Figure 3. MPL receptor on vascular ECs contributes to JAK2^V617F HSPC maintenance/expansion in MPN.

(A) MPL expression was increased by 46% in the JAK2^V617F ECs compared to JAK2^WT ECs (p = 0.003). MPL expression in JAK2^V617F ECs was shown as the relative ratio compared to its expression in JAK2^WT ECs which was set as “1”. (B) JAK2^V617F HSPC (Lin-cKit+) cell proliferation was significantly decreased when co-cultured with MPL^−/− EC, compared to when co-cultured with JAK2^WT ECs or JAK2^V617F ECs. Cell proliferation in EC co-culture was shown as the relative ratio compared to cell proliferation in SFEM which was set as “1”. (C) Left: experimental design of an in vitro competitive growth assay where JAK2^WT HSPCs (CD45.1) and JAK2^V617F HSPCs (CD45.2) were cultured together (1:1 mix) in the presence of ECCM collected from either WT EC or MPL^−/− EC. Right: while there were equal numbers of JAK2^WT HSPCs (CD45.1) and JAK2^V617F HSPCs (CD45.2) in the presence of WT ECCM, there were significantly more JAK2^WT HSPCs than JAK2^V617F HSPCs in the presence of MPL^−/− ECCM. The results were expressed as mean±s.e.m. (n=3). Data were from one of two independent experiments (with triplicates in each experiment) performed by two investigators (C.L. and H.Z.) that gave similar results.