3.2  Lower Tregs in IP sites of AA
HSCs have been mainly detected in close proximity to vasculature and the endosteum, which is the osteoblast niche9. Tregs accumulate in bone marrow and provide protection for HSCs via IP mechanisms. Therefore, FoxP3/CD4 should be able to indicate the immune tolerance status provided by IP to HSCs, especially in the area near endosteum, which has been identified as an IP site, overlapping with the osteoblast niche. We observed FoxP3+, CD4+ and CD3+ cells near the endosteum(Figure 1). FoxP3+/CD4+were significantly lower in AA than in normal controls (18.09%±5.38%vs . 21.72%±4.21%, P <0.05) and MDS (18.09%±5.38% vs . 22.63%±5.98%, P <0.05) (Figure 2A). Taking the low number of Tregs in bone marrow into account, we tried to use the absolute count of FoxP3+ cells to represent bone marrow immune tolerance status. The average number of Tregs in AA was also significantly lower than that in normal control (4.07±1.41 vs . 5.25±1.86 cells/HP, P <0.05) and MDS (4.07±1.41 vs . 5.30±1.49 cells/HP, P <0.05) (Figure 2B). No matter FoxP3+/CD4+or FoxP3+ absolute count, there was no statistically significant difference between MDS patients and normal controls (P >0.05).