Clinical validation of an assay for type 1 interferon in systemic lupus erythematosus

Systemic Lupus Erythematosus (SLE) is a multi-organ autoimmune disease characterised by the breakdown of immune tolerance, and a defect in the clearance of apoptotic material. There is an unmet need for better biomarkers to diagnose and monitoring of SLE. Type I interferon (IFN-I) has a crucial role in pathogenesis of SLE and IFN-I varies between patients. IFN-I has previously been measured using a signature of IFN-I-inducible genes but these have not been applied in routine clinical practice due to lack of validated assays and clinical validation. Tetherin (CD317) is an interferon-inducible protein expressed on the cell surface, and therefore, amenable to measurement using flow cytometry. Measurement of tetherin in specific cell subsets appeared to be a useful biomarker in SLE in discovery studies. In this study tetherin protein expression was assessed on whole blood and peripheral blood mononuclear cells using flow cytometry. It was determined that a whole blood assay reporting median tetherin MFI was convenient and applicable in a routine diagnostic laboratory. IFN-I inducible gene expression was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR) for comparison with tetherin. Healthy control (n=20), SLE-remission (n=66) and SLE-flare (n=65) groups were recruited from Leeds Lupus Clinic and tetherin levels were compared. Tetherin was increased in SLE patients with sensitivity (65.56%) and specificity (70%), with similar findings for gene expression. Tetherin expression on memory B cells, but not monocytes, predicted flare in patients in remission. However, repeat measurement of tetherin at follow up in flare patients (n=15) did not significantly reduce. The potential use of tetherin and IFN-I inducible gene expression to stratify patients to appropriate biologics was explored. These results demonstrate the potential value of tetherin as a biomarker in a routine clinical practice setting. Results have been used to design definitive validation studies.