Increased plasma levels of Gas6 and its soluble tyrosine kinase receptors Mer and Axl are associated with immunological activity and severity of lupus nephritis

Objective Growth arrest-specific 6 (Gas6) and its receptors have been shown to play a crucial role in the homeostasis of the innate immune system by regulating apoptosis and inflammation. We aimed to verify whether an impairment of this system is associated with systemic lupus erythematosus (SLE) disease activity and with lupus nephritis (LN). Methods Plasma Gas6 and the soluble cleaved form of the receptors MerTK (sMer) and Axl (sAxl) concentrations were measured in n=59 SLE patients (n=44 with nephritis, 75%) and analysed in relationship to clinical and laboratory data. Results Patients with LN were characterised by higher Gas6 (19.0 ng/mL [16.8–24.5] vs. 16.5 ng/mL [13.89–18.91]; p=0.03) and sAxl plasma levels than those without LN (31.36 ng/mL [25.1–41.4] vs. 20.2 ng/mL [15.6–30.7]; p=0.03); conversely sMer plasma concentrations were similar between groups. All the three biomarkers studied were directly correlated to creatinine and daily proteinuria, being inversely related to creatinine clearance. 39 patients had a proteinuria level of <0.5 mg/day, 14 between 0.5 and 3.5 mg/day and 5 had ≥3.5 g/day; Gas6, sAxl and sMer plasma concentrations significantly increased for increasing degree of proteinuria (test for trend p=0.0002; p=0.02; p=0.009, respectively).These correlations were confirmed in multiple linear regression analysis models accounting for gender, age, disease duration and concomitant treatment. Conclusion Plasma Gas6, sAxl and sMer concentrations are associated with the severity of LN in patients affected by SLE. The excess cleavage of TAM receptors might contribute to LN pathogenesis.