Antibodies in breast milk: Pro‐bodies designed for healthy newborn development

Summary

This manuscript sheds light on the impact of maternal breast milk antibodies on infant health. Milk antibodies prepare and protect the newborn against environmental exposure, guide and regulate the offspring's immune system, and promote transgenerational adaptation of the immune system to its environment. While the transfer of IgG across the placenta ceases at birth, milk antibodies are continuously replenished by the maternal immune system. They reflect the mother's real‐time adaptation to the environment to which the infant is exposed. They cover the infant's upper respiratory and digestive mucosa and are perfectly positioned to control responses to environmental antigens and might also reach their circulation. Maternal antibodies in breast milk play a key role in the immune defense of the developing child, with a major impact on infectious disease susceptibility in both HIC and LMIC. They also influence the development of another major health burden in children—allergies. Finally, emerging evidence shows that milk antibodies also actively shape immune development. Much of this is likely to be mediated by their effect on the seeding, composition and function of the microbiota, but not only. Further understanding of the bridge that maternal antibodies provide between the child and its environment should enable the best interventions to promote healthy development.

1. INTRODUCTION

A newborn's immune system is largely naive to the antigens they will encounter in their environment, including pathogens, commensals, and food antigens. It also has a modus operandi linked to different constraints than adults, such as prioritizing energy allocation to tissue development rather than the immune system or promoting tissue and barrier development rather than effector mechanisms. ¹ , ² , ³ , ⁴ , ⁵ , ⁶ This means that infant immune responses to antigens may be too slow or unsuitable to control a pathogen and be fatal, or too inflammatory and inappropriate, as in allergic reactions or necrotising enterocolitis (NEC) in response to an allergen or commensal, respectively. Mothers help their offspring's immune system by transferring antibodies in utero, ensuring that the infant is protected in the first few months against infections caused by pathogens the mother has previously encountered. Years of evolution have also shaped a special fluid, breast milk, designed to meet a developing infant's unique and ever‐changing needs. Breast milk influences offspring development in many ways other than as a source of energy. Here we review the evidence that antibodies in breast milk not only complement the infant's developing immune system, but also actively promote its development, with potential implications for the next generation.

2. THE UNIQUE MATERNAL IMMUNE SYSTEM‐MAMMARY GLAND AXIS

More than 100 years ago, Ehrlich first showed that maternal immunization and subsequent breastfeeding conferred significant protection to the offspring against the toxic effects of ingested ricin and abrin. ⁷ This was the first demonstration of vertical transfer of immunity from mother to newborn through breast milk. ⁷ It took another 70 years to discover one of the main biological compounds associated with the transfer of immunity through breast milk: maternal antibodies, the majority of which are IgA, while this class is present in very low levels in serum. ⁸ , ⁹ Further studies showed that, unlike serum IgA, IgA in breast milk was associated with a secretory piece, hence the name secretory IgA (SIgA). ¹⁰ SIgAs were then also identified in all secreted body fluids, like sweat, tears, urine, saliva, bile, and the mucous of the intestinal, genital, and respiratory tracts ¹¹ (Figure 1, Box 1).

FIGURE 1.

Personalized protection afforded by breast milk antibodies. Breast milk contains immunoglobulins (Ig) trafficked through the mammary gland (MG) epithelium by Fc receptors: PIGR transports IgA and IgM polymers, while FcRn binds IgG and IgE. These Ig have a dual origin: (1) upon lactation, the MG epithelial cells recruit CCR10+ plasma cells, primarily from mucosal sites, by secreting CCL28. These antibody‐secreting cells, in vast majority IgA+, constitute a resident population in the MG throughout lactation and the primary source of milk Ig. (2) Circulating serum Ig (mostly IgG), produced by bone marrow plasma cells, generated by past and current systemic responses, can be transferred to the milk through their binding to FcRn. The milk Ig repertoire reflects the mother's (and by extension, her infant's) environment, and is thus tailored to shape their microbiome and protect them from infection, allergy and gut inflammation. Image created with BioRender.

BOX 1. Antibodies in breast milk—main characteristics.

Targets

• ‐ Geographical and inter‐individual heterogeneity.

• ‐ Microbial antigens, commensal or pathogens, as well as food and respiratory allergens.

• ‐ Antigens present in the maternal mucosa, mostly from the gut, but also from the respiratory and urogenital tracts—favor IgA response in milk.

• ‐ Antigens entering the maternal circulation, for example, from invasive pathogens or parenteral immunization—favor IgG response in milk.

• ‐ Antigens from infant saliva.

• ‐ Specific epitopes targeted compared to antibodies produced at other maternal sites.

Production

• ‐ Real‐time.

• ‐ The severity of the disease determines the milk Ig classes, their quantity and repertoire.

• ‐ Locally by mammary antibody‐secreting cells (IgA and IgM), systemically (IgG and IgE), or by milk B cells.

• ‐ Transfer into milk by PIGR (IgA and IgM) and FcRn (IgG and maybe IgE).

• ‐ Specific glycosylation pattern.

Function

• ‐ Act locally in the infant's mucosa; may also act systemically—IgA during the first 3 days of life and IgG throughout the lactation period.

• ‐ Protect infants from infections by preventing mucosal tissue colonization, damage, and invasion by pathogens via (1) binding to pathogen antigens and neutralizing their toxin and/or ability to damage and invade cells; opsonisation, (2) promoting gut ILC3 ontogeny, thus contributing to promoting a strong barrier and anti‐pathogen responses (3) promoting microbiota seeding, which will compete for space and nutrients with pathogens (4) may also promote systemic immune response by active transfer of IgG‐pathogen antigen immune complex across the infant gut barrier.

• ‐ Protect against allergy by (1) excluding allergens, (2) promoting immunoregulatory tone in the gut probably directly by adjusting levels of regulatory T cells, inhibiting T cell activation, and indirectly by targeting specific commensals, (3) active transfer of IgG across the infant gut barrier may also promote systemic immune regulation by controlling B cell activation and IgG‐allergen immune complexes by inducing Tregs.

• ‐ Protect against malnutrition by influencing infant microbiota to prevent gut inflammation and promote nutrient absorption.

• ‐ Control the seeding and function of the gut microbiota by targeting specific commensals resulting in (1) the formation of biofilms which promote niche colonization and enhance microbial stability, (2) prevent commensals tissue invasion and inflammatory stimulation of immune system (3) influence their gene expression towards an anti‐inflammatory profile.

• ‐ Actively shape the infant immune system by multiple mechanisms such as the shaping of gut microbiota, the development of ILC3, the control of CD4 T cell activation and the levels of Tregs, or the development of antigen specific T cells.

Human milk contains all the different isotypes of immunoglobulins (IgA, IgG, IgM, IgD, IgE), with a strong predominance of IgA. A recent study summarizing the available data on levels of Ig in breastmilk showed that IgA levels are the highest in colostrum, ranging from 10 to 100 mg/mL, and representing 90% of Ig, and then decline to levels around 1 mg/mL in mature milk and representing 80% of Ig. ¹² IgG and IgM levels are constant throughout the different stages of lactation and average 0.1 to 1 mg/mL or 5% of milk Ig. IgE and IgD have been detected, but very rarely and in very low amount. ¹² Genetics, environmental exposure, and nutrition influence their levels such as regions with high pathogen exposure showing higher levels of milk Ig. ¹² , ¹³ , ¹⁴ The general nutritional status also seems to play a role with both undernutrition and obesity being associated with lower levels of milk Ig. ¹² , ¹⁴ Mouse studies were able to isolate malnutrition from other confounding factors and showed a dramatic effect on milk IgA. ¹⁵ More subtle dietary changes, such as prebiotic intake, may influence milk IgG levels, as recently shown in a randomized control trial where we found that mothers supplemented with prebiotics during pregnancy and lactation had lower milk IgG1 levels. ¹⁶

Human milk antibody production can be either local, by tissue‐resident plasma cells for IgA and IgM, or remote, that is, from the bone marrow for IgG and IgE, and transported through the blood. Milk antibodies might also be produced in the milk itself as a significant proportion of breast milk B cells have undergone terminal plasma cell differentiation. ¹⁷ In all tissues, antibody trafficking across the epithelial barrier is mediated by cells expressing Fc receptors, which mediate antibody capture and transcytosis. In the mammary gland, the luminal cells that secrete the milk also ensure the transfer of antibodies into the lumen. The polymeric immunoglobulin receptor (PIGR) allows the transport of IgA and IgM after binding of the J chain, which forms IgA dimers and IgM pentamers. After cleavage of the PIGR, its association with the J chain forms the secretory component that protects the antibodies from degradation by the enzymes of the newborn's digestive tract and increases their half‐life. ¹⁸ IgA and IgM cover infants' upper respiratory and gut mucosa and may also reach the circulation in the first days of life when gut permeability is still very high. ¹⁹ The expression of the neonatal Fc receptor (FcRn) on mammary gland epithelial cells mediates the transport of IgG. ²⁰ Recently, it has been proposed that maternal IgE could also be transported through milk by binding to FcRn, particularly in the case of allergy. ²¹ , ²² FcRn expression on the apical side of enterocytes suggests that the milk IgG may reach the newborn circulation. ²³

If IgG or IgE appear to be transported from the maternal serum, IgM and IgA are secreted locally within the mammary gland by a tissue‐resident population of antibody‐secreting cells (ASCs). These are very rare in the mammary gland before pregnancy, build up from late pregnancy, and increase sharply with the onset of lactation. ²⁴ Luminal cells are responsible for the recruitment of these ASCs through the secretion of the chemokine CCL28, which is upregulated in the mammary gland during lactation. ²⁵ The induction of CCL28 is likely triggered by hormonal signals as the injection of a combined regimen of progesterone, estrogen, and prolactin in virgin mice was enough to induce the migration of ASCs to the mammary gland. ²⁶ CCR10, the receptor of CCL28, is mainly expressed by ASCs generated in the mucosa‐associated lymphoid tissues (MALT). Several studies have shown through clonal relationship or tracing experiments that the gut‐associated lymphoid tissues, which collectively constitute the largest inductive site of the body, are the major source of the mammary plasma cells. ²⁷ , ²⁸ A large proportion of breast milk antibodies target the maternal microbiota through diversification of memory B cell clones, as opposed to recruitment of new B cell clones. ²⁷ This helps to maintain a stable host‐microbiota interaction and to shape the offspring microbiota to be similar to the maternal one (see below). In the case of mucosal infections, whether intestinal, respiratory or genital, new clones are recruited to both the inductive mucosal sites and the mammary gland, providing a dynamic and adapted immune response to pathogens that protects both mother and offspring ²⁷ , ²⁹ , ³⁰ , ³¹ , ³² (See below). The lactating mammary gland thus becomes a distal effector site, harboring a large population of tissue‐resident plasma cells derived from multiple inductive sites. Furthermore, the mother may not always need to be infected to produce Ig in breast milk. A recent study showed that the mammary gland connects the maternal immune system to the infant intestinal tract, and this bridge can work both ways: enteric viruses that replicated in the salivary glands of the offspring were transmitted to the maternal mammary gland through the saliva, eliciting the local rapid production of specific antibodies in the milk. ³³

In addition to microbial mucosal antigens, antibodies are produced against noninfectious antigens such as food and respiratory allergens (see below). Importantly, antibodies against parenteral immunizations performed during pregnancy or lactation are also transferred, implying the transfer of serum immunoglobulins and/or the recruitment of ASCs from nonmucosal inductive sites as well. ³⁴

Finally, a further level of specificity of milk antibodies is provided by posttranslational modifications such as glycosylation, which modulate their binding to Fc receptors, their ability to fix complement or their half‐life. ³⁵ A recent study showed that during late pregnancy/lactation, deacetylation of the sialic acid of maternal IgG alters its binding to CD22 and enhances immunity to intracellular bacteria in the newborn. ³⁶ In addition, the glycosylation patterns of breast milk IgA differ from those of saliva or serum IgA from the same women, most likely reflecting adaptation to the specific needs of the newborn. ³⁷ These findings show that the metabolism of plasma cells, and thus the antibodies they secrete, is finely regulated by the environment (hormones and tissues), giving breast milk antibodies unique properties. This was further demonstrated by the specific transcriptional signature of the mammary tissue‐resident plasma cells. ³⁸

Thus, through the unique maternal immune system‐mammary gland axis, the antibody repertoire in breast milk combines specificities against a very wide range of antigens, fully reflecting the environment of the mother and hence the newborn, demonstrating the precision medicine offered by breast milk.

3. A HIGHLY DYNAMIC AND PERSONALIZED FIRST LINE OF DEFENSE AGAINST PATHOGENS

In 2019, 5.30 million children under 5 years of age died in the 194 WHO Member States mostly in low‐ and middle‐income countries. ³⁹ Diarrhoeal diseases, lower respiratory infections, and sepsis were among the leading causes of death. The role of breastfeeding has been the focus of intense research to assess its potential beneficial impact on this high burden of disease and the results are impressive. Overall, it is estimated that between 0.8 and 1.3 million under‐five deaths could be prevented by increasing breastfeeding, making breast milk the most effective way to prevent child mortality. ⁴⁰ , ⁴¹ Moreover, early initiation of breastfeeding, that is, before the first 24 h, in LMIC is critical for the prevention of neonatal mortality, especially from infectious disease, with a 40 to 50% lower risk of neonatal mortality being observed upon early breastfeeding initiation. ⁴² , ⁴³ Interestingly, a protective effect of early breastfeeding was observed up to 6 months of age. ⁴³ A recent comprehensive review reports that, based on 66 different studies, half of the diarrheal diseases and a third of the respiratory infections would be prevented by breastfeeding. Even stronger is the protective effect against hospitalization, preventing more than 50% of admissions for diarrhea and respiratory infections. ⁴¹ Breastfeeding also improves the lives of children in high‐income countries with a 30% decrease in the risk of otitis media. ⁴⁴ Importantly, in a USA cohort, breastfeeding was associated with a 30% decreased risk of perinatal deaths ⁴⁵ and prolonged breastfeeding protected against infections requiring hospitalization in the first year of life in a large analysis conducted in Denmark. ⁴⁶ The mechanisms by which breast milk protects against infectious diseases are multiple, including increasing the barrier function of the infant's epithelia through its high levels of growth factors; transferring antimicrobial factors such as lactoferrin, lysozyme, oligosaccharides; promoting the growth of commensals through HMOs and transfer of its microbiota; and transferring maternal antigen‐specific immunity, which include lymphocytes and antibodies. SIgA and IgG most probably play a major role in the transfer of passive immunity against pathogens in the mother–child environment. They may also contribute to prime immunity in the breastfed child (Figure 1, Box 1).

Several studies have shown the presence of SIgA in human breast milk or in the feces of breastfed infants, which are directed against pathogens present in the mother–child environment. Two recent studies analyzed the levels and the reactivity of IgA and IgG antibodies against pathogens in mothers' milk from HIC and LMIC. Higher concentrations of milk antibodies with broader specificity against multiple pathogens were measured in LMIC compared to HIC. ¹³ , ¹⁴ In comparison to HIC, milk from LMIC contained more IgA against enteric pathogens, such as Campylobacter, cholera, Cryptosporidium, diarrheagenic E. coli, Salmonella, and Shigella. IgA antibodies also recognized a larger number of respiratory pathogens antigens from influenza A/B, Bordetella pertussis, Streptococcus pneumoniae, and Mtb. The Ig classes found in breast milk also revealed adaptation to the severity of the disease. Shigella induced great IgG and IgA responses in milk in LMIC but only IgA responses in HIC, probably reflecting the more invasive infections in LMIC resulting in IgG responses, whereas IgA responses, were equally distributed between LMIC and HIC. After HIV infection, there was also a higher frequency of detection of HIV‐specific IgG than IgA, further suggestingg the importance of the systemic distribution of the pathogen in promoting IgG in breast milk. ⁴⁷ , ⁴⁸ The mother's environment, exposure to pathogens and their nature, and the severity of the disease therefore determine the milk Ig classes, their quantity and repertoire. The memory of plasma cells homing to mammary glands needs further studies. Mothers who had migrated from Asia to the UK for years still carried milk Ig with specificities from their country of origin, suggesting it may last for years. ²⁹ Yet, boosting by environmental exposure might be required for optimal protection. During the recent pandemic, the restrictive measures imposed against SARS‐CoV‐2 blocked the circulation of common respiratory viruses. IgG and IgA against RSV declined in the maternal milk and a surge of infant RSV infections and hospitalization was reported in all countries. ⁴⁹ Importantly, findings in mice suggest that mother could even protect their infants against pathogens they have not experiences through milk antibodies. In an elegant study, Zheng et al. ⁵⁰ found that maternal IgG directed against her intestinal commensals were transferred to the newborn via breast milk and helped protect them against pathogenic intestinal infection with enterotoxigenic E. coli.

The recent COVID‐19 pandemic also brilliantly illustrated how milk adapts to the emergence of a new virus. A systematic review from 2022, showed that about 80% of breastmilk of infected mothers contained anti‐SARS‐CoV‐2 IgA or IgG in human and about 50% IgM. ⁵¹ Interestingly, most noninfected mothers or prepandemic milk samples also had detectable levels of anti‐SARS‐CoV‐2 IgA but very rarely IgG or IgM. ⁵² , ⁵³ , ⁵⁴ These results suggest that, independently of any contact with the virus, human milk contains broadly reactive IgA, probably generated by the response to other Coronaviruses, that cross‐react with SARS‐CoV‐2. The exceptional speed of the antibody response in breast milk may also be critical for infant protection. In a group of mothers infected at the time of delivery, Spike‐specific IgA was detectable in breast milk only two days after diagnosis, whereas IgG was not found in either breast milk or serum. ⁵⁵

The causal role of breast milk Ig in preventing infection is supported by several lines of evidence, such as higher milk antibody concentrations in human milk being associated with protection against Shigella, ⁵⁶ norovirus ⁵⁷ or rotavirus. ¹⁴ Mouse studies further establish the causality of breast milk antibodies and in particular IgG for pathogen clearance in offspring. ⁵⁰ , ⁵⁸ In vitro assays also give insight into the protective properties of milk antibodies. Pullen et al. ⁵⁹ characterized the functionality of SARS‐CoV‐2‐specific antibodies in maternal blood and breastmilk after SARS‐CoV‐2 infection. Although infection elicited polyfunctional IgG in the blood, breast milk contained predominantly neutralizing and neutrophil‐activating IgA, IgM and IgG, suggesting a selection of specific, noninflammatory antibody subsets in breast milk. Interestingly, many studies have evaluated the association between antibody levels and pathogen‐neutralizing activity and found very inconsistent results. ³⁴ We found that although maternal infection with SARS‐CoV‐2 increased levels of both SARS‐CoV‐2 IgA and IgG, neutralizing activity correlated only with levels of SARS‐CoV‐2 IgA. While studies have shown that purified breast milk IgGs have SARS CoV2 neutralizing activity, ⁶⁰ we found that levels in breast milk are probably too low to exert this activity. By depleting IgA from breast milk, we completely abolished the SARS CoV2 neutralizing activity of breast milk from infected mothers, demonstrating the primary role of IgA in SARS‐CoV‐2 neutralizing activity. ⁵⁴ In agreement with this observation, it has been shown that a monoclonal antibody generated against SARS‐CoV2 also neutralizes SARS‐CoV‐2 when converted to secretory IgA. ⁶¹ Among antibodies cloned from SARS‐CoV‐2 infected patients, dimeric IgA antibodies were 15–75 times more potent than their monomeric counterparts and protected mice from infection when administered intranasally. ⁶² , ⁶³ For other pathogens, however, IgG may be more important. This has been shown for HIV neutralization by milk IgG but not IgA ⁴⁷ and is also suggested for the prevention of RSV infection. The concentration of IgG antibodies that target the pre‐F protein and neutralize the virus was lower in the milk of infants with RSV acute respiratory illness than in the milk of mothers with uninfected infants. ⁶⁴

While most evidence suggests that milk antibodies act locally, inhibiting the attachment of pathogens to tissues or neutralizing their toxin or tissue‐damaging activity, there is also evidence that they can act systemically in the offspring. IgA transport across the gut barrier was described in the first 3 days of breastfed mature newborns ⁶⁵ , ⁶⁶ and premature neonates. ³⁰ There is also evidence from rodent models that milk IgG may impact offspring systemic immunity by FcRn mediated transport across the gut barrier. ⁶⁷ , ⁶⁸ , ⁶⁹ An adult mouse model showed that immune complexes made of IgG bound to pathogen antigen (citrobacter rodentium) were responsible for improved antigen presentation and pathogen defense mechanisms. ⁷⁰

Further studies are needed to clarify the relative roles of IgA and IgG in breast milk in protecting against infectious diseases, as this may have implications for vaccine strategies. By better understanding the antibody responses in breast milk to infection, we will be better able to design maternal vaccines that induce IgA and/or IgG for the best protection of infants.

4. PROTECTING FROM ALLERGIC REACTIONS AND POTENTIALLY EDUCATING THE IMMUNE SYSTEM TOWARDS TOLERANCE TOWARDS ALLERGENS

In addition to antibodies to pathogens and commensals (see below), mothers transfer antibodies to both airborne and food allergens through breast milk. There is evidence from multiple studies on the presence of IgA in breast milk against major food allergens such as cow's milk allergen beta‐lactoglobulin, ⁷¹ , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ , ⁷⁶ , ⁷⁷ , ⁷⁸ egg allergen ovalbumin, ⁷³ , ⁷⁶ , ⁷⁹ or wheat allergen gliadin. ⁷⁶ , ⁷⁸ Different specificities of IgA against Gliadin were found in serum, saliva, and breast milk, suggesting that the local IgA‐secreting cells are functionally different in the various tissues of the organisms. ⁷⁶ There is a correlation between food intake and the levels of IgA to food antigens in breast milk. ⁷⁷ , ⁸⁰ IgG to food antigens are also described but at much lower levels and with less consistency than IgA. ⁷⁶ , ⁷⁸ , ⁷⁹ , ⁸⁰ , ⁸¹ In contrast to IgA, breast milk IgG mirror the profile of IgG reactivity in the blood, ²¹ suggesting their serum origins. In addition to antibodies to food allergens, a few studies have demonstrated the presence of both IgA and IgG against respiratory allergens including cat ⁷³ and house dust mite allergens. ⁸² , ⁸³ , ⁸⁴ Finally, a study using microarray technology showed a much wider range of specificities for both food and respiratory allergens of breast milk IgG, including reactivity to cockroach, cat, grass, tree pollen, walnut, peanut, peach, latex. ²¹ IgE were also detected in the breast milk of highly sensitized mothers ²¹ (Figure 1, Box 1).

Whether the presence of antibodies to allergens in breast milk plays a role in breastfed infants' short‐ and long‐term susceptibility to allergies has been the focus of both human and animal studies. Human studies showed an inversed association between the levels of total IgA and cow's milk allergen‐specific IgA and the symptoms of cow's milk allergy in breastfed infants ⁷¹ as well as with the later development of cow's milk allergy during the first year. ⁷² , ⁸⁵ The association was the strongest with colostrum. In contrast, the cow's milk allergen levels did not correlate with the symptoms. ⁷¹ , ⁷² These observations suggest that IgA in breast milk can influence immune responses to allergens in the infant's diet, either from mixed feeding or from breast milk itself, ⁷⁴ and both decrease the immediate allergic reaction and actively modulate immune education to food allergens. IgA in breast milk may also influence the risk of allergies other than food allergy such as the risk of developing atopic dermatitis. ⁸⁶ No data in humans are available on the associated risk of developing respiratory allergies, Data in mouse models demonstrated that IgA in breast milk were not required for protection from asthma by allergen‐exposed immunized mothers. ⁸⁷ IgA antibodies in colostrum and human milk may provide immune exclusion and prevent antigen entry at the intestinal surface of the breastfed infant as suggested by in vitro experiments. ⁸⁰ This may be particularly important in the first days of life, when gut permeability is high and the newborn's immune system is more prone to inflammatory responses than immune tolerance. ⁵ This may explain why low levels of IgA, particularly in the colostrum that is produced during the first 3 days of life, may be predisposing to allergy. ⁷² , ⁸⁵ A low level of IgA in human milk may then result in the defective exclusion of food antigens, leading to allergic sensitisation and predisposing the offspring to develop food allergy on subsequent exposure to the allergen that would otherwise have resulted in oral tolerance. IgA could also play an immunoregulatory role that is allergen‐specific or nonallergen specific. ⁸⁸ The latter is suggested by the observations that inverse associations have been found between the levels of total IgA in breast milk and allergic sensitisation in offspring ⁷² , ⁸⁵ but not with allergen‐specific IgA. This is also supported by the finding that total IgA levels in breast milk are inversely associated with the risk of developing atopic dermatitis in infants. ⁸⁶ The nonallergen‐specific immunoregulatory role of milk IgA could be explained by its effects in setting the levels of Tregs in the gut mucosa as suggested in mice experiments ²⁸ or by modulating the microbiota (see below).

To our knowledge, only one human study examined the potential impact of maternal IgG in breastmilk on the risk of allergic sensitization in offspring. ⁸⁹ In a study investigating levels of allergen‐specific IgG in maternal blood and breast milk by microarray, Lupinek et al. ⁸⁹ found that infants of mothers with high levels of allergen‐specific IgG in serum and breast milk did not show sensitisation to the allergen at 5 years. However, it is difficult to disentangle in this study the potential role of maternal IgG transferred in utero versus via breast milk on infant allergic sensitisation and the causality of the findings. Studies in mice may provide insightful information in this regard.

For four decades, rodent experiments have explored the effects of in‐utero and milk transfer of IgG to offspring on allergy sensitisation and their mechanisms of action. ⁹⁰ Experimental data in rodents suggest that maternal allergen‐specific IgG transferred by placenta and/or breastfeeding prevents allergic sensitisation in the offspring. ⁹¹ , ⁹² , ⁹³ , ⁹⁴ , ⁹⁵ , ⁹⁶ , ⁹⁷ , ⁹⁸ , ⁹⁹ Cross‐fostering experiments showed that breast milk IgG transfer was sufficient to promote allergy prevention. ⁸⁷ , ⁹⁶ Breast milk IgGs can exert their effect both locally in the gut and systemically after transfer into the circulation by the neonatal FcR receptor. Like what has been described for preventing infection, breast milk IgG can protect against allergy by binding to allergens and promoting their clearance by phagocytosis. ⁹⁰ , ¹⁰⁰ They can also block IgE binding to allergens and inhibit mast cell degranulation. ⁹⁰ , ¹⁰⁰ In addition to this immediate action, there is evidence that IgG can promote immune regulation. Thus, IgG can bind to inhibitory receptor FcγRIIb on newborn B lymphocytes or dendritic cells, consequently inhibiting the production of IgE in response to allergens. ⁹⁰ , ¹⁰⁰ Finally, and probably most importantly, there is strong evidence from rodent experiments that antigen‐IgG immune complexes in breast milk may be critical in the imprinting of immune reactivity in the offspring, including allergy susceptibility. Allergen‐IgG immune complexes have been detected in human milk. ⁸¹ , ¹⁰¹ Oral exposure to OVA‐IgG immune complexes via breast milk resulted in the induction of OVA‐specific regulatory T cells (Tregs), which were responsible for prolonged tolerance to OVA in offspring, subsequently leading to the prevention of respiratory and food allergies. ⁷⁹ , ⁸¹ , ⁸⁷ This appeared to result from protected transport of OVA across the intestinal barrier and enhanced presentation by dendritic cells, both dependent on the use of the neonatal Fc receptor (FcRn), ⁸⁷ whereas FcgRIIb was not involved. ⁸⁷

Understanding how maternal antibodies influence allergy in the offspring will provide new opportunities to prevent allergy, for example by modulating their levels through dietary intervention or immunization.

5. A BLUEPRINT FOR A BALANCED MICROBIOME

The temporal development of the human gut microbiota plays a critical role in shaping the gut mucosal immune system during early life, with systemic implications beyond the gut. Interactions between the gut mucosal immune system and the microbiome occur within a critical window, which coincides with immune development. Both animal and human studies show that altering the gut microbiome in early life has a significant impact on the risk of asthma, allergies, metabolic disorders, and other inflammatory conditions in adulthood. ¹⁰² , ¹⁰³ , ¹⁰⁴ , ¹⁰⁵ Human milk is recognized as the strongest factor influencing microbiota composition and function ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ¹⁰⁹ with persistent metabolic consequences. ¹¹⁰ , ¹¹¹ A multitude of milk factors contribute to the shaping of the gut microbiota including the transfer of human milk oligosaccharides (HMOs) promoting the growth of beneficial microbes, the milk microbiota itself, natural anti‐microbial molecules such as Lactoferrin and Immunoglobulins. ¹⁰⁸ , ¹¹² , ¹¹³ Among all these factors, the transfer of maternal secretory IgA (SIgA) stands out due to the significant impact of SIgA in regulating the gut microbiome ¹¹³ (Figure 1, Box 1).

Evidence for the coating of commensal bacteria in the gut by IgA has been available for almost 3 decades both in adults ¹¹⁴ and breastfed neoneates. ¹¹⁵ Recently, there has been a surge of interest in breast milk SIgAs and their role in regulating the composition and function of symbiotic commensals in the small intestine and colon, in addition to providing passive immunity against pathogens. ¹¹³ As we will describe in detail, by binding to bacteria, SIgA can inhibit or promote bacteria seeding and proliferation and hence the diversity and richness of the gut microbiota, which will influence epithelial and immune cell signaling. SIgA can also influence bacteria gene expression and thus their production of metabolites that affect the local and systemic regulation of immune responses. These influences of maternal SIgAs, through their role in shaping the infant microbiome during the suckling period, have a lasting impact on the microbiome's development, with significant consequences for immune function and disease susceptibility throughout life.

In a mouse model, Rogier et al. ¹¹⁶ showed that by the age of weaning, mice that received maternal SIgA in breast milk had significantly different gut microbiota compared to those that did not, with these differences becoming even more pronounced by adulthood. The role of maternal SIgA is further supported by a translational study of Planer et al., ¹¹⁷ which showed that twins had more similar SIgA coating patterns compared to unrelated infants, suggesting a strong maternal influence on SIgA‐mediated microbiota shaping. The gradual generalization of SIgA targeting as infants age indicates that early maternal SIgA sets the stage for microbiota colonization, which subsequently matures as the infant's immune system begins producing its own SIgA. ¹¹⁷

The importance of breast milk SIgA in controlling susceptibility to inflammatory disease by their effect on the gut microbiota is suggested by a few studies. The importance of breast milk IgA in modulating inflammatory tone is highlighted in NEC, where reduced SIgA targeting of Enterobacteriaceae enables their overgrowth. ¹¹⁸ In a mouse model of NEC, offspring fed by IgA‐deficient dams were more susceptible to disease, with outcomes similar to formula‐fed pups. ¹¹⁸ Abbott et al. ¹¹⁹ showed that maternal SIgA is crucial for suppressing Enterobacteriaceae colonization and promoting beneficial microbes like Lactobacillus. Pups nursed by IgA‐deficient dams had dysbiotic microbiota and increased inflammatory Th17 cell responses, which persisted into adulthood as memory T cells, potentially driving inflammation and contributing to chronic immune‐related conditions. Another study found that neonatal mice nursed by dams deficient in the polymeric immunoglobulin receptor (and thus lack sIgA in breast milk) exhibited long‐lasting alterations in their microbial composition and increased susceptibility to later gut inflammatory disease, along with increased translocation of intestinal microbes to the mesenteric lymph nodes. ¹¹⁶ In a mouse model, Perruzza et al. ¹⁵ demonstrated that breastfeeding by malnourished mothers replicated environmental enteric dysfunction. Milk from undernourished mothers was deficient in SIgA and pups showed gut dysbiosis, including a reduction in beneficial microbes like Akkermansia muciniphil, which is preferentially coated by SIgA in a healthy gut. ¹²⁰ Dysbiosis was associated with impaired intestinal barrier function, increased inflammation, and metabolic dysfunction, characterized by stunted growth and poor nutrient absorption, thus perpetuating the state of undernutrition. The importance of coating specific bacteria with IgA in preventing malnutrition was further demonstrated in a study in which germ‐free mice developed a diet‐dependent enteropathy when given SIgA‐coated bacteria from malnourished children with kwashiorkor. ¹²¹ In addition to the importance of commensal SIgA‐coated bacteria in gut inflammatory disease and malnutrition, human studies further support the importance of proper maternal SIgA‐coated microbiota in early life for allergy prevention. Dzidic et al. ¹²² found that infants who later developed asthma and allergies had lower proportions of SIgA‐coated bacteria by 12 months of age compared to healthy infants. These differences were not due to absolute SIgA levels or bacterial load, but rather to altered SIgA targeting patterns. Altered IgA recognition patterns in children developing allergy were evident as early as 1 month of age when the IgA antibodies are predominantly maternally derived in breastfed children. Similar findings were found in children developing coeliac disease. ¹²³ Taken together, these findings suggest that bacterial targets of maternal IgA have implications for childhood malnutrition and inflammatory disease susceptibility. A better understanding of how to regulate maternal SIgA and the pattern of bacterial coating may lead to novel approaches to prevent the burden of childhood disease.

The mechanisms through which SIgA influence the microbiota have mainly been studied in adults and remain to be verified in the context of establishing microbiota and breast milk SIGA. One mechanism is the aggregation of bacteria into biofilms, enhancing mucosal adhesion and stabilizing microbial presence. Bollinger et al. ¹²⁴ observed that SIgA and mucin promote biofilm formation and gut colonization by commensal microbiota, facilitating the formation of biofilms by nonpathogenic E. coli on epithelial cell monolayers grown in vitro. This interaction helps promote niche colonization and enhance microbial stability and nutrient exchange, which is crucial for maintaining a healthy gut. Another key mechanism is the coating of commensal bacteria, where SIgA controls their niche colonization and prevents their penetration into deeper tissues. ¹²⁵ This regulation allows commensal bacteria to perform their metabolic functions properly without activating inflammatory genes in the epithelium (and interfering with their nutrient uptake function) or being translocated to the local lymph nodes and being overly targeted by the host immune system. IgA binding can also regulate the microbiota by altering microbial gene expression. Mouse models have demonstrated that SIgA binding to Bacteroides thetaiotaomicron impacts its gene expression, reducing its inflammatory potential, promoting colonization, and modulating its metabolic functions, ultimately enhancing its fitness within the gut niche. ¹²⁶ , ¹²⁷ , ¹²⁸ Nakajima et al. ¹²⁸ further showed that B. theta, when coated by SIgA, induced an expansion of Clostridiales, which was associated with increased short‐chain fatty acid production and protection from colitis. These findings suggest that SIgA targeting of a single organism can influence not only that specific microbe but also the broader microbial community through complex microbial interactions.

In addition to SIgA, Breast milk IgG and IgM may play complementary roles in shaping the microbiota and infant immune response. In a mouse model, Koch et al. ¹²⁹ showed that microbiota‐specific breast milk SIgG limited the activation of mucosal T follicular helper cells and subsequent germinal center B cell responses in newborn mice, preventing excessive immune activation in response to newly colonizing commensal microbes and ensuring proper immune education. In a birth cohort, Janzon et al. ¹³⁰ found that the vast majority of bacteria coating was with IgA and that IgM coated the same bacteria as IgA. IgG coated a smaller fraction and a different set of bacteria. Despite this collaboration between multiple isotypes, Catanzaro et al. ¹³¹ demonstrated in IgA‐deficient individuals that SIgA plays a critical and nonredundant role in shaping the human gut microbiota. Despite the secretion of compensatory IgM into the gut lumen, SIgA‐deficient subjects showed an altered gut microbiota composition compared to healthy controls. These changes were characterized by a trend towards reduced overall microbial diversity as well as significant shifts in the relative abundances of specific microbial taxa. Although these findings were observed in adults, they translate to the unique role of breast milk SIgA in infants, where promoting microbial diversity and stability is crucial for proper immune system development and long‐term intestinal homeostasis.

6. A LASTING AND INTERGENERATIONAL IMPACT ON CHILD HEALTH

The major impact of breastmilk Ig on offspring immune development was stressed in a pioneer study in mice by Kramer et al. ¹¹⁵ The authors showed that in the absence of milk Ig, the offspring undergo an accelerated development of natural IgA responses that include germinal center reactions in both Peyer's patches and mesenteric lymph nodes. ¹¹⁵ Similarly, Torow et al. ¹³² found that CD4 T cells expanded in the neonatal small intestine but remained immature throughout the postnatal period despite antigen exposure and the rapid emergence of the microbiota. Again, the authors found that active suppressive mechanisms were in place to prevent CD4 T cell activation, which relied on the transfer of maternal IgA. ¹³² How do maternal antibodies actively modulate immune development is the topic of many ongoing research and involves multiple mechanisms such as microbiota shaping, microbiota metabolite transfer, antigen‐immune complex formation, and probably many more yet to be discovered (Figure 2, Box 1).

FIGURE 2.

Transgenerational immune imprinting. Maternal antibodies in breastmilk prepare and protect the newborn against environmental exposure, guide and regulate the offspring's immune system, and promote transgenerational adaptation of the immune system to its environment via (1) SIgA in combination with other immunoglobulins directly bind to pathogen antigens neutralizing their toxin and/or ability to damage and invade epithelial cells, and exclude dietary and environmental allergens thus protecting against the development of allergy, (2) promote the shaping of a healthy microbiota, via positively selecting for symbiotic commensals, promoting niche colonization via forming biofilms for symbiotic commensals, and influencing microbial gene expression towards an anti‐inflammatory profile by preventing the production of microbial metabolites or surface proteins that could be harmful to the host, (3) influence immune development by shaping the microbiota (SIgA and IgG), inhibiting T helper effector responses and regulating the immunoregulatory tone of the gut environment (SIgA), controlling B cell activation (IgG), the generation of antigen specific Tregs (IgG‐antigen immune complexe), and the development of ILC3 (IgA‐ maternal microbiota metabolite immune complexe). Image created with BioRender.

Given how SIgA dictates the composition of the gut microbiome and shapes adaptive immune responses against it, maternal SIgA can be considered the first signal that teaches the infant gut to differentiate between beneficial commensals and harmful pathogens. By selectively coating the microbiota or neutralizing bacterial pathogens based on the mother's microbial history, maternal SIgA provides the infant's developing immune system with critical cues on which microbes to tolerate and which to defend against. This transgenerational mechanism likely evolved to enable the host immune system to establish tolerance towards commensal microbes that are likely to be encountered later in life. ¹³³

Indirect effects of the maternal Ig response to microbiota also actively influence immune development in offspring. Gomez de Agüero et al. ¹³⁴ found that bacteria in the mother's gut during pregnancy drive later innate maturation of the newborn, specifically an increase in the population of intestinal ILC3, which we know is important for gut microbiota commensalism. This maternal effect on offspring gut immunity did not require the microbiota itself, but rather the transfer of specific maternal bacterial metabolites in utero and via breast milk, using maternal IgG‐ and IgA‐mediated transfer, respectively. ¹³⁴

Interestingly, Ramanan et al. ²⁸ showed that vertical transmission of maternal SIgA enabled transgenerational immune imprinting in mice. After meticulously ruling out other possibilities such as genetic and epigenetic factors, microbiota variation, and differences in milk‐derived metabolites, the authors showed that maternal milk IgA were the main factor controlling offspring's frequency of colonic RORgT Tregs, which are critical for controlling barrier function and inflammatory response to commensals. By a double‐negative‐feedback loop, this effect is stable over many generations. ²⁸ Collective evidence regarding transgenerational immune priming opens up the possibility of transmission of inflammatory phenotypes via maternal SIgA as well as tolerogenic ones. ¹³⁵

Finally, as described above, in addition to setting the tone for immune regulation, maternal antibodies may contribute to antigen‐specific immune education in the offspring. This has been shown in mice where IgG‐OVA immune complexes in breast milk promote immune tolerance and prevent allergy. ⁸¹ , ⁸⁷ The role of Ig‐pathogen antigen immune complexes in promoting defense has been demonstrated in adult mice. Preliminary data on infants breastfed by SARS‐CoV2 infected mothers suggest that this may also be the case in breastfed infants, ¹³⁶ but further studies are warranted.

AUTHOR CONTRIBUTIONS

VV drafted the content of the whole manuscript, focusing on allergy and immune imprinting; JT focused on the maternal immune system‐mammary gland axis, RC on infection prevention and BT on the microbiota part.

FUNDING INFORMATION

VV work is supported by the Family Larsson‐Rosenquist Foundation and BT by the Swiss National Science Foundation Postdoc Mobility.

CONFLICT OF INTEREST STATEMENT

The authors have no conflict of interest to declare for the content of this manuscript.

Verhasselt V, Tellier J, Carsetti R, Tepekule B. Antibodies in breast milk: Pro‐bodies designed for healthy newborn development. Immunol Rev. 2024;328:192‐204. doi: 10.1111/imr.13411

DATA AVAILABILITY STATEMENT

Not applicable.