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Open Access Research article

TAM receptor ligands in lupus: Protein S but not Gas6 levels reflect disease activity in systemic lupus erythematosus

Chang-Hee Suh12, Brendan Hilliard1, Sophia Li1, Joan T Merrill3 and Philip L Cohen1*

Author Affiliations

1 Section of Rheumatology, Department of Medicine, Temple University School of Medicine, 3322 North Broad Street, Room 205, Philadelphia, PA 19140, USA

2 Department of Allergy-Rheumatology, Ajou University School of Medicine, Woncheon-dong San 5, Youngtong-gu, Suwon 443-721, Korea

3 Clinical Pharmacology Research Program, Oklahoma Medical Research Foundation, 825 N.W. 13th Street, Oklahoma City, OK 73106, USA

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Arthritis Research & Therapy 2010, 12:R146  doi:10.1186/ar3088


See related editorial by Silverman, http://arthritis-research.com/content/12/5/144

Published: 16 July 2010

Abstract

Introduction

The TAM (tyro 3, axl, mer) kinases are key regulators of innate immunity and are important in the phagocytosis of apoptotic cells. Gas6 and protein S are ligands for these TAM kinases and bind to phosphatidyl serine residues exposed during apoptosis. In animal models, absence of TAM kinases is associated with lupus-like disease. To test whether human systemic lupus erythematosus (SLE) patients might have deficient levels of TAM ligands, we measured Gas 6 and protein S levels in SLE.

Methods

107 SLE patients were recruited. Of these, 45 SLE patients were matched age, gender and ethnicity with normal controls (NC). Gas6 and free protein S were measured with sandwich enzyme linked immunosorbent assays (ELISAs).

Results

Overall, the plasma concentrations of Gas6 and free protein S were not different between 45 SLE patients and 45 NC. In SLE patients, the levels of free protein S were positively correlated with age (r = 0.2405, P = 0.0126), however those of Gas6 were not. There was no correlation between the concentrations of Gas6 and free protein S in individuals. Levels of free protein S were significantly lower in SLE patients with a history of serositis, neurologic disorder, hematologic disorder and immunologic disorder. Gas6 levels were elevated in patients with a history of neurologic disorder. The SLE patients with anti-Sm or anti-cardiolipin IgG showed lower free protein S levels. Circulating free protein S was positively correlated with complement component 3 (C3) (r = 0.3858, P < 0.0001) and complement component 4 (C4) (r = 0.4275, P < 0.0001). In the patients with active BILAG hematologic involvement, the levels of free protein S were lower and those of Gas6 were higher.

Conclusions

In SLE, free protein S was decreased in patients with certain types of clinical history and disease activity. Levels of free protein S were strongly correlated with C3 and C4 levels. Gas6 levels in SLE patients differed little from levels in NC, but they were elevated in the small numbers of patients with a history of neurological disease. The correlation of decreased protein S levels with lupus disease activity is consistent with a role for the TAM receptors in scavenging apoptotic cells and controlling inflammation. Protein S appears more important functionally in SLE patients than Gas6 in this regard.