Abstract
The anti-infective properties of breast milk have been known for decades. In recent years, an increasing number of papers have described the variety of bioactive compounds that are present in breast milk with varying degrees of antiviral activity. However, to date, the totality of the properties of these compounds is not fully understood and, above all, their synergistic interaction is not yet known. The purpose of this review is to describe the current knowledge about the antiviral compounds in breast milk, both with specific and non-specific action against pathogens. Due to the current pandemic situation from SARS-CoV-2 (Severe acute respiratory syndrome Coronavirus-2), research has focused on a multitude of potential antiviral substances, taking breast milk as a biological model of reference. Future research is needed to expand the knowledge of these compounds, which will hopefully assist in the development of therapies applicable even at later ages.
1. Introduction
Breast milk is now known as an incomparable source of bioactive substances that contribute to the newborn’s development and health. In recent years, there has been increased interest in human milk’s numerous compounds with promising immunological and antimicrobial effects [1]. It is now well known that breast milk has an overall protective effect on the newborn, particularly regarding the development of immunity and protection from infections [2].
The first systematic review describing the link between breastfeeding and the protection of newborns from infectious diseases was published in 1935 [3]. Since then, a growing amount of literature has demonstrated that the breastfed newborn is at lower risk of having a gastrointestinal viral disease or respiratory infection [4,5,6,7]. In addition, human milk feeding was correlated with protection against urinary tract infections, ear infections and many other diseases [8]. However, despite the identification of various compounds with antimicrobial effects, research has failed to fully understand human milk’s protective effect. This may be due not only to the presence of innumerable substances, but above all to a still lacking understanding of the hypothesized interactions between these substances and pathogens, such as for human milk oligosaccharides (HMOs) [9] and glycoproteins [10].
At birth, the newborn has not yet reached full immunological competency due to the adaptive immunity components’ immaturity. This immaturity derives from limited exposure to antigens in utero and impaired functions of B and T cells [11]. Therefore, the protection of the neonate against infections mainly depends upon the passively acquired antibodies transferred from the mother, bioactive antimicrobial compounds and the innate immune system [11]. To counteract this susceptibility, breast milk has evolved to concentrate on immunomodulating substances, antibodies and molecules with a direct antibacterial and antiviral function. This protective function is so marked that it has been speculated that the mammary gland could have ontogenically originated from the immune system. This would account for the functional aspect of the colostrum, which has an immunological function even before a nutritional one.
A recent review by Vorbach et al. has speculated about the innate immunity origin of the mammary gland by observing that many of the molecules found in breast milk also have immunological properties (Table 1). The authors suggest that milk could have evolved from a mucus secretion containing many evolutionarily conserved, protective molecules [12]. The combination of these mechanisms is so powerful that early exclusive breastfeeding may reduce the risk of postnatal human immunodeficiency virus infection 1 (HIV-1) transmission from HIV-positive mothers in ongoing treatment with antiretroviral therapy [13,14]. Recently, it has also been seen that the passage of Cytomegalovirus (CMV) through breast milk is strongly inhibited by some substances in the mother’s milk itself, in fact hindering the vertical transmission of the virus [15].
Table 1.
The main groups of antimicrobial compounds in human milk.
Thanks to the current pandemic situation caused by SARS-CoV-2 (Severe acute respiratory syndrome Coronavirus-2), breast milk’s antiviral properties have returned to be under the spotlight for many researchers worldwide. However, the complex interaction between human milk and viruses is yet to be understood. Recent studies have demonstrated the importance of the viral microbiota (virome) to the infant’s health [16]. In addition to the child’s intestinal bacterial colonization, there is a population of bacteriophages that modulate and regulate bacterial growth [17]. This virome has a dual origin and a sequential course. Liang et al. (2020) investigated how the newborn is first colonized mainly by bacteriophages, which are induced by the first bacteria constituting the microbiota. Subsequently, at around four months of life, human viruses with intracellular replication are also found in the infant’s intestine. Therefore, the quantity and pathogenicity of these human intracellular viruses are reduced if the infant is breastfed [18].
Emerging evidence shows that human milk viruses are also transmitted from the mother to the infant via breastfeeding [16]. The human milk virome has been only briefly investigated, but researchers are beginning to demonstrate a vertical transmission of bacteriophage, suggesting that the mammary gland provides both the bacterial and viral parts of the microbiome [19]. Therefore, it could be speculated that breast milk favors the survival and transmission of viral particles favorable to the neonate’s health and, at the same time, hinders the transmission and replication of pathogenic viral strains.
The purpose of this narrative review is to provide a brief description of the known antiviral compounds in human milk.
4. Conclusions
The current pandemic situation has renewed interest in compounds with known or supposed antiviral activity. As is often the case for other topics, breast milk is used as a biological model to be studied to extract its secrets, which have been obtained over hundreds and thousands of years. This narrative review cannot be defined as complete and conclusive. Despite the fact that the bioactive compounds of breast milk have been the subject of research for many years now, we still cannot say that we fully understand all their properties or, moreover, their peculiar interactions resulting in human milk having outstanding properties. This discussion aims to underline the importance of breast milk as a functional food for the infant and to be a stimulus for future research projects aimed at isolating and exploring the components of the most polyvalent compound—breast milk.
Author Contributions
Conceptualization, D.M., B.L.C. and M.L.G.; Writing—Original draft preparation, D.M., J.C. and G.V.; Writing—Review and editing, A.C., M.L.G. and F.C.; supervision, M.L.G., L.C. and F.M. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
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