Progesterone-induced blocking factor inhibits degranulation of natural killer cells

PROBLEM: During the first trimester of pregnancy, nonclassical (CD3(-), CD5 6(+), CD16(-), perforin [p](bright+)) natural killer (NK) cells comprise th e major decidual lymphocyte population. These cells, in spite of their high perforin content, exert a low cytolytic activity. Peripheral blood lymphoc ytes of healthy pregnant women produce progesterone-induced blocking factor (PIBF), which inhibits NK activity. PIBF-producing cells are likely to be present in decidua and might contribute to low decidual NK activity. METHOD OF STUDY: Decidual cells obtained from elective pregnancy terminatio n were double labeled for CD56 and PIBF. We tested the effect of PIBF on pe rforin liberation by activated peripheral blood NK cells. RESULTS: Sixty percent of decidual lymphocytes were CD56 + and expressed PI BF at the same time. PIBF-treated and untreated peripheral blood NK cells w ere incubated with K-562 cells, and perforin content of target conjugated N K cells was detected with immunocytochemistry. PIBF treatment of peripheral blood lymphocytes significantly reduced lysis of K-562 cells. Among target bound lymphocytes in PIBF-treated samples, we found a significantly (P < 0 .01) higher rate of P+ cells than in untreated samples. CONCLUSIONS: These data suggest that PIBF inhibits cytotoxicity of NK cells via a block of degranulation, and since decidual NK cells are PIBF+, it ca nnot be ruled out that this effect of PIBF contributes to low decidual NK a ctivity.