The expression and differentiation pattern of cell antigens and adhesion molecules on the nonadherent cell population in canine long-term marrow culture: a biphasic development of myeloid and lymphoid cells.

During maturation of normal hematopoietic progenitors, there appears to be differentiation-dependent expression of adhesion receptors, which may contribute to the homing and lodging of stem progenitor cells into the marrow and for the eventual release of mature effector cells from the marrow cavity. Using a model of long-term marrow culture, we studied the expression pattern of different lineage cell antigens and cell adhesion molecules on the nonadherent cells in canine long-term marrow culture. CD4+ cells became a major proportion of both the small and large cell subsets by day 8 of culture. Small CD4+ cells, the majority of which are negative for other T-cell antigens during the first 2 weeks of culture, express low levels of CD4 (CD4lo) and coexpress granulocytic and/or monocytic markers. These CD4lo cells have progenitor potential as measured by the long-term culture-initiating cell assay and differentiate into myeloid (first wave) and lymphoid (second wave) cells. The T cells, which appear in 2-week-old long-term marrow culture, respond to Con A but not to alloantigen. At the same time, most of the large cells are CD14+, CD11b+, DLA-DR+ and CD4lo+, while granulocytes are not observed. This phenotypic pattern closely resembles that found on myelomonocytic cells developing from fetal thymic and fetal liver CD34+ precursors in a model of human fetal thymic organ culture. The cell adhesion molecules--CD44, CD18, L-selectin, CD11a-c as well as VLAalpha4, dramatically increase from the first week of long-term marrow culture, while ICAM-1 and a new beta2 cell adhesion molecule, alpha dbeta2, increased slightly from the third week on. In the large ("monocytic") cell population, alpha dbeta2 was exclusively expressed by CD4+ cells. The differentiation pattern of T-cell antigens and adhesion molecules seen in the canine long-term marrow culture appear to mimic those required for developmental interactions between leukocytes and endothelial cells; that is, an early expression of L-selectin and CD44, followed by the integrins, and later on by ICAM-1 and alpha dbeta2. Our data support the view that in a model of canine long-term marrow culture hematopoietic precursors retain their intrinsic developmental potential.