Suppression of lymphocyte alloreactivity by early gestational human decidua: I. Characterization of suppressor cells and suppressor molecules

Abstract

We examined the immunosuppressor role of the first trimester human decidua on lymphocyte alloreactivity in vitro in order to identify (1) the major cell classes in the decidua mediating the suppressor effect; (2) the stages in the lymphocyte alloreactive responses susceptible to the suppressor effects of the decidua; and (3) the precise nature of the suppressor molecules. Irradiated (2800 R), Ficoll-Paque-separated nucleated cells of the collagenase-dispersed early gestational (6.5–9.5 weeks menstrual age) decidua containing 70–94% typical decidual cells (identified on the basis of distinctive morphology and numerous cytoplasmic or surface markers) or their plastic-nonadherent fractions further enriched for decidual cells (~96% pure) caused a strong dose-dependent suppression of the one way mixed lymphocyte reaction (MLR, i.e., proliferative response measured on Days 3, 4, or 5), when added at the onset of the mixed lymphocyte cultures (MLC). As few as 10 3 decidual cells caused a detectable inhibition of the MLR exhibited by 10 5−1.5 × 10 5 responder lymphocytes. A smaller degree of suppression was noted with the plastic-adherent fractions of the early decidua (which retained all macrophages and granulocytes, but still included many decidual cells) or unfractionated cells of later gestational (10–13 weeks) decidua containing a higher incidence of leukocytes, granulocytes, and macrophages in particular, or the plastic-adherent fraction thereof, enriched for macrophages. Thus, decidual cells seem to represent an important suppressor cell class in the early gestational human decidua; however, suppression by decidual leukocytes, macrophages in particular, was also evident. The suppressor effect was unrelated to the major histocompatibility phenotype of the responder or the stimulator cells. It was not caused by cell crowding, since an equivalent number of irradiated K562 erythroleukemia cells had little effect on the MLR. The effect was exerted during both the initiation and the progression of the MLR. A delay in the addition of regulator cells progressively minimized the effect on the Day 4 MLR, but did not abolish it completely even when added as late as on Day 3. The major class of mediator molecules was identified as prostaglandins, primarily PGE2, on the basis of the following results: (1) the presence of indomethacin (10 5 M) or varying dilutions of an anti-PGE 2 antibody abrogated this suppression substantially or completely. (2) Addition of pure PGE 2 (3 × 10 −7to 1.1 × 10 −5 M), but not PGF 2α, reproduced a dose-dependent suppressor effect. (3) PGE 2 levels measured in the Day 4 MLC wells containing decidual cells were positively correlated with the decidual cell dose or the degree of suppression. Presence of decidual cells or PGE2 at the onset of the MLC also inhibited the generation of cytotoxicity (CTL activity, examined on Day 7) in the MLC wells in a similar manner. The decidua-mediated inhibition of CTL generation was substantially relieved with indomethacin and completely relieved with anti-PGE 2 antibody. These results strongly suggest a key immunoregulatory role of the human decidua for the protection of the semiallogeneic conceptus, mediated by a local release of PGE 2.