PDCD1 and myeloid sarcoma: It was also discovered that the proportion of T follicular helper (Tfh) cells, PD-1+ Tfh cells, and PD-1+ inducible costimulator (ICOS)+ Tfh cells increased in the cerebrospinal fluid (CSF) of MS patients, with PD1+ Tfh cells showing a positive correlation with the number of CSF plasma cells and disease progression [21].