TNFRSF9 and systemic sclerosis: In keeping with the role of co-stimulatory pathways in shaping the immunological responses in SSc, the CD137 (TNFRSF9) receptor is highly expressed on circulating monocytes, and upon its stimulation, a shift towards a metabolically more active phenotype with the upregulation of both glycolysis and OXPHOS occurs, as well as an increase in monocyte proinflammatory activity and phagocytosis [31].