Lack of detection of agonist activity by antibodies to platelet-derived growth factor receptor alpha in a subset of normal and systemic sclerosis patient sera.

Objective. To investigate whether agonist antiplatelet-derived growth factor receptor alpha (anti-PDGFR alpha) antibodies are present in the serum of patients with systemic sclerosis (SSc; scleroderma). Methods. Sera were obtained from healthy subjects and scleroderma patients. An electrochemiluminescence binding assay was performed for detection of serum autoantibodies to PDGFR alpha, PDGFR beta, epidermal growth factor receptor (EGFR), and colony-stimulating factor receptor 1 (CSFR1). Serum immunoglobulin was purified by protein A/G chromatography. To assess Ig agonist activity, PDGFR alpha-expressing cells were incubated with pure Ig and the level of receptor phosphorylation determined in an enzyme-linked immunoassay, as well as by Western blotting. Ig agonist activity was also assessed in a mitogenic assay and by MAP kinase activation in a PDGFR alpha-expressing cell line. Results. Sera from 34.3% of the healthy subjects and 32.7% of the SSc patients contained detectable autoantibodies to PDGFR alpha and PDGFR beta, but not EGFR or CSFRL Purified Ig from these sera was shown to retain PDGFR binding activity and, at 200-1,000 mu g/ml, exhibited no agonist activity in a cell-based PDGFR alpha phosphorylation assay and did not stimulate a mitogenic response or MAP kinase activation in a PDGFR alpha-expressing cell line. Two purified Ig samples that were unable to bind PDGFRa did exhibit binding activity to a nonglycosylated form of PDGFR alpha. Conclusion. Although approximately one-third of sera from scleroderma patients contained detectable autoantibodies to PDGFR, these antibodies were not specific to scleroderma, since they were also detected in a similar percentage of samples from normal subjects. PDGFR alpha agonist activity was not demonstrated when purified Ig from these sera was tested in cell-based assays.