Elevated Membrane and Soluble CD64: A Novel Marker Reflecting Altered FcγR Function and Disease in Early Rheumatoid Arthritis That Can Be Regulated by Anti-Rheumatic Treatment

Abstract

Objectives: Fc receptors (FcR) interacting with immune complexes (ICs) is a central event in the immune pathogenesis of rheumatoid arthritis (RA). Here we asked if a specific FcR is linked to RA pathogenesis and if FcR activities relate to disease and treatment outcome in early RA.

Material and methods: Twenty autoantibody-positive RA patients and 33 HC were included. The patients were evaluated before and after treatment with methotrexate and prednisolone. At follow-up, the EULAR response criteria were applied to determine the individual treatment outcomes. Serum immunoglobulin levels were measured and the expression of FcR for IgG (FcγR) and IgA (FcαR) on peripheral blood monocytes were determined by flow cytometry. The monocytic FcγR function was evaluated by human IgG1 and IgG3 IC-binding and TNFα stimulated release. Plasma levels of soluble FcRs (sFcRs) were determined with ELISA.

Results: The IgG1 and IgG3 levels were elevated in the RA sera. The RA monocytes expressed more CD64 and cell surface-bound IgG than HC monocytes, and showed an impaired FcγR function as reflected by changes in IC-binding and decreased IC-stimulated TNFα secretion. These findings correlated significantly with different disease activity markers. Furthermore, sFcRs were elevated in the patient plasma, and sCD64 was specific for RA (compared with a reference group of patients with active psoriatic arthritis). Following treatment, immunoglobulins and sFcR levels were reduced, whereas membrane CD64 was only decreased in patients with good response to treatment.

Conclusions: Early RA patients display increased membrane and soluble CD64 and an impaired FcγR function correlating with joint disease activity. Beneficial responses of anti-rheumatic treatment in patients reduce CD64. These data suggest sCD64 as an important objective biomarker in RA.

Fig 1. FcγR and Igs are increased in early RA.

Expression of FcRs on monocytes (A-B), monocyte bound IgG (C) and IgG serum levels (D) in DMARD- and steroid naive early RA patients (n = 20) compared with healthy controls (HC) (n = 33). (MFI = mean fluorescence intensity).

Fig 2. Altered FcγR function in early RA.

IgG1 and IgG3 IC-binding (A-B), ratio of IgG1-IC/IgG3-IC binding (C) and IgG1- and IgG3-IC stimulated TNFα-release (index) (D-E) in DMARD- and steroid naive early RA patients (n = 20) and in healthy controls (HC) (n = 33).

Fig 3. Effect of methotrexate and prednisolone treatment on monocytes and FcR receptors.

Frequency and expression of CD14 (A), CD64 (B-C), IgG bound to CD64+ cells (D-E) and CD89 (F-G) on monocytes before and after 3–4 months of anti-rheumatic treatment in DMARD- and steroid naive early RA patients (n = 20). (FV = first visit, FU = follow up).

Fig 4. Reduction of CD64 expression after anti-rheumatic treatment in good responders.

The monocyte expressions of CD64 (A), IgG bound CD64+ cells (B), CD16 (C) and CD89 (D) before and after treatment with methotrexate and prednisolone in early RA patients, defined as good responders (n = 7) or non responders (n = 8). (FV = first visit, FU = follow up, MFI = mean fluorescent intensity).

Fig 5. Characteristics of good and non responders in early RA.

CRP-concentrations (A), the activating:inhibiting FcR expression ratio (B), and the monocyte CD32b expression (C) in good (n = 7) versus non responders (n = 8) before and after treatment with methotrexate and prednisolone in early RA. (the activating:inhibiting FcR expression ratio was defined as the ratio of the sum of the MFIs of the activating CD64 + CD32a + CD16a divided by the MFI of the inhibiting CD32b).

Fig 6. Soluble FcR levels in early RA are regulated by anti-rheumatic treatment.

The plasma levels of sCD89 (A), sCD64 (B) and sCD16a (C) in healthy controls (HC)(n = 20), in patients with active polyarticular psoriatic arthritis (PsA) (n = 22), and in early RA patients (n = 19) before and after treatment with anti-rheumatics. (FV = first visit, FU = follow-up).