Leukocyte Populations in Human Preterm and Term Breast Milk Identified by Multicolour Flow Cytometry

The Effects of Different Storage Conditions on Leukocytes in Human Breast Milk

Objectives

Breast milk is the best baby food because it contains various nutrients and important factors for the baby's immune system, including leukocytes. This study aimed to determine the effects on morphology, number of cells and breast milk leukocytes count of various ways of storing breast milk based on different temperatures and storage durations.

Methods

This study was conducted at the Biochemistry Laboratory, Integrated Laboratory and the Histology Laboratory, Universitas Indonesia, Jakarta, Indonesia from September 2022 to February 2023. Transitional breast milk samples from 7 breastfeeding mothers were utilised in the study. A total of 50 mL was divided into 4 tubes of 12.5 mL each and treated based on temperature, storage time and method of thawing frozen breast milk based on the Centers for Disease Control and Prevention's (CDC) recommendations for breast milk storage. The breast milk cells were isolated to calculate the cell number and leukocyte population. Subsequently, the breast milk cells were stained with haematoxylin and eosin to analyse the number and morphology of leukocytes.

Results

The findings showed a significant decrease in the breast milk's total number and population and changes in the morphology of breast milk leukocytes after storage.

Conclusion

This study indicates that CDC storage recommendations do not affect the quantity of the CD45+ leukocyte population; however, there is a decrease in the total number of leukocytes and alterations in their microscopic morphology. Thus, additional research is recommended to determine whether these modifications influence the function of the breast milk cells.

Characterization and comparison of human and mouse milk cells

Recent data has characterized human milk cells with unprecedented detail and provided insight into cell populations. While such analysis of freshly expressed human milk has been possible, studies of cell functionality within the infant have been limited to animal models. One commonly used animal model for milk research is the mouse; however, limited data are available describing the composition of mouse milk. In particular, the maternal cells of mouse milk have not been previously characterized in detail, in part due to the difficulty in collecting sufficient volumes of mouse milk. In this study, we have established a method to collect high volumes of mouse milk, isolate cells, and compare the cell counts and types to human milk. Surprisingly, we found that mouse milk cell density is three orders of magnitude higher than human milk. The cell types present in the milk of mice and humans are similar, broadly consisting of mammary epithelial cells and immune cells. These results provide a basis of comparison for mouse and human milk cells and will inform the most appropriate uses of mouse models for the study of human phenomena.

Frequencies of activated T cell populations increase in breast milk of HCMV-seropositive mothers during local HCMV reactivation

Background

Human cytomegalovirus (HCMV) can reactivate in the mammary gland during lactation and is shed into breast milk of nearly every HCMV-IgG-seropositive mother of a preterm infant. Dynamics of breast milk leukocytes during lactation, as well as blood leukocytes and the comparison between both in the context of HCMV reactivation is not well understood.

Methods

Here, we present the BlooMil study that aimed at comparing changes of immune cells in blood and breast milk from HCMV-seropositive- vs -seronegative mothers, collected at four time ranges up to two months post-partum. Viral load was monitored by qPCR and nested PCR. Multiparameter flow cytometry was used to identify leukocyte subsets.

Results

CD3+ T cell frequencies were found to increase rapidly in HCMV-seropositive mothers' milk, while they remained unchanged in matched blood samples, and in both blood and breast milk of HCMV-seronegatives. The activation marker HLA-DR was more strongly expressed on CD4+ and CD8+ T cells in all breast milk samples than matched blood samples, but HCMV-seropositive mothers displayed a significant increase of HLA-DR+ CD4+ and HLA-DR+ CD8+ T cells during lactation. The CD4+/CD8+ T cell ratio was lower in breast milk of HCMV-seropositive mothers than in the blood. HCMV-specific CD8+ T cell frequencies (recognizing pp65 or IE1) were elevated in breast milk relative to blood, which might be due to clonal expansion of these cells during local HCMV reactivation. Breast milk contained very low frequencies of naïve T cells with no significant differences depending on serostatus.

Conclusion

Taken together, we conclude that the distribution of breast milk leukocyte populations is different from blood leukocytes and may contribute to the decrease of breast milk viral load in the late phase of HCMV reactivation in the mammary gland.

Changes in the Immunology of Breast Milk From Obese or Overweight Women: a Brief Review

A systematic search was carried out through search platforms and specialized databases, such as Academic Google, PubMed, and Scopus, using thesauri: breast feeding, obesity, immunology, and human milk in English and Spanish, and those articles published from January 2000 to December 2021, in both languages. Only those reports that included quantitative data on immunological components in the milk of normal-weight and overweight women were considered. The PRISMA 2020 guides were used, and a total of 306 articles were reviewed, of which a total of 33 were included, according to the basic inclusion criteria. It was observed that in obese mothers, there is an increase in certain immune cells, such as B lymphocytes, cytotoxic T lymphocytes, and NK cells, and cytokines, such as IL-6 and IFN-γ; other alterations included the bacterial population and proteins with antibacterial action. Also, a decrease in growth factors such as TGF-β and IFG-1 was documented in overweight women. Immunoglobulin concentrations did not show substantial changes. This brief review shows that maternal overweight is associated with changes in the biochemical and immunological parameters of milk.

Preterm Delivery in Obese Mothers Predicts Tumor Necrosis Factor-α Levels in Breast Milk

Background: Breast milk (BM) is a nutritive fluid that is rich in bioactive components such as hormones and cytokines that can shape the newborn's feeding habits and program the newborn's immature immune system. BM components can change under different scenarios that include maternal body mass index (BMI) and premature birth. This study aimed to study the interaction of premature status or maternal obesity on the hormonal and cytokine profile in BM according to the sex of the offspring. Materials and Methods: We recruited 31 women with preterm births from the Centro de Alta Especialidad Dr. Rafael Lucio in Mexico. Luminex multiplexing assay was used for quantifying cytokine profile of monocyte chemoattractant protein-1, tumor necrosis factor (TNF)-α, interferon-γ, interleukin (IL)1-β, IL-2, IL-4, IL-6, IL-7, and hormones insulin, ghrelin, leptin, and glucagon in mature BM samples. Biological modeling was performed to predict the interaction between cytokines and hormones, maternal BMI status, infant birth sex, parity, and gestational age. Results: BM multiplex analysis showed positive correlations for TNF-α and increasing prematurity and for higher maternal BMI and IL-2, IL-4, and IL-6 cytokines. Multiple regression models identified an interaction between maternal BMI and gestational weeks in male infants that is associated to TNF-α accumulation in BM. Biological modeling predicts that preterm delivery in mothers with obesity modulates TNF- α levels in mature BM of women with male offspring. Conclusion: Prematurity and obesity modify BM's immune profile. TNF- α expression increases as prematurity increases, and maternal BMI correlates positively with increases in IL-2, IL-6, and IL-4. Our multiple regression model also shows that maternal BMI and gestational weeks in male infants predict TNF-α.

Anti-Inflammatory and Anti-Allergic Properties of Colostrum from Mothers of Full-Term and Preterm Babies: The Importance of Maternal Lactation in the First Days

Our narrative review focuses on colostrum components, particularly those that influence the neonatal immune system of newborns. Colostrum is secreted in small volumes by the alveolar cells of the breast during the first two to five days after birth. Colostrum is poor in fat and carbohydrates, with larger protein and bioactive compounds than mature milk. It plays a crucial role in driving neonates' immunity, transferring those immunological factors which help the correct development of the neonatal immune system and support establishing a healthy gut microbiome. The newborn has an innate and adaptive immune system deficiency, with a consequent increase in infection susceptibility. In particular, neonates born prematurely have reduced immunological competencies due to an earlier break in the maternal trans-placenta transfer of bioactive components, such as maternal IgG antibodies. Moreover, during pregnancy, starting from the second trimester, maternal immune cells are conveyed to the fetus and persist in small quantities post-natal, whereby this transfer is known as microchimerism (MMc). Thus, preterm newborns are deficient in this maternal heritage, and have their own immune system under-developed, but colostrum can compensate for the lack. Early breastfeeding, which should be strongly encouraged in mothers of preterm and full-term babies, provides those immunomodulant compounds that can act as a support, allowing the newborn to face immune needs, including fronting infections and establishing tolerance. Moreover, making mothers aware that administering colostrum helps their infants in building a healthy immune system is beneficial to sustain them in the difficult post-partum period.

Immunomodulatory Effect of Infectious Disease of a Breastfed Child on the Cellular Composition of Breast Milk

Recent studies suggest that the content of immune components in milk is influenced by the mother's health and also by the infant she feeds. We aimed to evaluate the effect of a child's respiratory tract infection on the cellular composition of breast milk (neutrophils, monocytes, eosinophils, lymphocytes, and their subpopulations). Twenty-six breastfeeding mothers whose children were hospitalized for respiratory tract infections were enrolled in the study. The control group consisted of 23 mothers of healthy children. Regarding the children, baseline laboratory blood tests were performed, and nasal swabs were taken for the presence of RS virus. In the next step, milk samples were collected from the mothers to assess the cellular composition of the milk, including neutrophils, monocytes, eosinophils, lymphocytes, and their subpopulations. Significantly higher percentages of T lymphocytes (helper and cytotoxic lymphocytes) were observed in the milk of the studied mothers. There was a significantly higher percentage of milk lymphocytes in the group of affected children with confirmed RSV etiology than in children with excluded RSV etiology. A significant positive correlation was observed between the duration of infection and the percentage of milk NK cells and between milk CD19 lymphocytes and the child's serum leukocytosis. This study may provide evidence of a link between cells in breast milk and disease in the breastfed infant. The severity of the infection, its duration, and the etiological agent of the infection may affect the cellular composition of milk.

Milk Transmission of Mammalian Retroviruses

The transmission of viruses from one host to another typically occurs through horizontal or vertical pathways. The horizontal pathways include transmission amongst individuals, usually through bodily fluids or excretions, while vertical transmission transpires from mother to their offspring, either during pregnancy, childbirth, or breastfeeding. While there are more than 200 human pathogenic viruses to date, only a small number of them are known to be transmitted via breast milk, including cytomegalovirus (CMV), human immunodeficiency virus type 1 (HIV-1), and human T cell lymphotropic virus type 1 (HTLV-1), the latter two belonging to the family Retroviridae. Breast milk transmission is a common characteristic among mammalian retroviruses, but there is a lack of reports summarizing our knowledge regarding this route of transmission of mammalian retroviruses. Here, we provide an overview of the transmission of mammalian exogenous retroviruses with a focus on Orthoretrovirinae, and we highlight whether they have been described or suspected to be transmitted through breast milk, covering various species. We also elaborate on the production and composition of breast milk and discuss potential entry sites of exogenous mammalian retroviruses during oral transmission.

Lactation-associated macrophages exist in murine mammary tissue and human milk

Macrophages are involved in immune defense, organogenesis and tissue homeostasis. Macrophages contribute to the different phases of mammary gland remodeling during development, pregnancy and involution postlactation. Less is known about the dynamics of mammary gland macrophages in the lactation stage. Here, we describe a macrophage population present during lactation in mice. By multiparameter flow cytometry and single-cell RNA sequencing, we identified a lactation-induced CD11c⁺CX3CR1⁺Dectin-1⁺ macrophage population (liMac) that was distinct from the two resident F4/80^hi and F4/80^lo macrophage subsets present pregestationally. LiMacs were predominantly monocyte-derived and expanded by proliferation in situ concomitant with nursing. LiMacs developed independently of IL-34, but required CSF-1 signaling and were partly microbiota-dependent. Locally, they resided adjacent to the basal cells of the alveoli and extravasated into the milk. We found several macrophage subsets in human milk that resembled liMacs. Collectively, these findings reveal the emergence of unique macrophages in the mammary gland and milk during lactation.

The hormonal physiology of immune components in breast milk and their impact on the infant immune response

During pregnancy, the maternal body undergoes a considerable transformation regarding the anatomy, metabolism, and immune profile that, after delivery, allows for protection and nourishment of the offspring via lactation. Pregnancy hormones are responsible for the development and functionality of the mammary gland for breast milk production, but little is known about how hormones control its immune properties. Breast milk composition is highly dynamic, adapting to the nutritional and immunological needs that the infant requires in the first months of life and is responsible for the main immune modeling of breastfed newborns. Therefore, alterations in the mechanisms that control the endocrinology of mammary gland adaptation for lactation could disturb the properties of breast milk that prepare the neonatal immune system to respond to the first immunologic challenges. In modern life, humans are chronically exposed to endocrine disruptors (EDs), which alter the endocrine physiology of mammals, affecting the composition of breast milk and hence the neonatal immune response. In this review, we provide a landscape of the possible role of hormones in the control of passive immunity transferred by breast milk and the possible effect of maternal exposure to EDs on lactation, as well as their impacts on the development of neonatal immunity.

Parental factors that impact the ecology of human mammary development, milk secretion, and milk composition-a report from "Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN)" Working Group 1

The goal of Working Group 1 in the Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN) Project was to outline factors influencing biological processes governing human milk secretion and to evaluate our current knowledge of these processes. Many factors regulate mammary gland development in utero, during puberty, in pregnancy, through secretory activation, and at weaning. These factors include breast anatomy, breast vasculature, diet, and the lactating parent's hormonal milieu including estrogen, progesterone, placental lactogen, cortisol, prolactin, and growth hormone. We examine the effects of time of day and postpartum interval on milk secretion, along with the role and mechanisms of lactating parent-infant interactions on milk secretion and bonding, with particular attention to the actions of oxytocin on the mammary gland and the pleasure systems in the brain. We then consider the potential effects of clinical conditions including infection, pre-eclampsia, preterm birth, cardiovascular health, inflammatory states, mastitis, and particularly, gestational diabetes and obesity. Although we know a great deal about the transporter systems by which zinc and calcium pass from the blood stream into milk, the interactions and cellular localization of transporters that carry substrates such as glucose, amino acids, copper, and the many other trace metals present in human milk across plasma and intracellular membranes require more research. We pose the question of how cultured mammary alveolar cells and animal models can help answer lingering questions about the mechanisms and regulation of human milk secretion. We raise questions about the role of the lactating parent and the infant microbiome and the immune system during breast development, secretion of immune molecules into milk, and protection of the breast from pathogens. Finally, we consider the effect of medications, recreational and illicit drugs, pesticides, and endocrine-disrupting chemicals on milk secretion and composition, emphasizing that this area needs much more research attention.

Metabolomic Diversity of Human Milk Cells over the Course of Lactation-A Preliminary Study

Human milk (HM) is a complex biofluid containing a wide cell variety including epithelial cells and leukocytes. However, the cellular compositions and their phenotypic properties over the course of lactation are poorly understood. The aim of this preliminary study was to characterize the cellular metabolome of HM over the course of lactation. Cells were isolated via centrifugation and the cellular fraction was characterized via cytomorphology and immunocytochemical staining. Cell metabolites were extracted and analyzed using ultra-performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UPLC-QqTOF-MS) in the positive and negative electrospray ionization modes. Immunocytochemical analysis revealed a high variability of the number of detected cells with relative median abundances of 98% of glandular epithelial cells, 1% of leukocytes, and 1% of keratinocytes. Significant correlations between the milk postnatal age with percentage of epithelial cells and leukocytes, and with total cell count were observed. Results from the Hierarchical Cluster Analysis of immunocytochemical profiles were very similar to those observed in the analysis of the metabolomic profiles. In addition, metabolic pathway analysis showed alterations in seven metabolic pathways correlating with postnatal age. This work paves the way for future investigations on changes in the metabolomic fraction of the cellular compartment of HM.

Spike-specific T cells are enriched in breastmilk following SARS-CoV-2 mRNA vaccination

Human breastmilk is rich in T cells; however, their specificity and function are largely unknown. We compared the phenotype, diversity, and antigen specificity of T cells in breastmilk and peripheral blood of lactating individuals who received SARS-CoV-2 messenger RNA (mRNA) vaccination. Relative to blood, breastmilk contained higher frequencies of T effector and central memory populations that expressed mucosal-homing markers. T cell receptor sequence overlap was limited between blood and breastmilk. Overabundant breastmilk clones were observed in all individuals, were diverse, and contained complementarity-determining regions in three sequences with known epitope specificity, including to SARS-CoV-2 spike. SARS-CoV-2 spike-specific T cell receptors were more frequent in breastmilk compared to blood and expanded in breastmilk following a 3rd mRNA vaccine dose. Our observations indicate that the lactating breast contains a distinct T cell population that can be modulated by maternal vaccination with potential implications for passive infant protection.

A cross-sectional study evidences regulations of leukocytes in the colostrum of mothers with obesity

BACKGROUND

Breastmilk is a dynamic fluid whose initial function is to provide the most adapted nutrition to the neonate. Additional attributes have been recently ascribed to breastmilk, with the evidence of a specific microbiota and the presence of various components of the immune system, such as cytokines and leukocytes. The composition of breastmilk varies through time, according to the health status of mother and child, and altogether contributes to the future health of the infant. Obesity is a rising condition worldwide that creates a state of systemic, chronic inflammation including leukocytosis. Here, we asked whether colostrum, the milk produced within the first 48 h post-partum, would contain a distinct leukocyte composition depending on the body mass index (BMI) of the mother.

METHODS

We collected peripheral blood and colostrum paired samples from obese (BMI > 30) and lean (BMI < 25) mothers within 48 h post-partum and applied a panel of 6 antibodies plus a viability marker to characterize 10 major leukocyte subpopulations using flow cytometry.

RESULTS

The size, internal complexity, and surface expression of CD45 and CD16 of multiple leukocyte subpopulations were selectively regulated between blood and colostrum irrespective of the study groups, suggesting a generalized cell-specific phenotype alteration. In obesity, the colostrum B lymphocyte compartment was significantly reduced, and CD16⁺ blood monocytes had an increased CD16 expression compared to the lean group.

CONCLUSIONS

This is the first characterization of major leukocyte subsets in colostrum of mothers suffering from obesity and the first report of colostrum leukocyte subpopulations in Latin America. We evidence various significant alterations of most leukocyte populations between blood and colostrum and demonstrate a decreased colostrum B lymphocyte fraction in obesity. This pioneering study is a stepping stone to further investigate active immunity in human breastmilk.

The human milk proteome and allergy of mother and child: Exploring associations with protein abundances and protein network connectivity

Background

The human milk proteome comprises a vast number of proteins with immunomodulatory functions, but it is not clear how this relates to allergy of the mother or allergy development in the breastfed infant. This study aimed to explore the relation between the human milk proteome and allergy of both mother and child.

Methods

Proteins were analyzed in milk samples from a subset of 300 mother-child dyads from the Canadian CHILD Cohort Study, selected based on maternal and child allergy phenotypes. For this selection, the definition of "allergy" included food allergy, eczema, allergic rhinitis, and asthma. Proteins were analyzed with non-targeted shotgun proteomics using filter-aided sample preparation (FASP) and nanoLC-Orbitrap-MS/MS. Protein abundances, based on label-free quantification, were compared using multiple statistical approaches, including univariate, multivariate, and network analyses.

Results

Using univariate analysis, we observed a trend that milk for infants who develop an allergy by 3 years of age contains higher abundances of immunoglobulin chains, irrespective of the allergy status of the mother. This observation suggests a difference in the milk's immunological potential, which might be related to the development of the infant's immune system. Furthermore, network analysis showed overall increased connectivity of proteins in the milk of allergic mothers and milk for infants who ultimately develop an allergy. This difference in connectivity was especially noted for proteins involved in the protein translation machinery and may be due to the physiological status of the mother, which is reflected in the interconnectedness of proteins in her milk. In addition, it was shown that network analysis complements the other methods for data analysis by revealing complex associations between the milk proteome and mother-child allergy status.

Conclusion

Together, these findings give new insights into how the human milk proteome, through differences in the abundance of individual proteins and protein-protein associations, relates to the allergy status of mother and child. In addition, these results inspire new research directions into the complex interplay of the mother-milk-infant triad and allergy.

The Effect of Infant Gastric Digestion on Human Maternal Milk Cells

SCOPE

Human breast milk contains a variety of cell types that have potential roles in infant immunity and development. One challenge associates with defining the purpose(s) of milk cells in the infant is a poor understanding of the effect of digestion on cell fate.

METHODS AND RESULTS

This study first demonstrates that milk cell death occurs after gastric digestion in mice. Then flow cytometry and RT-PCR are used to understand the mechanism of human milk cell death and quantify live cell types before and after simulated gastric digestion. This study finds that digestion in simulated gastric fluid for 30 min reduces cell viability from 72% to 27%, with most cell death is caused by the acidic pH. The primary mechanism of cell death is caspase-mediated apoptosis. The non-cellular components of milk offer only mild protection against cell death from stomach acid.

CONCLUSIONS

Gastric digestion does not select for a specific resilient cell population to survive-most cell types die in equal proportions in the gastric environment. Taken together, these results provide a foundation with which to understand the fate of human breast milk cells in the infant's intestine and beyond.

Spike-specific T cells are enriched in breastmilk following SARS-CoV-2 mRNA vaccination

Human breastmilk is rich in T cells; however, their specificity and function are largely unknown. We compared the phenotype, diversity, and antigen specificity of T cells in the breastmilk and peripheral blood of lactating individuals who received SARS-CoV-2 mRNA vaccination. Relative to blood, breastmilk contained higher frequencies of T effector and central memory populations that expressed mucosal-homing markers. T cell receptor (TCR) sequence overlap was limited between blood and breastmilk. Overabundan t breastmilk clones were observed in all individuals, were diverse, and contained CDR3 sequences with known epitope specificity including to SARS-CoV-2 Spike. Spike-specific TCRs were more frequent in breastmilk compared to blood and expanded in breastmilk following a third mRNA vaccine dose. Our observations indicate that the lactating breast contains a distinct T cell population that can be modulated by maternal vaccination with potential implications for infant passive protection.

One-Sentence Summary

The breastmilk T cell repertoire is distinct and enriched for SARS-CoV-2 Spike-specificity after maternal mRNA vaccination.

Assessment of Anti m-RNA SARS-CoV-2 (BNT162b2) Antibody Titres in Mother and Child Pairs of Breastfeeding Women Vaccinated Post-Delivery

The global response to the COVID-19 pandemic has been successfully driven by efforts to ramp up access to vaccines. Pregnant or breastfeeding women and their children have not benefited from the vaccines despite their susceptibility to the virus. We investigated whether women who were offered vaccination after delivery passively transferred protective antibodies to their infants via breast milk. Serum was collected from breast feeding mother–infant pairs and analysed for levels of antibodies to the SARS-CoV-2 spike protein using the CLIA chemiluminescence technique. Data were analysed for the significance of the differences using the Mann–Whitney U test and the Spearman’s rank correlation coefficient to determine the strength of the correlation. A total of 13 mothers, mean age 34.86 (95%CI = 33.21–36.48) years and their infants, mean age 15.77 (95%CI = 11.24–20.29) months were enrolled. The mothers had completed their courses of the mRNA BNT162b2 SARS-CoV-2 vaccine during breastfeeding, 8.3 (95%CI = 7.24–9.36) months before the study. All 13 mothers had detectable antibodies to the SARS-CoV-2 spike protein, mean 1252, (95%CI = 736–1769) BAU/mL. Antibodies were detected in 3/13 (23%) breast-fed infants mean 322, (95%CI = 252–897) BAU/mL. There was no correlation between the maternal and infant IgG antibody titres. The time-lag since full vaccination did not correlate to the presence of antibodies in infant sera. Maternal or infant ages did not correlate to the presence of antibodies. Although some children (23%) had anti-SARS-CoV-2 antibodies, there was no association between vaccine-induced COVID-19 spike protein specific maternal IgG antibody titres and the presence of antibodies in the breastfed infants. The data show that the transfer of passive immunity to infants following post-partum vaccination with the mRNA BNT162b2 SARS-CoV-2 vaccine may be infrequent in this population.

Profiling of mature-stage human breast milk cells identifies six unique lactocyte subpopulations

Breast milk is chock-full of nutrients, immunological factors, and cells that aid infant development. Maternal cells are the least studied breast milk component, and their unique properties are difficult to identify using traditional techniques. Here, we characterized the cells in mature-stage breast milk from healthy donors at the protein, gene, and transcriptome levels. Holistic analysis of flow cytometry, quantitative polymerase chain reaction, and single-cell RNA sequencing data identified the predominant cell population as epithelial with smaller populations of macrophages and T cells. Two percent of epithelial cells expressed four stem cell markers: SOX2, TRA-1-60, NANOG, and SSEA4. Furthermore, milk contained six distinct epithelial lactocyte subpopulations, including three previously unidentified subpopulations programmed toward mucosal defense and intestinal development. Pseudotime analysis delineated the differentiation pathways of epithelial progenitors. Together, these data define healthy human maternal breast milk cells and provide a basis for their application in maternal and infant medicine.

Vaccine Protection Through Placenta and Breastfeeding: The Unmet Topic in COVID-19 Pandemic

The coronavirus disease 2019 (COVID-19) pandemic has turned pregnant women’s healthcare into a worldwide public health challenge. Although initial data did not demonstrate pregnancy as a more susceptible period to severe outcomes of acute severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) infection, there are an increasing number of reports showing that not only pregnant women might be at significantly higher risk than non-pregnant women by COVID-19 but also the fetus. These findings may be related to adaptive changes that occur during pregnancy, such as the reduction in the residual respiratory capacity, the decrease in viral immune responses, and the increased risk for thromboembolic events. Additionally, despite the SARS-CoV-2 vertical transmission evidence being uncommon, maternal illness severity might reflect serious perinatal and neonatal outcomes. Thus, protecting the maternal–fetal dyad against COVID-19 is critical. Even though pregnant women initially were excluded from vaccine trials, several studies have provided safety and efficacy of the overall vaccine COVID-19 platforms. Vaccination during pregnancy becomes a priority and can generate benefits for both the mother and newborn: maternal neutralizing antibodies are transmitted through the placenta and breastfeeding. Moreover, regarding passive immunization, human milk contains other bioactive molecules and cells able to modulate the newborn’s immune response, which can be amplified after the vaccine. Nonetheless, many issues remain to be elucidated, considering the magnitude of the protective immunity transferred, the duration of the induced immunity, and the optimal interval for pregnant immunization. In this review, we assessed these unmet topics supported by literature evidence regarding the vaccine’s immunogenicity, pregnancy immune heterogeneity, and the unique human milk antiviral features.

Advantages and Challenges of Differential Immune Cell Count Determination in Blood and Milk for Monitoring the Health and Well-Being of Dairy Cows

A key challenge of the 21st century will be to provide the growing world population with a sustainable and secure supply of food. Consequently, the dairy farming’s primary task is to lower milk losses and other inefficiencies associated with diseased cows. Moreover, a shift from curative to preventive health management would be desirable for mastitis and a wide variety of other infectious and non-infectious cattle diseases, some of which are known to have profound negative effects on the performance and well-being of cows. Differential cell counting (DCC), a procedure that aims to determine the proportions of different somatic cell types in raw milk samples, has not only the potential to optimize mastitis diagnostics, but it could furthermore serve as a diagnostic tool for monitoring the general and overall health status of dairy cows. Based on a broad search of the literature, the practical utility of various types of DCC is summarized and discussed in this review. Since it might be of advantage to interpret DCC with the aid of data from studies in humans, differences between the immune systems of humans and dairy cattle, with a special focus on surface marker expression profiles and γδ (gamma delta) T-cell characteristics, are also described.

Breast Milk and the Importance of Chrononutrition

During pregnancy the human fetus receives timed cues from the circadian rhythms of temperature, metabolites, and hormones from the mother. This influence is interrupted after parturition, the infant does not secrete melatonin and their circadian rhythms are still immature. However, evolution provided the solution to this problem. The newborn can continue receiving the mother's timed cues through breastmilk. Colostrum, transitional, and mature human milk are extraordinary complex biofluids that besides nutrients, contain an array of other non-nutritive components. Upon birth the first milk, colostrum, is rich in bioactive, immunological factors, and in complex oligosaccharides which help the proper establishment of the microbiome in the gut, which is crucial for the infants' health. Hormones, such as glucocorticoids and melatonin, transfer from the mother's plasma to milk, and then the infant is exposed to circadian cues from their mother. Also, milk components of fat, proteins, amino acids, and endogenous cannabinoids, among others, have a markedly different concentration between day and night. In the present review, we give an overview of nutritive and non-nutritive components and their daily rhythms in human milk and explore their physiological importance for the infant. Finally, we highlight some interventions with a circadian approach that emphasize the importance of circadian rhythms in the newborn for their survival, proper growth, and development. It is estimated that ~600,000 deaths/year are due to suboptimal breastfeeding. It is advisable to increase the rate of exclusive breastfeeding, during the day and night, as was established by the evolution of our species.

Impact of IgG Isotype on the Induction of Antibody-Dependent Cellular Phagocytosis of HIV by Human Milk Leukocytes

Approximately 100,000 mother-to-child transmission (MTCT) events of HIV via human milk feeding occur each year. However, only about 15% of infants milk-fed by untreated HIV+ mothers become infected, suggesting a protective effect of the milk itself. Infants ingest 105-108 maternal leukocytes daily via milk, which remain functional beyond ingestion. Such function may be elicited by maternal milk antibody (Ab). Though IgA is dominant in milk, most HIV-specific milk Abs are of the IgG subclass, highlighting the importance of investigating the function of each IgG isotype in the milk context. Though Ab effector function mediated by the constant (Fc) domain via interaction with Fc Receptors (FcRs), such as Ab-dependent cellular phagocytosis (ADCP), are critical in protecting against HIV infection, ADCP is largely unexplored as it relates to mitigation of MTCT. Presently we report the ADCP activity of milk leukocytes against HIV particles and immune complexes (ICs), using 57 unique samples from 34 women, elicited by IgG1/2/3/4 of monoclonal (m)Ab 246-D. Granulocyte ADCP of HIV was most potent compared to other phagocytes when elicited by IgG1/3/4. IgG1/3 activated granulocytes similarly, exhibiting 1.6x-4.4x greater activity compared to IgG2/4, and a preference for virus compared to ICs. Notably, CD16- monocyte ADCP of a given target were unaffected by isotype, and CD16+ monocytes were poorly stimulated by IgG1. IgG2/4 elicited potent IC ADCP, and in terms of total leukocyte IC ADCP, IgG4 and IgG3 exhibited similar function, with IgG4 eliciting 1.6x-2.1x greater activity compared to IgG1/IgG2, and CD16+ monocytes most stimulated by IgG2. These data contribute to a more comprehensive understanding of Fc-mediated functionality of milk leukocytes, which is critical in order to develop therapeutic approaches to eliminating this route of MTCT, including mucosal administration of mAbs and/or a maternal vaccination aimed to elicit a potent milk Ab response.

Milk Immune Cell Composition in Dromedary Camels With Subclinical Mastitis

Mastitis represents one of the most important infectious diseases in camels with heavy economic losses due to reduced milk quantity and quality. Balanced immune cell composition and function in the mammary gland are essential for effective immune response to mastitis pathogens. The objective of the present study was to characterize the cellular immune response to subclinical mastitis in the mammary gland of dromedary camels. Therefore, immunostaining and flow cytometry were used to compare the cellular composition, leukocyte phenotype, and cell viability in camel milk from healthy she-camels (n = 8) and she-camels with subclinical mastitis (SCM; n = 6). In addition, the ex vivo phagocytic activity of milk phagocytes was compared between healthy and affected animals. The health status of the mammary gland was evaluated based on the California Mastitis Test (CMT) score. SCM (CMT score of ≥3 in the absence of clinical signs of mastitis) was found in six of the 56 sampled quarters (10.7 %) with only one affected quarter per animal. In comparison to milk from healthy camels, milk from SCM animals showed higher somatic cell count (SCC), higher numbers of CD45+ leukocytes with an expanded fraction of CD172a+ myeloid cells. Within the myeloid cell population, there was an increase in the percentage of granulocytes (CD172a⁺CD14^low) with a decreased percentage of macrophages (CD172a⁺CD14^high) in milk from affected animals compared to healthy animals. The decrease in lymphoid cells in SCM milk was mainly due to the decreased fraction of CD4+ helper T cells. Camel SCM was also associated with a stimulated phenotype, increased cell viability, and enhanced phagocytic activity of the milk phagocytes, macrophages and granulocytes. Collectively, the present study identified significant changes in SCC, leukocyte count, phenotype, viability, and function in association with subclinical mastitis in camels. The results of the present study support a better understanding of host-pathogen interaction mechanisms in the camel mammary gland.

Cellular and transcriptional diversity over the course of human lactation

Human breast milk (hBM) is a dynamic fluid that contains millions of cells, but their identities and phenotypic properties are poorly understood. We generated and analyzed single-cell RNA-sequencing (scRNA-seq) data to characterize the transcriptomes of cells from hBM across lactational time from 3 to 632 d postpartum in 15 donors. We found that the majority of cells in hBM are lactocytes, a specialized epithelial subset, and that cell-type frequencies shift over the course of lactation, yielding greater epithelial diversity at later points. Analysis of lactocytes reveals a continuum of cell states characterized by transcriptional changes in hormone-, growth factor-, and milk production-related pathways. Generalized additive models suggest that one subcluster, LC1 epithelial cells, increases as a function of time postpartum, daycare attendance, and the use of hormonal birth control. We identify several subclusters of macrophages in hBM that are enriched for tolerogenic functions, possibly playing a role in protecting the mammary gland during lactation. Our description of the cellular components of breast milk, their association with maternal–infant dyad metadata, and our quantification of alterations at the gene and pathway levels provide a detailed longitudinal picture of hBM cells across lactational time. This work paves the way for future investigations of how a potential division of cellular labor and differential hormone regulation might be leveraged therapeutically to support healthy lactation and potentially aid in milk production.

Human Breast Milk: From Food to Active Immune Response With Disease Protection in Infants and Mothers

Breastfeeding is associated with long-term wellbeing including low risks of infectious diseases and non-communicable diseases such as asthma, cancer, autoimmune diseases and obesity during childhood. In recent years, important advances have been made in understanding the human breast milk (HBM) composition. Breast milk components such as, non-immune and immune cells and bioactive molecules, namely, cytokines/chemokines, lipids, hormones, and enzymes reportedly play many roles in breastfed newborns and in mothers, by diseases protection and shaping the immune system of the newborn. Bioactive components in HBM are also involved in tolerance and appropriate inflammatory response of breastfed infants if necessary. This review summarizes the current literature on the relationship between mother and her infant through breast milk with regard to disease protection. We will shed some light on the mechanisms underlying the roles of breast milk components in the maintenance of health of both child and mother.

Study of Stem Cells in Human Milk

Stem cells are cells that have the ability to self-renew into an undifferentiated cell state, which can further delineate into distinct cell types. There are various sources of stem cells in the human body; some of them include cord blood, placental tissue, bone marrow, adipose tissue, dental pulp, etc. 

Breast milk could become an important source of stem cells in the near future because of its non-invasive isolation technique. Based on this nature, this study was conducted to isolate stem cells from breast milk and to show further potential implications of these cells. The total number of cells isolated from the milk ranged from 1.5 × 10⁵ cells to 3 × 10⁵ cells. As there was prolongation in the lactation period, the number of cells in the milk lowered significantly. There was no significant difference in the cell count in various gestational age groups. The cytochemistry analysis of these cells with their specific cell markers confirmed the presence of a homogenous population of mesenchymal stem cells. Further differentiation of these breast milk stem cell analyses showed transformation into adipocytes, chondrocytes, and osteoblasts in different culture mediums. So the presence of mesenchymal stem cells in human milk, which are multipotent in nature, makes it an important source of stem cells for further regenerative therapies, tissue culture techniques, and gene therapies. Due to this nature, these cells can be redirected to produce various tissues in the human body.

Human milk: From complex tailored nutrition to bioactive impact on child cognition and behavior

Human milk is a highly complex liquid food tailor-made to match an infant's needs. Beyond documented positive effects of breastfeeding on infant and maternal health, there is increasing evidence that milk constituents also impact child neurodevelopment. Non-nutrient milk bioactives would contribute to the (long-term) development of child cognition and behavior, a process termed 'Lactocrine Programming'. In this review we discuss the current state of the field on human milk composition and its links with child cognitive and behavioral development. To promote state-of-the-art methodologies and designs that facilitate data pooling and meta-analytic endeavors, we present detailed recommendations and best practices for future studies. Finally, we determine important scientific gaps that need to be filled to advance the field, and discuss innovative directions for future research. Unveiling the mechanisms underlying the links between human milk and child cognition and behavior will deepen our understanding of the broad functions of this complex liquid food, as well as provide necessary information for designing future interventions.

Testing the effects of processing on donor human Milk: Analytical methods

Holder pasteurization is the current recommended method for donor human milk treatment. This method effectively eliminates most life-threatening contaminants in donor milk, but it also greatly reduces some of its biological properties. Consequently, there is a growing interest for developing novel processing methods that can ensure both microbial inactivation and a higher retention of the functional components of donor milk. Our aim was to offer a comprehensive overview of the analytical techniques available for the evaluation of such methods. To suggest an efficient workflow for the analysis of processed donor milk, a safety analytical panel as well as a nutritional value and functionality analytical panel are discussed, together with the principles, benefits, and drawbacks of the available techniques. Concluding on the suitability of a novel method requires a multifactorial approach which can be achieved by a combination of analytical targets and by using complementary assays to cross-validate the obtained results.

Maternal vaccination as an additional approach to improve the protection of the nursling: Anti-infective properties of breast milk

Human milk constitutes a secretion with unique functions of both nourishing the nursling and providing protection against enteric and respiratory infections, mainly due to its content of secretory IgA antibodies but also due to the presence of a plethora of bioactive factors. Specific IgA antibodies are produced locally by plasma cells derived from B lymphocytes that migrate from other mucosae to the mammary gland during lactation, particularly from the gastrointestinal and respiratory tracts. Therefore, here, the authors will provide a comprehensive review of the content and functions of different nutritional and bioactive anti-infectious components from breast milk, such as oligosaccharides, lactoferrin, haptocorrin, α-lactalbumin, k-casein, lysozyme, lactoperoxidase, mucin, fatty acids, defensins, cytokines and chemokines, hormones and growth factors, complement proteins, leukocytes and nucleic acids, including microRNAs, among many others, and the induction of antibody responses in breast milk after maternal vaccination with several licensed vaccines, including the anti-SARS-CoV-2 vaccine preparations used worldwide. Currently, in the midst of the pandemic, maternal vaccination has re-emerged as a crucial source of passive immunity to the neonate through the placenta and breastfeeding, considering that maternal vaccination can induce specific antibodies if performed during pregnancy and after delivery. There have been some reports in the literature about milk IgA antibodies induced by bacterial antigens or inactivated virus vaccines, such as anti-diphtheria-tetanus-pertussis, anti-influenza viruses, anti-pneumococcal and meningococcal polysaccharide preparations. Regarding anti-SARS-CoV-2 vaccines, most studies demonstrate elevated levels of specific IgA and IgG antibodies in milk with virus-neutralizing ability after maternal vaccination, which represents an additional approach to improve the protection of the nursling during the entire breastfeeding period.

Immunomonitoring of Human Breast Milk Cells During HCMV-Reactivation

Background

Breast milk leukocytes may play a role in protecting the infant from pathogens. The dynamics and the role of lymphocytes in human cytomegalovirus (HCMV)-seropositive mothers shedding HCMV into breast milk during the first months postpartum (p.p.) are mostly unclear.

Methods

Breast milk cells were analyzed by Pappenheim panoptic and alpha-naphthyl acetate esterase staining as well as by imaging and polychromatic flow cytometry to simultaneously establish their morphological and phenotypic properties. The latter were characterized in HCMV-seropositive and seronegative mothers´ breast milk cells at different time points p.p.

Results

Panoptic staining of breast milk cells revealed the presence of monocytes/macrophages, granulocytes and lymphocytes. Imaging flow cytometry data combining phenotypic and morphological analysis identified NKT-like cells, NK cells, epithelial cells, T cells and monocytes/macrophages. HCMV-seropositive but not -seronegative mothers had significantly higher T cell frequencies in mature milk.

Conclusions

The presence of lymphocyte subsets in breast milk may be more influenced by the HCMV-seropositivity of the mother than previously recognized.

Specialized Pro-Resolving Lipid Mediators in Neonatal Cardiovascular Physiology and Diseases

Cardiovascular disease remains a leading cause of mortality worldwide. Unresolved inflammation plays a critical role in cardiovascular diseases development. Specialized Pro-Resolving Mediators (SPMs), derived from long chain polyunsaturated fatty acids (LCPUFAs), enhances the host defense, by resolving the inflammation and tissue repair. In addition, SPMs also have anti-inflammatory properties. These physiological effects depend on the availability of LCPUFAs precursors and cellular metabolic balance. Most of the studies have focused on the impact of SPMs in adult cardiovascular health and diseases. In this review, we discuss LCPUFAs metabolism, SPMs, and their potential effect on cardiovascular health and diseases primarily focusing in neonates. A better understanding of the role of these SPMs in cardiovascular health and diseases in neonates could lead to the development of novel therapeutic approaches in cardiovascular dysfunction.

Maternal Microbiota, Early Life Colonization and Breast Milk Drive Immune Development in the Newborn

The innate immune system is the oldest protection strategy that is conserved across all organisms. Although having an unspecific action, it is the first and fastest defense mechanism against pathogens. Development of predominantly the adaptive immune system takes place after birth. However, some key components of the innate immune system evolve during the prenatal period of life, which endows the newborn with the ability to mount an immune response against pathogenic invaders directly after birth. Undoubtedly, the crosstalk between maternal immune cells, antibodies, dietary antigens, and microbial metabolites originating from the maternal microbiota are the key players in preparing the neonate’s immunity to the outer world. Birth represents the biggest substantial environmental change in life, where the newborn leaves the protective amniotic sac and is exposed for the first time to a countless variety of microbes. Colonization of all body surfaces commences, including skin, lung, and gastrointestinal tract, leading to the establishment of the commensal microbiota and the maturation of the newborn immune system, and hence lifelong health. Pregnancy, birth, and the consumption of breast milk shape the immune development in coordination with maternal and newborn microbiota. Discrepancies in these fine-tuned microbiota interactions during each developmental stage can have long-term effects on disease susceptibility, such as metabolic syndrome, childhood asthma, or autoimmune type 1 diabetes. In this review, we will give an overview of the recent studies by discussing the multifaceted emergence of the newborn innate immune development in line with the importance of maternal and early life microbiota exposure and breast milk intake.

Resolving Human Lactation Heterogeneity Using Single Milk-Derived Cells, a Resource at the Ready

Single cell RNA sequencing (scRNAseq) of human milk-derived cells (HMDCs) makes highly detailed analyses of the biology of human lactation possible. We explore this powerful application as an exciting tool to inspect the cellular composition of human milk. We point out some important challenges unique to this approach and highlight the importance of collaborations between biologists and well-trained bioinformaticians to utilize these data to their maximum potential. We extend this focus by discussing the first two such studies that describe HMDCs via scRNAseq and a variety of important questions in the field that warrant attention through further research. The stage is set to apply scRNAseq in human lactation biology, potentially leading to new insights regarding the molecular and cellular diversity of human secretory mammary epithelial cells.

The Gut‒Breast Axis: Programming Health for Life

The gut is a pivotal organ in health and disease. The events that take place in the gut during early life contribute to the programming, shaping and tuning of distant organs, having lifelong consequences. In this context, the maternal gut plays a quintessence in programming the mammary gland to face the nutritional, microbiological, immunological, and neuroendocrine requirements of the growing infant. Subsequently, human colostrum and milk provides the infant with an impressive array of nutrients and bioactive components, including microbes, immune cells, and stem cells. Therefore, the axis linking the maternal gut, the breast, and the infant gut seems crucial for a correct infant growth and development. The aim of this article is not to perform a systematic review of the human milk components but to provide an insight of their extremely complex interactions, which render human milk a unique functional food and explain why this biological fluid still truly remains as a scientific enigma.

IgA and FcαRI: Versatile Players in Homeostasis, Infection, and Autoimmunity

Mucosal surfaces constitute the frontiers of the body and are the biggest barriers of our body for the outside world. Immunoglobulin A (IgA) is the most abundant antibody class present at these sites. It passively contributes to mucosal homeostasis via immune exclusion maintaining a tight balance between tolerating commensals and providing protection against pathogens. Once pathogens have succeeded in invading the epithelial barriers, IgA has an active role in host-pathogen defense by activating myeloid cells through divers receptors, including its Fc receptor, FcαRI (CD89). To evade elimination, several pathogens secrete proteins that interfere with either IgA neutralization or FcαRI-mediated immune responses, emphasizing the importance of IgA-FcαRI interactions in preventing infection. Depending on the IgA form, either anti- or pro-inflammatory responses can be induced. Moreover, the presence of excessive IgA immune complexes can result in continuous FcαRI-mediated activation of myeloid cells, potentially leading to severe tissue damage. On the one hand, enhancing pathogen-specific mucosal and systemic IgA by vaccination may increase protective immunity against infectious diseases. On the other hand, interfering with the IgA-FcαRI axis by monovalent targeting or blocking FcαRI may resolve IgA-induced inflammation and tissue damage. This review describes the multifaceted role of FcαRI as immune regulator between anti- and pro-inflammatory responses of IgA, and addresses potential novel therapeutic strategies that target FcαRI in disease.

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Can breastfeeding protect against antimicrobial resistance?

BACKGROUND

The proportion of infections among young children that are antimicrobial-resistant is increasing across the globe. Newborns may be colonized with enteric antimicrobial-resistant pathogens early in life, which is a risk factor for infection-related morbidity and mortality. Breastfeeding is actively promoted worldwide for its beneficial impacts on newborn health and gut health. However, the role of breastfeeding and human milk components in mitigating young children's carriage of antimicrobial-resistant pathogens and antibiotic resistance genes has not been comprehensively explored.

MAIN BODY

Here, we review how the act of breastfeeding, early breastfeeding, and/or human milk components, such as the milk microbiota, secretory IgA, human milk oligosaccharides, antimicrobial peptides, and microRNA -bearing extracellular vesicles, could play a role in preventing the establishment of antimicrobial-resistant pathogens in young children's developing gut microbiomes. We describe findings from recent human studies that support this concept.

CONCLUSION

Given the projected rise in global morbidity and mortality that will stem from antimicrobial-resistant infections, identifying behavioral or nutritional interventions that could decrease children's susceptibility to colonization with antimicrobial-resistant pathogens may be one strategy for protecting their health. We suggest that breastfeeding and human milk supplements deserve greater attention as potential preventive measures in the global effort to combat antimicrobial resistance, particularly in low- and middle-income settings.

Single Cell RNA Sequencing of Human Milk-Derived Cells Reveals Sub-Populations of Mammary Epithelial Cells with Molecular Signatures of Progenitor and Mature States: a Novel, Non-invasive Framework for Investigating Human Lactation Physiology

Cells in human milk are an untapped source, as potential "liquid breast biopsies", of material for investigating lactation physiology in a non-invasive manner. We used single cell RNA sequencing (scRNA-seq) to identify milk-derived mammary epithelial cells (MECs) and their transcriptional signatures in women with diet-controlled gestational diabetes (GDM) with normal lactation. Methodology is described for coordinating milk collections with single cell capture and library preparation via cryopreservation, in addition to scRNA-seq data processing and analyses of MEC transcriptional signatures. We comprehensively characterized 3740 cells from milk samples from two mothers at two weeks postpartum. Most cells (>90%) were luminal MECs (luMECs) expressing lactalbumin alpha and casein beta and positive for keratin 8 and keratin 18. Few cells were keratin 14⁺ basal MECs and a small immune cell population was present (<10%). Analysis of differential gene expression among clusters identified six potentially distinct luMEC subpopulation signatures, suggesting the potential for subtle functional differences among luMECs, and included one cluster that was positive for both progenitor markers and mature milk transcripts. No expression of pluripotency markers POU class 5 homeobox 1 (POU5F1, encoding OCT4) SRY-box transcription factor 2 (SOX2) or nanog homeobox (NANOG), was observed. These observations were supported by flow cytometric analysis of MECs from mature milk samples from three women with diet-controlled GDM (2-8 mo postpartum), indicating a negligible basal/stem cell population (epithelial cell adhesion molecule (EPCAM)^-/integrin subunit alpha 6 (CD49f)⁺, 0.07%) and a small progenitor population (EPCAM⁺/CD49f⁺, 1.1%). We provide a computational framework for others and future studies, as well as report the first milk-derived cells to be analyzed by scRNA-seq. We discuss the clinical potential and current limitations of using milk-derived cells as material for characterizing human mammary physiology.

Morphological Analysis of Human Milk Membrane Enclosed Structures Reveals Diverse Cells and Cell-like Milk Fat Globules

Over the past decade, the cellular content of human milk has been a focus in lactation research due to the benefit a potential non-invasive stem cell compartment could provide either to the infant or for therapeutic applications. Despite an increase in the number of studies in this field, fundamental knowledge in regard to milk cell identification and characterisation is still lacking. In this project, we investigated the nature, morphology and content of membrane enclosed structures (MESs) and explored different methods to enrich human milk cells (HMCs) whilst reducing milk fat globule (MFG) content. Using both flow cytometry and immunofluorescence imaging, we confirmed previous reports and showed that nucleated HMCs make up a minority of milk-isolated MESs and are indistinguishable from MFGs without the use of a nuclear stain. HMC heterogeneity was demonstrated by differential uptake of nuclear stains Hoechst 33258 and DRAQ5™ using a novel technique of imaging milk MESs (by embedding them in agar), that enabled examination of both extracellular and intracellular markers. We found that MESs often contain multiple lipid droplets of various sizes and for the first time report that late post-partum human milk contains secretory luminal binucleated cells found across a number of participants. After investigation of different techniques, we found that viably freezing milk cells is an easy and effective method to substantially reduce MFG content of samples. Alternatively, milk MESs can be filtered using a MACS® filter and return a highly viable, though reduced population of milk cells. Using the techniques and findings we've developed in this study; future research may focus on further characterising HMCs and the functional secretory mammary epithelium during lactation.

Granulocytic Myeloid-Derived Suppressor Cells in Breast Milk (BM-MDSC) Correlate with Gestational Age and Postnatal Age and Are Influenced by Infant's Sex

BACKGROUND

Infections are the main cause of death in preterm infants. Causative agents often descend from the intestinal flora of the infected neonate, indicating insufficient protection by the mucosal barrier. Breast milk (BM) contains different subsets of immune cells. We recently showed that BM contains significant numbers of myeloid-derived suppressor cells (MDSC)-immune cells that actively suppress pro-inflammatory immune responses-and hypothesized that the transfer of BM-MDSC may modulate the mucosal immunity of the newborn.

METHODS

Percentages of MDSC in the BM from mothers of 86 preterm infants between 23 + 0 and 36 + 6 weeks of gestation during their first five postnatal weeks were analyzed by flow cytometry and correlated with maternal and infant characteristics.

RESULTS

Percentages of BM-MDSC positively correlated with gestational age and postnatal age. The expression of activation markers on BM-MDSC did not change with gestational age, but it decreased with postnatal age. Mothers who received antepartum tocolytics had lower percentages of BM-MDSC, and infant's sex strongly influenced percentages of BM-MDSC.

CONCLUSION

Our results point toward a role of BM-MDSC for immune regulation in the neonatal gut, making them a potential target of immune-based therapies shortly after birth.

In utero and early life exposures in relation to endometriosis in adolescents and young adults

OBJECTIVE

Endometriosis is a common gynecologic disorder often associated with severe pelvic pain and infertility with few known modifiable risk factors. We investigated whether in utero and early life exposures are associated with surgically diagnosed endometriosis among adolescents and young adults.

STUDY DESIGN

This case-control study, including 295 laparoscopically-confirmed endometriosis cases and 309 population-based controls aged <25 years, was conducted using data from The Women's Health Study: From Adolescence to Adulthood which enrolled participants from 2012 to 2018. Information on in utero and early life factors were collected using a modified WERF EPHect questionnaire at enrollment, including their mother's age at delivery, birthweight, gestation length, exposure to smoking in utero and secondhand smoke during childhood up to age 13, and if the participant was breastfed.

RESULTS

Median age at enrollment was 17 years (range 12-24) in cases and 22 years (range 7-24) in controls, with 83 % and 68 % non-Hispanic whites, respectively. The majority of cases (95 %) were rASRM stage I or II at diagnostic surgery. Exposure to breastfeeding in early life was associated with lower odds of endometriosis diagnosis (OR = 0.39, 95 % CI = 0.21-0.74). Exposure to secondhand smoke during childhood due to maternal smoking was associated with increased odds of endometriosis diagnosis (OR = 2.70, 95 % CI = 1.11-6.60).

CONCLUSIONS

Among adolescents and young adults, our data suggest exposures to breastfeeding in early life and secondhand smoke during childhood may be associated with endometriosis risk, providing insight into etiologic pathways to be explored in this young population.

T Cells in Preterm Infants and the Influence of Milk Diet

Preterm infants born before 32 weeks gestational age (GA) have high rates of late onset sepsis (LOS) and necrotizing enterocolitis (NEC) despite recent improvements in infection control and nutrition. Breast milk has a clear protective effect against both these outcomes likely due to multiple mechanisms which are not fully understood but may involve effects on both the infant's immune system and the developing gut microbiota. Congregating at the interface between the mucosal barrier and the microbiota, innate and adaptive T lymphocytes (T cells) participate in this interaction but few studies have explored their development after preterm delivery. We conducted a literature review of T cell development that focuses on fetal development, postnatal maturation and the influence of milk diet. The majority of circulating T cells in the preterm infant display a naïve phenotype but are still able to initiate functional responses similar to those seen in term infants. T cells from preterm infants display a skew toward a T-helper 2(Th2) phenotype and have an increased population of regulatory cells (Tregs). There are significant gaps in knowledge in this area, particularly in regards to innate-like T cells, but work is emerging: transcriptomics and mass cytometry are currently being used to map out T cell development, whilst microbiomic approaches may help improve understanding of events at mucosal surfaces. A rapid rise in organoid models will allow robust exploration of host-microbe interactions and may support the development of interventions that modulate T-cell responses for improved infant health.

Large Stem/Progenitor-Like Cell Subsets can Also be Identified in the CD45- and CD45+/High Populations in Early Human Milk

BACKGROUND

Stem/progenitor cells have been identified in human milk. However, characterization and percentages of cell subsets in human milk using hematopoietic stem and progenitor cell markers according to the differential expression of CD45, i.e., as CD45^dim/+ (mainly hematopoietic stem/progenitor cells) and CD45^- (mainly non-hematopoietic stem/progenitor cells), have not been assessed to date.

RESEARCH AIM

To characterize stem/progenitor-like cell phenotypes in human milk and to report the percentages of these cells at two different lactation stages compared to peripheral blood.

METHODS

Human milk samples paired with peripheral blood samples (N = 10) were analyzed by flow cytometry using CD45, CD34, CD133, CD38, and lineage-negative markers. The percentage of cell subsets was analyzed in colostrum (Day 3 postpartum) and transitional milk (Day 5/6 postpartum) and compared with the peripheral blood counterpart.

RESULTS

The percentage of CD45^-CD34⁺ cells was predominant in both colostrum and transitional milk. The percentage of CD45^+/highCD133⁺ cells was high in colostrum while the percentage of CD45^-CD133⁺ cells was high in transitional milk. Furthermore, the median percentages of the CD45^-CD34⁺, CD45^-CD133⁺, and CD45^dimCD133⁺ cell subsets were higher in colostrum than its peripheral blood counterpart (0.11% vs. 0.002%; 0.17% vs. 0.0005%; 0.09% vs. 0.05%, p = .04, respectively); also CD45^-CD34^-CD133⁺ and CD45^dimCD34^-CD133⁺ cell subsets were higher in colostrum than peripheral blood (1.32% vs. 0.0% and 2.4% vs. 0.06%, p = .04), respectively).

CONCLUSION

Early human milk is an abundant reservoir of hematopoietic stem/progenitor-like cells in the CD45^+/high population and non-hematopoietic stem/progenitor-like cells in the CD45^- population.

Macrophage profile and homing into breast milk in response to ongoing respiratory infections in the nursing infant

Studies have shown that immune components of human milk can be changed during an infection in the nursing infant. Macrophages are abundant in human milk and they are classified into inflammatory (CD16^-) and noninflammatory (CD16⁺) subsets. This study investigated CD16⁺ and CD16^- macrophage homing into breast milk in response to ongoing infections in nursing infants. Peripheral blood and mature milk were collected from 33 healthy mothers of nursing infants with respiratory infections (Group I) and from 26 healthy mothers of healthy nursing infants (Group H). Blood and milk total, CD16^- and CD16⁺ monocyte (Mo)/macrophage (Mφ) subsets, respectively, and CCR2 and CX3CR1 expression and cytokine levels were analyzed by flow cytometry. CCL2 and CX3CL1 were quantified by ELISA and cytokines by flow cytometry in serum and milk. There was an increase of total and CD16⁺ Mφ, and, also a decrease of CD16- Mφ frequencies in maternal milk from Group I compared to Group H, but absolute numbers analyses showed higher numbers of all subpopulations of milk Mφ in Group I compared to Group H. Higher numbers of CX3CR1⁺CD16⁺ and double-staining of CCR2 and CX3CR1 in both CD16⁺ and CD16^- cells were observed in milk during infant infection, which weren't observed in the blood. CCR2 expression was hardly found in milk CD16^- Mφ in both groups. CCL2 and CX3CL1 were both higher in milk than in blood from both groups, but Group I showed higher levels of these chemokines in milk than Group H. Breast milk showed higher IL-6 and IL-8 concentrations than serum, and infant infection caused an increase in these cytokines only in milk. Our findings suggest that milk Mφ profiles are different from blood Mo, and the ongoing infection in the nursing infant could change milk Mφ to a more anti-inflammatory profile compared to that in the healthy group, possibly as an additional strategy of infant protection.

Maternal Leukocytes and Infant Immune Programming during Breastfeeding

The fetal immune system develops in a rather sterile environment relative to the outside world and, therefore, lacks antigenic education. Soon after birth, the newborn is exposed to the hostile environment of pathogens. Recently, animal- and limited human-based studies have indicated that help from the mother, upon transfer of leukocytes and their products via breast milk feeding, greatly assists the newborn's immune system. Here, I discuss the newest advances on how milk leukocytes impact early life immunity, with an emphasis on the development of the infant T cell repertoire and early immune responses in the periphery and gut-associated lymphoid tissue. A deeper understanding of these novel mechanistic insights may inform potential translational approaches to improving immunity in infants.

A Review of the Immunomodulating Components of Maternal Breast Milk and Protection Against Necrotizing Enterocolitis

Breast milk contains immunomodulating components that are beneficial to newborns during maturation of their immune system. Human breast milk composition is influenced by an infant's gestational and chronological age, lactation stage, and the mother and infant's health status. Major immunologic components in human milk, such as secretory immunoglobulin A (IgA) and growth factors, have a known role in regulating gut barrier integrity and microbial colonization, which therefore protect against the development of a life-threatening gastrointestinal illness affecting newborn infants called necrotizing enterocolitis (NEC). Breast milk is a known protective factor in the prevention of NEC when compared with feeding with commercial formula. Breast milk supplements infants with human milk oligosaccharides, leukocytes, cytokines, nitric oxide, and growth factors that attenuate inflammatory responses and provide immunological defenses to reduce the incidence of NEC. This article aims to review the variety of immunomodulating components in breast milk that protect the infant from the development of NEC.

Host Microbe Interactions in the Lactating Mammary Gland

The bacteria present in human milk constitute the human milk microbiome (hMM). Both the older culture-based work and the more recent studies using molecular detection of bacterial DNA have reached similar conclusions: the hMM mostly consists of commensal staphylococci such as Staphylococcus epidermidis, and streptococci. The prevalence of other bacterial groups such lactobacilli varies widely, while the abundance and prevalence of bifidobacteria is generally low. Recently, the hMM became accepted as a part of a physiologically normal state with suggested potential health benefits. Most research on the hMM has focused on its composition and potential effect on the breastfed infant. A major role as a microbiome inoculum for the infant gut has been proposed, but remains to be clearly demonstrated. Herein, we also discuss the emerging connection between the hMM and mammary gland physiology and lactation. Similarities between the mammary gland and mucosal interfaces are considerable, and in particular mucosal-like immune attributes of mammary gland. The potential role of hMM-host interactions in the mammary gland in maternal health is explored with a primary focus on lactational mastitis.

Characterization of Stem Cells and Immune Cells in Preterm and Term Mother's Milk

BACKGROUND

Human milk is known to be rich in cellular components, including stem cells and immune cells. However, the dynamics of these cellular components at different lactation stages, and the differences between milk for preterm and term infants, are poorly understood.

RESEARCH AIM

To identify changes in the cellular components of human milk at different lactation stages, and to explore the associations of these changes with maternal and infant characteristics.

METHODS

Forty mothers of newborns of different gestational ages were enrolled. Colostrum, transitional, and mature milk samples were collected. Stem cell and immune cell molecule markers were detected using flow cytometry. Pluripotent genes (SOX2, NANOG, OCT4, and KLF4) were detected via quantitative real-time PCR.

RESULTS

Human milk contained some stem cells but more immune cells. The percentages of hemopoietic stem cells were significantly higher in mature milk than in colostrum, and the percentages of total immune cells were lower in mature milk than in colostrum. The percentages of hemopoietic stem cells in colostrum and transitional milk were influenced by gestational age. Some minor differences in the cell composition of human milk could be explained by maternal body mass index, the mode of delivery, and parity.

CONCLUSION

Our results again confirmed that human milk contains stem cells. Additionally, the percentages of hemopoietic stem cells and major immune cells changed dynamically at different lactation stages and were associated with gestational age at delivery.

Stem cells in human breast milk

Recent studies have demonstrated that breast milk contains a population of cells displaying many of the properties typical of stem cells. This review outlines progress made in this newly emerging field of stem cell biology and provides an analysis of the available data on purification, propagation and differentiation of certain types of progenitor cells from breast milk. The possible fates of breast milk cells, including microchimerism caused by their transmission to the distant organs of the infant, are also discussed. Unique properties of breast milk-derived stem cells, such as their unusually low tumorigenic potential and their negligible ability to form teratomas, are highlighted as obvious advantages for using these cells in regenerative therapy.