Abstract
As we have shown previously, cocultures of bone marrow cells with large numbers of syngeneic thymocytes enhance erythroid colony formation in plasma clots, whereas cocultures with low numbers of thymocytes suppress erythroid colony formation. In this article, we present evidence that the enhancing and suppressing functions of thymocytes are most likely mediated by at least two separate subpopulations. When thymocytes were fractionated on the basis of cell surface density of the theta antigen, enhancing cells were limited to the high theta density fraction, whereas suppressing cells were accumulated in the low theta density fraction. When thymocytes were fractionated on the basis of negative surface charge, the enhancing cells were recovered among the more negatively charged cells. A short in vitro incubation with polyamino acids selectively abrogated the suppressor function.
