|Ph.D Student||Dowery Reem|
|Subject||Homeostatic Mechanisms Controlling B Lymphopoiesis in|
|Department||Department of Medicine||Supervisor||Professor Doron Melamed|
|Full Thesis text|
Generally, the number of peripheral B cells does not change significantly throughout life, as generation of new B cells in the Bone marrow (BM) is balanced by death in the periphery, a process refers to as cellular homeostasis. This balance between cell input and output within the B cell compartment is important for the integrity of the organism and for mounting an effective immune response. Yet, cellular homeostasis adapts to physiological changes. A good example for such adaptation is the dramatic changes in the B cell compartment that occur with aging, where B cell production declines and memory B cells accumulate in the periphery. So far, the mechanism by which cellular homeostasis in the B lineage is established in aging is still unknown.
Previous studies in our lab have demonstrated that removal of peripheral B cells reactivates B lymphopoiesis in the BM and rejuvenates the peripheral compartment in old mice and humans. These studies suggested the existence of a feedback mechanism by which peripheral B cells suppress B lymphopoiesis in aging. In the present study we aim to identify molecular mechanisms and/or molecules that mediate this cross-talk mechanism. To test this, we used an in vitro bone marrow culture system to grow B lineage cells from progenitors. We found that B cells purified from old mice suppressed B lymphopoiesis when cultured in mixed-culture or in transwell system. To support this, we transferred old splenic B cells into young hCD20Tg mice that were treated for B cell depletion. Bone marrow analysis for B lymphopiesis revealed a significant suppression relative to hCD20Tg mice that were injected with young splenic B cells. Furthermore, we found that B lymphopoiesis is effectively suppressed in the presence of sera from old mice. These findings suggest that old B cells inhibit B lymphopoiesis and that this inhibition is mediated by a soluble factor in blood.
Using proteomic analysis of human plasma samples we identified Insulin-like Growth Factor-1 (IGF-1) as a potent mediator for such a cross-talk. Our in vitro and in vivo experiments reveled that IGF-1 is capable to stimulate B lymphopoiesis in aged mice.
In further studies we show that peripheral B cells in aged mice and humans regulate IGF-1 levels through the secretion of TNF-a, which stimulates production of IGF-binding protein-1 (IGFBP-1). Thus, TNF-a levels, which increase in aged mice and humans, drop in old-depleted mice and humans to levels that are found in the young counterparts. Lastly, we found that IGFBP-1 levels in blood increase in mice and human with aging but decrease after B cell depletion. Hence, our proposed model is that in aging, old peripheral B cells secret high levels of TNF-a that stimulate the liver to secrete more IGFBP-1. IGFBP-1 binds more IGF-1 to sequester its activity, eventually resulting in a low B lymphopoiesis in BM.
Collectively, our results suggest that in aged individuals B lymphopoiesis is subjected to homeostatic feedback mechanisms imposed by mature B cells in the peripheral compartment, and that this feedback is mediated by a TNF-a/IGFBP-1/IGF-1 axis.