Immune System-stimulating Proteins Influence the Development of the Neonatal Microbiome and Immune System

  • Interactions between the immune system and gut microbiota play an important role in early immune development and maturation.
  • A new study investigates the role of certain immune system-stimulating proteins found at high levels in breast milk—S100A8 and S100A9—in the development of the neonatal gut microbiota and immune system.
  • S100A8 and S100A9 are also found at high fecal levels in infants compared with adults, and are crucial host factors influencing the co-development of beneficial intestinal microbiota and gut immunity in infants.
  • Intestinal S100A8 and S100A9 deficiency increases the risk of newborn individuals developing unfavorable gut microbiota and associated long-term disorders such as obesity.
  • Feeding mice S100A8 at birth prevented many of the unfavorable outcomes of low intestinal S100A8 and S100A9, and the researchers suggest that nutritional supplementation with these proteins could potentially help immune development and prevent microbiota-related disorders in preterm infants.


The saying “you are what you eat” may be especially true for newborns. For instance, breastfeeding is known to have long-term impacts on health and immunity, and several components present in breast milk are known to influence both infant immunity and their gut microbiome.

In a new study, Dr. Dorothee Viemann of Hannover Medical School and her colleagues investigated the role of certain proteins found at high levels in breast milk, S100A8 and S100A9, in the development of the microbiome and early immune responses [1]. Breast milk contains extremely high levels of S100A8 and S100A9, and these proteins are also found at high levels in healthy breast-fed infants [2,3]. Physiologically, these proteins form a complex (S100A8-A9) known as calprotectin [4,5].

Structure of the S100A9 protein.

Early immune development and maturation are critical to infant health, and it has become increasingly clear that the infant gut microbiome plays a crucial role in these processes. As microbes initially colonize the infant gut, their interactions with the intestinal immune system help in its early development and maturation [6,7]. If all goes well, the gut microbiome and host immune system develop a balance, known as “homeostasis,” but disruptions in this balance can increase an individual’s risk of inflammatory and metabolic diseases [8–12]. However, it’s still unclear what host factors influence the interactions between intestinal immunity and initial gut colonization to ensure homeostasis.

Previous studies showed that in healthy neonates high serum concentrations of S100A8-A9 can dampen immune reactions to microbes in a process known as “stress tolerance” [13–15]. However, when S100A8-A9 is released later in life in inflammatory settings, it has an amplifying effect, and fecal calprotectin serves as a biomarker of inflammation in Inflammatory Bowel Disease [16].

To elucidate the role that calprotectin plays in the development of infant intestinal immunity and microbial colonization, Viemann and her colleagues collected stool samples from both full-term and preterm infants over different time points during the first year of life and analyzed fecal microbiomes and levels of S100A8–A9. They also studied the development of the neonatal intestinal microbiota and immune system in mice lacking calprotectin.

The researchers found that fecal calprotectin levels were significantly higher in healthy term babies during the first three months of life than in adults. Initial fecal S100A8-A9 levels were significantly lower in preterm infants than in term infants but increased during the first month of life. In both preterm and term infants, fecal S100A8-A9 levels were higher after vaginal delivery than after a Caesarian section delivery.

The researchers first looked at how S100A8-A9 influenced the microbiome. High fecal S100A8-A9 levels during infancy—from 30 days to 1 year of age—relative to the cut-off for normal adults of 50 µg/g correlated with an increase in certain beneficial gut microbiota such as Bifidobacteriaceae and reduction of other microbiota such as Enterobacteriaceae that have been linked to chronic inflammation and disease. These taxonomic changes suggest that an increase in calprotectin is linked to an increase in favorable infant gut microbiota and beneficial changes to gut metabolism, including an increase in the availability of short-chain fatty acids. Additionally, the researchers found a higher abundance of Enterobacteriaceae in the fecal microbiomes of mice lacking S100A8-A9.

The researchers then explored how S100A8-A9 influences intestinal immunity. They found that S100A8 and S100A9 proteins were expressed by immune cells known as lamina propria macrophages (LPMPs) in intestinal tissues from infants at higher levels than in intestinal tissues from adults. In addition, high levels of fecal S100A8-A9 in the mouse neonatal gut influenced LPMPs to increase levels of immune cells known as T regulatory cells. T regulatory cells can in turn promote favorable changes to the gut microbiota, thus providing a mechanistic link between S100A8-A9 levels and the microbiota.

To understand the clinical relevance of S100A8-A9 fecal levels, the researchers also investigated the effects of S100A8-A9 on necrotizing enterocolitis and late-onset sepsis, two diseases of preterm infants associated with dysregulation of the gut microbiota, known as gut dysbiosis. They found virtually no S100A8-A9 expression in the intestine of necrotizing enterocolitis patients, and low fecal S100A8-A9 levels reliably predicted a high risk of late-onset sepsis, suggesting that fecal S100A8-A9 deficiency is associated with necrotizing enterocolitis and sepsis.

In addition, a low level of S100A8 and S100A9 proteins in infant fecal samples was associated with the development of sepsis and obesity by age 2, suggesting a link between these proteins and long-term dysbiosis-associated diseases. Previous studies have shown that long-term gut dysbiosis is a risk factor for the development of obesity [5,17,18].

When the researchers looked at neonatal mice lacking S100A8-A9, they found that these were also more likely to weigh more and develop fatal sepsis than normal mice. However, feeding mice S100A8 at birth reduced body weight and death from neonatal sepsis and beneficially changed the gut microbiome and intestinal immunity.

The findings indicate that S100A8-A9 plays a clinically relevant role in regulating newborn intestinal immunity and could help prevent dysbiosis-related diseases. In addition, data from mouse studies indicate the potential for nutritional supplementation of S100A8-A9 to prevent such diseases. The researchers suggest the need for clinical studies to evaluate whether S100A8 and S100A9 supplementation might aid in the development of gut immunity in preterm infants and prevent dysbiosis-associated disorders later in life.


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Contributed by
Dr. Sandeep Ravindran
Freelance Science Writer