Transfer of immunoglobulins

The immunoglobulins (Ig) G (IgG), whose molecular mass is approximately 160 kDa, use the specific transporter neonatal Fc receptor (FcRn) to transfer from maternal to fetal circulation³². The passage of Ig through the placenta starts at week 13 of gestation and increases proportionally with gestational age. It peaks at the beginning of the third trimester³³, coinciding with the period when the thickness of the placental barrier diminishes. The presence of the fraction Fab (fragment and antigen binding), the fraction Fc (fraction, crystallizable), and the complete IgG of maternal origin, has been detected in fetal circulation³⁴, while FcRn has been shown in the syncytiotrophoblast³⁵. This is correlated with greater levels of Fc and complete IgG, compared with Fab. It is known that these transporters have a ubiquitous distribution, since they are also found in the blood-brain barrier, in the endothelium of the renal glomerulus and in the intestinal epithelium, whose function is to promote the host’s defenses and to control the deposit of immune complexes³⁶.

Under normal conditions, newborn babies maintain the IgG of maternal origin in their circulation for approximately 6 months, but in newborn babies of women with SLE, it could be longer, since IgGs form immune complexes and these may participate in organic damage mainly related to maternal autoantibodies, such as the case of fetal heart block³⁷. Table 4 shows the frequency with which circulating autoantibodies characteristic of women with SLE are identified, being the anti-dsDNA antibodies the most abundant³⁸,³⁹. Almost all autoantibodies that can cross the placental barrier, attach themselves to the NMDA (N-Metil-D-Aspartate) receptors present in cortical cells⁴⁰, inducing apoptosis in these cells in the fetus. This has been related to the development of neurological disorders in postnatal life⁴¹. Despite the fact that large amounts of anti-dsDNA antibodies have been found circulating in the children of mothers with SLE, there does not seem to be any correlation with the development of CVD. However, an association has been found with these antibodies and the risk of CVD in adults with SLE, such as thromboembolic events and atherosclerosis. This discrepancy may be related with the disease activity or with a particular inflammatory response characterized by enhancement of the expression of pro-inflammatory molecules (VCAM1, ICAM1, metalloproteinases, and chemoattractant molecules)⁴².

The maternal autoantibodies anti-Ro/SSA and anti-La/SSb are detected in the fetal circulation from weeks 18 to 24 of gestation and are associated with atrio-ventricular blockage (AV block), due to affectation of the AV node. This can occur in babies born to mothers with SLE, Sjögren’s syndrome, or even asymptomatic carriers. Even though AV block is the primary cardiac alteration described in children of mothers with SLE, it occurs in only 2% of cases with maternal anti-Ro/SSA antibodies positivity and 3% of cases with both anti-Ro/SSA and anti-La-SSB antibodies positivity⁴³. The apoptosis of cardiomyocytes occurs normally during the formation of the heart but it could be increased in mothers with SLE and positive anti-Ro and/or anti-La antibodies. In the other hand, it is known that these antibodies bind to intracellular antigens Ro52, Ro60, and La48, which could be over-expressed in the embryonic cells, and thus form immune complexes, coupled with de fact that cardiomyocyte apoptosis can also result in expression of Ro/SS-A and La/SS-B on the cell surface. The immune complexes are captured by FcYRIIA receptors in the phagocyte and are transported to the endosomic compartment, where they are recognized by TLRs.⁴⁴. As a consequence, pro-inflammatory cytokines such as INF-γ, TNF-α, and IL-6 could be released. In addition, there can be an increase in the production of STAT1 dependent molecules which are able to cause the trans differentiation of cardiac cells to fibroblasts⁴⁵, which will finally produce fibrosis of the AV node⁴⁶. This mechanism also can explain the development of other alterations related to cardiogenesis, such as endocardial fibroelastosis and dilated cardiomyopathy⁴⁷.

Nevertheless, these are not the only autoantibodies related to fetal heart block. This is also associated with other antibodies that modulate several cellular pathways such as anti-calreticulin antibodies, which recognize calreticulin, a protein involved in the storage of intracellular calcium, anti-muscarinic acetylcholine receptor M1 antibodies, as well as anti-aFodrin antibodies which recognize fodrin, a membrane protein related to apoptosis⁴⁸,⁴⁹.

The antiphospholipid antibodies (aPL) present in SLE are associated with obstetric complications such as recurrent miscarriages and preeclampsia, as well as affectation of postnatal cognitive development. The main target of aPL is the b2-glycoprotein-I located in endothelial cells⁵⁰ and platelets⁵¹; they are associated with prothrombotic events and repeated fetal losses, but they also interact with fetal neuronal antigens, causing alterations in postnatal cognitive behavioral development⁵². Likewise, an increase in aPL is related to changes in the circulating concentrations of fms-tyrosine-like kinase-1 (sFlt-1), placenta-like growth factor (PlGF) and soluble endoglin (sEng), and other placental factors that could be working together affecting vascular endothelium development and, as a consequence, placental-mediated vascular insufficiency, and hypoxia may be present⁵³.

The fetal cardiovascular system is in constant change, and the effect of maternal autoantibodies mainly targets the placenta, but does not seem to match with early developing heart. Cardiac septation occurs around the sixth week of gestation⁵⁴, and the transplacental movement of IgG starts in the 13^th week thus, one possible explanation of why maternal autoantibodies do not produce frequent morphological alterations in the fetal heart, is due major organogenesis of the heart happens before the placenta can permit the transference of Ig.

Placental transference of cytokines

The elevated concentration of pro-inflammatory cytokines in the circulation of patients with SLE, allows one to consider the possibility that these cytokines cross the placental barrier. Nevertheless, there exists great controversy on this issue. In a model of perfusion, bidirectional transference of IL-6 was observed in the placenta of pregnancies at term⁵⁵, but another study did not find IL-6 to be capable of crossing the placental barrier of healthy women, nor was transference of TNFα, IL-1b⁵⁶ or IL-8⁵⁷ detected. In one rat model treated with lipopolysaccharide (LPS) through intraperitoneal injection at the end of the gestation period, only a minimal transference of maternal IL-1 to the fetus was observed⁵⁸. There are not studies that clearly show the passage of cytokines from the mother to the fetus throughout pregnancy rather this has been studied only at the end of pregnancy. Given this, and due to the fact that placental barrier structure is being constantly modified, it is not possible to dismiss the idea that cytokines may be able to be transferred to the fetus during the first and second trimester of gestation. The existing verified evidence only shows that at the end of the pregnancy, there is some transference of maternal cytokines to the fetus, but it remains in controversy. Therefore, it has been difficult to confirm the role of maternal cytokines in the prenatal baby and if they could have any influence on the children postnatal development. In addition, when a case of inflammatory process is identified, it is not clear whether this was generated by the tissues of the product itself⁵⁹, or whether it was secondary to the passage of maternal cytokines or another inflammatory stuffs.

Placental transference of cells

Microchimerism refers to the trafficking of cells or DNA molecules in the circulation that are genetically different from the host’s own. This phenomenon is observed during pregnancy and is due to a bidirectional transference of maternal and fetal cells that are able to continue existing in the circulation for years⁶⁰,⁶¹. The "alien" cells can exist without being perceived by the host immune system and without generating any apparent alteration; however, they could also be associated with the development of autoimmune diseases such a systemic sclerosis. This paradoxical response seems to depend particularly on the extent of similarity between the HLA antigen of the chimeric cells and those of the host⁶². The maternal cells are similar to immune grafts and persist in the circulation of children with severe immunodeficiencies⁶³. These have also been observed in the pancreas of patients with Neonatal Lupus (NL) and Type I Diabetes⁶⁴. In a study carried out in necropsies of newborn babies with NL who died due to arrhythmia secondary to the illness, cells of maternal origin were found attached to the fetal cardiomyocytes⁶⁵. It is possible that this conditions the development of a local inflammatory response which later favors fibrosis of the conduction system. This may then manifest as AV Block or may generate apoptosis in the cardiomyocytes of any region. One possible explanation is that the presence of maternal cells in the host (child) may result in a rupture of the immune tolerance. Another possibility is that a small population of these cells may regulate a decrease in the immune response of the host, which over the long run generates a reaction of rejection. It appears that fetal microchimerism can persist silently for decades, and may gradually initiate pathological processes related to autoimmunity⁶⁶.

Inflammatory profile of children of mothers with SLE and its effect on the heart

Endothelial fetal cells are the ones that first establish contact with the maternal antibodies and inflammatory molecules that cross over the placental barrier. However, there is still limited evidence regarding the effect of this on fetal inflammatory states and of the response of these cells to the presence of such substances. In a research carried out by our group, we studied the expression of pro-inflammatory molecules in human umbilical vein endothelial cells (HUVEC) of healthy children of mothers with SLE, and the results showed a lower expression of IL-8, TLR9 and of adhesion molecules (E-Selectin, I-CAM, and V-CAM), compared to the HUVEC of healthy children of healthy mothers. However, there were no differences in the expression of IL-6⁶⁷. Nevertheless, when the HUVEC were stimulated with TNF-alfa, the response was proportionally similar in both groups. This indicates that despite being basally depressed, the HUVEC of SLE had not lost the capacity to become activated in pro-inflammatory conditions. It is important to consider that both genetic background and regulation due to environmental conditions affect the activation of the endothelium in postnatal life. In another study that looked at the family history of heart disease in healthy pregnant women whose children were healthy on birth, it was observed that the basal expression of pro-inflammatory cytokines in HUVEC was greater in women with some family history of CVD, than those with little or no history of CVD⁶⁸. This shows that the inflammatory response in children from mothers with pro-inflammatory conditions, or with some risk of maternal illness, enhances the risk in the short or long term to develop some degenerative or autoimmune disease, with consequences in adult life. Furthermore, in other study with the goal of investigating the inflammatory profile of children of mothers with SLE and their association with alterations in cognitive and neurological development, it was observed that they had a greater incidence of autoimmune diseases such as glomerulonephritis, asthma, thyroid diseases, vitiligo, and cryoglobulinemia compared to children of healthy mothers. They also showed greater levels of IL1β, IL2, IL4, IL5, IL6, IL10, INFγ, and TNFα⁶⁹. Since these patients were between the ages of 8 and 17 at the time of this study, it is hard to know if there was any relationship with the interaction with maternal environment during gestation or due to placental transference; however, it still confirms the pro-inflammatory situation suffered by children of mothers with SLE, over the long term and this may also cause a predisposition to the development of CVD. The evidence shows that the main cardiac affectation of children from mothers with SLE is AV blockage; however, we cannot dismiss other alterations that may affect cardiac function over the medium to long range. The administration of LPS in animal models allows for a simulation of maternal pro-inflammatory states, as it has been shown in female pregnant rats, LPS produces a decrease of the contractile function and increased remodeling of the left ventricle of the pups⁷⁰, and in rats, the offspring presents cardiomegaly, myocardial fibrosis, and hypertension⁷¹,⁷². It is known that LPS is an activator of the innate immune system and its effects are mediated through its interaction with receptors of the TLR family⁷³. Most of the cells of the cardiovascular tissue present receptors for TLRs, and the TLR activity has been related to diseases such as atherosclerosis, cardiac dysfunction in sepsis, and congestive heart failure⁷⁴. Therefore, fetal cardiomyocytes may be an important target during gestational heart development. There are still no studies on the mechanisms of action of TLR in the development of heart disease in children from women with SLE, but the action of these receptors could favor the synthesis and secretion of cytokines, adhesion molecules, chemoattractants, and reactive species of oxygen, which, in turn, promote the activation of endothelial cells and leukocytes in patients with SLE⁷⁵.

On studying the structure and function of the myocardium in children with active SLE, a high frequency of global dysfunction of the left ventricle was observed, even in the absence of clinical symptoms or signs of heart diseases. However, this is different in adults with active SLE, where there is a predominance of coronary artery disease due to premature atherosclerosis, hypertension, heart failure, and valvular disease⁷⁶. This allows for the observation that age, environment, and history of the disease, can have distinct effects on the development of cardiac disease. In a longitudinal study carried out for over 20 years, the accelerated development of CVD was observed in patients diagnosed with SLE, in comparison with control groups. Nevertheless, there were no differences in associated risk factors such as cholesterol levels or blood glucose⁷⁷. Likewise, their risk of developing subclinical atherosclerosis is similar to that of patients with rheumatoid arthritis and diabetes mellitus, which are associated with the activation of the P38-MAPK pathway, leading to the activation of NFkB as a response to the signals induced by aPL⁷⁸.

The inflammatory state of a woman with SLE may be the result of environmental interaction with the state of the disease itself, which implies a great variability in clinical manifestations. Epigenetics is the study of changes in genetic expression that are not related to mutations in the underlying DNA sequence, but rather to changes in DNA methylation, the modification of histones and the expression of miRNA, all of which are regulated by environmental conditions. In studying the transcription and methylation of the genome of PBMC cells of patients with SLE, various genes were found to be over- or under-expressed, and these are correlated to the state of methylation⁷⁹. Differential analysis revealed a significant affectation of the signaling pathways of INF and TLRs, such as the expression of MX1, GPR84, and E2F2 molecules which are related to pro-inflammatory signaling pathways in macrophages. These were found to be increased in patients who already had lupus nephritis, in comparison with those who did not, while the serum levels of the cytokines IL17A, IP-10, bFGF, TNF-α, IL-6, IL-15, GM-CSF, IL-1RA, IL-5, and IL-12p70 were found to be elevated in all patients. In addition, it was observed that the over-regulation of IL-15 was correlated with hypomethylated sites in the promoter region of this gene⁸⁰. It is known that apoptotic DNA is hypomethylated and produces an increase in the immune response through signaling by TLR9. This culminates with the production of anti-dsDNA antibodies and lupus nephritis in rats⁸¹.

Other environmental factors such as exposure to UV rays can cause a significant reduction in the global methylation of the DNA of peripheral blood mononuclear cells, phenomenon that is exaggerated in patients with SLE. Endogenous retrovirus can be reactivated through the hypomethylated state present in SLE, where the retroviral elements imitate autoantigens that induce a response from autoantibodies. Therefore, this state favors epigenetic modifications that can have repercussions throughout postnatal life, enhancing the susceptibility to develop cardiovascular and autoimmune diseases.