PDCD1 and macrophage activation syndrome: In studies on human populations, it is interesting to note that recently, a unique pathogenic cell subset, namely the CD4dim CD8+ T cells, which is significantly increased and expresses markers of activation/exhaustion such as CD38, human leukocyte antigen DR (HLA-DR), CD25, programmed cell death protein 1 (PD-1), CD95, and IFNγ, has been found in the peripheral blood of patients with MAS/sHLH (73).