HMO – recent developments to understand their biology

Editor(s): Norbert Sprenger.

HMO – recent developments to understand their biology

Norbert Sprenger

The early-life gut microbiota establishes and matures sequentially during infancy and early childhood. An age-appropriate microbiota maturation is important for normal digestive, immune competence and metabolic development.

The gut microbiome has to be seen as an eco-system, it’s not an individual bacteria or an individual bacterial group, they are working together, also with the host. The early life microbiota trajectory is dynamic. At 3–6 months of  age the infant gut harbours bifidobacteria dominated community types. These communities transit and change further until to about  2–3 years, when they resemble community types that are
reported for adults. The first years of an infant’s life are characterized by a very dynamic microbiota establishment and obviously when you have different community types dominanted by different microbiota taxa you have also different metabolic activity in the gut ecosystem.

That dynamic and this trajectory brings us to the windows of opportunity that are often discussed in early life, also including prenatal. (Fig.1) The microbes in microbial activity in the mother can have an influence on the fetus. There are metabolites that can go in utero and probably modulate also the development of the fetus.

In early life you have different relevant events so microbiota seeding at birth – probably the most important event.

With the introduction of complimentary diet, you have a diversification of nutrition. If you do not diversify your nutrients you do not diversify your microbiota with a higher risk again for diseases later in life.

Influencing factors
Delivery mode and maternal intrapartum antibiotic use are major covariates related with variation in microbiota development seen in delayed bifidobacterium development in C- section born infants.  (Shao et al., Nature 2019)

There is a higher risk of atopic disease or food allergy, asthma related to C-section birth. But there is not always an association, this is a mystery still and it must be stratified better to understand the different risk factors for many of these diseases. Also, the early live microbiome could have a role for overweight obesity. Same for antibiotic use: A large study indicates that antibiotic use between 6–24 months had no effect, but < 6 months there was an effect seen, indicating the importance of the early life microbiota development.

The nutrition influences on microbiota in breastfeeding period is relatively easily done because we can compare breastfed infants and formula fed infants. The breastfed infants have a lower diversity and then it increases, while the cow milk formula or soy milk fed infants have already higher diversity into microbiota early on. There is a bifidobacteria dominance in the breastfed infants and a delay in the bifidobacteria appearance in the formula fed infants. Possibly, the microbiota needs to grow slowly and have to have time to go through different phases.

Human Milk Oligosaccharides (HMO)
Among the major differences between breastmilk and formula milk are the human milk oligosaccharides (HMOs), structurally diverse elongations of the milk sugar lactose by enzymes that are also responsible for mucosal glycosylation. Breastmilk HMO composition is highly variable and understanding the underlying factors is key to have meaningful observational correlation studies with infant growth, development and health. HMOs shape the establishing early life gut microbiota and supposedly help the development of appropriate immune competence.

Factors influencing HMO composition in breastmilk  include:

Genetics (Secretor-, Lewis gene) Lactation stage
Physiological status of the mother (e.g. BMI)
Mode of delivery
Infant gestational age
Preterm and term delivery
Diet

HMO 2’FL and LNFP II are proxies for the fu-
cosyltransferases FUT2 and FUT3 genotypes and allow to stratify breastmilk into 4 milk groups in mothers. These most prominently affect HMO composition (Fig.2):
FUT2 / FUT3: pos/pos
FUT2 / FUT3: neg/pos
FUT2 / FUT3: pos/neg
FUT2 / FUT3: neg/neg

There are different pathways how the HMO works: one major way is through the bifido- bacteria that can have effects also to the host, then there are some reports on direct deflection or inhibition of pathogens, some reports on strengthening the gut barrier function and some reports on educating the developing immune system either through the bacteria or their metabolites. Or, directly through some HMOs that may go systemic. And then there is newer data that there might be modulation also of the nervous system and the brain function.

Clinical observations
Infants were observed to harbor microbiota community types with highest bifidobacteria abundance in presence of Bifidobacteria- containing the genes that can utilize fucosyl-
HMOs. The gut ecosystem of those infants also have lowest pH and highest amounts of acetate, a typical short chain fatty acid produced by bifidobacteria. (Matsuki et al., Nat Commun. 2016)

In a Finnish cohort of mother-infant pairs, 2’fucosylated HMO in breastmilk were seen to alleviate the early life dysbiosis of C-section born infants compared to vaginal born infants (Korpela et al., Scientific Reports 2018).

Among the microbiota changes the Bifidobacterium and specifically B. breve increased in abundance. Interestingly, B. breve strains were identified by Matsuki et al. 2018 to be able to utilise fucosyllactose. In the same Finnish cohort associations of 2’Fucosyl-HMOs to allergies up to 5 years were studied. A faster onset of allergy was observed in the C-section born infants at 2 years of age if they get breastmilk deficient in 2’Fucosyl-HMOs.

In another cohort in Dhaka, Bangladesh, there were no milk samples, but the genetics were used to define the FUT2 positive/negative groups of mothers and infants and the FUT3 positive/negative of mothers and infants. In the exclusive breastfeeding period up to 6 months an association was observed indicating a lower risk for acute respiratory infection (ARI) in breastfed infants receiving 2’Fucosyl- HMOs through breastmilk.

What do we know about HMO effects in formula fed infants?

The primary objective of a clinical intervention study with HMO supplemented with HMO was safety growth up to 4 months and then followed up to 1 year of growth in the intervention and in the control group. (Puccio et al., JPGN 2017). Stool samples were collected to study gut microbiota composition at 3 months and 12 months of age.

The infant formula supplemented with 2’FL and LNnT is safe, well-tolerated and supports age-appropriate growth. Among the a prioridefined secondary outcome findings, a reduced reported morbidity risk (primarily in the lower respiratory tract) and medication use was observed in infants fed the HMO supplemented formula. As somewhat expected, the 2 HMOs shifted the early gut microbiota in formula fed infants towards that of breastfed infants.The main driver to this end are bifido- bacteria, which increased in abundance upon the two HMOs feeding at 3 months of age.

Looking at the stratified data set of C-section and vaginal born infants, the effect of the 2 HMOs was strongest in C-section born infants, this both for the reduced risk for lower respiratory tract morbidity (LRTI ) and for the microbiota changes.

Conclusion

Clinical observation studies with breastfed infants suggests that specific 2’Fucosyl-HMOs may act through the microbiota, especially in situations of dysbiosis like upon C-section delivery, to help immune protection and development.

RCT with formula fed infants sug- gests the 2 HMOs 2’FL and LNnT help protect from LRTI and antibiotic use, possibly through their effects on the early gut microbiome maturation and activity.