A few studies shed light on the possible ways breast milk vaccination may work and provide the optimal route to activate the newborn’s immune system (Fig. 1).

Besides providing important but rapidly weaning passive immunity, breastmilk may actively provoke a long-lasting immune response by the transfer of low doses of microbial antigens in an infant-tailored vehicle. Breastmilk contains microbial antigens as a whole or (partially) pre-digested, presented by milk antigen-presenting cells or captured in extracellular vesicles or immune complexes. These antigens are delivered to the infant mucosa in conjunction with milk bioactive molecules and milk microbiota that can function as infant-tailored adjuvants. This may allow breastmilk to activate the neonatal immune system to induce protective, long-lasting immunity against pathogens. Image created with BioRender.

The very low amounts of antigens in human milk may fit the specific requirements for activation of the developing immune system. Neonates mount an efficient cytotoxic immune response to a viral dose 10,000 times lower than an adult, while higher doses are unable to activate an appropriate immune response9. The levels of P. falciparum antigen histidine-rich protein 2 and hepatitis B e antigen are 10-100-fold lower, and hepatitis B surface antigens are 30,000 times lower in human milk than in serum7,10. These low levels may be an important cue for the developing immune system of the neonate.

Exogenous proteins in human milk are pre-digested within the mammary gland. This process might be key for the generation of immunogenic peptides. As the newborn has only limited digestive abilities, predigesting pathogen-specific proteins in breast milk may be important for the newborn to generate a long-lasting and protective response9.

The presence of both maternal pathogen-specific antibodies and pathogens in breast milk highly suggests the presence of pathogen antigen-immune complexes. Antigen-IgG antibody immune complexes improve the transport of pathogen across the gut barrier using the neonatal Fc receptor and enhance the stimulation of effector immune responses by antigen-presenting cells9.

Antigen-presenting cells present in milk might play a role in the induction of a pathogen-specific immune response. Interestingly, the proportion of leucocytes in human milk increases upon maternal infection, which may increase pathogen-derived antigen presentation by milk antigen-presenting cells9. Human milk extracellular vesicles also express major histocompatibility complex molecules, which could contribute to the induction of antigen-specific immune responses in breastfed infants9.

Finally, microbes and microbial antigens in breast milk are surrounded by thousands of immune modulatory factors. The breast milk milieu contains molecules including antibodies and enzymes that can alter, weaken or reduce the viability of microbes present in the milk. This could lead to the generation of live attenuated pathogens that are fit to immunize the infant without infecting them. Among the bioactive compounds in breast milk are also potential strong adjuvants such as cytokines, the milk microbiota, soluble CD14 and Toll-like receptors.

The complex and dynamic composition of breast milk may have specifically been selected for and adapted to the newborn’s situation, in order to effectively promote immune defence upon microbial antigen transfer in the milk.