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Problem:  One immunoregulatory pathway that has received little attention is placental exosome release. In normal pregnancy, as factors linked with early immunomodulation decline, placental exosomes become key in modulating T-cell activation, maintaining the absence of effector T-cells by enhancing T cell apoptosis and loss of CD3-z.

Method of Study:  Placental exosomes were isolated from the maternal peripheral circulation by a chromatographic/MACS procedure developed to specifically purify exosomes of placental origin. Exosomal FasL was identified by immunoelectron microscopy (IEM) and quantified by ELISA. Exosomal suppression of T cell signaling molecules and induction of apoptosis was determined by flow cytometry and DNA fragmentation, respectively. The role of FasL in these events was defined by use of FasL blocking antibody. The differential effect of exosomes on CD3-z on T subsets was analyzed by western immunoblot (WB).

Results:  When T cells were co-incubated with placental exosomes, CD3-z was down-regulated, with placental exosomes treated cells expressing 18,900 ± 7,000 MESF units of zeta, while control fraction-treated cells expressed 49,000 ± 4,800 MESF units of zeta (P < 0.001). This down-regulation of CD3-z was partially reversed by pre-incubating T cells with ZB4 antibody, suggesting a role for FasL. IEM revealed FasL-antibody complexes located on the exterior of placental exosomes; however, only 20/27 preparations were FasL+. When placental exosomes were added to T cells, the level of CD3-z expression on CD8+ cells was 7.1%, (8.7% vs 59.7%) inhibited 1.43-fold more than in CD4+ cells (85.5 P < 0.01). On CD4+CD25+ cells, CD3-z was not significantly inhibited. (P < 0.05).

Conclusions:  While all term placental exosome preparations tested induced apoptosis, exosomes obtained from 7/27 patients were negative for FasL but were able to induce DNA degradation. These results indicate that, while exosomal FasL is apoptogenic, additional exosome components are capable of inducing apoptosis, but these factors exhibit differential effect on T subsets.