Natural suppressor cells in spleens of irradiated, bone marrow-reconstituted mice and normal bone marrow: lack of Sca-1 expression and enrichment by depletion of Mac1-positive cells.

We have recently reported the development of natural suppressor (NS) cells in lethally irradiated, bone marrow-reconstituted mice during the early weeks after bone marrow transplantation (BMT). These cells were shown to be derived primarily from the syngeneic marrow component in recipients of mixed allogeneic plus syngeneic (host type) marrow, and it was speculated that they might be responsible for the anti-GVHD effect previously described for T-cell-depleted syngeneic marrow. It was therefore of interest to look for such suppressive activity in normal adult bone marrow, which might serve as an obtainable source of such cells if they were to be isolated and used clinically. Such activity has indeed been found in normal adult bone marrow and its characteristics compared to that in spleens of early BMT recipients. Suppressive cells from both sources were similar in their specificity patterns and radiosensitivity, and were of the null (i.e., non-T, non-B, nonmacrophage) cell phenotype. Suppression from either source can be enriched by removal of Mac1-positive cells, providing a possible approach to obtaining NS-enriched populations for in vitro expansion and adoptive transfer studies. Such depletion of Mac1-positive cells was associated with a threefold enrichment of Thy1-positive cells, of which one half were CD4- and CD8-negative, similar to the reported phenotype of cultured NS cell lines. Even when enriched in this manner, the contribution of Thy1-positive cell populations did not reach statistical significance. A recent report has suggested that NS cells might actually be pluripotent hematopoietic stem cells. In contrast, we report here that depletion of Sca1-positive pluripotent hematopoietic stem cells with monoclonal antibody plus immunomagnetic beads does not remove NS activity.