Intravenous administration of splenocytes in early pregnancy changes the implantation window in mice

To clarify the role of immune cells in the reproductive phenomenon during early pregnancy, we investigated the effect of splenocytes obtained from mice in early pregnancy on embryo implantation. We prepared splenocytes from 5 week old pregnant ICR mice on day 4 (pregnancy day 4 splenocytes; P4-spl) and day 8 (pregnancy day 8 splenocytes; P8-spl), from 5 week old pseudopregnant ICR mice on day 4 (pseudopregnancy day 4 splenocytes; PP4-spl) and from 5 week old di-oestrous virgin mice (di-oestrous splenocytes; Di-spl). The supernatants (P4-sup, P8-sup, PP4-sup and Di-sup) derived from the suspensions of P4-spl, P8-spl, PP4-spl and Di-spl respectively, were used in the control groups. The recipient pseudopregnant mice (6 weeks of age) were injected i.v. with splenocytes (2 x 10(7) cells) or supernatants as controls, and embryo transfer was performed on day 2. Embryo implantation was evaluated 7 days later under laparotomy. The successful implantation rate per embryo was markedly higher compared with the control groups in the P4-spl (30.5 +/- 8.55 versus 1.0 +/- 1.37%, n = 200, P < 0.01) and P8-spl (20.1 +/- 10.3 versus 1.17 +/- 1.62%, n = 200, P < 0.05) groups. On the other hand, a slight but not significant increase was observed in the PP4-spl (8.33 +/- 8.80% versus 1.5 +/- 1.37%, n = 200) and Di-spl (9.0 +/- 5.76% versus 2.3 +/- 2.28%, n = 200) groups. Among the splenocyte administration groups, the implantation rate in the P4-spl group was significantly higher than in the PP4-spl and Di-spl groups (P < 0.01). These findings indicate that i.v. administration of splenocytes can change the murine implantation window. Since the splenocytes during early pregnancy are most effective on embryo implantation, peripheral immune cells in early pregnancy may be involved in embryo implantation.