Milk isn't just milk. The pasteurized cow milk that we can purchase in the grocery store has been cleaned, processed, and in many cases chemically scrubbed of fat. Unique among mammals, we Western humans stubbornly persist in our consumption of dairy products well into adulthood regardless of whether or not our guts like it. But the cow milk in the store is very different from the fresh milk humans nurse their newborns with. Fresh human milk contains growth factors, vitamins, an astounding density of calories, IgA antibodies, and also antigen. Disregarding the controversial and impassioned debate surrounding breast milk vs. infant formula, research has found that the immunological molecules secreted in breast milk are important for the developing immune system of the infant.
Verhasselt and Julia et al have demonstrated that antigen secreted in a mother's breast milk significantly impacts the later development of allergic asthma in her infants. In effect, this is immunological programming.
They used an elegant experimental system wherein they took nursing dams (mouse mother) and exposed them to antigens (OVA) without their pups, then placed them back with their pups to nurse. Later on when the pups had reached adulthood, they sensitized the mice per the immunology canon and tested their allergic asthmatic (hereafter refered to as atopic) response. In mice breastfed by OVA-exposed dams it was found that, in comparison to those breastfed by non-exposed dams, airway hyperreactivity, pulmonary eosinophilia, cellular infiltrate, and mucus deposition were all decreased towards mice not challenged with OVA (normal, negative controls). Moreover, the classical TH2 cytokines (IL-4, IL-5, IL-10, and IL-13) that have been associated widely with atopic responses were decreased in OVA-breastfed mice, as were the frequencies of the lung CD4+ T-cells that secrete them. Overall, this points to OVA-breastfed mice having a significantly weakened allergenic response to an allergen, or put another way, these mice tolerated the prescence of the allergen much better.
As an aside, OVA is an abbreviation for ovalbumin, which is a key molecule in eggs and is a very sticky molecule. In every study of allergy or asthma that I can remember reading, OVA was used to sensitize mice and produce an allergic response to it. This is done by injecting a solution of OVA into the peritoneal cavity of the mice, waiting 2 weeks to let OVA-specific CD4+ T-cells develop, and then challenging the mice by squirting an OVA aerosol up their noses. This model has proven to reliably produce a strong and specific allergenic response.
But how did the authors determine that it was secreted antigen itself that was inducing the tolerance?
To address this, they took normal, wild-type C57 mouse pups and gave them to lactating uMT and RAG-2 transgenic dams*. Both these transgenic strains of mice are completely unable to mount an adaptive immune response to anything; they do not and cannot make antibodies. When these mice were exposed to OVA and then nursed the wild-type pups, the same effects of the OVA-breastfeeding inducing allergen tolerance were observed. This was also replicated with Balb/c mice (which is important because the C57 strain is known to skew towards a TH1 response phenotype while Balb/c skews more to TH2, which is better characterized in the pathophysiology of atopy).
But then Verhasselt and Julia took a look at some important and specific immunoregulatory molecules: TGFb and IL-10. Both of these molecules are broad suppresors of inflammatory immune responses regardless of that response's cellular phenotype. IL-10 transgenic dams did not alter observed results when exposed to OVA, but TGFb knockdown dams did in that the pups they nursed were reactive to OVA. This strongly suggests that TGFb must accompany antigen in the breast milk in order for the infants' immune systems to recognize that antigen as a harmless allergen and, in effect, program itself not to react against it. Luckily for human mothers, though, breast milk already contains TGFb.
At a wider level, this suggests that mothers who are exposed to many allergens will pass on tolerance to those allergens to any breast-feeding infants they may have. Perhaps this, coupled with the hygiene hypothesis, is a call for more moms to teach their infants how to make mud pies even earlier.
*I can't help but wonder how many times they did this and found all the pups had been killed off by their adoptive mothers, as this is what stressed out rodent dams tend to do.
Verhasselt, V., Milcent, V., Cazareth, J., Kanda, A., Fleury, S., Dombrowicz, D., Glaichenhaus, N., & Julia, V. (2008). Breast milk–mediated transfer of an antigen induces tolerance and protection from allergic asthma Nature Medicine, 14 (2), 170-175 DOI: 10.1038/nm1718
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