The researchers also confirmed in separate in vitro analyses that human breast milk contains these complement components, which demonstrated similar activity in targeting specific bacteria.
Taken together, these findings shed light on the mechanisms of how breast milk functions to provide protection from certain bacterial infections.
The study was published in the journal Cell.
”These findings reveal a critical role for breast milk complement proteins in shaping offspring’s gut microbe compositions and protecting against bacterial infection in the gut in early life,” says study senior author Fengyi Wan, PhD, a professor in the Bloomberg School’s Department of Biochemistry and Molecular Biology. “This represents an important expansion of our understanding of breast milk’s protective mechanisms.”
The study’s first author is Dongqing Xu, PhD, an assistant scientist in Wan’s research group.
Breastfeeding Benefits and Complement Proteins
Breastfeeding has many known and suspected benefits. It provides excellent nutrition to infants and appears to protect against some short-term or long-term illnesses. Breast milk is also known to help protect against common infections by sharing antibodies and white blood cells from the mother.
Breast milk also contains complement proteins that can work with, or “complement,” antibodies in attacking bacteria. While complement proteins that circulate in the blood have been the focus of much research, complement proteins in breast milk have been far less studied, and until now their role has been unclear.
In the new study, Wan and his team used engineered mice that lacked critical complement genes. They found that milk from female mice of this type left several-weeks-old mouse pups—even those with normal complement genes—highly susceptible to colitis, often lethal, from Citrobacter rodentium infections. By contrast, pups feeding on normal, complement-containing milk showed only minor and transient signs of gut infection.
The team discovered that this protective effect of breast milk complement proteins depends on their capacity in shaping infant gut microbiota. The complement proteins kill certain gut bacterial species, and this culling of microbes creates an overall gut environment in which harmful inflammation is much less likely in the presence of Citrobacter rodentium.
“Gut microbiota is of great importance to health,” says Wan. “Breast milk complement proteins contribute crucially to the establishment of a ‘protective’ gut microbiota during the early stages of development, promoting infant health and defending against pathogens.”
Implications and Future Directions
The study also appears to mark an advance in basic immunology. Complement proteins in blood, although known to be capable of causing direct damage to bacterial cells, have been thought to typically work in partnership with antibodies in a specific immune response. However, Wan and his team showed that this breast milk complement activity against bacteria does not require antibodies and is a nonspecific immune response.
“This opens the door to a lot of new investigations, for example, elucidating the specific complement biology in breast milk and comparing that to complement biology in the blood, and assessing the role of complement beyond the antibody-dependent specific immune system,” Wan says.
Reference: “Complement in breast milk modifies offspring gut microbiota to promote infant health” by Dongqing Xu, Siyu Zhou, Yue Liu, Alan L. Scott, Jian Yang and Fengyi Wan, 18 January 2024, Cell.
By JOHNS HOPKINS BLOOMBERG SCHOOL OF PUBLIC HEALTH
New Protective Benefits of Breast Milk Uncovered
