SPECIFICITY OF HEMATOPOIETIC STEM CELL HOMING

Homing of hematopoietic stem cells (HSC) may be defined as the cells’ ability to seek marrow stroma selectively, to interact with it and subsequently to lodge within it to initiate hematopoiesis. This complex process is no doubt mediated through multiple recognition/adhesion events. Homing may proceed through one of several alternative mechanisms, however, such as through physical trapping of stem cells by marrow ultrastructural elements or through the providing of a selective survival and/or proliferative advantage by marrow. A third alternative that provides for the central element of stem cell homing—its high degree of specificity—is through the action of a specific homing protein in HSC. There are data to support this latter mechanism of stem cell homing as the correct one, and the nature of this protein may be similar to that of the lymphocyte homing receptors that are lectin‐like molecules. Lectin–carbohydrate interactions are known to provide enormous specificity to cell recognition processes and to participate in cellular targeting. Leukemic cells have recently been demonstrated to home to marrow stroma and proliferate in the same way as normal stem cells. Thus, identification of proteins or other adhesion molecules that participate in normal and malignant cell homing could lead to more specific recruitment regimens for tumour‐free collections.