CXCR4 and Parkinson disease: M-MDSCs (defined as CD3-CD19-CD56-CD33 + HLA-DRlo/negCD11b + CD14 + ) were unchanged in frequency among myeloid cells in PD subjects (Fig. 1c), however there was a significant decrease in the proportion of these cells expressing the peripheral tissue-homing marker CXCR4 (Fig. 1d).