French scientists studying lactating mice say they can add an important piece of evidence to a charged debate as to whether breastfeeding helps protect a child against asthma.
In a paper published online on Sunday by the journal Nature Medicine, a National Institute for Health and Medical Research (Inserm) team exposed lactating mice to airborne dust containing ovalbumin, a well-known asthma allergen that is found in egg whites.
The mother mice transmitted the allergen to their newborn through the milk, helping the offspring to develop an immunological tolerance to the irritant.
The tolerance was induced thanks to the presence of TGF beta, an important signalling protein, in the breast milk.
Breastfed mice whose mother had been exposed to ovalbumin were far less likely to develop wheezing, airway mucus and other asthma symptoms than non-breastfed counterparts.
"Breast-feeding induced tolerance may rely on both the chronic administration of an antigen at a low dose, a setting known to promote tolerance induction, and the presence of milk-borne TGF beta," they suggest.
Asthma is a worsening health problem that affects 300 million people worldwide, although the causes for it are complex.
One suspected source is exposure to allergens such as tobacco smoke, pollen and mites while in childhood.
These allergens in later life are identified as intruders by the immune system's T helper type-2 cells. They go into overdrive, causing the airways to inflame and constrict.
Some research, conducted among populations rather than in the lab, has suggested that newborns can develop a tolerance to airborne antigens through breastfeeding.
Other research, though, has found no difference. Some studies have even suggested breastfeeding may accentuate the risk of asthma.
But the Inserm researchers note that these studies do not take into account the mother's exposure to airborne allergens at the time when they were breastfeeding -- nor did they measure levels of antigens in breast milk.
That avenue of exploration should now be opened up in the light of the new findings, they say.