• Immune surveillance;
  • implantation;
  • natural killer cells

PROBLEM: The immunomodulatory effect of progesterone (P) in pregnancy manifested via a protein named the P-induced blocking factor (PIBF) was previously reported. The goal of this study was to measure and compare the PIBF expression on lymphocytes between pregnant and non-pregnant women especially in early pregnancy.

METHODS: PIBF expression was determined by immunocytochemistry using a PIBF-specific polyclonal antibody. Levels were assessed during the mid-cycle, luteal phase, and first trimester of pregnancy.

RESULTS: PIBF expression was found in 24.9% of mid-cycle sera, 49% of luteal phase sera of women who failed to conceive, and 75% of luteal phase sera of women who conceived.

CONCLUSIONS: These data indicate that the percentage of PIBF expressing lymphocytes increases as a result of pregnancy and that the stimulus for PIBF induction occurs soon after implantation. These data support the concept that PIBF may play an important role in early implantation possibly by inhibiting the destructive function of natural killer lymphocytes.