IMMUNOACTIVE PRODUCTS OF PLACENTA .5. IMMUNOREGULATORY PROPERTIES OF A LOW-MOLECULAR-WEIGHT COMPOUND OBTAINED FROM HUMAN PLACENTAL CULTURES

PROBLEM: We have previously shown that supernatants from short-term cu ltures of human placental explants (HPS) are immunosuppressive in vitr o as well as in vivo. They contain a low M.W factor endowed with immun oregulatories activities (Filtrate of such with a 5 kDa cut off). In t his paper, we wanted to assess whether this low M.W. material accounts for most, if not all, of the immunosuppressive properties of crude HP S and begin to investigate its mode of action. RESULTS : The filtrate is active across species barrier and inhibits human and murine PHA dri ven lymphocyte proliferation, Mixed Lymphocyte Reaction, and Natural K iller activity as did crude TIPS. It does not affect CTL lytic functio n at effector stage. Its cross species activity allowed us to study it s effects in vivo. It corrects resorbtions in the CBA x DBA/2 murine s pontaneous abortion model, and suppresses local and general GVH reacti ons in a model (A cells into irradiated A x B Fls) relevant to a clini cal use, e.g., bone marrow transplantation. To ensure that such surviv al of the recipients was due to donor cells in the latter, surviving e xperimental animals were analysed by FAGS for repopulating lymphocytes phenotype, which was indeed of donor origin. To elucidate the mechani sm(s) of action of the active TIPS moiety, we first tested various mal ignant cell lines for the minimal incubation time required for maximal lymphocyte inhibition. In the same vein, we verified that lymphocytes stimulated by PHA and simultaneously treated with filtrate were unres ponsive to a second PHA challenge. The effects of the material was rev ersible if cells were washed out of it early enough before otherwise e ntering a cycle leading ultimately to cell death in vitro. Finally, we tested several second messenger pathways, none of which were modified . CONCLUSION: These data suggest that the filtrate contains an entity that represents the main, if not all, the immunosuppressive molecules present in TIPS. In addition, they suggest that the material acts only on activated T cells and requires to be present early in the replicat ion activation cycle. Altogether, the in vitro data strongly suggest t hat the material is acting by inducing clonal deletion in activated (T ) cells.