Prednisolone reduces preconceptual endometrial natural killer cells in women with recurrent miscarriage. 
To test the hypothesis that high numbers of uterine natural killer (uNK) cells in the endometrium of women with recurrent miscarriage (RM) could be reduced with prednisolone.  A before and after study.  A tertiary referral teaching hospital.  Eighty-five women with idiopathic RM recruited from all over the UK and 18 women attending for sterilization (controls).  An endometrial sample was taken on day 21 +/- 2 of the menstrual cycle.  Immunohistochemistry was used to identify uNK (CD56+, CD16-, CD3-).  Twenty-nine women with RM and >5% uNK agreed to take 20 mg oral prednisolone daily from day 1 to 21 of their menstrual cycle, when a second biopsy was obtained and analyzed.  The percentage of stromal cells that were uNK.  The normal range was defined using control samples as <5%.  Women with RM had significantly more uNK than the controls (P=.008).  Prednisolone treatment significantly reduced the number of CD56 cells in the endometrium, from a median of 14% (before) to 9% (after) (P=.0004).  We have demonstrated that high numbers of uNK in preimplantation endometrium of women with recurrent miscarriage can be reduced with administration of prednisolone.