In Vitro Stimulation of Whole Milk Specimens: A Field-Friendly Method to Assess Milk Immune Activity

Maternal and infant predictors of proinflammatory milk immune activity in Kilimanjaro, Tanzania

OBJECTIVES

The immune system of milk (ISOM) creates a mother-infant immune axis that plays an important role in protecting infants against infectious disease (ID). Tradeoffs in the immune system suggest the potential for both protection and harm, so we conceive of two dimensions via which the ISOM impacts infants: promotion of protective activity and control of activity directed at benign targets. High variability in ISOM activity across mother-infant dyads suggests investment the ISOM may have evolved to be sensitive to maternal and/or infant characteristics. We assessed predictors of appropriate and misdirected proinflammatory ISOM activity in an environment of high ID risk, testing predictions drawn from life history theory and other evolutionary perspectives.

METHODS

We characterized milk in vitro interleukin-6 (IL-6) responses to Salmonella enterica (a target of protective immune activity; N = 96) and Escherichia coli (a benign target; N = 85) among mother-infant dyads in rural Kilimanjaro, Tanzania. We used ordered logistic regression and mixture models to evaluate maternal and infant characteristics as predictors of IL-6 responses.

RESULTS

In all models, IL-6 responses to S. enterica increased with maternal age and decreased with gravidity. In mixture models, IL-6 responses to E. coli declined with maternal age and increased with gravidity. No other considered variables were consistently associated with IL-6 responses.

CONCLUSIONS

The ISOM's capacities for appropriate proinflammatory activity and control of misdirected proinflammatory activity increases with maternal age and decreases with gravidity. These findings are consistent with the hypothesis that the mother-infant immune axis has evolved to respond to maternal life history characteristics.

Predictors of milk cortisol in North American women

OBJECTIVES

Human milk content varies across mother-child dyads, environments, and populations. Among the hormones in milk is cortisol, a glucocorticoid; its impact on the breastfeeding child is unknown. Milk cortisol may constitute a signal to the child's developing physiology which can shape characteristics (e.g., growth, temperament) to prevailing environmental conditions. This exploratory study evaluated the maternal, breastfeeding, and infant characteristics associated with milk cortisol.

METHODS

We evaluated archived milk specimens for cortisol using enzyme immunoassay and employed an information-theoretic approach to assess associations between milk cortisol and participant characteristics with linear regression modeling. Because we employed secondary data, information for some variables likely to impact milk cortisol variation (e.g., time of day, socioeconomic status, maternal or infant body mass index, milk energy density) was unavailable.

RESULTS

Participants were 48 lactating mothers from upstate New York, aged 21-40 years. Milk cortisol ranged from 0.098 to 1.007 μg/dL. Child age ranged from 1 to 26 months. In linear regression employing best fit modeling criteria, milk cortisol increased with child age (B: 0.069; p: .000; a 7.1% increase in milk cortisol for each month of child age), while child symptoms of illness (B: -0.398; p: .057; a 33% decrease) and consumption of complementary foods (B: -.525; p: .020; a 41% decrease) were associated with lower milk cortisol.

CONCLUSIONS

We speculate that increasing milk cortisol with child age plays a role in signaling development (e.g., as increasing independence increases risk for injury and other negative health outcomes), independent of the maternal stressors we could capture.

Associations between milk cortisol and activity of the immune system of milk

OBJECTIVES

Both the immune system of human milk and milk cortisol have complex short- and long-term effects on child health and development. As understanding continues to grow of the independent effects of each of these components of milk, it is also important to investigate their intersection, including how milk cortisol affects the immune system of milk. We began this important endeavor through secondary analyses of archived milk specimens.

METHODS

Participants were 31 lactating mothers from upstate New York. We estimated milk cortisol concentrations via enzyme immunoassay. We assessed milk proinflammatory cytokine (interleukin-6, IL-6) responses to pathogenic (Salmonella) and commensal (Escherichia, Lactobacillus, Bifidobacterium) bacteria via in vitro stimulation. We estimated ordered logistic regression models to assess associations between milk cortisol and IL-6 responses to bacteria.

RESULTS

Milk cortisol ranged from 0.098 to 1.007 μg/dL. Milk cortisol was positively associated with IL-6 responses to S. enterica (B: 4.035; 95% CI: 0.674, 7.395) and B. breve (B: 3.675; 95% CI: 0.426, 6.924); this association persisted after controlling for child age. Results were less clear for associations between milk cortisol and IL-6 responses to L. acidophilus (B: 2.318; 95% CI: -1.224, 5.859) and E. coli (B: 2.366; 95% CI: -0.960, 5.692).

CONCLUSIONS

Complex interactions between cortisol and the immune system extend to milk. Milk cortisol was positively associated with proinflammatory responses to some bacteria in vitro. This may suggest that milk cortisol is causally upstream of protective immune activity.

Does the immune system of milk increase activity for infants experiencing infectious disease episodes in Kilimanjaro, Tanzania?

INTRODUCTION

Multiple studies have reported that milk immune content increases for infants experiencing infectious disease (ID) episodes, suggesting that the immune system of milk (ISOM) offers enhanced protection when needed to combat ID.

METHODS

To test the hypothesis that ISOM content and/or activity increases during an infant's ID episode, we characterized milk secretory immunoglobulin A (sIgA; a major ISOM constituent) and in vitro interleukin-6 (IL-6) responses to Salmonella enterica and Escherichia coli, as system-level biomarkers of ISOM activity, in a prospective study among 96 mother-infant dyads in Kilimanjaro, Tanzania.

RESULTS

After control for covariates, no milk immune variables (sIgA, Coef: 0.03; 95% CI -0.25, 0.32; in vitro IL-6 response to S. enterica, Coef: 0.23; 95% CI: -0.67, 1.13; IL-6 response to E. coli, Coef: -0.11; 95% CI: -0.98, 0.77) were associated with prevalent ID (diagnosed at the initial participation visit). Among infants experiencing an incident ID (diagnosed subsequent to the initial participation), milk immune content and responses were not substantially higher or lower than the initial visit (sIgA, N: 61; p: 0.788; IL-6 response to S. enterica, N: 56; p: 0.896; IL-6 response to E. coli, N: 36; p: 0.683); this was unchanged by exclusion of infants with ID at the time of initial participation.

CONCLUSION

These findings are not consistent with the hypothesis that milk delivers enhanced immune protection when infants experience ID. In environments with a high burden of ID, dynamism may be less valuable to maternal reproductive success than stability in the ISOM.

Immunomodulatory Effects of Bacterial Toll-like Receptor Ligands on the Phenotype and Function of Milk Immune Cells in Dromedary Camel

(1) Toll-like receptors (TLR) are a family of pattern recognition receptors that sense distinct molecular patterns of microbial origin. Although the immune cell composition of camel milk has been recently described, host-pathogen interaction studies in the camel mammary gland are still scarce. The present study aimed to use a whole milk stimulation assay for investigating the modulatory effect of selected Toll-like receptor (TLR) ligands on the phenotype and function of milk immune cells. (2) Methods-camel milk samples (n = 7) were stimulated in vitro with the TLR4 ligand LPS or the TLR2/1 ligand Pam3CSK4, and separated milk cells were evaluated for stimulation-induced shape change, the expression of cell surface markers, phagocytosis, apoptosis, ROS production, and NETosis. Stimulation with PMA was used as a control stimulation. (3) Results-all stimulants induced shape change in milk cells, change in the expression of several cell markers, and increased cell apoptosis and NETosis. In addition, stimulation with Pam3CSK4 and PMA was associated with enhanced ROS production, while only PMA stimulation resulted in enhanced bacterial phagocytosis by milk immune cells. (4) Conclusions-our data indicates selective modulating effects of the TLR ligands LPS and Pam3CSK4 on camel milk phagocytes. These results may have implications for the use of synthetic TLR agonists as immunomodulatory adjuvants of the immune response to intra-mammary vaccines against mastitis pathogens.

Tradeoffs in milk immunity affect infant infectious disease risk

Background and objectives

The human immune system has evolved to balance protection against infection with control of immune-mediated damage and tolerance of commensal microbes. Such tradeoffs between protection and harm almost certainly extend to the immune system of milk.

Methodology

Among breastfeeding mother–infant dyads in Kilimanjaro, Tanzania, we characterized in vitro proinflammatory milk immune responses to Salmonella enterica (an infectious agent) and Escherichia coli (a benign target) as the increase in interleukin-6 after 24 h of incubation with each bacterium. We characterized incident infectious diseases among infants through passive monitoring. We used Cox proportional hazards models to describe associations between milk immune activity and infant infectious disease.

Results

Among infants, risk for respiratory infections declined with increasing milk in vitro proinflammatory response to S. enterica (hazard ratio [HR]: 0.68; 95% confidence interval [CI]: 0.54, 0.86; P: 0.001), while risk for gastrointestinal infections increased with increasing milk in vitro proinflammatory response to E. coli (HR: 1.44; 95% CI: 1.05, 1.99; P: 0.022). Milk proinflammatory responses to S. enterica and E. coli were positively correlated (Spearman’s rho: 0.60; P: 0.000).

Conclusions and implications

These findings demonstrate a tradeoff in milk immune activity: the benefits of appropriate proinflammatory activity come at the hazard of misdirected proinflammatory activity. This tradeoff is likely to affect infant health in complex ways, depending on prevailing infectious disease conditions. How mother–infant dyads optimize proinflammatory milk immune activity should be a central question in future ecological–evolutionary studies of the immune system of milk.