Increased expression of FcγRI/CD64 on circulating monocytes parallels ongoing inflammation and nephritis in lupus

INTRODUCTION The high-affinity receptor for IgG Fcgamma/CD64 is critical for the development of lupus nephritis (LN). Cross-linking Fc receptor on recruited monocytes by IgG-containing immune complexes is a key step in immune-complex-mediated nephritis in systemic lupus erythematosus (SLE). The goal of this study was to determine whether expression of Fc receptor (FcgammaR) I on circulating monocytes is associated with systemic inflammation and renal disease in SLE patients. METHODS We studied 205 SLE patients (132 with LN and 73 without LN) along with 74 healthy control individuals. Surface expression of CD14 (monocytes), FcgammaRI/CD64, FcgammaRII/CD32, and FcgammaRIII/CD16 was evaluated by flow cytometry. Monocyte function was assessed by determining the migratory capacity and the ability to produce CCL2 (monocyte chemotractic protein 1). High-sensitivity C-reactive protein, C3 and C4 were measured by nephelometry. RESULTS There was little difference in the expression of FcgammaRIII/CD16 or FcgammaRIII/CD32 on circulating monocytes between patients with SLE and control individuals. In contrast, FcgammaRI/CD64 expression was significantly higher in SLE patients and even higher in patients with LN. FcgammaRI/CD64 expression was positively associated with serum creatinine and indicators of systemic inflammation. Monocytes from patients with high FcgammaRI/CD64 expression also exhibited increased chemotaxis and capacity to produce monocyte chemotractic protein 1. CONCLUSIONS Increased FcgammaRI/CD64 expression on circulating monocytes parallels systemic inflammation and renal disease in SLE patients. We propose that circulating monocytes activated by immune complexes and/or proinflammatory mediators upregulate surface expression of FcgammaRI/CD64 in SLE. The enhanced chemotactic and inflammatory potential of the activated monocytes may participate in a vicious cycle of immune cell recruitment and renal injury in SLE.