Problem: To determine if pregnancy is recognized by the immune system and if inadequate recognition of fetal antigens might result in failed pregnancy.
Method of study: Review of literature and current data.
Results: In the decidua gamma/delta TCR positive cells significantly increase in number. A subset of gamma/delta T cells reacts with nonpolymorphic Class I or Class I like molecules. Trophoblast recognition is mediated by the V gamma 1 subset which recognize a conserved mammalian sequence on the trophoblast. Almost all gamma/delta T cells in the decidua are activated and use the V delta 1 chain, whereas the majority of human peripheral gamma/delta lymphocytes expresses V gamma 9/V delta 2 TCR. Peripheral gamma/delta T cells of healthy pregnant women preferentially use V gamma V delta 1 chains, on the other hand, those of recurrent aborters use the V gamma 9V delta 2 combination. Signaling via the V gamma 1.4V delta 1 receptor induces a Th2 type response, whereas activation of the lymphocytes via the V gamma 9V delta 2 receptor results in increased IL-12 production and natural killer (NK) activity. In the presence of progesterone, activated lymphocytes synthesize the progesterone induced blocking factor (PIBF), which inhibits NK activity and exerts an anti abortive effect in vivo. Decidual CD56+ and gamma delta+ cells are to a high extent the same population.
Conclusion: All decidual CD56+ cells express PIBF, thus it cannot be excluded that local production of this substance contributes to low decidual NK activity and thus to the success of the pregnancy.