The role of IL-7 on early B cell development in mouse bone marrow

The last decade has witnessed a marked increase in our understanding of early B cell development. However, several important issues regarding the earliest stages at which interleukin (IL)-7 acts and its precise functions in regulating early B cell development are still unclear. In this thesis project, IL-7 KO mice were used as the main animal model with which to answer these questions. The results showed that early B-lineage development in BM of IL-7 KO mice is blocked at the transition between late pro-B and early pre-B cells. in contrast the generation of pre-pro-B cells and pro-B cells appeared to be normal, as judged by total cell numbers, proliferative indices, D-JH and V-DJH gene rearrangements, and mRNA for RAG-1, RAG-2, TdT, Igμ, λ5 and VpreB. However, pro-B cells from IL-7 KO mice failed to up-regulate the expression of cμ, TdT and IL-7Rα chain. Similar defects were present in γc and Jak3 KO mice but not λ5 or (excluding cμ) RAG-1 KO mice. Consequently, the normal development of pro-B cells requires signaling through the high affinity IL-7R, but not the pre-BCR. Furthermore, the defects in pro-B cell development in IL-7 KO mice could be corrected by in vivo reconstitution with rIL-7. This treatment also enabled pro-B cells from IL-7 KO mice to generate pre-B cells and immature B cells in vivo in approximately 7–12 days, and to respond to IL-7 in vitro. However, IL-7 by itself had none of these properties in vitro unless additional signaling from a BM stromal cell layer or conditioned medium (CM) therefrom was provided. In each instance, the expression of IL-7Rα was up-regulated. Most importantly, purified PPBSF (a heterodimeric complex of IL-7 and a 30 KDa stromal cell-derived cofactor) from CM could prime pro-B cells from IL-7 KO mice to respond to rIL-7 in vitro. Thus, these results strongly indicate that IL-7 is necessary but not sufficient for early B-lineage development in IL-7 KO mice. Rather, PPBSF appears to be the operative form of IL-7 that regulates B-lineage development from the earliest committed progenitor to the “transitional” pro-B cell stage.