IL-7 activates the phosphatidylinositol 3-kinase/AKT pathway in normal human thymocytes but not normal human B cell precursors

IL-7 signaling culminates in different biological outcomes in distinct lymphoid populations, but knowledge of the biochemical signaling pathways in normal lymphoid populations is incomplete. We analyzed CD127/IL-7Rα expression and function in normal (nontransformed) human thymocytes, and human CD19⁺ B-lineage cells purified from xenogeneic cord blood stem cell/MS-5 murine stromal cell cultures, to further clarify the role of IL-7 in human B cell development. IL-7 stimulation of Cl)34⁺ immature thymocytes led to phosphorylation (p-) of STATS, ERK1/2, AKT, and glycogen synthase kinase-3 β, and increased AKT enzymatic activity. In contrast, IL-7 stimulation of CD34^- thymocytes (that included CD4 ⁺/CD8⁺ double-positive, and CD4⁺ and CD8 ⁺ single-positive cells) only induced p-STAT5. IL-7 stimulation of CD19⁺ cells led to robust induction of p-STAT5, but minimal induction of p-ERKl/2 and p-glycogen synthase kinase-3 β. However, CD19⁺ cells expressed endogenous p-ERKl/2, and when rested for several hours following removal from MS-5 underwent de-phosphorylation of ERK1/2. IL-7 stimulation of rested CD19⁺ cells resulted in robust induction of p-ERKl/2, but no induction of AKT enzymatic activity. The use of a specific JAK3 antagonist demonstrated that all IL-7 signaling pathways in CD34⁺ thymocytes and CD19⁺ B-Iineage cells were JAK3-dependent. We conclude that human CD34⁺ thymocytes and CD19⁺ B-lineage cells exhibit similarities in activation of STATS and ERK1/2, but differences in activation of the PI3K/AKT pathway. The different induction of PI3K/AKT may at least partially explain the different requirements for IL-7 during human T and B cell development.