B lymphocytes are pleiotropic cells belonging to the adaptive arm of the immune system. Although B cells were classically regarded for their capacity to produce antibodies, in the recent years, several other functions were attributed to these cells. B cells can uptake, process and present antigens as well as produce several cytokines that further influence immunity.Mammalian pregnancies represent a fascinating phenomenon in which the maternal immune system must be able to ‘tolerate’ the semi-allogenic fetus while simultaneously protecting the mother and the fetus against external pathogens. This requires a finely regulated balance between immune activation and tolerance. In this regard, B cells and the antibodies they produced were shown to actively participate in both, pregnancy well-being as well as pregnancy-associated pathologies.We discuss here the currently available information concerning the role of B cells in the context of pregnancy.
B lymphocytes can be defined as a population of cells that express clonally diverse cell surface immunoglobulin (Ig) receptors recognizing specific antigenic epitopes. B cell origin can be traced to the evolution of adaptive immunity in jawed vertebrates beginning more than 500 million years ago. Beside their essential role in humoral immunity, B cells regulate many other functions that are important for immune homeostasis. They contribute to the regulation, development, and activation of lymphoid architecture and produce cytokines that in turn regulate the function of dendritic and T-cells.[2, 3]
During mammalian pregnancy, the maternal immune system is challenged to create a transient status to the one hand tolerate the foreign-growing fetus and on the other hand be alert against pathogens. In this regard, pregnancy success depends on the appropriate regulation of cellular and humoral immune responses to fulfill both opposite needs.
In this review, we will focus on the current knowledge concerning the participation of known B cell sub-populations in pregnancy outcome.
B cell development and classification
B cells can be divided in two different sub-populations: B1 and B2 cells. These two main populations differ not only in their functionality but also in their ontogeny.
B-2 cells are continuously produced during postnatal life from precursors present in the bone marrow (BM); this process culminates in the generation of surface immunoglobulin M (sIgM)-expressing cells which then leave the BM, migrate to the spleen, where they undergo further maturation into follicular or marginal-zone B cells. Upon antigen exposure and signals from T-helper cells, follicular B cells can undergo Igs class switching, somatic hypermutation, and differentiation into plasma and memory B cells.
Unlike B2, B1 cells development occurs primarily during fetal and perinatal life from precursors present in the fetal liver. B1 cells can be further subdivided into B-1a and B-1b cells based on the expression of the cellular marker CD5. B-1a but not B-1b cells express CD5. For humans, Griffin and co-authors have recently extended the phenotype of B-1a B cells and showed that in addition to CD5, human B-1a B cells express CD27 and CD43.
When considering their localization, it should be noted that B1 cells are differently distributed when comparing humans with mouse. In the murine system, B1 cells are predominantly found in the peritoneal cavity but are also present in the pleural cavities, the spleen, and various parts of the intestine. In humans, unlike in the mouse, B1 cells mainly circulate in peripheral blood.
Beside their differences concerning cellular markers, B1 cells subsets (B-1a and B-1b) have also different functions within the immune system. While B-1b cells produce adaptive antibodies upon antigen stimulation, which are implicated in the generation of long-lasting immunity, B-1a cells produce the so-called natural antibodies. Unlike adaptive antibodies, produced mainly by B2 cells and in low proportion by B-1b cells, natural antibodies are produced in the absence of antigenic stimuli. Natural antibodies are characterized by their low-affinity, poly-reactivity, and self-reactivity. B-1a cells and their poly-reactive antibodies have been long suspected to be involved in the onset of autoimmune diseases.
B cells: Friend or foe to pregnancy?
The function of B cells in pregnancy has been so far indirectly approached by their capacity to produce antibodies. Thus, protective antibodies were shown to account for pregnancy well-being, while auto-antibodies were associated with pregnancy complications. We will recapitulate here the participation of B cells (‘the good’ and ‘the bad’) in the production of these antibodies affecting pregnancy outcome.
The good: B cells produce protective antibodies that help pregnancy to be established
The participation of protective antibodies, directed against paternal antigens during pregnancy, was a matter of extensive investigation in the field of reproductive immunology. Pioneer works during the 1970s have elegantly demonstrated the presence of anti-paternal lymphocytotoxic antibodies in normal pregnant women.[12-14] Taylor & Hancock first and then Kolb et al. have nicely showed that the cytotoxic effect of maternal lymphocytes on cultured trophoblast was completely prevented by the presence of maternal serum. Notably, if the immunoglobulin G (IgG) fraction was removed from the serum, the protective effect was significantly reduced. These experiments supported the idea that the presence of anti-paternal antibodies in maternal serum has a protective effect for pregnancy. Similarly, it has been proposed that the modulation of human leukocyte antigen (HLA) expression on trophoblast cells by cytotoxic antibodies may be the important factor in normal allogeneic pregnancies, rather than a cellular immunity blocking antibody. In this regard, Beard et al., have showed that the lack of maternal blocking antibodies directed to paternal HLA antigens is a dominant feature of recurrent spontaneous abortions (RSA), and this was further confirmed by other groups.
Simultaneously, in the 1970s, it has also been demonstrated that during the course of a humoral immune response, there is a proportion of antigen-specific IgGs that binds to antigens with relative high affinity, but lacks the capacity to form insoluble antigen-antibody complexes. These IgGs, called asymmetric antibodies, are therefore unable to activate immune effector functions, such as complement fixation, phagocytosis, and cytotoxicity. These protective antibodies are characterized by a post-transcriptional modification represented by the addition of an oligosaccharide group of the high mannose type in only one of the two Fab fragments. The combination of the corresponding antibody site with the antigen is hindered by the oligosaccharide group and, as consequence, the molecule behaves as univalent.
Malan Borel and co-authors have showed that pregnant women display much higher titers of asymmetric antibodies than non-pregnant women in serum. Some years later, the same group was able to demonstrate that patients with RSA have significantly lower levels of asymmetric antibodies in serum than normal pregnant women. These results led to the suggestion that during normal pregnancies, asymmetric antibodies directed against paternal antigens are synthesized as a protective mechanisms. The inability to produce these asymmetric antibodies results in pregnancy failure.
Whether protective antibodies are produced by B1 or B2 cells subsets was not yet deeply investigated. Canellada and co-authors were able to show that B cells isolated from human term placenta and further stimulated in vitro with CD40L and a combination of cytokines (IL-10, IL4 and IL-19) synthesized large amounts of pregnancy-protective antibodies. Similarly, Kelemen et al., have demonstrated that placental produced PIBF (progesterone-induced blocking factor) induced in vitro and in vivo the production of pregnancy-protective antibodies by B cells.
The fact that CD40L engagement is necessary for B2, but not for B1 antibody production, suggests that at least those B cells isolated from placental tissue represents a B2 linage. Nevertheless, a more extensive and detailed characterization of systemic B cells responsible for the production of protective antibodies during pregnancy is still missing.
The bad: B cells can produce auto-antibodies that provoke pregnancy disturbances
Contrary to the beneficial role of protective antibodies during pregnancy, the presence of auto-antibodies was extensively demonstrated to exert a detrimental effect in pregnancy outcome. In this regard, antiphospholipid antibodies (aPL) have been associated with several obstetrics complications, such as intrauterine fetal death, spontaneous abortions, deep vein thrombosis, and pre-eclampsia.[26-29] Antiphospholipid antibodies include lupus anticoagulant (LAC), anticardiolipin antibodies (aCL), and anti-beta2-glycoprotein-I. The presence of these antibodies is a characteristic and a pathological cause of the antiphospholipid syndrome (APS). This autoimmune disease is characterized by a hypercoagulable state that provokes blood clots (thrombosis) both, in arteries and veins, which in the context of pregnancy may lead to high pregnancy morbidity. Due to the central role auto-antibodies exert in the development of this auto-immune disease, APS syndrome was conceived as a model for B cell-induced autoimmunity. As in the case of protective antibodies, the B cell subsets responsible for the production of auto-antibodies related to APC and subsequently to pregnancy failures are not well defined. However, Velasquillo et al., have showed that high numbers of CD5+ B cells (B-1a cells) in peripheral blood of patients with APC positively correlates with high levels of aPL auto-antibodies.
Beside aPL, other auto-antibodies were shown to be generated in pregnant women and account for pregnancy complications. Among others, the auto-antibody against the angiotensin II type I receptor (AT1-AA) has been extensively investigated. Because Wallukat and co-authors have shown the presence of this agonistic auto-antibody in pre-eclamptic patients, many features of the disease were attributed to its presence. The physiological effect of AT1-AA mimics the natural ligand of the angiotensin type I receptor. Namely, AT1-AA binds, with relative high affinity, to the AT1 receptor highly expressed in the placenta, leading to the activation of an intracellular cascade which in turn induces the production of anti-angiogenic factors (s-Flt1 and Endoglin), both highly reported to be involved in the onset of the disease.[32, 33]Strong evidences from animal experiments clearly confirmed the role of the AT1-AA in the development of pre-eclampsia (PE). The injection of AT1-AAs isolated from pre-eclamptic women into pregnant mice induces the key features of pre-eclampsia, such as hypertension, proteinuria, glomerular endotheliosis, placental abnormalities, and embryonic defects.
Despite the very well-described impact AT1-AAs have in the development of PE, it was not until very recently that the B cell subset responsible for their production was described.
We observed that CD19+CD5+ B-1a cells but not CD19+CD5− B2 cells isolated from peripheral blood of non-pregnant women and further stimulated in vitro with serum from patients with PE produced AT1-AA auto-antibodies. Interestingly, it has been already proposed that these CD19+CD5+ B-1a cells may be involved in pregnancy disturbances. Bhat et al., showed that CD5 expressing B cell (B-1a cells) proportions significantly decrease during human pregnancies. The authors have proposed that, as B-1a cells are the main producers of poly-reactive antibodies, the decrease in circulating B-1a cells during human pregnancy may be one of the mechanisms responsible for maintaining the immunologic balance involved in preventing pregnancy complications. We have recently corroborated and further expanded these results. We confirmed that CD19+CD5+ levels significantly drop toward the third trimester in pregnant women developing normal pregnancies while their levels remain very high in pre-eclamptic patients. Reinforcing the idea of B cells been involved in the pathophysiology of PE, LaMarca and co-authors have showed that the depletion of B cells in a rat model of PE correlates with a lower levels of AT1-AA antibodies and diminished score of PE symptoms.
Eutherian mammalian evolution was accompanied by a fine adaptation of both, cellular and humoral immunity in term of allowing the semi-allogenic fetus to grow within the maternal body and at the same time to protect the mother against infections. We have discussed in this review how different B cells populations and antibodies produced by them participate in the process of pregnancy establishment as well as influence pregnancy complications (Fig. 1). Taking into consideration, the critical role B cells have in orchestrating an immune response, further investigations are demanded to better understand how B cells behave at different pregnancy stages. Understanding the mechanisms leading to the production of either protective or auto-antibodies in the context of pregnancy will help in the future not only to predict pregnancy failures but generate strategies to treat them.