Reintroduction of tocilizumab elicited macrophage activation syndrome in a patient with adult-onset Still's disease with a previous successful tocilizumab treatment.

Macrophage activation syndrome (MAS) is a form of secondary hemophagocytic lymphohistiocytosis and is a rapidly progressive, life-threatening complication of adult-onset Still's disease (AOSD). An anti-IL-6 receptor monoclonal antibody, tocilizumab, has shown to be effective in the treatment of AOSD but may precipitate MAS in patients with AOSD. The precise mechanism of MAS developed during anti-cytokine biologic agents remains unknown, but selective inhibition of a subset of pathways could impact other immune signalling pathways and trigger MAS. We herein describe a case of AOSD with the opposite outcomes of tocilizumab therapy, remission and development of MAS, after tocilizumab treatment at the initial flare and the relapse. From the comparison of clinical characteristics and concomitant treatment around the time of starting tocilizumab in both flares, the type and intensity of concomitant immunosuppressive therapy might strongly affect MAS development during tocilizumab therapy.