Human Hematopoietic Regeneration Dynamics Following Highly Intense Cytotoxic Perturbations

Eliezer Shochat, Salomon M. Stemmer, Lee Segel

Hematopoesis is comprised of multiple stages, originating from pluripotent stem cells through intermediate progenitors to mature differentiated cells. Consequently, during the development of blood cells numerous sites are potentially exposed to the intense perturbations induced by anticancer chemotherapy. In particular, high dose chemotherapy (HDCT) employed in the treatment of selected malignances followed by autologous PBSC transplantation and G-CSF support introduces a violent perturbation to the hematopoietic system. However, little is known about human hematopoietic regeneration dynamics following such intense cytotoxic perturbations. Here we define a new detailed mathematical model of hematopoesis that reconstruct the complex in-vivo dynamics of hematopoietic populations, including the elusive pluripotent stem cells. This enables us to estimate the in-vivo density of the stem cell and progenitor content and their activation rate in human bone marrow prior to and following these highly cytotoxic treatments.

The direct and detailed observation of the intrinsic bone marrow kinetics with multiple and frequent bone marrow biopsies has a very low yield during the acute regeneration phase and is clinically unjustified. Yet, the readily available peripheral blood count kinetics may also reflect on the underlying bone marrow stem cell and progenitor behavior during these perturbations. Thus to calibrate the model we use extensive clinical data of the frequent routine examinations of the peripheral blood counts during the transplant setting. The bone marrow kinetic parameters were estimated by using white blood cell counts routinely collected in patients during high dose chemotherapy (HDCT) followed by autologous peripheral blood stem cell (PBSC) transplantation and granulocyte colony stimulating factor (G-CSF) injections. Studying the model performance under a wide variety of parameter values reveals that bone marrow is surprisingly robust in the physiologically feasible parameter space.

We infer that the human hematopoietic pluripotent stem cell density is approximately 1 in 2*10^5 mononuclear cells and that most of these cells are quiescent, dividing once in 3-4 weeks. This low steady state activation rate may have implications for effective stem cell gene therapy. In addition, by utilizing knowledge of the quantity of CD34+ cells that were infused to the patients we extrapolate the concentration of the true stem cells in the autologous graft. Our results suggest a significant regenerative capacity of the re-infused transplant (stem cell density of 1/300 of CD34+ cells) which contributes to both the long-term marrow re-population as well as to short-term support. The proposed model gives indication of the bone marrow dynamics over a wide range of perturbations caused by clinical interventions. It suggests that in most patients, the pluripotent population recovers within 4 months following HDCT. In addition it directs to possible effects of G-CSF manipulation and of ex-vivo graft expansion in improving transplantation procedures.