Thymic influence of proliferation and interaction of fractionated spleen cells of mice.

Cells from the spleens of mice were separated into seven subpopulations by centrifugation on a discontinuous density gradient of dextran. The lightest fraction (containing stem cells), the three heaviest fractions (containing small lymphocytes), or a mixture of these, were injected into thymectomized or sham-thymectomized irradiated mice together with sheep red blood cells as antigen. After 8 days, the spleens of treated mice were tested for their content of cells lysing sheep red blood cells in agar (PFC) or cells mounting graft-versus-host reaction in newborn mice (GVHR). Heavy fractions were depleted of progenitors of PFC, but contained progenitors of cells mounting GVHR. The light fraction was depleted of both. If light cells were given together with heavy ones, the potential to generate PFC was restored, but only if the recipients were without their thymus; by contrast, the potential to generate GVH-reactive cells was strongly suppressed in the absence of a thymus. It is concluded that, in this model, stem cells probably interact with immunocompetent cells, and that the interaction depends on the thymus.