PDCD1 and myeloid sarcoma: This increase in soluble PD-L1 levels negatively correlated with disease disability and activity measures, determined by EDSS score and MRI studies, supporting the hypothesis that increased PD-1/PD-L1 co-regulatory signalling in MS patients might be associated with a stable MS phenotype.52,53 Future studies will be needed to determine whether this correlation is due to a failure of co-regulatory mechanisms in patients with RRMS.