Structural Basis for Norovirus Inhibition by Human Milk Oligosaccharides

Probiotics and Human Milk Oligosaccharides in Premature Infants

Intestinal dysbiosis precedes and is a likely causative factor in necrotizing enterocolitis (NEC) and many cases of late-onset sepsis. Randomized controlled trials and observational cohort studies demonstrate decreased risk of NEC, sepsis, and death with the administration of probiotic microbes and decreased risk of NEC and sepsis with feeding of human milk. Animal studies suggest promising mechanisms by which probiotic microbes and human milk oligosaccharides alter the composition of the intestinal microbiota and may prevent disease in premature infants. Inclusion of parents in discussions of the risks and benefits of human milk and probiotics for premature infants is essential.

Inhibitory Effects of Laminaria japonica Fucoidans Against Noroviruses

Norovirus is the leading cause of nonbacterial foodborne disease outbreaks. Human noroviruses (HuNoVs) bind to histo-blood group antigens as the host receptor for infection. In this study, the inhibitory effects of fucoidans from brown algae, Laminaria japonica (LJ), Undaria pinnatifida and Undaria pinnatifida sporophyll, were evaluated against murine norovirus (MNoV), feline calicivirus (FCV) and HuNoV. Pretreatment of MNoV or FCV with the fucoidans at 1 mg/mL showed high antiviral activities, with 1.1 average log reductions of viral titers in plaque assays. They also showed significant inhibition on the binding of the P domains of HuNoV GII.4 and GII.17 to A- or O-type saliva and the LJ fucoidan was the most effective, reaching 54-72% inhibition at 1 mg/mL. In STAT1-/- mice infected with MNoV, oral administration of the LJ fucoidan, composed of mainly sulfated fucose and minor amounts of glucose and galactose, improved the survival rates of mice and significantly reduced the viral titers in their feces. Overall, these results provide the LJ fucoidan can be used to reduce NoV outbreaks.

Human milk oligosaccharides: Shaping the infant gut microbiota and supporting health

Human milk oligosaccharides (HMO) are complex sugars which are found in breast milk at significant concentrations and with unique structural diversity. These sugars are the fourth most abundant component of human milk after water, lipids, and lactose and yet provide no direct nutritional value to the infant. Recent research has highlighted that HMOs have various functional roles to play in infant development. These sugars act as prebiotics by promoting growth of beneficial intestinal bacteria thereby generating short-chain fatty acids which are critical for gut health. HMOs also directly modulate host-epithelial immune responses and can selectively reduce binding of pathogenic bacteria and viruses to the gut epithelium preventing the emergence of a disease. This review covers current knowledge related to the functional biology of HMOs and their associated impact on infant gut health.

Like mother, like microbe: human milk oligosaccharide mediated microbiome symbiosis

Starting shortly after parturition, and continuing throughout our lifetime, the gut microbiota coevolves with our metabolic and neurological programming. This symbiosis is regulated by a complex interplay between the host and environmental factors, including diet and lifestyle. Not surprisingly, the development of this microbial community is of critical importance to health and wellness. In this targeted review, we examine the gut microbiome from birth to 2 years of age to characterize the role human milk oligosaccharides play in early formation of microbial flora.

Nanobody-Mediated Neutralization Reveals an Achilles Heel for Norovirus

Human norovirus frequently causes outbreaks of acute gastroenteritis. Although discovered more than five decades ago, antiviral development has, until recently, been hampered by the lack of a reliable human norovirus cell culture system. Nevertheless, a lot of pathogenesis studies were accomplished using murine norovirus (MNV), which can be grown routinely in cell culture. In this study, we analyzed a sizeable library of nanobodies that were raised against the murine norovirus virion with the main purpose of developing nanobody-based inhibitors. We discovered two types of neutralizing nanobodies and analyzed the inhibition mechanisms using X-ray crystallography, cryo-electron microscopy (cryo-EM), and cell culture techniques. The first type bound on the top region of the protruding (P) domain. Interestingly, this nanobody binding region closely overlapped the MNV receptor-binding site and collectively shared numerous P domain-binding residues. In addition, we showed that these nanobodies competed with the soluble receptor, and this action blocked virion attachment to cultured cells. The second type bound at a dimeric interface on the lower side of the P dimer. We discovered that these nanobodies disrupted a structural change in the capsid associated with binding cofactors (i.e., metal cations/bile acid). Indeed, we found that capsids underwent major conformational changes following addition of Mg²⁺ or Ca²⁺ Ultimately, these nanobodies directly obstructed a structural modification reserved for a postreceptor attachment stage. Altogether, our new data show that nanobody-based inhibition could occur by blocking functional and structural capsid properties.IMPORTANCE This research discovered and analyzed two different types of MNV-neutralizing nanobodies. The top-binding nanobodies sterically inhibited the receptor-binding site, whereas the dimeric-binding nanobodies interfered with a structural modification associated with cofactor binding. Moreover, we found that the capsid contained a number of vulnerable regions that were essential for viral replication. In fact, the capsid appeared to be organized in a state of flux, which could be important for cofactor/receptor-binding functions. Blocking these capsid-binding events with nanobodies directly inhibited essential capsid functions. Moreover, a number of MNV-specific nanobody binding epitopes were comparable to human norovirus-specific nanobody inhibitors. Therefore, this additional structural and inhibition information could be further exploited in the development of human norovirus antivirals.

Immunomodulation by Human Milk Oligosaccharides: The Potential Role in Prevention of Allergic Diseases

The prevalence and incidence of allergic diseases is rising and these diseases have become the most common chronic diseases during childhood in Westernized countries. Early life forms a critical window predisposing for health or disease. Therefore, this can also be a window of opportunity for allergy prevention. Postnatally the gut needs to mature, and the microbiome is built which further drives the training of infant's immune system. Immunomodulatory components in breastmilk protect the infant in this crucial period by; providing nutrients that contain substrates for the microbiome, supporting intestinal barrier function, protecting against pathogenic infections, enhancing immune development and facilitating immune tolerance. The presence of a diverse human milk oligosaccharide (HMOS) mixture, containing several types of functional groups, points to engagement in several mechanisms related to immune and microbiome maturation in the infant's gastrointestinal tract. In recent years, several pathways impacted by HMOS have been elucidated, including their capacity to; fortify the microbiome composition, enhance production of short chain fatty acids, bind directly to pathogens and interact directly with the intestinal epithelium and immune cells. The exact mechanisms underlying the immune protective effects have not been fully elucidated yet. We hypothesize that HMOS may be involved in and can be utilized to provide protection from developing allergic diseases at a young age. In this review, we highlight several pathways involved in the immunomodulatory effects of HMOS and the potential role in prevention of allergic diseases. Recent studies have proposed possible mechanisms through which HMOS may contribute, either directly or indirectly, via microbiome modification, to induce oral tolerance. Future research should focus on the identification of specific pathways by which individual HMOS structures exert protective actions and thereby contribute to the capacity of the authentic HMOS mixture in early life allergy prevention.

Biotechnologically produced fucosylated oligosaccharides inhibit the binding of human noroviruses to their natural receptors

Norovirus infections cause severe gastroenteritis in millions of people every year. Infection requires the recognition of histo-blood group antigens (HBGAs), but such interactions can be inhibited by human milk oligosaccharides (HMOs), which act as structurally-similar decoys. HMO supplements could help to prevent norovirus infections, but the industrial production of complex HMOs is challenging. Here we describe a large-scale fermentation process that yields several kilograms of lacto-N-fucopentaose I (LNFP I). The product was synthesized in Escherichia coli BL21(DE3) cells expressing a recombinant N-acetylglucosaminyltransferase, β(1,3)galactosyltransferase and α(1,2)fucosyltransferase. Subsequent in vitro enzymatic conversion produced HBGA types A1 and B1 for norovirus inhibition assays. These carbohydrates inhibited the binding of GII.17 virus-like particles (VLPs) to type A1 and B1 trisaccharides more efficiently than simpler fucosylated HMOs, which were in turn more effective than any non-fucosylated structures. However, we found that the simpler fucosylated HMOs were more effective than complex molecules such as LNFP I when inhibiting the binding of GII.17 and GII.4 VLPs to human gastric mucins and mucins from human amniotic fluid. Our results show that complex fucosylated HMOs can be produced by large-scale fermentation and that a combination of simple and complex fucosylated structures is more likely to prevent norovirus infections.

Breast Milk, a Source of Beneficial Microbes and Associated Benefits for Infant Health

Human breast milk is considered the optimum feeding regime for newborn infants due to its ability to provide complete nutrition and many bioactive health factors. Breast feeding is associated with improved infant health and immune development, less incidences of gastrointestinal disease and lower mortality rates than formula fed infants. As well as providing fundamental nutrients to the growing infant, breast milk is a source of commensal bacteria which further enhance infant health by preventing pathogen adhesion and promoting gut colonisation of beneficial microbes. While breast milk was initially considered a sterile fluid and microbes isolated were considered contaminants, it is now widely accepted that breast milk is home to its own unique microbiome. The origins of bacteria in breast milk have been subject to much debate, however, the possibility of an entero-mammary pathway allowing for transfer of microbes from maternal gut to the mammary gland is one potential pathway. Human milk derived strains can be regarded as potential probiotics; therefore, many studies have focused on isolating strains from milk for subsequent use in infant health and nutrition markets. This review aims to discuss mammary gland development in preparation for lactation as well as explore the microbial composition and origins of the human milk microbiota with a focus on probiotic development.

Human Milk Oligosaccharides: Health Benefits, Potential Applications in Infant Formulas, and Pharmacology

The first months of life are a special time for the health development and protection of infants. Breastfeeding is the natural and best way of feeding an infant, and positively influences their development and health. Breast milk provides the ideal balance of nutrients for the infant and contains countless bioactive ingredients such as immunoglobulins, hormones, oligosaccharides and others. Human milk oligosaccharides (HMOs) are a very important and interesting constituent of human milk, and are the third most abundant solid component after lactose and lipids. They are a structurally and biologically diverse group of complex indigestible sugars. This article will discuss the mechanisms of action of HMOs in infants, such as their anti-adhesive properties, properties modulating the immune system, and impact on bacterial flora development. Many health benefits result from consuming HMOs. They also may decrease the risk of infection by their interactions with viruses, bacteria or protozoa. The commercial use of HMOs in infant formula, future directions, and research on the use of HMOs as a therapy will be discussed.

Infant growth, neurodevelopment and gut microbiota during infancy: which nutrients are crucial?

PURPOSE OF REVIEW

To update the role of specific nutrients during infant development.

RECENT FINDINGS

Several bioactive nutrients such as long-chain polyunsaturated fatty acids (LC-PUFAs), iron, vitamins, proteins, or carbohydrates have been identified to exert an important role during the first 1000 days of life on infant growth, neurodevelopment, and gut microbiota establishment and maturation. LC-PUFAs are structural constituents of the central nervous system (CNS), being essential in retinal development or hippocampal plasticity. Recently, components of the milk fat globule membrane (MFG) are being added to infant formulas because of their key role in infant's development. A high intake of proteins induces a faster weight gain during infancy which correlates with later obesity. Digestible carbohydrates provide glucose, crucial for an adequate functioning of CNS; nondigestible carbohydrates [e.g. human milk oligosaccharides (HMOs)] are the main carbon source for gut bacteria. Iron-deficiency anemia during infancy has been associated with alterations of mental and psychomotor development. Folate metabolism, closely related to vitamins B6 and B12, controls epigenetic changes, whereas inadequate status of vitamin D affects bone development, but may also increase intestinal permeability and alter gut microbiota composition.

SUMMARY

LC-PUFAs, proteins, carbohydrates, iron, and vitamins during early life are critical for infant's growth, neurodevelopment, and the establishment and functioning of gut microbiota.

Human Breast Milk Promotes the Secretion of Potentially Beneficial Metabolites by Probiotic Lactobacillus reuteri DSM 17938

Human breast milk (HBM) may have beneficial effects on Lactobacillus reuteri DSM 17938 (LR 17938) -mediated immunomodulation. We aimed to determine the effects of HBM on proliferation of LR 17938 in vitro and its associated proteins and metabolites in culture, in order to provide mechanistic insights into the health benefits of LR 17938. LR 17938 was cultured anaerobically in MRS bacterial culture media, HBM (from 6 mothers), and 2 types of cow-milk formula. The colony-forming unit (CFU) was calculated to evaluate LR 17938 growth. Sixteen-hour-fermented supernatants were used for metabolomics, and bacterial lysates were used for proteomics analysis. We found that growth of LR 17938 was 10 times better in HBM than in formula. We detected 261/452 metabolites upregulated when LR 17938 cultured in HBM compared to in formula, mainly participating in the glyoxylate cycle (succinate), urea cycle (citrulline), methionine methylation (N-acetylcysteine), and polyamine synthesis (spermidine). The significantly up-regulated enzymes were also involved in the formation of acetyl-CoA in the glyoxylate cycle and the antioxidant N-acetylcysteine. In conclusion, HBM enhances the growth of LR 17938 compared to formula and promotes LR 17938-associated metabolites that relate to energy and antioxidant status, which may be linked to the physiological effects of L. reuteri.

Infectious Agents in Bovine Red Meat and Milk and Their Potential Role in Cancer and Other Chronic Diseases

Red meat and dairy products have frequently been suggested to represent risk factors for certain cancers, chronic neurodegenerative diseases, and autoimmune and cardiovascular disorders. This review summarizes the evidence and investigates the possible involvement of infectious factors in these diseases. The isolation of small circular single-stranded DNA molecules from serum and dairy products of Eurasian Aurochs (Bos taurus)-derived cattle, obviously persisting as episomes in infected cells, provides the basis for further investigations. Gene expression of these agents in human cells has been demonstrated, and frequent infection of humans is implicated by the detection of antibodies in a high percentage of healthy individuals. Epidemiological observations suggest their relationship to the development multiple sclerosis, to heterophile antibodies, and to N-glycolylneuraminic acid (Neu5Gc) containing cell surface receptors.

The Antigenic Topology of Norovirus as Defined by B and T Cell Epitope Mapping: Implications for Universal Vaccines and Therapeutics

Human norovirus (HuNoV) is the leading cause of acute nonbacterial gastroenteritis. Vaccine design has been confounded by the antigenic diversity of these viruses and a limited understanding of protective immunity. We reviewed 77 articles published since 1988 describing the isolation, function, and mapping of 307 unique monoclonal antibodies directed against B cell epitopes of human and murine noroviruses representing diverse Genogroups (G). Of these antibodies, 91, 153, 21, and 42 were reported as GI-specific, GII-specific, MNV GV-specific, and G cross-reactive, respectively. Our goal was to reconstruct the antigenic topology of noroviruses in relationship to mapped epitopes with potential for therapeutic use or inclusion in universal vaccines. Furthermore, we reviewed seven published studies of norovirus T cell epitopes that identified 18 unique peptide sequences with CD4- or CD8-stimulating activity. Both the protruding (P) and shell (S) domains of the major capsid protein VP1 contained B and T cell epitopes, with the majority of neutralizing and HBGA-blocking B cell epitopes mapping in or proximal to the surface-exposed P2 region of the P domain. The majority of broadly reactive B and T cell epitopes mapped to the S and P1 arm of the P domain. Taken together, this atlas of mapped B and T cell epitopes offers insight into the promises and challenges of designing universal vaccines and immunotherapy for the noroviruses.

Norovirus antivirals: Where are we now?

Human noroviruses inflict a significant health burden on society and are responsible for approximately 699 million infections and over 200 000 estimated deaths worldwide each year. Yet despite significant research efforts, approved vaccines or antivirals to combat this pathogen are still lacking. Safe and effective antivirals are not available, particularly for chronically infected immunocompromised individuals, and for prophylactic applications to protect high-risk and vulnerable populations in outbreak settings. Since the discovery of human norovirus in 1972, the lack of a cell culture system has hindered biological research and antiviral studies for many years. Recent breakthroughs in culturing human norovirus have been encouraging, however, further development and optimization of these novel methodologies are required to facilitate more robust replication levels, that will enable reliable serological and replication studies, as well as advances in antiviral development. In the last few years, considerable progress has been made toward the development of norovirus antivirals, inviting an updated review. This review focuses on potential therapeutics that have been reported since 2010, which were examined across at least two model systems used for studying human norovirus or its enzymes. In addition, we have placed emphasis on antiviral compounds with a defined chemical structure. We include a comprehensive outline of direct-acting antivirals and offer a discussion of host-modulating compounds, a rapidly expanding and promising area of antiviral research.

2'-Fucosyllactose Is Well Tolerated in a 100% Whey, Partially Hydrolyzed Infant Formula With Bifidobacterium lactis: A Randomized Controlled Trial

Human milk oligosaccharides are important components of breast milk. We evaluated feeding tolerance of the human milk oligosaccharide 2′-fucosyllactose (2′FL) in a 100% whey, partially hydrolyzed infant formula with the probiotic Bifidobacterium animalis ssp lactis strain Bb12 (B lactis; Test) as compared with the same formula without 2′FL (Control) in a randomized controlled trial of healthy infants enrolled at 2 weeks of age (±5 days). After 6 weeks of feeding the assigned formula, the primary outcome of tolerance was assessed using the Infant Gastrointestinal Symptom Questionnaire. Stooling, vomiting, spit-up, crying, and fussing were compared between groups. Seventy-nine infants were enrolled and 63 completed the study per protocol (30 Test, 33 Control). Infant Gastrointestinal Symptom Questionnaire scores were similar between groups (Test 20.9 ± 4.8, Control 20.7 ± 4.3, P = .82). Partially hydrolyzed infant formula with 2′FL and B lactis is tolerated well, as confirmed by a validated multi-symptom index.

Metabolism of Milk Oligosaccharides in Preterm Pigs Sensitive to Necrotizing Enterocolitis

Human milk oligosaccharides (HMO) are major components of breast milk that may have local effects in the gastrointestinal tract and systemic functions after being absorbed, both depending on their metabolism. Using preterm pigs, we investigated the metabolic fate of HMO in three experiments with two different HMO blends. In addition, we examined effects on the colonic microbiota in the presence or absence of necrotizing enterocolitis (NEC). Thus, preterm pigs (n = 112) were fed formula without or with HMO supplementation (5-10) g/L of a mixture of 4 (4-HMO) or >25 HMO (25-HMO) for 5 (Experiment 1 and 2) or 11 days (Experiment 3). Individual HMO were quantified in colon contents and urine using MALDI-TOF-MS (matrix-assisted laser desorption ionization mass spectrometry) and HPAEC-PAD (high-performance anion-exchange chromatography with pulsed amperometric detection). Microbial colonization was analyzed by sequencing of 16S rRNA gene tags. Intestinal permeability was measured by lactulose to mannitol ratio in urine. HMO supplemented to formula were detected in urine and colon contents in preterm piglets after 5 and 11 days in all three experiments. The amount of HMO excreted via the gut or the kidneys showed large individual variations. Microbial diversity in the colon changed from high levels of Firmicutes (dominated by Clostridium) at day 5 (Exp 2) to high levels of Proteobacteria dominated by Helicobacter and Campylobacter at day 11 (Exp 3). Colonic microbiota composition as well as HMO excretion pattern varied greatly among piglets. Interestingly, the 5-day supplementation of the complex 25-HMO blend led to low concentrations of 3-fucosyllactose (FL) and lacto-N-fucopentaose (LNFP) I in colonic contents, indicating a preferred utilization of these two HMO. Although the interpretation of the data from our piglet study is difficult due to the large individual variation, the presence of Bifidobacteria, although low in total numbers, was correlated with total HMO contents, and specifically with 2'FL levels in colonic content. However, early supplementation of formula with HMO did not affect NEC incidence.

Structural Basis of Nanobodies Targeting the Prototype Norovirus

The discovery of vulnerable regions on norovirus particles is instrumental in the development of effective inhibitors, particularly for GI noroviruses that are genetically diverse. Analysis of these GI.1-specific Nanobodies has shown that similar to GII norovirus particles, the GI particles have vulnerable regions. The only known cofactor region, the HBGA binding pocket, represents the main target for inhibition. With a combination treatment, i.e., the addition of Nano-7 or Nano-94 with 2′FL, the effect of inhibition was increased. Therefore, combination drug treatments might offer a better approach to combat norovirus infections, especially since the GI genotypes are highly diverse and are continually changing the capsid landscape, and few conserved epitopes have so far been identified.

Human norovirus infections are a major disease burden. In this study, we analyzed three new norovirus-specific Nanobodies that interacted with the prototype human norovirus (i.e., genogroup I genotype 1 [GI.1]). We showed that the Nanobodies bound on the side (Nano-7 and Nano-62) and top (Nano-94) of the capsid-protruding (P) domain using X-ray crystallography. Nano-7 and Nano-62 bound at a similar region on the P domain, but the orientations of these two Nanobodies clashed with the shell (S) domain and neighboring P domains on intact particles. This finding suggested that the P domains on the particles should shift in order for Nano-7 and Nano-62 to bind to intact particles. Interestingly, both Nano-7 and Nano-94 were capable of blocking norovirus virus-like particles (VLPs) from binding to histo-blood group antigens (HBGAs), which are important cofactors for norovirus infection. Previously, we showed that the GI.1 HBGA pocket could be blocked with the soluble human milk oligosaccharide 2-fucosyllactose (2′FL). In the current study, we showed that a combined treatment of Nano-7 or Nano-94 with 2′FL enhanced the blocking potential with an additive (Nano-7) or synergistic (Nano-94) effect. We also found that GII Nanobodies with 2′FL also enhanced inhibition. The Nanobody inhibition likely occurred by different mechanisms, including particle aggregation or particle disassembly, whereas 2′FL blocked the HBGA binding site. Overall, these new data showed that the positive effect of the addition of 2′FL was not limited to a single mode of action of Nanobodies or to a single norovirus genogroup.

IMPORTANCE The discovery of vulnerable regions on norovirus particles is instrumental in the development of effective inhibitors, particularly for GI noroviruses that are genetically diverse. Analysis of these GI.1-specific Nanobodies has shown that similar to GII norovirus particles, the GI particles have vulnerable regions. The only known cofactor region, the HBGA binding pocket, represents the main target for inhibition. With a combination treatment, i.e., the addition of Nano-7 or Nano-94 with 2′FL, the effect of inhibition was increased. Therefore, combination drug treatments might offer a better approach to combat norovirus infections, especially since the GI genotypes are highly diverse and are continually changing the capsid landscape, and few conserved epitopes have so far been identified.

Human Norovirus Neutralized by a Monoclonal Antibody Targeting the Histo-Blood Group Antigen Pocket

Temporal changes in the GII.4 human norovirus capsid sequences occasionally result in the emergence of genetic variants capable of causing new epidemics. The persistence of GII.4 is believed to be associated with the recognition of numerous histo-blood group antigen (HBGA) types and antigenic drift. We found that one of the earliest known GII.4 isolates (in 1974) and a more recent epidemic GII.4 variant (in 2012) had varied norovirus-specific monoclonal antibody (MAb) reactivities but similar HBGA binding profiles. To better understand the binding interaction of one MAb (10E9) that had varied reactivity with these GII.4 variants, we determined the X-ray crystal structure of the NSW-2012 GII.4 P domain 10E9 Fab complex. We showed that the 10E9 Fab interacted with conserved and variable residues, which could be associated with antigenic drift. Interestingly, the 10E9 Fab binding pocket partially overlapped the HBGA pocket and had direct competition for conserved HBGA binding residues (i.e., Arg345 and Tyr444). Indeed, the 10E9 MAb blocked norovirus virus-like particles (VLPs) from binding to several sources of HBGAs. Moreover, the 10E9 antibody completely abolished virus replication in the human norovirus intestinal enteroid cell culture system. Our new findings provide the first direct evidence that competition for GII.4 HBGA binding residues and steric obstruction could lead to norovirus neutralization. On the other hand, the 10E9 MAb recognized residues flanking the HBGA pocket, which are often substituted as the virus evolves. This mechanism of antigenic drift likely influences herd immunity and impedes the possibility of acquiring broadly reactive HBGA-blocking antibodies.IMPORTANCE The emergence of new epidemic GII.4 norovirus variants is thought to be associated with changes in antigenicity and HBGA binding capacity. Here, we show that HBGA binding profiles remain unchanged between the 1974 and 2012 GII.4 variants, whereas these variants showed various levels of reactivity against a panel of GII.4 MAbs. We identified a MAb that bound at the HBGA pocket, blocked norovirus VLPs from binding to HBGAs, and neutralized norovirus virions in the cell culture system. Raised against a GII.4 2006 strain, this MAb was unreactive to a GII.4 1974 isolate but was able to neutralize the newer 2012 strain, which has important implications for vaccine design. Altogether, these new findings suggest that the amino acid variations surrounding the HBGA pocket lead to temporal changes in antigenicity without affecting the ability of GII.4 variants to bind HBGAs, which are known cofactors for infection.

Structural Basis for Human Norovirus Capsid Binding to Bile Acids

A recently developed human norovirus cell culture system revealed that the presence of bile enhanced or was an essential requirement for the growth of certain genotypes. Before this discovery, histo-blood group antigens (HBGAs) were the only well-studied cofactor known for human noroviruses, and there was evidence that several genotypes poorly bound HBGAs. Therefore, the purpose of this study was to investigate how human norovirus capsids interact with bile acids. We found that bile acids had low-micromolar affinities for GII.1, GII.10, and GII.19 capsids but did not bind GI.1, GII.3, GII.4, or GII.17. We showed that bile acid bound at a partially conserved pocket on the norovirus capsid-protruding (P) domain using X-ray crystallography. Amino acid sequence alignment and structural analysis delivered an explanation of selective bile acid binding. Intriguingly, we discovered that binding of the bile acid was the critical step to stabilize several P domain loops that optimally placed an essential amino acid side chain (Asp375) to bind HBGAs in an otherwise HBGA nonbinder (GII.1). Furthermore, bile acid enhanced HBGA binding for a known HBGA binder (GII.10). Altogether, these new data suggest that bile acid functions as a loop-stabilizing regulator and enhancer of HBGA binding for certain norovirus genotypes.IMPORTANCE Given that human norovirus virions likely interact with bile acid during a natural infection, our evidence that an HBGA nonbinder (GII.1) can be converted to an HBGA binder after bile acid binding is of major significance. Our data provide direct evidence that, like HBGAs, bile acid interaction on the capsid is an important cofactor for certain genotypes. However, more unanswered questions seem to arise from these new discoveries. For example, is there an association between the bile acid requirement and the prevalence of certain genotypes? That is, the GII.1 and GII.10 (bile acid binders) genotypes rarely caused outbreaks, whereas the GII.4 and GII.17 genotypes (bile acid nonbinders) were responsible for large epidemics. Therefore, it seems plausible that certain genotypes require bile acids, whereas others have modified their bile acid requirements on the capsid.

Quantifying the binding stoichiometry and affinity of histo-blood group antigen oligosaccharides for human noroviruses

Human noroviruses (HuNoVs) are a major cause of acute gastroenteritis. Many HuNoVs recognize histo-blood group antigens (HBGAs) as cellular receptors or attachment factors for infection. It was recently proposed that HuNoV recognition of HBGAs involves a cooperative, multistep binding mechanism that exploits both known and previously unknown glycan binding sites. In this study, binding measurements, implemented using electrospray ionization mass spectrometry (ESI-MS) were performed on homodimers of the protruding domain (P dimers) of the capsid protein of three HuNoV strains [Saga (GII.4), Vietnam 026 (GII.10) and VA387 (GII.4)] with the ethyl glycoside of the B trisaccharide (α-d-Gal-(1→3)-[α-l-Fuc-(1→2)]-β-d-Gal-OC2H5) and free B type 1 tetrasaccharide (α-d-Gal-(1→3)-[α-l-Fuc-(1→2)]-β-d-Gal-(1→3)-d-GlcNAc) in an effort to confirm the existence of new HBGA binding sites. After correcting the mass spectra for nonspecific interactions that form in ESI droplets as they evaporate to dryness, all three P dimers were found to bind a maximum of two B trisaccharides at the highest concentrations investigated. The apparent affinities measured for stepwise binding of B trisaccharide suggest positive cooperativity. Similar results were obtained for B type 1 tetrasaccharide binding to Saga P dimer. Based on these results, it is proposed that HuNoV P dimers possess only two HBGA binding sites. It is also shown that nonspecific binding corrections applied to mass spectra acquired using energetic ion source conditions that promote in-source dissociation can lead to apparent HuNoV-HBGA oligosaccharide binding stoichiometries and affinities that are artificially high. Finally, evidence that high concentrations of oligosaccharide can induce conformational changes in HuNoV P dimers is presented.

Biosynthesis of a Functional Human Milk Oligosaccharide, 2'-Fucosyllactose, and l-Fucose Using Engineered Saccharomyces cerevisiae

2'-fucosyllactose (2-FL), one of the most abundant human milk oligosaccharides (HMOs), has received much attention due to its health-promoting activities, such as stimulating the growth of beneficial gut microorganisms, inhibiting pathogen infection, and enhancing the host immune system. Consequently, large quantities of 2-FL are on demand for food applications as well as in-depth investigation of its biological properties. Biosynthesis of 2-FL has been attempted primarily in Escherichia coli, which might not be the best option to produce food and cosmetic ingredients due to the presence of endotoxins on the cell surface. In this study, an alternative route to produce 2-FL via a de novo pathway using a food-grade microorganism,  Saccharomyces cerevisiae, has been devised. Specifically, heterologous genes, which are necessary to achieve the production of 2-FL from a mixture of glucose and lactose, were introduced into S. cerevisiae. When the lactose transporter (Lac12), de novo GDP-l-fucose pathway (consisting of GDP-d-mannose-4,6-dehydratase (Gmd) and GDP-4-keto-6-deoxymannose-3,5-epimerase-4-reductase (WcaG)), and α1,2-fucosyltransferase (FucT2) were introduced, the resulting engineered strain (D452L-gwf) produced 0.51 g/L of 2-FL from a batch fermentation. In addition, 0.41 g/L of l-fucose was produced when α-l-fucosidase was additionally expressed in the 2-FL producing strain (D452L-gwf). To our knowledge, this is the first report of 2-FL and l-fucose production in engineered S. cerevisiae via the de novo pathway. This study provides the possibility of producing HMOs by a food-grade microorganism S. cerevisiae and paves the way for more HMO production in the future.

Supplementation With 2'-FL and scGOS/lcFOS Ameliorates Rotavirus-Induced Diarrhea in Suckling Rats

Rotavirus (RV) is considered to be the most common cause of gastroenteritis among infants aged less than 5 years old. Human milk bioactive compounds have the ability to modulate the diarrheic process caused by several intestinal pathogens. This study aimed to evaluate the potential protective role of a specific human milk oligosaccharide, 2′-fucosyllactose (2′-FL), a mixture of the prebiotic short-chain galacto-oligosaccharides and long-chain fructo-oligosaccharides 9:1 (GOS/FOS) and their combination (2′-FL+GOS/FOS) on RV-induced diarrhea in suckling rats. The nutritional intervention was performed from the second to the sixteenth day of life by oral gavage and on day 5 an RV strain was orally administered to induce infection. Fecal samples were scored daily to assess the clinical pattern of severity, incidence and duration of diarrhea. Blood and tissues were obtained at day 8 and 16 in order to evaluate the effects on the epithelial barrier and the mucosal and systemic immune responses. In the assessment of severity, incidence and duration of diarrhea, both 2′-FL and GOS/FOS displayed a beneficial effect in terms of amelioration. However, the mechanisms involved seemed to differ: 2′-FL displayed a direct ability to promote intestinal maturation and to enhance neonatal immune responses, while GOS/FOS induced an intestinal trophic effect and an RV-blocking action. The combination of 2′-FL and GOS/FOS showed additive effects in some variables. Therefore, it could be a good strategy to add these compounds in combination to infant formulas, to protect against human RV-induced diarrhea in children.

Sialic Acid and Sialylated Oligosaccharide Supplementation during Lactation Improves Learning and Memory in Rats

Sialic acids (Sia) are postulated to improve cognitive abilities. This study evaluated Sia effects on rat behavior when administered in a free form as N-acetylneuraminic acid (Neu5Ac) or conjugated as 6'-sialyllactose (6'-SL). Rat milk contains Sia, which peaks at Postnatal Day 9 and drops to a minimum by Day 15. To bypass this Sia peak, a cohort of foster mothers was used to raise the experimental pups. A group of pups received a daily oral supplementation of Neu5Ac to mimic the amount naturally present in rat milk, and another group received the same molar amount of Sia as 6'-SL. The control group received water. After weaning, rats were submitted to behavioral evaluation. One year later, behavior was re-evaluated, and in vivo long-term potentiation (LTP) was performed. Brain samples were collected and analyzed at both ages. Adult rats who received Sia performed significantly better in the behavioral assessment and showed an enhanced LTP compared to controls. Within Sia groups, 6'-SL rats showed better scores in some cognitive outcomes compared to Neu5Ac rats. At weaning, an effect on polysialylated-neural cell adhesion molecule (PSA-NCAM) levels in the frontal cortex was only observed in 6'-SL fed rats. Providing Sia during lactation, especially as 6'-SL, improves memory and LTP in adult rats.

Therapeutics and Immunoprophylaxis Against Noroviruses and Rotaviruses: The Past, Present, and Future

Background:

Noroviruses and rotaviruses are important viral etiologies of severe gastroenteritis. Noroviruses are the primary cause of nonbacterial diarrheal outbreaks in humans, whilst rotaviruses are a major cause of childhood diarrhea. Although both enteric pathogens substantially impact human health and economies, there are no approved drugs against noroviruses and rotaviruses so far. On the other hand, whilst the currently licensed rotavirus vaccines have been successfully implemented in over 100 countries, the most advanced norovirus vaccine has recently completed phase-I and II trials.

Methods:

We performed a structured search of bibliographic databases for peer-reviewed research litera-ture on advances in the fields of norovirus and rotavirus therapeutics and immunoprophylaxis.

Results:

Technological advances coupled with a proper understanding of viral morphology and replication over the past decade has facilitated pioneering research on therapeutics and immunoprophylaxis against noroviruses and rotaviruses, with promising outcomes in human clinical trials of some of the drugs and vaccines. This review focuses on the various developments in the fields of norovirus and rotavirus thera-peutics and immunoprophylaxis, such as potential antiviral drug molecules, passive immunotherapies (oral human immunoglobulins, egg yolk and bovine colostral antibodies, llama-derived nanobodies, and anti-bodies expressed in probiotics, plants, rice grains and insect larvae), immune system modulators, probiot-ics, phytochemicals and other biological substances such as bovine milk proteins, therapeutic nanoparti-cles, hydrogels and viscogens, conventional viral vaccines (live and inactivated whole virus vaccines), and genetically engineered viral vaccines (reassortant viral particles, virus-like particles (VLPs) and other sub-unit recombinant vaccines including multi-valent viral vaccines, edible plant vaccines, and encapsulated viral particles).

Conclusions:

This review provides important insights into the various approaches to therapeutics and im-munoprophylaxis against noroviruses and rotaviruses..

Review of the Clinical Experiences of Feeding Infants Formula Containing the Human Milk Oligosaccharide 2'-Fucosyllactose

Human milk oligosaccharides (HMOs) are the third most abundant solid component in human milk after lactose and lipids. Preclinical research has demonstrated that HMOs and specifically 2′-fucosyllactose (2′-FL) are more than a prebiotic and have multiple functions, including immune, gut, and cognition benefits. Previously, human milk has been the only source for significant levels of HMOs. The most abundant HMO in most mothers’ breast milk is 2′-FL. Recently, 2′-FL has been synthesized and shown to be structurally identical to the 2′-FL found in human milk. 2′-FL HMO is now available in some commercial infant formulas. The purpose of this narrative review was to summarize the clinical experiences of feeding infant formula supplemented with the HMO, 2′-FL. Most of these studies investigated standard intact milk protein-based infant formulas containing 2′-FL, and one evaluated a partially hydrolyzed whey-based formula. Collectively, these clinical experiences demonstrated that 2′-FL being added to infant formula was safe, well-tolerated, and absorbed and excreted with similar efficiency to 2′-FL in human milk. Further, infants that were fed formula with 2′-FL had immune benefits, fewer parent-reported respiratory infections, and improved symptoms of formula intolerance. Ultimately, infant formula with 2′-FL supports immune and gut health and is closer compositionally and functionally to human milk.

Human Milk Oligosaccharides and Immune System Development

Maternal milk contains compounds that may affect newborn immunity. Among these are a group of oligosaccharides that are synthesized in the mammary gland from lactose; these oligosaccharides have been termed human milk oligosaccharides (HMOs). The amount of HMOs present in human milk is greater than the amount of protein. In fact, HMOs are the third-most abundant solid component in maternal milk after lactose and lipids, and are thus considered to be key components. The importance of HMOs may be explained by their inhibitory effects on the adhesion of microorganisms to the intestinal mucosa, the growth of pathogens through the production of bacteriocins and organic acids, and the expression of genes that are involved in inflammation. This review begins with short descriptions of the basic structures of HMOs and the gut immune system, continues with the beneficial effects of HMOs shown in cell and animal studies, and it ends with the observational and randomized controlled trials carried out in humans to date, with particular emphasis on their effect on immune system development. HMOs seem to protect breastfed infants against microbial infections. The protective effect has been found to be exerted through cell signaling and cell-to-cell recognition events, enrichment of the protective gut microbiota, the modulation of microbial adhesion, and the invasion of the infant intestinal mucosa. In addition, infants fed formula supplemented with selected HMOs exhibit a pattern of inflammatory cytokines closer to that of exclusively breastfed infants. Unfortunately, the positive effects found in preclinical studies have not been substantiated in the few randomized, double-blinded, multicenter, controlled trials that are available, perhaps partly because these studies focus on aspects other than the immune response (e.g., growth, tolerance, and stool microbiota).

Avidity of α-fucose on human milk oligosaccharides and blood group-unrelated oligo/polyfucoses is essential for potent norovirus-binding targets

There is agreement with respect to norovirus infection routes in humans regarding binding of the pathogen to gastrointestinal epithelia via recognition of blood group-active mucin-typeO-glycans as the initiating and essential event. Among food additives playing a potential role in applications to protect newborns, human milk oligosaccharides (HMOs) as competitors are of major importance. By focusing on fractions of high-molecular mass HMOs with high fucose contents, we attempted to identify the structural elements required for norovirus GII.4 (Sydney 2012, JX459908) capsid binding in neoglycolipid-based arrays. We provide evidence that HMO fractions with the strongest binding capacities contained hepta- to decasaccharides expressing branches with terminal blood group H1 or Lewis-b antigen. H2 antigen, as recognized by UEA-I lectin, is apparently not expressed in high-mass HMOs. Beyond affinity, sterical and valency effects contribute more to virus-like particle binding, as revealed for oligovalent fucose conjugates of α-cyclodextrin and oligofucoses from fucoidan. Accordingly, high-mass HMOs with oligovalent fucose can exhibit stronger binding capacities compared with monovalent fucose HMOs. The above features were revealed for the most clinically relevant and prevalent GII.4 strain and are distinct from other strains, like GII.10 (Vietnam 026, AF504671), which showed a preference for blood group Lewis-a positive glycans.

Atomic Structure of the Murine Norovirus Protruding Domain and Soluble CD300lf Receptor Complex

Human noroviruses are the leading cause of acute gastroenteritis in humans. Noroviruses also infect animals, such as cows, mice, cats, and dogs. How noroviruses bind and enter host cells is still incompletely understood. Recently, the type I transmembrane protein CD300lf was identified as the murine norovirus receptor, yet it is unclear how the virus capsid and receptor interact at the molecular level. In this study, we determined the X-ray crystal structure of the soluble CD300lf (sCD300lf) and the murine norovirus capsid protruding domain complex at a 2.05-Å resolution. We found that the sCD300lf-binding site is located on the topside of the protruding domain and involves a network of hydrophilic and hydrophobic interactions. sCD300lf locked nicely into a complementary cavity on the protruding domain that is additionally coordinated with a positive surface charge on sCD300lf and a negative surface charge on the protruding domain. Five of six protruding domain residues interacting with sCD300lf were maintained between different murine norovirus strains, suggesting that sCD300lf was capable of binding to a highly conserved pocket. Moreover, a sequence alignment with other CD300 paralogs showed that the sCD300lf-interacting residues were partially conserved in CD300ld but variable in other CD300 family members, consistent with previously reported infection selectivity. Overall, these data provide insights into how a norovirus engages a protein receptor and will be important for a better understanding of selective recognition and norovirus attachment and entry mechanisms.IMPORTANCE Noroviruses exhibit exquisite host range specificity due to species-specific interactions between the norovirus capsid protein and host molecules. Given this strict host range restriction, it has been unclear how the viruses are maintained within a species between relatively sporadic epidemics. While much data demonstrate that noroviruses can interact with carbohydrates, recent work has shown that expression of the protein CD300lf is both necessary and sufficient for murine norovirus infection of mice and binding of the virus to permissive cells. Importantly, the expression of this murine protein by human cells renders them fully permissive for murine norovirus infection, indicating that at least in this case, host range restriction is determined by molecular events that control receptor binding and entry. Defining the atomic-resolution interactions between the norovirus capsid protein and its cognate receptor is essential for a molecular understanding of host-range restriction and norovirus tropism.

The Double Face of Mucin-Type O-Glycans in Lectin-Mediated Infection and Immunity

Epithelial human blood group antigens (HBGAs) on O-glycans play roles in pathogen binding and the initiation of infection, while similar structures on secretory mucins exert protective functions. These double-faced features of O-glycans in infection and innate immunity are reviewed based on two instructive examples of bacterial and viral pathogens. Helicobacter pylori represents a class 1 carcinogen in the human stomach. By expressing blood group antigen-binding adhesin (BabA) and LabA adhesins that bind to Lewis-b and LacdiNAc, respectively, H. pylori colocalizes with the mucin MUC5AC in gastric surface epithelia, but not with MUC6, which is cosecreted with trefoil factor family 2 (TFF2) by deep gastric glands. Both components of the glandular secretome are concertedly up-regulated upon infection. While MUC6 expresses GlcNAc-capped glycans as natural antibiotics for H. pylori growth control, TFF2 may function as a probiotic lectin. In viral infection human noroviruses of the GII genogroup interact with HBGAs via their major capsid protein, VP1. HBGAs on human milk oligosaccharides (HMOs) may exert protective functions by binding to the P2 domain pocket on the capsid. We discuss structural details of the P2 carbohydrate-binding pocket in interaction with blood group H/Lewis-b HMOs and fucoidan-derived oligofucoses as effective interactors for the most prevalent norovirus strains, GII.4 and GII.17.

Human Milk Oligosaccharides as Promising Antivirals

Human milk oligosaccharides (HMOs) are diverse unconjugated carbohydrates that are highly abundant in human breast milk. These glycans are investigated in the context of exhibiting multiple functions in infant growth and development. They seem to provide protection against infectious diseases, including a number of poorly manageable viral infections. Although the potential mechanism of the HMO antiviral protection is rather broad, much of the current experimental work has focused on studying of HMO antiadhesive properties. HMOs may mimic structures of viral receptors and block adherence to target cells, thus preventing infection. Still, the potential of HMOs as a source for new antiviral drugs is relatively unexploited. This can be partly attributed to the extreme complexity of the virus-carbohydrate interactions and technical difficulties in HMO isolation, characterization, and manufacturing procedures. Fortunately, we are currently entering a period of major technological advances that have enabled deeper insights into carbohydrate mediated viral entry, rational selection of HMOs as anti-entry inhibitors, and even evaluation of individual synthetic HMO structures. Here, we provide an up-to-date review on glycan binding studies for rotaviruses, noroviruses, influenza viruses, and human immunodeficiency viruses. We also discuss the preventive and therapeutic potential of HMOs as anti-entry inhibitors and address challenges on the route from fundamental studies to clinical trials.

Diversity of Human Milk Oligosaccharides and Effects on Early Life Immune Development

One of the well-known features of human milk, is the capacity to protect against the risk and impact of neonatal infections, as well as to influence the onset of allergic and metabolic disease manifestations. The major objective of this review is to provide a detailed overview regarding the role of human milk, more specifically the diversity in human milk oligosaccharides (HMOS), on early life immune development. Novel insights in immune modulatory effects of HMOS obtained by in vitro as well as in vivo studies, adds to the understanding on how early life nutrition may impact immune development. Extensive description and analysis of single HMOS contributing to the diversity within the composition provided during breastfeeding will be discussed with specific emphasis on immune development and the susceptibility to neonatal and childhood infections.

Norovirus Binding to Ligands Beyond Histo-Blood Group Antigens

Histo-blood group antigens (HBGAs) are commonly accepted as the cellular receptors for human norovirus. However, some human noroviruses have been found not to bind any HBGA ligand, suggesting potential additional co-factors. Some ligands have been found to bind noroviruses and have the potential to be additional cellular receptors/attachment factors for human norovirus or inhibitors of the HBGA interaction. The studies identifying these mostly characterize different chemical, human, food, or bacterial components and their effect on norovirus binding and infection, although the mechanism of interaction is unknown in many cases. This review seeks to supplement the already well-covered HBGA-norovirus literature by covering non-HBGA human norovirus ligands and inhibitors to provide investigators with a more comprehensive view of norovirus ligands.

Breastfeeding Behaviors and the Innate Immune System of Human Milk: Working Together to Protect Infants against Inflammation, HIV-1, and Other Infections

The majority of infants’ breastfeeding from their HIV-infected mothers do not acquire HIV-1 infection despite exposure to cell-free virus and cell-associated virus in HIV-infected breast milk. Paradoxically, exclusive breastfeeding regardless of the HIV status of the mother has led to a significant decrease in mother-to-child transmission (MTCT) compared with non-exclusive breastfeeding. Although it remains unclear how these HIV-exposed infants remain uninfected despite repeated and prolonged exposure to HIV-1, the low rate of transmission is suggestive of a multitude of protective, short-lived bioactive innate immune factors in breast milk. Indeed, recent studies of soluble factors in breast milk shed new light on mechanisms of neonatal HIV-1 protection. This review highlights the role and significance of innate immune factors in HIV-1 susceptibility and infection. Prevention of MTCT of HIV-1 is likely due to multiple factors, including innate immune factors such as lactoferrin and elafin among many others. In pursuing this field, our lab was the first to show that soluble toll-like receptor 2 (sTLR2) directly inhibits HIV infection, integration, and inflammation. More recently, we demonstrated that sTLR2 directly binds to selective HIV-1 proteins, including p17, gp41, and p24, leading to significantly reduced NFκB activation, interleukin-8 production, CCR5 expression, and HIV infection in a dose-dependent manner. Thus, a clearer understanding of soluble milk-derived innate factors with known antiviral functions may provide new therapeutic insights to reduce vertical HIV-1 transmission and will have important implications for protection against HIV-1 infection at other mucosal sites. Furthermore, innate bioactive factors identified in human milk may serve not only in protecting infants against infections and inflammation but also the elderly; thus, opening the door for novel innate immune therapeutics to protect newborns, infants, adults, and the elderly.

Nanobodies targeting norovirus capsid reveal functional epitopes and potential mechanisms of neutralization

Norovirus is the leading cause of gastroenteritis worldwide. Despite recent developments in norovirus propagation in cell culture, these viruses are still challenging to grow routinely. Moreover, little is known on how norovirus infects the host cells, except that histo-blood group antigens (HBGAs) are important binding factors for infection and cell entry. Antibodies that bind at the HBGA pocket and block attachment to HBGAs are believed to neutralize the virus. However, additional neutralization epitopes elsewhere on the capsid likely exist and impeding the intrinsic structural dynamics of the capsid could be equally important. In the current study, we investigated a panel of Nanobodies in order to probe functional epitopes that could trigger capsid rearrangement and/ or interfere with HBGA binding interactions. The precise binding sites of six Nanobodies (Nano-4, Nano-14, Nano-26, Nano-27, Nano-32, and Nano-42) were identified using X-ray crystallography. We showed that these Nanobodies bound on the top, side, and bottom of the norovirus protruding domain. The impact of Nanobody binding on norovirus capsid morphology was analyzed using electron microscopy and dynamic light scattering. We discovered that distinct Nanobody epitopes were associated with varied changes in particle structural integrity and assembly. Interestingly, certain Nanobody-induced capsid morphological changes lead to the capsid protein degradation and viral RNA exposure. Moreover, Nanobodies employed multiple inhibition mechanisms to prevent norovirus attachment to HBGAs, which included steric obstruction (Nano-14), allosteric interference (Nano-32), and violation of normal capsid morphology (Nano-26 and Nano-85). Finally, we showed that two Nanobodies (Nano-26 and Nano-85) not only compromised capsid integrity and inhibited VLPs attachment to HBGAs, but also recognized a broad panel of norovirus genotypes with high affinities. Consequently, Nano-26 and Nano-85 have a great potential to function as novel therapeutic agents against human noroviruses.

We determined the binding sites of six novel human norovirus specific Nanobodies (Nano-4, Nano-14, Nano-26, Nano-27, Nano-32, and Nano-42) using X-ray crystallography. The unique Nanobody recognition epitopes were correlated with their potential neutralizing capacities. We showed that one Nanobody (Nano-26) bound numerous genogroup II genotypes and interacted with highly conserved capsid residues. Four Nanobodies (Nano-4, Nano-26, Nano-27, and Nano-42) bound to occluded regions on the intact particles and impaired normal capsid morphology and particle integrity. One Nanobody (Nano-14) bound contiguous to the HBGA pocket and interacted with several residues involved in binding HBGAs. We found that the Nanobodies delivered multiple inhibition mechanisms, which included steric obstruction, allosteric interference, and disruption of the capsid stability. Our data suggested that the HBGA pocket might not be an ideal target for drug development, since the surrounding region is highly variable and inherently suffers from lack of conservation among the genetically diverse genotypes. Instead, we showed that the capsid contained other highly susceptible regions that could be targeted for virus inhibition.

Pediatric norovirus GII.4 infections in Nicaragua, 1999-2015

OBJECTIVES

Investigate clinical and epidemiological factors of pediatric GII.4 norovirus infections in children with acute gastroenteritis (AGE) in Nicaragua between 1999 and 2015.

METHODS

We retrospectively analyzed laboratory and epidemiologic data from 1,790 children≤7years with AGE from 6 hospitals in Nicaragua (n=538), and 3 community clinics (n=919) and households (n=333) in León, between 1999 and 2015. Moreover, asymptomatic children from community clinics (n=162) and households (n=105) were enrolled. Norovirus was detected by real-time PCR and genotyped by sequencing the N-terminal and shell region of the capsid gene.

RESULTS

Norovirus was found in 19% (n=338) and 12% (n=32) of children with and without AGE, respectively. In total, 20 genotypes including a tentatively new genotype were detected. Among children with AGE, the most common genotypes were GII.4 (53%), GII.14 (7%), GII.3 (6%) and GI.3 (6%). In contrast, only one (1.4%) GII.4 was found in asymptomatic children. The prevalence of GII.4 infections was significantly higher in children between 7 and 12months of age. The prevalence of GII.4 was lowest in households (38%), followed by community clinics (50%) and hospitals (75%). Several different GII.4 variants were detected and their emergence followed the global temporal trend.

CONCLUSIONS

Overall our study found the predominance of pediatric GII.4 norovirus infections in Nicaragua mostly occurring in children between 7 and 12months of age, implicating GII.4 as the main norovirus vaccine target.

Human milk oligosaccharides and infant gut bifidobacteria: Molecular strategies for their utilization

Breast milk is the gold standard in infant nutrition. In addition to provide essential nutrients for the newborn, it contains multiple bioactive molecules that provide protection and stimulate proper development. Human milk oligosaccharides (HMO) are complex carbohydrates abundant in breast milk. Intriguingly, these molecules do not provide energy to the infant. Instead, these oligosaccharides are key to guide and support the assembly of a healthy gut microbiome in the infant, dominated by beneficial gut microbes such as Bifidobacterium. New analytical methods for glycan analysis, and next-generation sequencing of microbial communities, have been instrumental in advancing our understanding of the positive role of breast milk oligosaccharides on the gut microbiome, and the genomics and molecular strategies of Bifidobacterium to utilize these oligosaccharides. Moreover, novel approaches to simulate the impact of HMO on the gut microbiome have been described and successfully validated, including the incorporation of synthetic HMO and bovine milk oligosaccharides to infant formula. This review discusses recent advances regarding the influence of HMO in promoting a healthy gut microbiome, with emphasis in the molecular basis of the enrichment in beneficial Bifidobacterium, and novel approaches to replicate the effect of HMO using synthetic or bovine oligosaccharides.

Milk Oligosaccharides Inhibit Human Rotavirus Infectivity in MA104 Cells

Background: Oligosaccharides in milk act as soluble decoy receptors and prevent pathogen adhesion to the infant gut. Milk oligosaccharides reduce infectivity of a porcine rotavirus strain; however, the effects on human rotaviruses are less well understood.Objective: In this study, we determined the effect of specific and abundant milk oligosaccharides on the infectivity of 2 globally dominant human rotavirus strains.Methods: Four milk oligosaccharides-2'-fucosyllactose (2'FL), 3'-sialyllactose (3'SL), 6'-sialyllactose (6'SL), and galacto-oligosaccharides-were tested for their effects on the infectivity of human rotaviruses G1P[8] and G2P[4] through fluorescent focus assays on African green monkey kidney epithelial cells (MA104 cells). Oligosaccharides were added at different time points in the infectivity assays. Infections in the absence of oligosaccharides served as controls.Results: When compared with infections in the absence of glycans, all oligosaccharides substantially reduced the infectivity of both human rotavirus strains in vitro; however, virus strain-specific differences in effects were observed. Compared with control infections, the maximum reduction in G1P[8] infectivity was seen with 2'FL when added after the onset of infection (62% reduction, P < 0.01), whereas the maximum reduction in G2P[4] infectivity was seen with the mixture of 3'SL + 6'SL when added during infection (73% reduction, P < 0.01). The mixture of 3'SL + 6'SL at the same ratio as is present in breast milk was more potent in reducing G2P[4] infectivity (73% reduction, P < 0.01) than when compared with 3'SL (47% reduction) or 6'SL (40% reduction) individually. For all oligosaccharides the reduction in infectivity was mediated by an effect on the virus and not on the cells.Conclusions: Milk oligosaccharides reduce the infectivity of human rotaviruses in MA104 cells, primarily through an effect on the virus. Although breastfed infants are directly protected, the addition of specific oligosaccharides to infant formula may confer these benefits to formula-fed infants.

Human Genetic Determinants of Viral Diseases

Much progress has been made in the identification of specific human gene variants that contribute to enhanced susceptibility or resistance to viral diseases. Herein we review multiple discoveries made with genome-wide or candidate gene approaches that have revealed significant insights into virus-host interactions. Genetic factors that have been identified include genes encoding virus receptors, receptor-modifying enzymes, and a wide variety of innate and adaptive immunity-related proteins. We discuss a range of pathogenic viruses, including influenza virus, respiratory syncytial virus, human immunodeficiency virus, human T cell leukemia virus, human papilloma virus, hepatitis B and C viruses, herpes simplex virus, norovirus, rotavirus, parvovirus, and Epstein-Barr virus. Understanding the genetic underpinnings that affect infectious disease outcomes should allow tailored treatment and prevention approaches in the future.

Structural features of glycan recognition among viral pathogens

Recognition and binding to host glycans present on cellular surfaces is an initial and critical step in viral entry. Diverse families of host glycans such as histo-blood group antigens, sialoglycans and glycosaminoglycans are recognized by viruses. Glycan binding determines virus-host specificity, tissue tropism, pathogenesis and potential for interspecies transmission. Viruses including noroviruses, rotaviruses, enteroviruses, influenza, and papillomaviruses have evolved novel strategies to bind specific glycans often in a strain-specific manner. Structural studies have been instrumental in elucidating the molecular determinants of these virus-glycan interactions, aiding in developing vaccines and antivirals targeting this key interaction. Our review focuses on these key structural aspects of virus-glycan interactions, particularly highlighting the different strain-specific strategies employed by viruses to bind host glycans.

Histo-blood group antigens as mediators of infections

The critical first step of a microbial infection is usually the attachment of pathogens to host cell glycans. Targets on host tissues are in particular the histo-blood group antigens (HBGAs), which are present in rich diversity in the mucus layer and on the underlying mucosa. Recent structural and functional studies have revealed significant new insight into the molecular mechanisms, explaining why individuals with certain blood groups are at increased risk of some infections. The most prominent example of blood-group-associated diseases is cholera, caused by infection with Vibrio cholerae. Many other microbial pathogens, for example Pseudomonas aeruginosa infecting the airways, and enterotoxigenic Escherichia coli (ETEC) causing traveler's diarrhea, also bind to histo-blood group antigens, but show a less clear correlation with blood group phenotype. Yet other pathogens, for example norovirus and Helicobacter pylori, recognize HBGAs differently depending on the strain. In all cases, milk oligosaccharides can aid the hosts' defenses, acting as natural receptor decoys, and anti-infectious therapy can be designed along similar strategies. In this review, we focus on important infections of humans, but the molecular mechanisms are of general relevance to a broad range of microbial infections of humans and animals.

Human norovirus inhibition by a human milk oligosaccharide

Human noroviruses are the leading cause of outbreaks of acute gastroenteritis. Norovirus interactions with histo-blood group antigens (HBGAs) are known to be important for an infection. In this study, we identified the HBGA binding pocket for an emerging GII genotype 17 (GII.17) variant using X-ray crystallography. The GII.17 variant bound the HBGA with an equivalent set of residues as the leading pandemic GII.4 variants. These structural data highlights the conserved nature of HBGA binding site between prevalent GII noroviruses. Noroviruses also interact with human milk oligosaccharides (HMOs), which mimic HBGAs and may function as receptor decoys. We previously showed that HMOs inhibited the binding of rarely detected GII.10 norovirus to HBGAs. We now found that an HMO, 2'-fucosyllactose (2'FL), additionally blocked both the GI.1 and GII.17 noroviruses from binding to HBGAs. Together, these findings provide evidence that 2'FL might function as a broadly reactive antiviral against multiple norovirus genogroups.

Influence of Gestational Age, Secretor, and Lewis Blood Group Status on the Oligosaccharide Content of Human Milk

OBJECTIVES

Human milk oligosaccharides (HMOs) are considered to play an important role for the infant. As the biotechnical production of some HMOs is feasible today and clinical studies are being designed, the individual variation of the total amount of HMOs and of single components is of particular importance. Our objectives were to investigate whether differences exist between term and preterm milk, milk from mothers with secretor or nonsecretor status, and a Lewis blood group (a+b-), (a-b+), or (a-b-) pattern.

METHODS

Within a longitudinal study 96 milk samples (colostrum, transitional, and mature milk) from 32 mothers with preterm (n = 18) and term (n = 14) infants were collected. Delipidated and deproteinized milk was subjected to porous graphitized carbon cartridges followed by high pH anion exchange chromatography with pulsed amperometric detection.

RESULTS

Quantitation of 16 single HMOs revealed changes during the first weeks of lactation without discrepancies between term and preterm milk. Significant differences occurred between "secretor" and "nonsecretor" milk (median approximately 10 vs 5 g/L total HMOs). Lacto-N-tetraose (LNT) and lacto-N-fucopentaose (LNFP) II comprised > 55% of the total HMO content in Lewis blood group (a+b-), "nonsecretor" milk and LNT together with 2'fucosyllactose, LNFP I, and difucosyllactose approximately 60% in Lewis (a-b+), "secretor" milk. In Lewis (a-b-), "secretor" milk 80% of oligosaccharides are due to LNT, 2'fucosyllactose, and LNFP I.

CONCLUSIONS

There are marked differences in total HMOs and single HMOs in milk depending on Lewis blood group and secretor status, which need to be taken into account in clinical studies.

Human Norovirus Evolution in a Chronically Infected Host

The norovirus genogroup II genotype 4 (GII.4) variants have approximately 5% divergence in capsid amino acid identity and have dominated over the past decade. The precise reason(s) for the GII.4 emergence and persistence in the human population is still unknown, but some studies have suggested that chronically infected patients might generate novel variants that can cause new epidemics. We examined GII.4 noroviruses isolated from an immunocompromised patient with a long-term infection. Numerous norovirus capsid quasi-species were isolated during the 13-month study. The capsid quasi-species clustered into two genetic and antigenic types. However, the HBGA binding profiles were similar between the two antigenic clusters, indicating that the amino acid substitutions did not alter the HBGA binding interactions. The isolated sequences represented two new GII.4 variants, but similar sequences were not found in the database. These results indicated that chronically infected patients might not generate novel noroviruses that cause outbreaks.

Typically, human noroviruses cause symptoms of acute gastroenteritis for 2 to 4 days. Often, the virions are shed in stool for several days after the symptoms recede, which in turn can lead to further contamination and transmission. Moreover, a number of reports have considered that chronic norovirus infections, i.e., lasting months and years, might even function as reservoirs for the generation of novel strains that can escape the herd immunity or have modified binding interactions with histo-blood group antigens (HBGAs). In this study, we analyzed noroviruses isolated from a patient who has presented a chronic infection for more than 6 years. We found that the isolated capsid sequences clustered into two main genetic types (termed A and B), despite a plethora of capsid quasi-sequences. Furthermore, the two genetic types corresponded well with distinct antigenicities. On the other hand, we showed that numerous amino acid substitutions on the capsid surface of genetic types A and B did not alter the HBGA binding profiles. However, divergent binding profiles for types A and B were observed with human milk oligosaccharides (HMOs), which structurally mimic HBGAs and may act as natural antivirals. Importantly, the isolated capsid sequences only had approximately 90% amino acid identity with other known sequences, which suggested that transmission of these chronic noroviruses could be limited.

IMPORTANCE The norovirus genogroup II genotype 4 (GII.4) variants have approximately 5% divergence in capsid amino acid identity and have dominated over the past decade. The precise reason(s) for the GII.4 emergence and persistence in the human population is still unknown, but some studies have suggested that chronically infected patients might generate novel variants that can cause new epidemics. We examined GII.4 noroviruses isolated from an immunocompromised patient with a long-term infection. Numerous norovirus capsid quasi-species were isolated during the 13-month study. The capsid quasi-species clustered into two genetic and antigenic types. However, the HBGA binding profiles were similar between the two antigenic clusters, indicating that the amino acid substitutions did not alter the HBGA binding interactions. The isolated sequences represented two new GII.4 variants, but similar sequences were not found in the database. These results indicated that chronically infected patients might not generate novel noroviruses that cause outbreaks.

Glycan-protein interactions in viral pathogenesis

The surfaces of host cells and viruses are decorated by complex glycans, which play multifaceted roles in the dynamic interplay between the virus and the host including viral entry into host cell, modulation of proteolytic cleavage of viral proteins, recognition and neutralization of virus by host immune system. These roles are mediated by specific multivalent interactions of glycans with their cognate proteins (generally termed as glycan-binding proteins or GBPs or lectins). The advances in tools and technologies to chemically synthesize and structurally characterize glycans and glycan-GBP interactions have offered several insights into the role of glycan-GBP interactions in viral pathogenesis and have presented opportunities to target these interactions for novel antiviral therapeutic or vaccine strategies. This review covers aspects of role of host cell surface glycan receptors and viral surface glycans in viral pathogenesis and offers perspectives on how to employ various analytical tools to target glycan-GBP interactions.

Norovirus drug candidates that inhibit viral capsid attachment to human histo-blood group antigens

Human noroviruses are the leading causative agents of epidemic and sporadic viral gastroenteritis and childhood diarrhoea worldwide. Human histo-blood group antigens (HBGA) serve as receptors for norovirus capsid protein attachment and play a critical role in infection. This makes HBGA-norovirus binding a promising target for drug development. Recently solved crystal structures of norovirus bound to HBGA have provided a structural basis for identification of potential anti-norovirus drugs and subsequently performed in silico and in vitro drug screens have identified compounds that block norovirus binding and may thereby serve as structural templates for design of therapeutic norovirus inhibitors. This review explores norovirus therapeutic options based on the strategy of blocking norovirus-HBGA binding.

Antiviral targets of human noroviruses

Human noroviruses are major causative agents of sporadic and epidemic gastroenteritis both in children and adults. Currently there are no licensed therapeutic intervention measures either in terms of vaccines or drugs available for these highly contagious human pathogens. Genetic and antigenic diversity of these viruses, rapid emergence of new strains, and their ability to infect a broad population by using polymorphic histo-blood group antigens for cell attachment, pose significant challenges for the development of effective antiviral agents. Despite these impediments, there is progress in the design and development of therapeutic agents. These include capsid-based candidate vaccines, and potential antivirals either in the form of glycomimetics or designer antibodies that block HBGA binding, as well as those that target essential non-structural proteins such as the viral protease and RNA-dependent RNA polymerase. In addition to these classical approaches, recent studies suggest the possibility of interferons and targeting host cell factors as viable approaches to counter norovirus infection. This review provides a brief overview of this progress.

Human Norovirus Interactions with Histo-Blood Group Antigens and Human Milk Oligosaccharides

Human noroviruses interact with both human histo-blood group antigens (HBGAs) and human milk oligosaccharides (HMOs). The former are believed to be important for a virus infection, while the latter might act as natural decoys in the host during an infection. However, certain noroviruses are known to bind poorly to HBGAs and yet still cause infections; some interact with numerous HBGA types but are nonprevalent; and yet others bind HBGAs and seem to be increasing in prevalence. HBGAs and HMOs can be found as soluble antigens in humans, can be structurally alike, and can interact with equivalent residues at identical binding pockets on the capsid. In this Gem, we discuss HBGA and HMO binding studies for human noroviruses, concentrating on the clinically important genogroup II noroviruses. In short, the roles of HBGA and HMO interactions in norovirus infections are still unclear.