Amplification of the proliferative response to alloantigen by a factor present in an extract of syngeneic thymic lymphoid cells.

A synergistic interaction in the proliferative response to alloantigen has been previously noted when intact thymus cells are cultured with post-thymic (peripheral) lymphoid cells. In the present study, a factor extracted from the thymus has been shown to similarly enhance the reactivity of syngeneic lymph node cells and thus to retain the amplifier activity of intact thymus cells. The factor has no effect on lymphoid cell proliferation in the absence of alloantigen. Cells with amplifier activity are found in highest concentration in the thymus but also may be detected in spleen cells that are nonadherent to nylon wool. The factor is shown in these experiments to be derived from thymic lymphoid cells and to act primarily upon post-thymic (peripheral) lymphoid cells. As such, this factor appears to be distinct from various other thymus factors that have been localized to thymic reticuloepithelial elements and that are thought to effect predominantly the differentiation of T-cell precursors.