Soluble factors secreted by naturally occurring suppressor cells that interfere with in vivo graft-vs.-host disease and with T cell responsiveness in vitro.

A potent immunosuppressive factor (SUF) is found in the supernatant of short-term cultures of unstimulated thymocytes or spleen cells of neonatal mice and rats and in culture medium of hybridoma cell lines established by fusing neonatal mouse spleen cells with T lymphoma cells (the BW 5147 line). In vitro incubation of spleen cells with SUF suppresses the acute in vivo graft-vs.-host disease, normally induced by allogeneic spleen cells in lethally irradiated mice. Incubation of bone marrow cells with SUF does not affect the hemopoietic stem cells. The addition of SUF to mixed lymphocyte reaction cultures strongly suppresses lymphocyte proliferation. The non-species-restricted inhibition of cell proliferation induced by SUF is shown not to be due to toxicity or nonspecific interference with DNA synthesis. Molecular size fractionation of crude SUF revealed two active moieties: a large moiety of molecular mass greater than 100 kDa and a small moiety of less than 3 kDa. The high kDa moiety mediates T cell unresponsiveness both in vivo and in vitro. In vitro studies revealed that this moiety primarily affects an early event in the proliferative response to alloantigen and mitogen, that prevents interleukin 2 (IL 2) receptor expression and, consequently, blastogenesis and DNA duplication. It does not affect, however, the synthesis of IL 2. The suppressive activity of the low kDa moiety can be demonstrated only in in vitro systems. Pre-treatment of donor lymphocytes with this fraction cannot prevent graft-vs.-host disease mortality. The inhibition of cell proliferation induced by this fraction in vitro is most likely due to interference with the utilization of IL 2, as suggested by its suppressive effect on the proliferation of CTLL-2 cells (an IL 2-dependent cell line).