78 INTERFERON-τ SECRETION OF CRYOPRESERVED BOVINE TROPHOBLASTIC VESICLES DERIVED FROM IN VIVO

Abstract

The co-transfer of bovine trophoblastic vesicles (bTVs) prepared from in vivo recovered conceptuses is known to promote the successful implantation of embryos, which expected lower viability, through the effects of interferon-τ (IFN-τ) secreted by bTVs. We have reported that the pregnancy rate was improved for co-transferred embryos with frozen-thawed bTVs using the direct-transfer technique (Hashiyada et al. 2008, 41st SSR). However, the IFN-τ secretion level from cryopreserved bTVs is not well known. The objective of the present study was to measure concentration of IFN-τ released from frozen-thawed bTVs individually cultured in vitro. bTVs were prepared from elongating blastocysts 3 to 20 mm in length, following superstimulatory treatment and recovered on Day 16 post-AI, by dissection using a surgical blade. Each trophoblastic fragment, 1 to 1.5 mm in width, was cultured in a well of a 96-well plate using TCM-199 supplemented with 20% (v/v) fetal bovine serum and 0.1 mM β-mercaptoethanol. Formed vesicles after 24 or 48 h of culture were cryopreserved using D-PBS supplemented with 20% calf serum, 1.5 M ethylene glycol (EG) and 0.1 M sucrose or 1.8 M EG. After thawing, each bTVs was cultured for 2 days to compare IFN-τ secretion between the 2 cryoprotectants. Furthermore, transition of IFN-τ level was assessed in continuous culture until Day 10 (the day of thawing was defined as Day 0). The volume of culture medium was 100 μL well^–1 day^–1 until Day 2 and thereafter changed to 200 μL well^–1 day^–1 until termination. Exchange and collection of culture media were performed on Day 1, 2, 4, 6, 8 and 10. Collected culture media were stored at –30°C until use. IFN-τ was measured by RIA (Takahashi et al. 2005 Theriogenology 63, 1050–1060). Data were analysed by Student's t-test. Initial IFN-τ secretion from bTVs before cryopreservation did not differ between 24 and 48 h of culture period to form vesicles, 44.0 ± 2.9 (mean ± standard error of the mean, n = 64) and 52.8 ± 6.4 ng mL^–1 (n = 27), respectively. IFN-τ secretion was no difference between the 1.5 M EG group and the 1.8 M EG group on Day 1 (41.2 ± 4.9 ng mL^–1, n = 42 and 30.4 ± 2.2 ng mL^–1, n = 31) and on Day 2 (38.0 ± 5.4 and 38.2 ± 4.5 ng mL^–1), respectively. In the continuous culture group (n = 28), IFN-τ secretion tended to increase from Day 2 (25.2 ± 3.4 ng mL^–1) to Day 4 (51.8 ± 12.3 ng mL^–1) and 6 (55.4 ± 13.3 ng mL^–1) (P < 0.05). However, this amount of IFN-τ on Day 6 significantly decreased on Day 8 (25.6 ± 2.7 ng mL^–1; P < 0.05) and Day 10 (15.5 ± 2.2 ng mL^–1; P < 0.01), gradually. These results indicate that cryopreserved bTVs could secrete IFN-τ at the same level as fresh bTVs on Day 4 to 6 after thawing and then these amounts of IFN-τ significantly decrease in vitro.