Evidence for the in vivo production and release into the serum of a T-cell lymphokine, persisting-cell stimulating factor (PSF), during graft-versus-host reactions.

The T-cell lymphokine, persisting-cell stimulating factor (PSF or interleukin-3), was detected in the serum of mice undergoing graft-versus-host reactions (GVHR). Gel filtration under non-dissociating conditions indicated that the PSF in the serum had an apparent molecular weight of 34,000, a figure identical with that of PSF generated from activated T cells in vitro, indicating that PSF was not bound by serum proteins. The GVHR was accompanied by increases in the numbers in the bone marrow and spleen of precursors of PSF-dependent mast cells, and increases in the numbers of mast cells, megakaryocytes and immature and mature neutrophils in the spleen. These effects of GVHR on haemopoietic cells paralleled those seen when haemopoietic tissues were stimulated with pure PSF in vitro and closely resembled those induced in previous studies by the presence of a tumour that secreted PSF alone. These studies are the first to show that PSF can enter the circulation during immune reactions in vivo and suggest that much of the stimulation of haemopoietic cells seen in GVHR, can be accounted for by the release of PSF from activated T cells.