This study demonstrated that normal pregnancy is associated with activation of the plasma complement system as determined by increased maternal plasma C3a, C4a and C5a concentrations (). In addition, the concentrations of these anaphylatoxins did not change with advancing gestational age (from 20 weeks to term gestation) ().
The determination of plasma anaphylatoxin concentration was used, since these molecules are more sensitive markers of complement activation in vivo than total hemolytic complement activity (CH50) or native complement proteins.[9
] Previous studies have demonstrated that the concentration of complement proteins or CH50 in maternal blood during normal pregnancy is increased.[32
] Moreover, Johnson et al.[41
] reported that the concentrations of some complement proteins (C2, C4, C3, C5, C6, and Factors B and H) were increased during pregnancy, while others remained unchanged (C1q, C1r and P). However no study has compared the plasma anaphylatoxin concentrations of non-pregnant and normal pregnant women. Although Haeger et al.[42
] reported higher plasma C3a and C5a concentrations in preeclamptic patients than in normal pregnant women, no comparisons were performed between normal pregnant and non-pregnant women.
The hormonal regulation of complement tissue expression has been assessed in the endometrium of humans[43
] and mice.[44
] Hasty et al.[43
] determined that human endometrial C3 expression and synthesis was up-regulated during the luteal phase of the menstrual cycle. In contrast, in animal models, the up-regulation of C3 appeared to be influenced by estrogens.[44
] The increment in systemic anaphylatoxins during gestation could be attributed, in part, to a stimulation of their precursors by high levels of steroid hormones. Futures studies are required to evaluate the relationship between systemic complement activation and endocrine factors.
The plasma concentration of C1 esterase inhibitor (C1INH), which regulates the activation of the first component of the classical pathway, has been reported to be either low[45
] or unchanged[40
] during normal pregnancy. Its reduction may lead to the activation of the complement cascade and subsequent elevation of C3a, C4a and C5a in maternal plasma. Furthermore, a decrease in the expression of complement receptor 1 (CR1) and DAF on red blood cells has been observed during normal pregnancy.[48
] These findings could also explain high levels of complement split products in maternal plasma.
C5a, the most potent anaphylatoxin, can induce oxidative burst in neutrophils and stimulate oxygen radical species production,[49
] enhance phagocytosis,[49
] chemoattract granulocytes,[7
] and reduce neutrophil apoptosis.[53
] Interestingly, these findings have been described in normal pregnant women.[26
] Activated phagocytic cells have the ability to cleave C5 into C5a and C5b fragments.[55
] Vogt et al. showed that oxidants generated by PMN myeloperoxidase and kallikrein activity resulted in the cleavage of C5 and subsequent production of C5a.[56
] Additionally, the enzymatic activity of neutrophil elastase[57
] and macrophage serine protease[55
] can also lead to the generation of this anaphylatoxin. Considering that pregnancy is associated with an increase in the white blood cell count,[25
] as well as “activation” of granulocytes and monocytes,[26
] these may, at least in part, explain the increased C5a concentrations.
It is unknown when complement split products C3a, C4a and C5a begin to rise during normal gestation. However, our results indicate that their plasma concentrations remain unchanged from 20 weeks of gestation to term. Previous studies by Baines et al.[32
] and Kitzmiller et al.[34
] reported no difference in plasma CH50 concentration in the first trimester when compared to those of non-pregnant women. Stabile et al.[60
] did not observe a correlation between maternal complement protein concentrations (C3, C4, factor B) and gestational age during the second trimester, but the interval period of the study was short. Kovar et al.[61
] reported similar results during the third trimester. The weak positive correlation between plasma concentration of C3a and the other two anaphylatoxins studied (C4a and C5a) support the view that not all C3a peptides are generated after C4, and that other routes of complement activation may be involved (i.e., alternative pathway). Similarly, other factors besides C3 activation may be involved in the production of C5a, as previously described.[55
In conclusion, our study demonstrated that normal pregnancy is associated with increased maternal plasma C3a, C4a and C5a concentration. The activation of the complement system constitutes further evidence of activation of the innate immune system during normal pregnancy. We propose that complement activation may compensate for the decreased adaptive immunity observed in normal pregnancy, and is aimed to protect the host (mother and/or fetus) from microorganisms and other potential antigens.