Mouse embryonic cells become susceptible to CTL lysis after midgestation.

The susceptibility of murine embryonic cells to CTL lysis has been assessed. Zona-free blastocysts were not lysed during 6 hr incubation with CTL sensitized against the embryonic genotype and remained capable of implantation and normal development in vivo as well as outgrowth in vitro. Freshly dissociated cells from postimplantation embryos before the 13th day of gestation were not lysed in a 51Cr-release assay, but cells from 13- to 16-day embryos were susceptible to lysis. Cells from Day 12 embryos cultured for 40 hr in the presence or absence of interferon became susceptible to CTL lysis. Cultured cells from Day 10 embryos were only lysed following culture in medium containing interferon. Cell populations lysed in the CTL assay remained resistant to lysis by NK effector cells. These experiments indicate that the murine fetus is protected from the maternal immune system at least during the first half of pregnancy by an intrinsic ability of early embryonic cells to resist cell mediated lysis. Suppression of maternal CTL responses may not be essential for fetal survival until later stages of gestation.