Marrow cell suspensions of unprimed donor mice have been transplanted into X-irradiated syngeneic hosts. 5–46 days later, bone cavities and spleens contained regenerated cells of the immune system which required interaction with thymocytes (from intact donors) and antigen (SRBC) to form antigen-sensitive units (ASU) and to generate mature immunocytes. These cells were capable of differentiating either into direct or indirect hemolytic plaque-forming cells (PFC). The precursors of PFC regenerated earlier than the other cell type necessary for immunocompetence, the antigen-reactive cell (ARC). The latter was not found until 10 or more days after transplantation. Availability of ARC was inferred from PFC responses elicited by grafted mice challenged with SRBC at varying intervals.
In a second series of experiments, graded numbers of marrow cells (ranging from 107 to 5 x 107) were transplanted with 5 x 107 or 108 thymocytes into irradiated mice, and SRBC were given 18 hr later. After 9–12 days the recipient spleens contained all or some of the following immunocytes: direct and indirect PFC, and hemagglutinating cluster-forming cells. The frequency of each immune response varied independently of the others, but in relation to the number of marrow cells grafted. This was interpreted to indicate that ASU formed in irradiated mice by interaction of marrow and thymus cells were similar to those of intact mice. In particular, they were specialized for the molecular class (IgM or IgG) and function (lysis or agglutination) of the antibody to be secreted by their descendent immunocytes. Hence, class-differentiation appeared to be conferred upon ASU by their marrow-derived components.