Baby Backwash Can Trigger Immune Response in Milk

Written by: Lauren Milligan Newmark, Ph.D. | Issue # 110 | 2022

Mothers have many ways to tell if their infant might be sick—an increase in body temperature, a decrease in appetite, or even small changes in sleep habits could all hint that an illness is brewing. But milk researchers have long suspected that nursing infants have another, more direct means of communicating their health status to their mothers: backwash.

When a human infant suckles, their tongue movements create a vacuum that draws milk from the ducts to the mouth. Suckling also creates negative pressure that pulls fluids in the reverse direction, taking milk and saliva from the infant’s mouth through the nipple and into the milk ducts [1-3]. Researchers believe that if this milky backwash—called retrograde duct flow—contains information that signals an illness, it could trigger a local response in the mammary gland to increase the production of immune factors [4].

Can a sick infant’s backwash actually change the composition of their mother’s milk? A handful of studies [5–8] on human milk have attempted to test this intriguing hypothesis. Immune factors in milk from healthy mothers were found to increase during times of infant illness, supporting the idea that infants can communicate their health status to mammary gland receptors. However, these studies stopped short of singling out baby backwash as the source of communication (after all, infants can share a lot of bodily fluids with their mothers, especially during times of illness).

A new study [9] on suckling mice pups—who also backwash during nursing [10]—adds in the important piece missing from human research by tracking a single signal from infant saliva to the mammary gland, to milk. The investigators were interested in determining whether enteric viruses, such as norovirus, replicate in salivary glands. To demonstrate that saliva transmits these types of viruses, the study team performed a very clever set of experiments involving suckling pups, mammary glands, and milk composition [9].

In the first part of the study, mice pups were inoculated with either murine norovirus (MNV-1) or rotavirus (EDIM), but their mothers (dams) were not. After three days, the team identified a spike in the levels of secretory immunoglobulin A (sIgA) in the intestines of the pups and in the dam’s milk. Moreover, they found a 10⁵-fold increase in viral RNA in dam mammary glands (specifically in the cells lining the milk ducts) [9]. To rule out that the dams were infected by the infants through contact with fecal material, they also orally inoculated a group of dams that were pup-free. Although these dams’ intestines had an increase in viral RNA (signaling infection), there was no detectable viral RNA in their mammary glands nor was there a surge of sIgA in their milk [9].

To further demonstrate an oral route of transmission, the study team also conducted a cross-over experiment. A group of mouse pups (pups A) were orally inoculated with EDIM and allowed to stay with their mother (dam A) for one day. Then, the researchers switched dams; dam A was placed with uninoculated (healthy) pups (pups B), while dam B (also healthy) was placed in the cage with pups A. Two days later, the researchers identified a 10⁴-fold increase in viral genome levels in mammary glands from both dam A and dam B and a 10⁶-fold increase in the viral genome levels of both pups A and B [9]. Dam A showed no evidence of infection and is presumed to have passed on the virus to pups B via infected mammary glands (courtesy of nursing pups A for one day).

Both experiments demonstrate that saliva is a route of transmission for enteric viruses, but they are also the most clear-cut evidence to date that pathogens backflow from infants’ mouths to their mother’s mammary gland during nursing, and that the resulting localized infection in the mammary gland triggers an immune response in milk [9]. Perhaps even more incredible, this immune response was quicker than when the dam was infected, and milk antibodies were maternally derived via the entero-mammary pathway.

The benefits of passing on pathogens through retrograde duct flow to the infant are clear; the surge in immune factors (many that are specific to the pathogen causing the infection) likely contributes to helping them clear that infection. However, in a previous SPLASH! piece, lactation biologist Foteini Kakulas (né Hassiotou) made the important observation that the increase in immune cells in human milk not only functions to protect the infant, they also protect the lactating breast. Kakulas believes the surge in immune factors in milk following mammary infection could be an evolved response to maternal infection, infant infection, or maternal and infant infection. But regardless of the why, the response of the mammary gland provides yet another example of how milk protects infants, be they mice or (hu)man.

References

1. Geddes DT, Sakalidis VS. Breastfeeding: how do they do it? Infant sucking, swallowing, and breathing. Infant. 2015; 11(5): 146-50
2. Gardner H, Kent JC, Hartmann PE, Geddes DT. Asynchronous milk ejection in human lactating breast: case series. Journal of Human Lactation. 2015 May;31(2):254-9.
3. Geddes DT, Kent JC, Mitoulas LR, Hartmann PE. Tongue movement and intra-oral vacuum in breastfeeding infants. Early Human Development. 2008 Jul 1;84(7):471-7.
4. Bode L, McGuire M, Rodriguez JM, Geddes DT, Hassiotou F, Hartmann PE, McGuire MK. It’s alive: microbes and cells in human milk and their potential benefits to mother and infant. Advances in Nutrition. 2014 Sep;5(5):571-3.
5. Breakey AA, Hinde K, Valeggia CR, Sinofsky A, Ellison PT. Illness in breastfeeding infants relates to concentration of lactoferrin and secretory Immunoglobulin A in mother’s milk. Evolution, Medicine, and Public Health. 2015 Jan 1;2015 (1): 21-31.
6. Bryan DL, Hart PH, Forsyth KD, Gibson RA. Immunomodulatory constituents of human milk change in response to infant bronchiolitis. Pediatric Allergy and Immunology. 2007 Sep;18(6):495-502.
7. Hassiotou F, Hepworth AR, Metzger P, Lai CT, Trengove N, Hartmann PE, Filgueira L. Maternal and infant infections stimulate a rapid leukocyte response in breastmilk. Clinical & Translational Immunology. 2013 Apr;2(4), e3.
8. Riskin A, Almog M, Peri R, Halasz K, Srugo I, Kessel A. Changes in immunomodulatory constituents of human milk in response to active infection in the nursing infant. Pediatric Research. 2011 Feb;71(2): 220-5.
9. Ghosh S, Kumar M, Santiana M, Mishra A, Zhang M, Labayo H, Chibly AM, Nakamura H, Tanaka T, Henderson W, Lewis E. Enteric viruses replicate in salivary glands and infect through saliva. Nature. 2022 Jul;607(7918):345-50.
10. Ramsay DT, Kent JC, Owens RA, Hartmann PE. Ultrasound imaging of milk ejection in the breast of lactating women. Pediatrics. 2004 Feb;113(2):361-7.

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