Martin Frederik Laursen

Type 1 and type 3 γδ T cells develop in the embryonic thymus, yet it is not known whether they continue to differentiate after birth in tissues. Here, we show that in the first week of life, intestinal γδ T cells up-regulated many T cell activation and type 3 genes, while the postweaning period was dominated by induction of genes related to effector function, type 1 immunity, and cytotoxicity. During this period, γδ T cells protected the gut from fungal infection. In the adult intestine, type 3 interleukin-17–producing γδ T cells (γδT17 cells) had a distinct phenotype, a Tbet-dependent type 1 immune profile and enhanced T cell receptor (TCR) signaling. Last, we found a γδT17 cell–like population in the neonatal human gut. Collectively, our data suggest that γδ T cells continue to mature after they leave the thymus and, upon localization to the gut, a dynamic temporal differentiation program reshapes type 3 cells into Tbet ⁺ type 1 effectors with heightened TCR activity.

Bifidobacterium longum subsp. infantis (B. infantis ) is an important colonizer of the infant gut. This species is known to produce a variety of health-beneficial metabolites, including aromatic lactic acids (ALAs), supporting early-life immune development. However, the regulation of ALA production in B. infantis remains poorly understood. In this study, we investigated how environmental factors, such as substrate availability and pH, affect B. infantis DSM20088 growth and ALA production in vitro . Bacterial batch cultivations supplemented with different amino acid concentrations revealed a linear relationship between indole-3-lactic acid (ILA), 4-hydroxyphenyllactic acid (OH-PLA), and phenyllactic acid (PLA) production, and exogenous concentrations of tryptophan, tyrosine, and phenylalanine, respectively. Furthermore, chemostat cultivations at physiologically relevant pH levels (4.5, 5.5, and 6.5) showed that an acidic pH slowed growth and shifted ALA production by B. infantis . Overall, ALA concentrations correlated strongly with bacterial abundance across and within pH levels, showing that cell density is a major determinant of ALA production. After considering the growth-limiting effects of lower environmental pH, it had a limited impact on the total production of ALAs, but the relative production of PLA was stepwise enhanced at more acidic pH at the expense of ILA, independent of cell density. In conclusion, this study shows that cell density, substrate availability, and environmental pH determine aromatic lactic acid production by B. infantis . IMPORTANCE: Bifidobacterium longum subsp. infantis can be an abundant bacterial species in the infant gut microbiota, where it supports immune development, e.g., through the production of aromatic lactic acids. In this study, we cultured Bifidobacterium longum subsp. infantis at different concentrations of aromatic amino acids and pH levels to investigate the effect of these environmental factors on aromatic lactic acid production. We show that there is a linear relationship between aromatic lactic acid production and the exogenous aromatic amino acid concentration, that the production strongly follows cell density, and that pH regulates the preference toward producing different aromatic lactic acids. This knowledge may help inform strategies to enhance or direct beneficial aromatic lactic acid (ALA) production in the infant gut.

Intestinal transit time (TT) varies considerably between healthy individuals and affects gut microbiota composition and activity. Whether differences in the gut microbiota composition also affect the TT is not well elucidated. In this study, we conducted two animal experiments to explore causality between the gut microbiota and TT. In the first experiment, we transplanted two groups of female germ-free (GF) Swiss-Webster mice with fecal material from two healthy human donors with fast and slow TT phenotypes. Following transplantation with human feces, we observed a decrease in TT for both groups of GF recipient mice (from 300 min to 167 min, 95% CI: ±45; and from 369 vs 205 min, 95% CI: ±52) corresponding to reductions of approximately 45% in each group, supporting previous findings that the mere presence of a gut microbiota reduces TT. However, we found no differences in TT between the two recipient groups. In the second experiment, we transplanted two groups of female GF C57Bl/6J mice with cecal material from two different conventional C57Bl/6J mouse donor groups treated with the TT-increasing drug loperamide or a saline vehicle. Again, no differences in TT were observed between the two recipient groups. These findings indicate that either the transferred microbiota did not engraft effectively, or that gut microbiota composition itself is not the principal driver of inter-individual TT variation.

Human milk oligosaccharide (HMO)-degrading Bifidobacterium species are key early colonizers of the gut and influence gut and immune maturation. Loss of these taxa, particularly Bifidobacterium infantis , in many industrialized populations has raised concern. O’Brien et al. showed that supplementation with B. infantis EVC001 in exclusively breastfed U.S. infants aged 2–4 months leads to rapid and abundant colonization that persists 1 month after supplementation, demonstrating effective colonization beyond the neonatal period (C. E. O'Brien, S. A. Frese, K. Cernioglo, K. Damian-Medina, et al., mSphere e00518-25, 2025, https://doi.org/10.1128/msphere.00518-25 ). These findings align with observational cohort data showing that B. infantis can overcome priority effects and dominate the gut microbiome in breastfed infants by 2-3 months of age. Key questions remain regarding colonization in mixed- or formula-fed infants, the HMO thresholds required to sustain dominance during milk- and complementary feeding, and the critical developmental windows of B. infantis colonization for beneficial immune effects. Ongoing clinical trials with B. infantis will further clarify its role in disease prevention.

Early-life microbial exposures shape immune development and allergy risk. Food allergen sensitization, reflected by the presence of food allergen-specific immunoglobulin E (IgE), is an early indication of impaired immune tolerance. Here we show that early-life transmission of aromatic lactate-producing bifidobacteria strains in 147 children followed from birth to 5 years of age, facilitated by vaginal delivery, exposure to older siblings and exclusive breastfeeding for the first 2 months, led to increased levels of aromatic lactates in the infant gut. This microbiota–metabolite signature was inversely associated with the development of food allergen-specific IgE until 5 years and atopic dermatitis at 2 years. The observed effect was mediated by 4-hydroxy-phenyllactate, which inhibited IgE, but not IgG, production in ex vivo human immune cell cultures. Together, these findings define an early-life microbiota–metabolite–immune axis linking microbial transmission and feeding practices with reduced allergic sensitization.

Bifidobacterium longum subsp. infantis (B. infantis) is an important colonizer of the infant gut. This species is known to produce a variety of health beneficial metabolites, including aromatic lactic acids (ALAs), supporting early life immune development. However, the regulation of ALA production in B. infantis remains poorly understood. In this study, we investigated how environmental factors, such as substrate availability and pH affect B. infantis DSM20088 growth and ALA production in vitro . Bacterial batch cultivations supplemented with different amino acid concentrations revealed a linear relationship between indole-3-lactic acid (ILA), 4-hydroxyphenyllactic acid (OH-PLA), and phenyllactic acid (PLA) production and exogenous concentrations of tryptophan, tyrosine and phenylalanine, respectively. Furthermore, chemostat cultivations at physiological relevant pH levels (4.5, 5.5 and 6.5) showed that an acidic pH slowed growth and shifted ALA production by B. infantis . Overall, ALA concentrations correlated strongly with bacterial abundance across and within pH levels, showing that cell density is a major determinant of ALA production. After considering the growth limiting effects of lower environmental pH, it had limited impact on the total production of ALAs, but the relative production of PLA was stepwise enhanced at more acidic pH at the expense of ILA, independent of cell density. In conclusion, this study shows that cell density, substrate availability and environmental pH determine aromatic lactic acid production by B. infantis . Importance: Bifidobacterium longum subsp. infantis is an abundant bacterial species in the infant gut microbiota where it supports immune development e.g. though the production of aromatic lactic acids. In this study we cultured Bifidobacterium longum subsp. infantis at different concentrations of aromatic amino acids and pH levels to investigate the effect of these environmental factors on aromatic lactic acid production. We show that there is a linear relationship between aromatic lactic acid production and the exogenous aromatic amino acid concentration, that the production strongly follows cell density, and that pH regulates the preference towards producing different aromatic lactic acids. This knowledge may help inform strategies to enhance or direct beneficial ALA production in the infant gut.