MxA mRNA decrease preceding NA b detection in IFN β‐treated MS patients

Background Multiple sclerosis ( MS ) patients treated with interferon beta ( IFN β) are at risk of a declining response to treatment because of the production of IFN β‐neutralizing antibodies ( NA bs). The expression of Myxovirus resistance protein A (MxA) mRNA is regarded as a marker of IFN β bioactivity. Aims The aim of this study was to analyze the kinetics of MxA mRNA expression during long‐term IFN β treatment and assess its relationship to NA b production. Methods A prospective, observational, open‐label, non‐randomized study was designed in multiple sclerosis patients starting IFN β treatment. NA bs and MxA mRNA were monitored every six months. Results 119 patients were consecutively enrolled and 107 were included in the final analysis. Both the presence of NA bs and a decrease in MxA mRNA below the cut off were revealed in 15 patients, however, in six patients (40%) positivity for NA bs was preceded by the decrease in MxA mRNA . In addition, a further six patients showing a decline in MxA mRNA did not have detectable NAbs. Conclusion Our data indicate that quantification of MxA mRNA is a more sensitive identifier of loss of IFN β efficacy than the NA b positivity.