Determination of ligand binding capacity of soluble Fc gamma RII and Fc gamma RIII in sera of patients with SLE

BACKGROUND: Soluble, human low affinity Fcgamma receptors, such as sFcgammaRII and sFcgammaRIII, are known to play a pathologic role in different diseases. Sandwich ELISAs had previously been applied for the specific detection and determination of these soluble receptors. In these ELISAs, commercial monoclonal antibodies (Ab) were used as capture antibodies with monoclonal or polyclonal antibodies serving as detector Abs. Increased levels of cell-free FcgammaRIII have been detected in patients with lupus but the functions and levels of sFcgammaRII have not been fully characterized yet. OBJECTIVES: The aim of this work was to determine the ligand binding capacities and levels of soluble FcgammaRII and FcgammaRIII in sera of patients with systemic lupus erythematosus (SLE). Moreover, correlation between the levels of sFcgammaRII and sFcgammaRIII and the clinical activity of the disease were investigated. METHODS: Sera of 47 patients with SLE, and 51 healthy subjects were analyzed. In the newly developed indirect sandwich ELISAs commercial monoclonal anti-FcgammaRs are used as capture antibodies, and the ligand of FcgammaRII and FcgammaRIII, an artificial immune complex (IC), serves as a detection component replacing the second antibodies used in previous methods. RESULTS: The ligand binding capacity of both soluble FcgammaRII and sFcgammaRIII were elevated in the sera of SLE patients compared to control samples. This increase was significant in patients with the active disease (n = 30; p < 0.01). It was also revealed that a substantial part of the soluble Fcgamma receptors in these patients was bound in vivo to circulating IC. CONCLUSION: These newly developed ELISAs are probably more phisiologically relevant than other previous assays because they detect the circulating receptors on the basis their in vitro ligan binding capacities. Therefore this method can separately measure the levels of the soluble, free FcgammaRs and those bound circulating IC in vivo.