The ability of cell fractions of mouse spleen to repopulate the hemopoietic tissue and to mount graft-versus-host reaction.

Mouse spleen cells were separated into seven fractions on a discontinuous gradient of dextran. The lightest, uppermost fraction contained colony-forming units and was capable of repopulating the hemopoietic tissues damaged by irradiation. The fraction below it also contained the colony-forming units, but its repopulating ability was limited. Heavy fractions contained immunocompetent cells capable of mounting graft-versus-host reaction, but the performance of the cells depended on the type of assay, i.e. on whether the cells were assayed in irradiated or in non-irradiated recipients. It is concluded, firstly, that the technique used resolved the colony-forming units into two classes - those capable of self-renewal and those committed to differentiation - which were concentrated in different portions of the light part of the gradient; and secondly, that the cells implementing graft-versus-host reaction were probably also resolved into two classes - the inciting and the proliferating cells - which were concentrated in different portions of the heavy part of the gradient.