 141516. Interestingly, SLE patients also displayed higher plasma MBL levels compared to healthy controls with similar MBL2 genetic background 25. Earlier reports have shown association of low-producer MBL genotypes with susceptibility to SLE 2032343536. The discordance in observation has not been satisfactorily explained, and further highlights the complexity associated with the pathogenesis of SLE. The level of serum MBL in SLE patients can be variable depending upon the equilibrium between production and consumption 25. It stands to reason that the serum MBL values will vary depending on the time of sample collection. In our study, samples were collected from treatment-naive patients at the time of assessment. The distribution of plasma MBL in SLE patients (Figure 1A) was found to be variable with a mean level as low as 0.33ug/ml to a high 2.04 ug/ml. The levels correlated with specific clinical categories. Those with arbitrarily defined high and intermediate levels had major organ involvement, such as nephritis, in combination with carditis or serositis, and/or NPSLE. Most patients with musculoskeletal and cutaneous manifestations had low MBL.

Identification of a sensitive disease activity biomarker is essential in SLE management. Currently available biomarkers for disease activity are yet to be validated 37. However, markers like anti-dsDNA, C3 and C4 are being widely used in clinical practice 38394041. In the present study, plasma MBL levels correlated significantly with disease activity scores (SLEDAI). In addition, it correlated positively with anti-dsDNA and inversely with complement factors (C3 and C4). Complements play a pivotal role in the pathogenesis of SLE. They help opsonize and transport the immune complex, and maintain immune complex solubility by preventing formation of large insoluble aggregates 42. Their role is further highlighted by increased predisposition to develop SLE in persons with deficiency of early complement components. The failure to clear apoptotic bodies containing nuclear components leads to a break in self tolerance and the development of SLE 43. On the contrary, increased activation of complements mediates tissue damage. Low levels of C3 and C4 often correlate with disease flare 44. MBL is intimately associated with complement activation through the lectin pathway, resulting in low complement levels in patients with high MBL.

Anti-dsDNA antibodies are found in 60% of patients with SLE. They strongly correlate with renal involvement but are of limited value for other manifestations of disease. Interestingly, the levels may fall at the peak of disease activity, probably due to deposition of antibodies in tissue 45. In the current study we observed a positive correlation between MBL and anti-dsDNA antibodies. Tissue damage resulting from high levels of anti-dsDNA antibodies and MBL are distinct. Anti-ds DNA containing immune complex are deposited in tissue, resulting in tissue damage through the classical complement pathway 46, while MBL activates the lectin pathway of complement and causes tissue damage. Both appear to be independent variables of disease activity.

Lupus nephritis, a severe manifestation of SLE, affects nearly 50% of patients in the course of the disease 47, and 56% of the SLE patients in our series had nephritis. There are multiple mechanisms involved in the development of lupus nephritis. In the current study, proteinuria correlated positively with MBL levels (P < 0.0001). Patients with nephritis and other systemic involvement showed significantly higher levels of MBL (Table 2). Tissue damage in lupus nephritis is mediated by immune complexes or by binding of antibody to in vivo antigen, resulting in activation of the complement system and release of inflammatory cytokines. Previous studies have demonstrated deposition of MBL on glomerular tissue in SLE patients 48. The possible ligand for MBL in the kidney is G0 glycoprotein of IgG 49. Additionally, a significant galactose deficiency of IgG is also seen in lupus 50, which facilitates binding to MBL. Results from the current and previous studies indicate that nephritis may not directly correlate with plasma, but with tissue MBL levels.

Plasma MBL levels are elevated in SLE patients and significantly correlate with various parameters of disease activity. Furthermore, they also correlate with increased proteinuria, indicating a possible role for MBL in lupus nephritis. MBL appears to be a promising biomarker in assessing SLE disease activity.

ACR: American College of Rheumatology; AIHA: autoimmune hemolytic anemia; C3: complement component 3; C4: complement component 4; ELISA: enzyme-linked immunosorbent assay; HC: healthy controls; IgG: immunoglobulin G; MBL: mannose binding lectin; NPSLE: neuropsychiatric systemic lupus erythematosus; SLE: systemic lupus erythematosus; SLEDAI: systemic lupus erythematosus disease activity index.

The authors declare that they have no competing interests.

AKP contributed to the design, performed the ELISA, data interpretation and writing of the first draft of the manuscript. JRP and SSP were involved in sample collection, data management and clinical categorization of samples. RT performed all routine tests. RT, BR and BKD contributed to the design, data interpretation, work supervision and critical revision of the manuscript. All authors read and approved the manuscript.