Interaction of Anti-thymocyte Serum with Haematopoietic Stem Cells II. STIMULATION OF COLONY FORMATION <em>IN VITRO</em>.

When mouse haematopoietic cells are incubated in vitro in appropriate dilutions of horse anti-mouse thymocyte serum or globulin (HAMTS or HAMTG), the number of in vitro colonies formed by such cells is increased. There was a good correlation between the immunosuppressive potency and the colony-enhancing property of a given serum preparation. Correlation between the thymocytotoxicity titre and the colony enhancing property of the sera was poor. For colony enhancement, it was found necessary to have an added source of colony stimulating factor in the medium, whether in the form of mouse serum or human urine. Unlike the effect produced by exposure of bone marrow cells to certain antigens, HAMTG or HAMTS enhanced colonies in the absence of the alpha-globulin component of mouse serum. Thymus-derived immunocompetent cells are not involved in enhancement since this effect was also observed using foetal liver cells. It appears likely that the interaction of an antibody on the surface of some cell or cells involved in vitro colony formation permits a larger fraction of the colony-forming cells to proliferate in vitro. [ABSTRACT FROM AUTHOR]