Immunosuppressive effect of serum progesterone during pregnancy depends on the progesterone binding capacity of the lymphocytes.

Cytotoxic activity and progesterone binding capacity of the lymphocytes, together with serum progesterone concentrations, were determined in women with normal pregnancy or with a clinical diagnosis of threatened abortion or threatened premature labour. The lymphocytes of women with threatened abortion or threatened premature labour showed significantly higher cytotoxic activity (P less than 0.001) and significantly lower progesterone binding capacity (P less than 0.001) than did lymphocytes obtained from the healthy pregnant women. Significant inverse correlation was found between progesterone binding capacity and cytotoxic activity of the lymphocytes (P less than 0.001), but the progesterone concentration of the pregnancy serum appeared to have no influence on the other two parameters. The findings indicate that intact progesterone binding capacity of the lymphocytes is an essential factor for the manifestation of the blocking effect exerted by pregnancy serum on lymphocyte cytotoxicity in vitro.