The human gut harbors a vast number of microbes. These microbes are not passive bystanders. They are important in modulating the immune system. We have previously shown that early colonization with lactobacilli and Staphylococcus (S.) aureus differentially associates with allergy development and/or immune profile at early ages. Here we focus on understanding how these microbes modulate the response of intestinal epithelial cells and immune cells in vitro. In paper I, we investigated the impact of UV-killed and/or cell free supernatant (CFS) of different Lactobacillus (L.) species and S. aureus strains on cytokine production from intestinal epithelial cells (IEC) and immune cells. Enterotoxin-expressng S. aureus 161:2-CFS triggered CXCL-1/GRO-α and CXCL-8/IL-8 production by IEC. S. aureus-induced CXCL-8/IL-8 production was hampered by MyD88 gene silencing of IEC, indicating the importance of TLR signaling. Further, lactobacilli-CFS and S. aureus-CFS were able to induce the production of a number of cytokines by peripheral blood mononuclear cells (PBMC) from healthy donors, but only S. aureus triggered T-cell associated cytokines: IL-2, IL-17, IFN-γ and TNF-α; which were dampened by the co-treatment with S. aureus and any of the different Lactobacillus strains. Flow cytometry of the stimulated PBMC further verified IFN-γ and IL-17 production by T cells upon treatment with S. aureus-CFS, which also induced CTLA-4 expression and IL-10 production by Treg cells. In paper II, we investigated the influence of CFS of L. reuteri and S. aureus on the differentiation of monocyte to DC and subsequently how the generated DC influence T cell response. DC generated in the presence of L. reuteri exhibited an increase in expression of surface markers (HL-DR, CD86, CD83, CCR7) and cytokine production (IL-6, IL-10 and IL-23), but had a decreased phagocytic capacity compared with conventional Mo-DC, showing a more mature phenotype. However, upon LPS stimulation, DC generated in the presence of L. reuteri-CFS displayed a more regulatory phenotype, with a reduced cytokine response both at mRNA and protein levels. On the contrary, DC generated in the presence of S. aureus-CFS resembled the control Mo-DC both at mRNA and protein expression, but SA-DC was more efficient in inducing cytokine production in autologous T cells. In paper III, we studied the influence of L. reuteri-CFS on the retinoic acid (RA)-driven mucosal-like DCs’ phenotype and function to modulate T regulatory cells (Treg) in vitro. DC generated in the presence of RA showed a mucosal-like regulatory-DC phenotype with its CD103 expression, high IL10 production and decreased expression of genes associated with inflammation (NFκB1, RELB and TNF). Further, treatment with L. reuteri-CFS enhanced the regulatory phenotype of RA-DC by increasing the production of several chemokines, such as CXCL1, CXCL5, CCL3, CCL15 and CCL20, which are involved in gut homeostasis, while dampening the expression of most chemokine receptor genes. L. reuteri-CFS also increased CCR7 expression on RA-DC.  RA-DC co-cultured with T cell increased IL10 and FOXP3 expression in Treg. However L. reuteri-CFS pre-conditioning of the RA-DC did not improve the Treg phenotype. In conclusion, bacteria-CFS can have an impact on the response of IEC, differentiation and function of DC and, subsequently the T cell response, when taken together in the context of gut; these can have an impact on the health and disease of the host.

Alteration of gut microbiota has been associated with development of immune mediated diseases, such as allergy. In part, this could be due to the influence of microbes in shaping the immune response. In paper I, we investigated the association of early-life gut colonization with bacteria, and numbers of IL-4, IL-10 and IFN-γ producing cells at two years of age in response to PBMC stimulation with phytohemagglutinin (PHA) in vitro. Early Staphylococcus (S) aureus colonization was directly proportional to increased numbers of IL-4 and IL-10 secreting cells, while early co-colonization with lactobacilli and S. aureus associated with a decrease in IL-4, IL-10 and IFN-γ secreting cells compared to S. aureus alone. This was also confirmed in in vitro stimulations of PBMC with Lactobacillus and/or S. aureus strains, where S. aureus-induced IFN-γ production by Th cells was down regulated by co-stimulation with Lactobacillus. In paper II, we investigated the effects of UV-killed and/or culture supernatant (sn) of Lactobacillus strains and S. aureus strains on IEC and immune cell responses. IEC exposed to S. aureus-sn produced CXCL-1/GRO-α and CXCL-8/IL-8, while UV-killed bacteria had no effect. Further, PBMC from healthy donors exposed to Lactobacillus-sn and S. aureus-sn were able to produce a plethora of cytokines, but only S. aureus induced the T-cell associated cytokines: IL-2, IL-17, IFN-γ and TNF-α; which were down regulated by co-stimulation with any of the different Lactobacillus strains. Intracellular staining verified S. aureus-induced IFN-γ and IL-17 production by Th cells, and increased CTLA-4 expression and IL-10 production by T reg cells.

In conclusion, we show that colonization with gut microbiota at early age modulates the cytokine response in infancy. In addition, bacterial species influence cytokine response in a species-specific manner and we demonstrate that lactobacilli modulate S. aureus-induced immune response away from an inflammatory phenotype.

There seems to be a correlation between early gut microbiota composition and postnatal immune development. Alteration in the microbial composition early in life has been associated with immune mediated diseases, such as autoimmunity and allergy. We have previously observed associations between the presence of lactobacilli and Staphylococcus (S.) aureus in the early-life gut microbiota, cytokine responses and allergy development in children. Consistent with the objective to understand how bacteria modulate the cytokine response of intestinal epithelial cell (IEC) lines and immune cells, we exposed IEC lines (HT29, SW480) to UV-killed bacteria and/or culture supernatants (-sn) from seven Lactobacillus strains and three S. aureus strains, while peripheral blood mononuclear cells (PBMC) and cord blood mononuclear cells (CBMC) from healthy donors were stimulated by bacteria-sn or with bacteria conditioned IEC-sn. Although the overall IEC response to bacterial exposure was characterized by limited sets of cytokine and chemokine production, S. aureus 161: 2-sn induced an inflammatory response in the IEC, characterized by CXCL1/GROa and CXCL8/IL-8 production, partly in a MyD88-dependent manner. UV-killed bacteria did not induce a response in the IEC line, and a combination of both UV-killed bacteria and the bacteria-sn had no additive effect to that of the supernatant alone. In PBMC, most of the Lactobacillus-sn and S. aureus-sn strains were able to induce a wide array of cytokines, but only S. aureus-sn induced the T-cell associated cytokines IL-2, IL-17 and IFN-gamma, independently of IEC-produced factors, and induced up regulation of CTLA-4 expression and IL-10 production by T-regulatory cells. Notably, S. aureus-sn-induced T-cell production of IFN-gamma and IL-17 was down regulated by the simultaneous presence of any of the different Lactobacillus strains, while the IEC CXCL8/IL-8 response was unaltered. Thus these studies present a possible role for lactobacilli in induction of immune cell regulation, although the mechanisms need to be further elucidated.

Introduction: In early-life, the immature mucosal barrier allows contact between the gut microbiota and the developing immune system. Due to their strategic location and their ability to sample luminal antigen, dendritic cells (DC) play a central role in the interaction of microbes and immune cells in the gut. Here, we investigated how two bacteria associated with opposite immune profiles in children, that is, Lactobacillus (L.) reuteri and Staphylococcus (S.) aureus, influenced the differentiation of monocytes in vitro as well how the generated DC impacted T cell responses.

Methods: We exposed monocyte cultures to cell-free supernatants (CFS) from these bacteria during their differentiation to DC.

Results: The presence of L. reuteri-CFS during DC differentiation resulted in DC with a more mature phenotype, in terms of up-regulated surface markers (HLA-DR, CD86, CD83, CCR7) and enhanced cytokine production (IL6, IL10, and IL23), but had a reduced phagocytic capacity compared with non-treated monocyte-derived DC (Mo-DC). However, upon LPS activation, L. reuteri-CFS-generated DC displayed a more regulated phenotype than control Mo-DC with notable reduction of cytokine responses both at mRNA and protein levels. In contrast, S. aureus-CFS-generated DC were more similar to control Mo-DC both without and after LPS stimulation, but they were still able to induce responses in autologous T cells, in the absence of further T cell stimulation.

Conclusions: We show that bacterial signals during DC differentiation have a profound impact on DC function and possibly also for shaping the T cell pool.

Lactobacilli are widely used as probiotics with beneficial effects on infection-associated diarrhea, but also used in clinical trials of e.g., necrotizing enterocolitis and inflammatory bowel diseases. The possibility of using probiotic metabolic products, so-called postbiotics, is desirable as it could prevent possible side effects of live bacteria in individuals with a disturbed gut epithelial barrier. Here, we studied how Lactobacillus reuteri DSM 17938 cell-free supernatant (L. reuteri-CFS) influenced retinoic acid (RA)-driven mucosal-like dendritic cells (DC) and their subsequent effect on T regulatory cells (Treg) in vitro. RA clearly imprinted a mucosal-like DC phenotype with higher IL10 production, increased CD103 and CD1d expression, and a downregulated mRNA expression of several inflammatory-associated genes (NFκB1, RELB, and TNF). Treatment with L. reuteri-CFS further influenced the tolerogenic phenotype of RA-DC by downregulating most genes involved in antigen uptake, antigen presentation, and signal transduction as well as several chemokine receptors, while upregulating IL10 production. L. reuteri-CFS also augmented CCR7 expression on RA-DC. In cocultures, RA-DC increased IL10 and FOXP3 expression in Treg, but pre-treatment with L. reuteri-CFS did not further influence the Treg phenotype. In conclusion, L. reuteri-CFS modulates the phenotype and function of mucosal-like DC, implicating its potential application as postbiotic.

Breast milk contains pro- and anti-inflammatory cytokines and chemokines with potential to influence immunological maturation in the child. We have shown previously that country of birth is associated with the cytokine/chemokine profile of breast milk. In this study we have investigated how these differences in breast milk affect the cellular response of cord blood mononuclear cells (CBMCs) and intestinal epithelial cells (IECs, cell line HT-29) to microbial challenge. Ninety-five women were included: 30 from Mali in West Africa, 32 Swedish immigrants and 33 native Swedish women. CBMCs or IECs were stimulated in vitro with breast milk, alone or in combination with lipopolysaccharide (LPS) or peptidoglycan (PGN). Breast milk in general abrogated the LPS-induced down-regulation of surface CD14 and Toll-like receptor (TLR)-4 expression on CB monocytes, while inhibiting the PGN-induced TLR-2 up-regulation. However, breast milk from immigrant women together with LPS induced a lower CBMC release of interleukin (IL)-6 (P = 0·034) and CXCL-8/IL-8 (P = 0·037) compared with breast milk from Swedish women, while breast milk from Swedish women and Mali women tended to increase the response. The same pattern of CXCL-8/IL-8 release could be seen after stimulation of IECs (HT-29). The lower CBMC and IEC (HT-29) responses to microbial compounds by breast milk from immigrant women could be explained by the fact that breast milk from the immigrant group showed a divergent pro- and anti-inflammatory content for CXCL-8/IL-8, transforming growth factor-β1 and soluble CD14, compared to the other two groups of women. This may have implications for maturation of their children's immune responses.

Microbial exposure early in life influences immune maturation and potentially also the development of immune-mediated disease. Here we studied early-life gut colonization in relation to cytokine responses at two years of age. Fecal samples were collected from infants during the first two months of life. DNA was extracted from the fecal samples and Bifidobacterium (B.) adolescentis, B. breve, B. bifidum, a group of lactobacilli (L. casei, L. paracasei and L. rhamnosus) as well as Staphylococcus (S.) aureus were detected with real time PCR. Peripheral mononuclear cells were stimulated with phytohaemagglutinin (PHA) and numbers of IL-4-, IL-10- and IFN-γ secreting cells were evaluated using ELISpot. We further stimulated peripheral blood mononuclear cells with bacterial supernatants in vitro and assessed the IL-4-, IL-10- and IFN-γ inducing capacity by flow cytometry and ELISA. Early S. aureus colonization associated with higher numbers of IL-4- (p = 0.022) and IL-10 (p = 0.016) producing cells at two years of age. In contrast to colonization with S. aureus alone, co-colonization with lactobacilli associated with suppression of IL-4- (p = 0.004), IL-10- (p = 0.004) and IFN-γ (p = 0.034) secreting cells. In vitro stimulations of mononuclear cells with bacterial supernatants supported a suppressive role of L. rhamnosus GG on S. aureus-induced cytokine responses. We demonstrate that the early gut colonization pattern associates with the PHA-induced cytokine profile at two years of age and our in vitro findings support that specific bacterial species influence the T helper cell subsets. This suggests that dysbiosis in the early microbiota may modulate the risk of developing inflammatory conditions like allergy.

Due to a global increase in the range and number of infections caused by multi-resistant bacteria, 11 phage therapy is currently experiencing a resurgence of interest. However, there are a number of 12 well-known concerns over the use of phages to treat bacterial infections. In order to address concerns 13 over safety and the poorly understood pharmacokinetics of phages and their associated cocktails, 14 immunological characterization is required. In the current investigation, the immunogenicity of four 15 distinct phages and their interaction with donor derived peripheral blood mononuclear cells and 16 immortalized cell lines (HT-29 and Caco-2 intestinal epithelial cells) were investigated using 17 standard immunological techniques. When exposed to high phage concentrations (109 PFU/well), 18 cytokine driven inflammatory responses were induced from all cell types. Although phages appeared 19 to inhibit the growth of intestinal epithelial cell lines, they also appear to be non-cytotoxic. Despite 20 co-incubation with different cell types, phages maintained a high killing efficiency, reducing 21 extended-spectrum beta-lactamase-producing Escherichia coli numbers by 1-4 log10 compared to 22 untreated controls. Phages were also able to actively reproduce in the presence of human cells 23 resulting in an approximately 2 log10 increase in phage titer compared to the initial inoculum. 24 Through an increased understanding of the complex pharmacokinetics of phages, it may be possible 25 to address some of the safety concerns surrounding phage preparations prior to creating new 26 therapeutic strategies.

Due to a global increase in the range and number of infections caused by multi resistant bacteria, phage therapy is currently experiencing a resurgence of interest. However, there are a number of well-known concerns over the use of phages to treat bacterial infections. In order to address concerns over safety and the poorly understood pharmacokinetics of phages and their associated cocktails, immunological characterization is required. In the current investigation, the immunogenicity of four distinct phages (taken from the main families that comprise the Caudovirales order) and their interaction with donor derived peripheral blood mononuclear cells and immortalized cell lines (HT-29 and Caco-2 intestinal epithelial cells) were investigated using standard immunological techniques. When exposed to high phage concentrations (10(9) PFU/well), cytokine driven inflammatory responses were induced from all cell types. Although phages appeared to inhibit the growth of intestinal epithelial cell lines, they also appear to be non-cytotoxic. Despite co-incubation with different cell types, phages maintained a high killing efficiency, reducing extended-spectrum betalactamase-producing Escherichia colinumbers by 1-4 log(10) compared to untreated controls. When provided with a suitable bacterial host, phages were also able to actively reproduce in the presence of human cells resulting in an approximately 2 log10 increase in phage titer compared to the initial inoculum. Through an increased understanding of the complex pharmacokinetics of phages, it may be possible to address some of the safety concerns surrounding phage preparations prior to creating new therapeutic strategies.

The intestinal microbiota influences immune maturation during childhood, and is implicated in early-life allergy development. However, to directly study intestinal microbes and gut immune responses in infants is difficult. To investigate how different types of early-life gut microbiota affect immune development, we collected fecal samples from children with different allergic heredity (AH) and inoculated germ-free mice. Immune responses and microbiota composition were evaluated in the offspring of these mice. Microbial composition in the small intestine, the cecum and the colon were determined by 16S rRNA sequencing. The intestinal microbiota differed markedly between the groups of mice, but only exposure to microbiota associated with AH and known future allergy in children resulted in a T helper 17 (Th17)-signature, both systemically and in the gut mucosa in the mouse offspring. These Th17 responses could be signs of a particular microbiota and a shift in immune development, ultimately resulting in an increased risk of allergy.

Extremely preterm neonates are particularly susceptible to infections, likely because of severely impaired immune function. However, little is known on the composition of the T cell compartment in early life in this vulnerable population. We conducted a comprehensive phenotypic flow cytometry-based longitudinal analysis of the peripheral conventional T cell compartment of human extremely low gestational age neonates (ELGAN) with extremely low birth weight (ELBW; <1000 g) participating in a randomized placebo-controlled study of probiotic supplementation. PBMCs from ELGAN/ELBW neonates were collected at day 14, day 28, and postmenstrual week 36. Comparisons were made with full-term 14-d-old neonates. Total CD4(+) and CD8(+) T cell frequencies were markedly lower in the preterm neonates. The reduction was more pronounced among the CD8(+) population, resulting in an increased CD4/CD8 ratio. The preterm infants were also more Th2 skewed than the full-term infants. Although the frequency of regulatory T cells seemed normal in the ELGAN/ELBW preterm neonates, their expression of the homing receptors alpha 4 beta 7, CCR4, and CCR9 was altered. Notably, ELGAN/ELBW infants developing necrotizing enterocolitis before day 14 had higher expression of CCR9 in CD4(+)T cells at day 14. Chorioamnionitis clearly associated with reduced T regulatory cell frequencies and functional characteristics within the preterm group. Finally, probiotic supplementation with Lactobacillus reuteri did not impose any phenotypic changes of the conventional T cell compartment. In conclusion, notable immaturities of the T cell compartment in ELGAN/ELBW neonates may at least partially explain their increased susceptibility to severe immune-mediated morbidities.