Lack of the BH3-Only Proteins Bim, Bmf and Puma In Haematopoietic Stem and Progenitor Cells Facilitates Early Reconstitution and Long Term Haematopoiesis.

Abstract 1542

During haematopoietic stem cell transplantation (HSCT) the transplanted haematopoietic stem and progenitor cells (HSPC) suffer a transient deprivation of survival signals (i.e. cytokines or adhesion molecules) normally provided by the stem cell niche. Therefore, cell loss may occur prior to successful engraftment and thus restrict haematopoietic reconstitution.

Apoptosis induced in response to lack of cytokines or contact to the extracellular matrix is regulated by members of the Bcl-2 family. Bcl-2 family members can be divided into the pro-apoptotic ‘BH3-only’ proteins and anti-apoptotic Bcl-2 like proteins, the two subclasses antagonizing each other in their function. Using different gene-modified mouse models lacking the ‘BH3-only’ proteins Bim, Bmf or Puma, all implicated in leukocyte homeostasis, we aimed to delineate which one of these Bcl-2 family proteins is critically involved in limiting successful reconstitution of the haematopoietic system.

Our results demonstrate that HSPC lacking Bim show accelerated reconstitution of lethally irradiated recipient mice. Moreover, competitive reconstitution experiments reveal that wild type haematopoiesis is completely displaced in wt:bim^-/- and strongly suppressed in wt:bmf^-/- and wt:puma^-/- bone marrow chimeras. The effects of Bim are comparable to those observed upon induced overexpression of Bcl-2 and cannot be enhanced further by additional loss of Puma or Bmf, identifying Bim as the major BH3-only protein limiting cell survival during haematopoiesis. Since both lymphopoiesis and myelopoiesis are similarly affected, a direct role of Bim on early progenitor or even stem cells can be assumed. However, wild type lymphocytes are additionally outcompeted by bim^-/- and bcl-2 tg lymphocytes during differentiation, since the balance between Bim and Bcl-2 is critically involved in many selection processes occurring during a lymphocyte's life (i.e. negative selection in the thymus).

In summary, inhibition of apoptosis in HSPC by transiently interfering with Bim-function may be a promising strategy to increase the efficacy of HSCT and reduce transplantation-related morbidity.