Beverly Torok-Storb

Regulation of hematopoiesis.

The long-term maintenance of blood cell production requires that a balance between stem cell replication and differentiation be maintained regardless of the physiological demands for mature, functional cells. Even in cases of extreme hematopoietic demand, exemplified by total body irradiation and stem cell transplantation, a cohort of stem cells will remain undifferentiated, yet presumably divide in order to replenish the stem cell pool. Although the mechanisms responsible for preventing stem cell differentiation have not been identified, studies conducted to understand this phenomenon suggest that regulatory signals involve cellular interactions with direct cell-cell contact as well as indirect interactions mediated by factors or matrix, both of which occur within the marrow microenvironment (ME).

For several decades, primary long-term cultures (LTC) of marrow stroma have been used to approximate the ME. The functional complex as defined by LTC consists of several heterogeneous populations of cells including endothelial cells, fibroblasts, fat cells, adventitial reticular cells, and macrophages. A properly functional LTC is capable of maintaining a population of stem cells for several months and at the same time supporting proliferation and at least myeloid differentiation. How this is accomplished is not clear.

One goal of our laboratory is to identify the functional components of the ME and identify activities that regulate stem cell function. To achieve this goal, we have generated immortalized cloned stromal cell lines that represent functionally distinct cell populations. Comparative analyses of known and unknown genes expressed by these different cell lines have identified molecules that may be associated with these distinct functions. By generating antibodies that block or peptides that mimic these molecules, we are now in a position to test their effects on stem cell proliferation and differentiation.

The assays developed to evaluate ME function have been applied to an analysis of marrow function in patients following stem cell transplantation. These studies have led to an appreciation of cellular interactions that facilitate engraftment, and a better understanding of how this process can be compromised. We now know that in addition to allograft rejection, marrow failure can occur as a consequence of viral infections of critical ME components or graft-versus-host reactions against host-derived ME cells. The fact that stromal cells of the marrow microenvironment are not derived from donor stem cells has remained controversial. However recent data indicate that even decades after stem cell transplantation these cells are host in origin, an observation that calls into question the much proclaimed 'plasticity' of adult stem cells.