Toll-like receptor 2 enhances ZO-1-associated intestinal epithelial barrier integrity via protein kinase C

Lactobacillus plantarum WCFS1 and its host interaction: a dozen years after the genome

Lactobacillus plantarum WCFS1 is one of the best studied Lactobacilli, notably as its genome was unravelled over 12 years ago. L. plantarum WCFS1 can be grown to high densities, is amenable to genetic transformation and highly robust with a relatively high survival rate during the gastrointestinal passage. In this review, we present and discuss the main insights provided by the functional genomics research on L. plantarum WCFS1 with specific attention for the molecular mechanisms related to its interaction with the human host and its potential to modify the immune system, and induce other health-related benefits. Whereas most insight has been gained in mouse and other model studies, only five human studies have been reported with L. plantarum WCFS1. Hence NCIMB 8826 (the parental strain of L. plantarum WCFS1) in human trials as to capitalize on the wealth of knowledge that is summarized here.

Administration of probiotic mixture DM#1 ameliorated 5-fluorouracil-induced intestinal mucositis and dysbiosis in rats

OBJECTIVE

The use of probiotics to alleviate chemotherapy-induced intestinal mucositis is supported by clinical consensus. However, no studies to date, to our knowledge, have systematically analyzed the effects of a probiotic mixture on chemotherapy-induced mucositis or assessed changes in the intestinal microbiota after probiotic treatment. The aim of this study was to report the effects of a probiotic mixture, DM#1, on intestinal mucositis and dysbiosis of rats treated with 5-fluorouracil (5-FU).

METHODS

Twenty-eight male Sprague Dawley rats weighing 180 to 220 g were randomly divided into four groups: control, 5-FU, probiotic high (PH), and probiotic low (PL). Except for the control group, all other groups received intraperitoneal injections of 5-FU for 5 d, and the PH and PL groups received DM#1 intragastrically (1 × 10⁹ or 1 × 10⁸ colony-forming units/kg, respectively) for 8 d. One day after the last administration, rats were sacrificed and the ilea were removed for histopathologic assessment and evaluation of permeability, myeloperoxidase activity, levels of cytokines (interleukin [IL]-4, IL-6, tumor necrosis factor [TNF]-α), and mRNA of toll-like receptors (TLR; TLR2, TLR4, and TLR9). Additionally, intestinal microbiota profiles were analyzed by polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis and quantitative real-time PCR.

RESULTS

Treatment with DM#1 ameliorated 5-FU-induced intestinal mucosal injury in rats, possibly by reducing proinflammatory cytokine levels and neutrophil infiltration. The increased intestinal permeability caused by 5-FU was ameliorated. These results were closely associated with the reestablishment of intestinal microbial homeostasis and alteration of the TLR2/TLR4 signaling pathway.

CONCLUSIONS

Administration of the probiotic mixture DM#1 ameliorated 5-FU-induced intestinal mucositis and dysbiosis in rats.

Effects of Lactobacillus paracasei 01 fermented milk beverage on protection of intestinal epithelial cell in vitro

BACKGROUND

Intestinal protection is an important function of probiotics. However, there is no evidence that the potential probiotic Lactobacillus paracasei 01 is able to improve intestinal function. In the present study, the protective effect and underlying mechanisms of L. paracasei 01 on intestinal epithelial cell in vitro were investigated.

RESULTS

A fermented milk beverage containing L. paracasei 01 at 3 × 10(8) CFU mL(-1) was made from skim milk powder, glucose, sucrose and isomaltooligosaccharide. Results indicated that L. paracasei 01 fermented milk beverage could reduce macromolecule permeability caused by dextran sodium sulfate-induced damage. Besides, 50 mL L(-1) fermented milk beverage with live L. paracasei 01 in the cell medium could increase intestinal epithelial cell Caco-2 growth, decrease lipopolysaccharide/tumor necrosis factor-α (TNF-α)/interferon-γ (IFN-γ)-induced Caco-2 cell death and chemokine CCL-20 production. The protecting mechanisms involve promoting intestinal epithelial cell growth and intestinal epithelial integrity to strengthen the intestinal barrier against chemical and inflammatory stimuli-induced damage. Inhibition of inflammatory cytokines and chemokine such as IFN-γ, TNF-α and CCL-20 also contributes to the beneficial effects of this product on epithelial function.

CONCLUSION

Lactobacillus paracasei 01 fermented milk beverage may be a new functional food with intestinal protecting effects in vitro.

Barrier protection via Toll-like receptor 2 signaling in porcine intestinal epithelial cells damaged by deoxynivalnol

Intestinal barrier is the first line of defense inside the body and comprises intercellular tight junction (TJ) proteins that regulate paracellular permeability. Deoxynivalenol (DON), a fungal metabolite often found in the contaminated food of domestic animals, is known to impair intestinal barrier function and may be involved in intestinal inflammation. Unlike in humans and mice, the importance of Toll-like receptor (TLR) 2 expressed in porcine intestinal epithelial cells is largely unclear. Therefore, the aim of the present study was to investigate whether TLR2 stimulation enhances intestinal barrier function and protects against DON exposure. We found that the cells treated with TLR2 ligands decreased the epithelial barrier permeability and enhanced TJ protein expression in intestinal porcine epithelial cells (IPEC-J2). In addition, pretreatment with TLR2 ligand, including Pam3CSK4 (PCSK) and lipoteichoic acid from Bacillus subtilis, prevented DON-induced barrier dysfunction by increasing the expression of TJ proteins via the PI3K-Akt-dependent pathway. It is likely that the DON-disrupted intestinal barrier caused biological changes of immune cells in the lamina propria. Thus, we conducted co-culture of differentiated IPEC-J2 cells in the upper well together with peripheral blood mononuclear cells in the bottom well and found that apical TLR2 stimulation of IPEC-J2 cells could alleviate the reduction in cell survival and proliferation of immune cells. Conclusively, TLR2 signaling on intestinal epithelial cells may enhance intestinal barrier function and prevent DON-induced barrier dysfunction of epithelial cells.

Human oral isolate Lactobacillus fermentum AGR1487 induces a pro-inflammatory response in germ-free rat colons

Lactobacilli are thought to be beneficial for human health, with lactobacilli-associated infections being confined to immune-compromised individuals. However, Lactobacillus fermentum AGR1487 negatively affects barrier integrity in vitro so we hypothesized that it caused a pro-inflammatory response in the host. We compared germ-free rats inoculated with AGR1487 to those inoculated with another L. fermentum strain, AGR1485, which does not affect in vitro barrier integrity. We showed that rats inoculated with AGR1487 had more inflammatory cells in their colon, higher levels of inflammatory biomarkers, and increased colonic gene expression of pro-inflammatory pathways. In addition, our in vitro studies showed that AGR1487 had a greater capacity to activate TLR signaling and induce pro-inflammatory cytokines in immune cells. This study indicates the potential of strains of the same species to differentially elicit inflammatory responses in the host and highlights the importance of strain characterization in probiotic approaches to treat inflammatory disorders.

The effect of hydrolysates of proteins from rice milk on the physiological response of enterocytes and on the adhesion of bacteria from healthy and allergic people - an in vitro study

Designing an optimal diet requires knowledge of the biological activity of food products, particularly in relation to people with food allergies. The hypothesis, which constitutes the basis of this thesis, states that the peptides and glycopeptides released from proteins by enzymatic hydrolysis are able to change the quantity and quality of the human gastrointestinal ecosystem. Such substrates may interfere with adhesion to the intestinal epithelium microbiota and alter enterocytic metabolic activity. The aim of this study was to determine the effect of protein hydrolysates from rice milk substitute on gut epithelial cells and the intestinal microbiota of healthy people and ones suffering from an allergy to milk. The following experimental work applied systems that reflect the conditions occurring in the gastrointestinal tract.

Toll-Like Receptor Signalling and the Control of Intestinal Barrier Function

Epithelial barrier function and innate immunity are fundamental to the pathogenesis of inflammatory and infectious disease. Along with plasma membranes, epithelial cells are the primary cellular determinant of epithelial barrier function. The mechanism by which polarized epithelia form a permeability barrier is of fundamental importance to the prevention of many infectious and inflammatory diseases. Moreover, epithelial cells express Toll-like receptors (TLRs) which upon recognition of conserved microbial factors such as lipopolysaccharide (LPS) induce epithelial responses including epithelial cell proliferation, secretion of secretory IgA into the lumen and production mucins and antimicrobial peptides, thereby promoting intestinal barrier function. Understanding gut barrier integrity and regulation of permeability is crucial to increase our understanding of the pathogenesis of intestinal disease. A variety of tests have been developed to assess this barrier, including assessing intestinal epithelial cell proliferation or death, intestinal tight junction status and the consequence of intestinal barrier integrity loss such as increased intestinal permeability and susceptibility to bacterial infection. Using a mouse model, this chapter describes some of the methods to assess the functional integrity of this epithelial barrier and the part played by a TLR signalling pathway.

Breaking down the barriers: the gut microbiome, intestinal permeability and stress-related psychiatric disorders

The emerging links between our gut microbiome and the central nervous system (CNS) are regarded as a paradigm shift in neuroscience with possible implications for not only understanding the pathophysiology of stress-related psychiatric disorders, but also their treatment. Thus the gut microbiome and its influence on host barrier function is positioned to be a critical node within the brain-gut axis. Mounting preclinical evidence broadly suggests that the gut microbiota can modulate brain development, function and behavior by immune, endocrine and neural pathways of the brain-gut-microbiota axis. Detailed mechanistic insights explaining these specific interactions are currently underdeveloped. However, the concept that a "leaky gut" may facilitate communication between the microbiota and these key signaling pathways has gained traction. Deficits in intestinal permeability may underpin the chronic low-grade inflammation observed in disorders such as depression and the gut microbiome plays a critical role in regulating intestinal permeability. In this review we will discuss the possible role played by the gut microbiota in maintaining intestinal barrier function and the CNS consequences when it becomes disrupted. We will draw on both clinical and preclinical evidence to support this concept as well as the key features of the gut microbiota which are necessary for normal intestinal barrier function.

Biofilm-associated bacterial amyloids dampen inflammation in the gut: oral treatment with curli fibres reduces the severity of hapten-induced colitis in mice

Background/objectives:

A disruption of epithelial barrier function can lead to intestinal inflammation. Toll-like receptor (TLR) 2 activation by microbial products promotes intestinal epithelial integrity and overall gut health. Several bacterial species, including enteric bacteria, actively produce amyloid proteins as a part of their biofilms. Recognition of amyloid fibres found in enteric biofilms, termed curli, by the Toll-like receptor (TLR)2/1 complex reinforces barrier function. Here, we investigated the effect of purified curli fibres on inflammation in a mouse model of acute colitis.

Methods:

Bone marrow–derived macrophages as well as lamina propria cells were treated with curli fibres of both pathogenic Salmonella enterica serovar Typhimurium and commensal Escherichia coli Nissle 1917 biofilms. Mice were given 0.1 or 0.4 mg of purified curli orally 1 day post administration of 1% 2,4,6-trinitrobenzene sulphonic acid (TNBS) enema. Histopathological analysis was performed on distal colonic tissue taken 6 days post TNBS enema. RNA extracted from colonic tissue was subjected to RT-PCR.

Results:

Here we show that curli fibres of both pathogenic and commensal bacteria are recognised by TLR2 leading to the production of IL-10, immunomodulatory cytokine of intestinal homeostasis. Treatment of mice with a single dose of curli heightens transcript levels of Il10 in the colon and ameliorates the disease pathology in TNBS-induced colitis. Curli treatment is comparable to the treatment with anti-tumour necrosis factor alpha (anti-TNFα) antibodies, a treatment known to reduce the severity of acute colitis in humans and mice.

Conclusion:

These results suggest that the bacterial amyloids had a role in helping to maintain immune homeostasis in the intestinal mucosa via the TLR2/IL-10 axis. Furthermore, bacterial amyloids may be a potential candidate therapeutic to treat intestinal inflammatory disorders owing to their remarkable immunomodulatory activity.

The importance of the Non Obese Diabetic (NOD) mouse model in autoimmune diabetes

Type 1 Diabetes (T1D) is an autoimmune disease characterized by the pancreatic infiltration of immune cells resulting in T cell-mediated destruction of the insulin-producing beta cells. The successes of the Non-Obese Diabetic (NOD) mouse model have come in multiple forms including identifying key genetic and environmental risk factors e.g. Idd loci and effects of microorganisms including the gut microbiota, respectively, and how they may contribute to disease susceptibility and pathogenesis. Furthermore, the NOD model also provides insights into the roles of the innate immune cells as well as the B cells in contributing to the T cell-mediated disease. Unlike many autoimmune disease models, the NOD mouse develops spontaneous disease and has many similarities to human T1D. Through exploiting these similarities many targets have been identified for immune-intervention strategies. Although many of these immunotherapies did not have a significant impact on human T1D, they have been shown to be effective in the NOD mouse in early stage disease, which is not equivalent to trials in newly-diagnosed patients with diabetes. However, the continued development of humanized NOD mice would enable further clinical developments, bringing T1D research to a new translational level. Therefore, it is the aim of this review to discuss the importance of the NOD model in identifying the roles of the innate immune system and the interaction with the gut microbiota in modifying diabetes susceptibility. In addition, the role of the B cells will also be discussed with new insights gained through B cell depletion experiments and the impact on translational developments. Finally, this review will also discuss the future of the NOD mouse and the development of humanized NOD mice, providing novel insights into human T1D.

Human Alpha-Defensin HNP1 Increases HIV Traversal of the Epithelial Barrier: A Potential Role in STI-Mediated Enhancement of HIV Transmission

Alpha-defensins, including human neutrophil peptides 1-3 (HNP1-3) and human defensin 5 (HD5), are elevated at the genital mucosa in individuals with sexually transmitted infections (STIs). The presence of STIs is associated with an increased risk of human immunodeficiency virus (HIV) transmission, suggesting there may be a role for defensins in early events of HIV transmission. HD5 has been demonstrated to contribute to STI-mediated increased HIV infectivity in vitro. HNPs exhibit anti-HIV activity in vitro. However, increased levels of HNPs have been associated with enhanced HIV acquisition and higher viral load in breast milk. This study found that HNP1, but not HD5, significantly disrupted epithelial integrity and promoted HIV traversal of epithelial barriers. Linear HNP1 with the same charges did not affect epithelial permeability, indicating that the observed effect of HNP1 on the epithelial barrier was structure dependent. These results suggest a role for HNP1 in STI-mediated enhancement of HIV transmission.

Feeding probiotic Lactobacillus rhamnosus (MTCC 5897) fermented milk to suckling mothers alleviates ovalbumin-induced allergic sensitisation in mice offspring

The neonatal period is often polarised to T helper (Th2) response at the time of birth, predisposing offspring to allergic disorders. Passive immunity through the mother's milk is critical for immune system development of newborns. Probiotics have been proposed to harmonise Th1/Th2 imbalance in allergic conditions in adults. In the present study, the anti-allergic effects of feeding probiotic Lactobacillus rhamnosus-fermented milk (PFM) either to dams during the suckling period or to their offspring after weaning individually or else in successive periods against ovalbumin (OVA)-induced allergy in newborns was analysed. After allergen sensitisation, physical symptoms of allergy, gut immune response, humoral immune response and cell-mediated response through interleukins were detected. Consumption of PFM by mothers and offspring showed a reduction (P<0·01) in physical allergic symptoms in newborns with an increase (P<0·01) in the numbers of goblet and IgA+ cells in the small intestine. Similarly, considerable (P<0·001) decreases in OVA-specific antibodies (IgE, IgG, IgG1) and ratios of IgE/IgG2a and IgG1/IgG2a in the sera of newborn mice were recorded. A decrease in IL-4 and an increase in interferon-γ levels further confirmed the shift from Th2 to Th1 pathway in PFM-fed mice. It is logical to conclude that the timing of PFM intervention in alleviating allergic symptoms is critical, which was found to be most effective when mothers were fed during the suckling period.

Primary breast cancer induces pulmonary vascular hyperpermeability and promotes metastasis via the VEGF-PKC pathway

The lung is one of the most frequent target organs for breast cancer metastasis. When breast cancer cells from a primary tumor do not colonize the lung, which we named the premetastatic phase, the microenvironment of the lung has already been influenced by the primary tumor. However, little is known about the exact premetastatic alteration and regulatory mechanisms of the lung. Here, we used 4T1 cells (a mouse breast cancer cell line which can specifically metastasize to the lung) to build a mouse breast cancer model. We found that primary breast tumor induced increased pulmonary vascular permeability in the premetastatic phase, which facilitated the leakage of rhodamine-dextran and the extravasation of intravenous therapy injected cancer cells. Furthermore, tight junctions (TJs) were disrupted, and the expression of zonula occludens-1(ZO-1), one of the most important components of tight junctions, was decreased in the premetastatic lung. In addition, elevated serum vascular endothelial growth factor (VEGF) was involved in the destabilization of tight junctions and the VEGF antagonist bevacizumab reversed the primary tumor-induced vascular hyperpermeability. Moreover, activation of the protein kinase C (PKC) pathway disrupted the integrity of TJs and accordingly, the disruption could be alleviated by blocking VEGF. Taken together, these data demonstrate that primary breast cancer may induce tight junction disruptions in the premetastatic lung via the VEGF-PKC pathway and promote pulmonary vascular hyperpermeability before metastasis. © 2015 Wiley Periodicals, Inc.

Short-term effects of Poly(I:C) on gut permeability

The intestinal barrier function depends on an adequate response to pathogens by the epithelium. Toll-like receptor 3 (TLR-3) recognizes double-stranded RNA, a virus-associated molecular pattern. Activation of TLR-3 with Poly(I:C), a synthetic agonist, modulates tissue repair and permeability in other epithelia; however, the effects of local luminal TLR-3 agonists on gut barrier function are unknown. The aim of this investigation was to evaluate short-term effects of Poly(I:C) on rat ileal and colonic permeability ex vivo. We also studied the acute effects of intrarectal administration of Poly(I:C) on colonic barrier function. Ileum tissues displayed decreased transepithelial electrical resistance (TEER) 1h after incubation with 200μg/mL Poly(I:C); however, the mucosa-to-serosa transit of macromolecules (4.4 and 40kDa dextrans - TD4.4 and FD40, respectively) remained unchanged. Conversely, colon tissue preparations stimulated with 200μg/mL Poly(I:C) showed a decreased thinning of the mucosal layer after 2h and a decreased transit of FD40 after 3h, in comparison to controls. There was no change in colonic TEER after 3h of treatment. In addition, colon tissue taken from rats 6h after an intrarectal administration of 100μg Poly(I:C) also showed decreased permeability to FD40 in the everted gut sac assay at 3h post-extraction. Tissue morphology remained unchanged. Our results suggest that an acute exposure to Poly(I:C) reduces colon permeability to macromolecules but increases ileum permeability to electrolytes/small molecules ex vivo. Although the mechanism associated to these effects needs further investigation, to our knowledge this is the first report of a direct effect of a TLR-3 ligand in intestinal barrier function and may be of significance to understand region-specific interactions between gut mucosa and microbiota.

A Selected Lactobacillus rhamnosus Strain Promotes EGFR-Independent Akt Activation in an Enterotoxigenic Escherichia coli K88-Infected IPEC-J2 Cell Model

Enterotoxigenic Escherichia coli (ETEC) are important intestinal pathogens that cause diarrhea in humans and animals. Although probiotic bacteria may protect against ETEC-induced enteric infections, the underlying mechanisms are unknown. In this study, porcine intestinal epithelial J2 cells (IPEC-J2) were pre-incubated with and without Lactobacillus rhamnosus ATCC 7469 and then exposed to F4+ ETEC. Increases in TLR4 and NOD2 mRNA expression were observed at 3 h after F4+ ETEC challenge, but these increases were attenuated by L. rhamnosus treatment. Expression of TLR2 and NOD1 mRNA was up-regulated in cells pre-treated with L. rhamnosus. Pre-treatment with L. rhamnosus counteracted F4+ ETEC-induced increases in TNF-α concentration. Increased PGE2. concentrations were observed in cells infected with F4+ ETEC and in cells treated with L. rhamnosus only. A decrease in phosphorylated epidermal growth factor receptor (EGFR) was observed at 3 h after F4+ ETEC challenge in cells treated with L. rhamnosus. Pre-treatment with L. rhamnosus enhanced Akt phosphorylation and increased ZO-1 and occludin protein expression. Our findings suggest that L. rhamnosus protects intestinal epithelial cells from F4+ ETEC-induced damage, partly through the anti-inflammatory response involving synergism between TLR2 and NOD1. In addition, L. rhamnosus promotes EGFR-independent Akt activation, which may activate intestinal epithelial cells in response to bacterial infection, in turn increasing tight junction integrity and thus enhancing the barrier function and restricting pathogen invasion. Pre-incubation with L. rhamnosus was superior to co-incubation in reducing the adhesion of F4+ ETEC to IPEC-J2 cells and subsequently attenuating F4+ ETEC-induced mucin layer destruction and suppressing apoptosis. Our data indicate that a selected L. rhamnosus strain interacts with porcine intestinal epithelial cells to maintain the epithelial barrier and promote intestinal epithelial cell activation in response to bacterial infection, thus protecting cells from the deleterious effects of F4+ ETEC.

Effects of Lactobacillus johnsonii and Lactobacillus reuteri on gut barrier function and heat shock proteins in intestinal porcine epithelial cells

Heat shock proteins (HSPs) are a set of highly conserved proteins that can serve as intestinal gate keepers in gut homeostasis. Here, effects of a probiotic, Lactobacillus rhamnosus GG (LGG), and two novel porcine isolates, Lactobacillus johnsonii strain P47-HY and Lactobacillus reuteri strain P43-HUV, on cytoprotective HSP expression and gut barrier function, were investigated in a porcine IPEC-J2 intestinal epithelial cell line model. The IPEC-J2 cells polarized on a permeable filter exhibited villus-like cell phenotype with development of apical microvilli. Western blot analysis detected HSP expression in IPEC-J2 and revealed that L. johnsonii and L. reuteri strains were able to significantly induce HSP27, despite high basal expression in IPEC-J2, whereas LGG did not. For HSP72, only the supernatant of L. reuteri induced the expression, which was comparable to the heat shock treatment, which indicated that HSP72 expression was more stimulus specific. The protective effect of lactobacilli was further studied in IPEC-J2 under an enterotoxigenic Escherichia coli (ETEC) challenge. ETEC caused intestinal barrier destruction, as reflected by loss of cell–cell contact, reduced IPEC-J2 cell viability and transepithelial electrical resistance, and disruption of tight junction protein zonula occludens-1. In contrast, the L. reuteri treatment substantially counteracted these detrimental effects and preserved the barrier function. L. johnsonii and LGG also achieved barrier protection, partly by directly inhibiting ETEC attachment. Together, the results indicate that specific strains of Lactobacillus can enhance gut barrier function through cytoprotective HSP induction and fortify the cell protection against ETEC challenge through tight junction protein modulation and direct interaction with pathogens.

Strengthening of the intestinal epithelial tight junction by Bifidobacterium bifidum

Epithelial barrier dysfunction has been implicated as one of the major contributors to the pathogenesis of inflammatory bowel disease. The increase in intestinal permeability allows the translocation of luminal antigens across the intestinal epithelium, leading to the exacerbation of colitis. Thus, therapies targeted at specifically restoring tight junction barrier function are thought to have great potential as an alternative or supplement to immunology-based therapies. In this study, we screened Bifidobacterium, Enterococcus, and Lactobacillus species for beneficial microbes to strengthen the intestinal epithelial barrier, using the human intestinal epithelial cell line (Caco-2) in an in vitro assay. Some Bifidobacterium and Lactobacillus species prevented epithelial barrier disruption induced by TNF-α, as assessed by measuring the transepithelial electrical resistance (TER). Furthermore, live Bifidobacterium species promoted wound repair in Caco-2 cell monolayers treated with TNF-α for 48 h. Time course (1)H-NMR-based metabonomics of the culture supernatant revealed markedly enhanced production of acetate after 12 hours of coincubation of B. bifidum and Caco-2. An increase in TER was observed by the administration of acetate to TNF-α-treated Caco-2 monolayers. Interestingly, acetate-induced TER-enhancing effect in the coculture of B. bifidum and Caco-2 cells depends on the differentiation stage of the intestinal epithelial cells. These results suggest that Bifidobacterium species enhance intestinal epithelial barrier function via metabolites such as acetate.

Immune function of biliary epithelial cells

Biliary epithelial cells (BECs) are the epithelial cells lining the bile duct, constituting the biliary system's first line of defense against pathogenic microorganisms. BECs can express many kinds of pathogen recognition receptors, activate intracellular signal transduction pathways, initiate the internal microbial defense system, including the release of pro-inflammatory cytokines and chemokines and antibacterial peptide synthesis, and maintain the integrity of the biliary epithelium. By expressing and releasing adhesion molecules and immune mediators, BECs interact with other cells in the liver, such as lymphocytes and Kupffer's cells. BECs are involved in a complex feedback mechanism of liver cells and thereby regulate the response to microbial infection. BECs actively participate in the biliary duct mucosal immunity and form an important component of liver immunity.

Reshaping faecal gut microbiota composition by the intake of trans-resveratrol and quercetin in high-fat sucrose diet-fed rats

Diet-induced obesity is associated to an imbalance in the normal gut microbiota composition. Resveratrol and quercetin, widely known for their health beneficial properties, have low bioavailability, and when they reach the colon, they are targets of the gut microbial ecosystem. Hence, the use of these molecules in obesity might be considered as a potential strategy to modulate intestinal bacterial composition. The purpose of this study was to determine whether trans-resveratrol and quercetin administration could counteract gut microbiota dysbiosis produced by high-fat sucrose diet (HFS) and, in turn, improve gut health. Wistar rats were randomised into four groups fed an HFS diet supplemented or not with trans-resveratrol [15 mg/kg body weight (BW)/day], quercetin (30 mg/kg BW/day) or a combination of both polyphenols at those doses. Administration of both polyphenols together prevented body weight gain and reduced serum insulin levels. Moreover, individual supplementation of trans-resveratrol and quercetin effectively reduced serum insulin levels and insulin resistance. Quercetin supplementation generated a great impact on gut microbiota composition at different taxonomic levels, attenuating Firmicutes/Bacteroidetes ratio and inhibiting the growth of bacterial species previously associated to diet-induced obesity (Erysipelotrichaceae, Bacillus, Eubacterium cylindroides). Overall, the administration of quercetin was found to be effective in lessening HFS-diet-induced gut microbiota dysbiosis. In contrast, trans-resveratrol supplementation alone or in combination with quercetin scarcely modified the profile of gut bacteria but acted at the intestinal level, altering the mRNA expression of tight-junction proteins and inflammation-associated genes.

Toll-like receptor mediated modulation of T cell response by commensal intestinal microbiota as a trigger for autoimmune arthritis

In autoimmune diseases, a disturbance of the balance between T helper 17 (Th17) and regulatory T cells (Tregs) is often observed. This disturbed balance is also the case in rheumatoid arthritis (RA). Genetic predisposition to RA confers the presence of several polymorphisms mainly regulating activation of T lymphocytes. However, the presence of susceptibility factors is neither necessary nor sufficient to explain the disease development, emphasizing the importance of environmental factors. Multiple studies have shown that commensal gut microbiota is of great influence on immune homeostasis and can trigger the development of autoimmune diseases by favoring induction of Th17 cells over Tregs. However the mechanism by which intestinal microbiota influences the Th cell balance is not completely understood. Here we review the current evidence supporting the involvement of commensal intestinal microbiota in rheumatoid arthritis, along with a potential role of Toll-like receptors (TLRs) in modulating the relevant Th cell responses to trigger autoimmunity. A better understanding of TLR triggering by intestinal microbiota and subsequent T cell activation might offer new perspectives for manipulating the T cell response in RA patients and may lead to the discovery of new therapeutic targets or even preventive measures.

Mechanisms underlying protective effects of probiotics on intestinal epithelial

Intestinal epithelial cells (IECs), the first line of defense against pathogens, are an initial point of contact between the host and intestinal microbes. Growing evidence suggests that the interactions between the host and intestinal microbes may lead to dysregulated immune responses, while probiotics can reinforce the barrier function and exert a modest stimulation of the immune system to prevent this situation. On one hand, probiotics exert antagonistic functions via competition for nutrients, metabolites, and occupying effect. Therefore, probiotics can regulate the endogenous gastrointestinal flora and restrain exogenous pathogenic bacteria. On the other hand, IECs recognize probiotics and their metabolites through pattern recognition receptors to stimulate the non-specific immune responses. In addition, probiotics can induce IECs to produce the mucus layer covering the entire intestinal tract and prevent attachment and invasion of various bacterial pathogens. Clinical trials have also shown beneficial effects of probiotics as a potential preventive method for inflammatory bowel disease such as Crohn disease and ulcerative colitis.

TLR signaling modulates side effects of anticancer therapy in the small intestine

Intestinal mucositis represents the most common complication of intensive chemotherapy, which has a severe adverse impact on quality of life of cancer patients. However, the precise pathophysiology remains to be clarified, and there is so far no successful therapeutic intervention. In this study, we investigated the role of innate immunity through TLR signaling in modulating genotoxic chemotherapy-induced small intestinal injury in vitro and in vivo. Genetic deletion of TLR2, but not MD-2, in mice resulted in severe chemotherapy-induced intestinal mucositis in the proximal jejunum with villous atrophy, accumulation of damaged DNA, CD11b(+)-myeloid cell infiltration, and significant gene alterations in xenobiotic metabolism, including a decrease in ABCB1/multidrug resistance (MDR)1 p-glycoprotein (p-gp) expression. Functionally, stimulation of TLR2 induced synthesis and drug efflux activity of ABCB1/MDR1 p-gp in murine and human CD11b(+)-myeloid cells, thus inhibiting chemotherapy-mediated cytotoxicity. Conversely, TLR2 activation failed to protect small intestinal tissues genetically deficient in MDR1A against DNA-damaging drug-induced apoptosis. Gut microbiota depletion by antibiotics led to increased susceptibility to chemotherapy-induced mucosal injury in wild-type mice, which was suppressed by administration of a TLR2 ligand, preserving ABCB1/MDR1 p-gp expression. Findings were confirmed in a preclinical model of human chemotherapy-induced intestinal mucositis using duodenal biopsies by demonstrating that TLR2 activation limited the toxic-inflammatory reaction and maintained assembly of the drug transporter p-gp. In conclusion, this study identifies a novel molecular link between innate immunity and xenobiotic metabolism. TLR2 acts as a central regulator of xenobiotic defense via the multidrug transporter ABCB1/MDR1 p-gp. Targeting TLR2 may represent a novel therapeutic approach in chemotherapy-induced intestinal mucositis.

Theaflavins enhance intestinal barrier of Caco-2 Cell monolayers through the expression of AMP-activated protein kinase-mediated Occludin, Claudin-1, and ZO-1

We investigated the effect of theaflavins (TFs) on membrane barrier of Caco-2 cells. For fluorescein-transport experiments, the apparent permeability (Papp) of fluorescein in Caco-2 cells pretreated with 20 μM TFs were significantly decreased compared with that in untreated cells. Although the respective monomeric catechins did not show any Papp reduction, purpurogallin pretreatment resulted in a significant Papp reduction similar to that of TF-3′-O-gallate (TF3′G) pretreatment. This indicates that the benzotropolone moiety may play a crucial role in the Papp reduction or tight junction (TJ)-closing effect induced by TFs. In TF-3′-O-gallate-pretreated Caco-2 cells, fluorescein transport was completely restored by compound C (AMPK inhibitor). In addition, TF3′G significantly increased both the mRNA and protein expression of TJ-related proteins (occludin, claudin-1, and ZO-1) as well as the phosphorylation of AMPK. It was, thus, concluded that TFs could enhance intestinal barrier function by increasing the expression of TJ-related proteins through the activation of AMPK in Caco-2 cells.

Protective effects of baicalin on LPS-induced injury in intestinal epithelial cells and intercellular tight junctions

AIMS

To investigate the protective effects and mechanisms of baicalin on lipopolysaccharide (LPS)-induced injury in intestinal epithelial cells and intercellular tight junctions.

METHODS

IEC-6 cells were stimulated with LPS (1.0 μg/mL), with or without baicalin, for 24 h. The levels of the inflammatory cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α were determined using ELISA. Quantitative real-time PCR was used for determining the mRNA expression level of claudin-3, occludin, and ZO-1; Western blot and immunofluorescence analysis were used for analyzing the expression level and the distribution patterns of ZO-1 protein.

RESULTS

Pretreatment with baicalin (10.0 μg/mL) improved LPS-stimulated cell viability and repressed IL-6 and TNF-α levels. In addition, pretreatment with baicalin up-regulated mRNA and protein expression levels of ZO-1 and kept the protein intact in IEC-6 cells injured with LPS.

CONCLUSION

Baicalin has the capacity to protect IEC-6 cells and the intercellular tight junctions from LPS-induced injury. The mechanisms may be associated with inhibiting the production of inflammatory cytokines, and up-regulating the mRNA and protein expression of ZO-1.

A method for high purity intestinal epithelial cell culture from adult human and murine tissues for the investigation of innate immune function

Intestinal epithelial cells (IECs) serve as an important physiologic barrier between environmental antigens and the host intestinal immune system. Thus, IECs serve as a first line of defense and may act as sentinel cells during inflammatory insults. Despite recent renewed interest in IEC contributions to host immune function, the study of primary IEC has been hindered by lack of a robust culture technique, particularly for small intestinal and adult tissues. Here, a novel adaptation for culture of primary IEC is described for human duodenal organ donor tissue as well as duodenum and colon of adult mice. These epithelial cell cultures display characteristic phenotypes and are of high purity. In addition, the innate immune function of human primary IEC, specifically with regard to Toll-like receptor (TLR) expression and microbial ligand responsiveness, is contrasted with a commonly used intestinal epithelial cell line (HT-29). Specifically, TLR expression at the mRNA level and production of cytokine (IFNγ and TNFα) in response to TLR agonist stimulation is assessed. Differential expression of TLRs as well as innate immune responses to ligand stimulation is observed in human-derived cultures compared to that of HT-29. Thus, use of this adapted method to culture primary epithelial cells from adult human donors and from adult mice will allow for more appropriate studies of IECs as innate immune effectors.

Toll-like receptor 2 mediates ischemia-reperfusion injury of the small intestine in adult mice

Toll-like receptor 2 (TLR2) recognizes conserved molecular patterns associated with both gram-negative and gram-positive bacteria, and detects some endogenous ligands. Previous studies demonstrated that in ischemia-reperfusion (I/R) injury of the small intestine, the TLR2-dependent signaling exerted preventive effects on the damage in young mice, but did not have a significant effect in neonatal mice. We investigated the role of TLR2 in adult ischemia-reperfusion injury in the small intestine. Wild-type and TLR2 knockout mice at 16 weeks of age were subjected to intestinal I/R injury. Some wild-type mice received anti-Ly-6G antibodies to deplete circulating neutrophils. In wild-type mice, I/R induced severe small intestinal injury characterized by infiltration by inflammatory cells, disruption of the mucosal epithelium, and mucosal bleeding. Compared to wild-type mice, TLR2 knockout mice exhibited less severe mucosal injury induced by I/R, with a 35%, 33%, and 43% reduction in histological grading score and luminal concentration of hemoglobin, and the numbers of apoptotic epithelial cells, respectively. The I/R increased the activity of myeloperoxidase (MPO), a marker of neutrophil infiltration, and the levels of mRNA expression of tumor necrosis factor-α (TNF-α), intercellular adhesion molecule-1 (ICAM-1), and cyclooxygenase-2 (COX-2) in the small intestine of the wild-type mice by 3.3-, 3.2-, and 13.0-fold, respectively. TLR2 deficiency significantly inhibited the I/R-induced increase in MPO activity and the expression of mRNAs for TNF-α and ICAM-1, but did not affect the expression of COX-2 mRNA. I/R also enhanced TLR2 mRNA expression by 2.9-fold. TLR2 proteins were found to be expressed in the epithelial cells, inflammatory cells, and endothelial cells. Neutrophil depletion prevented intestinal I/R injury in wild-type mice. These findings suggest that TLR2 may mediate I/R injury of the small intestine in adult mice via induction of inflammatory mediators such as TNF-α and ICAM-1.

Ontogeny of intestinal epithelial innate immune responses

Emerging evidence indicates that processes during postnatal development might significantly influence the establishment of mucosal host-microbial homeostasis. Developmental and adaptive immunological processes but also environmental and microbial exposure early after birth might thus affect disease susceptibility and health during adult life. The present review aims at summarizing the current understanding of the intestinal epithelial innate immune system and its developmental and adaptive changes after birth.

Immunology of inflammatory bowel disease and molecular targets for biologics

Inflammatory bowel disease (IBD) is an immune-mediated disease and involves a complex interplay of host genetics and environmental influences. Recent advances in the field, including data from genome-wide association studies and microbiome analysis, have started to unravel the complex interaction between host genetics and environmental influences in the pathogenesis of IBD. A drawback of current clinical trials is inadequate or lack of immune phenotyping of patients. However, recent advances in high-throughput technologies provide an opportunity to monitor the dynamic and complex immune system, which may to lead to a more personalized treatment approach in IBD.

Bacillus subtilis Protects Porcine Intestinal Barrier from Deoxynivalenol via Improved Zonula Occludens-1 Expression

Intestinal epithelial cells (IECs) forming the barrier for the first-line of protection are interconnected by tight junction (TJ) proteins. TJ alteration results in impaired barrier function, which causes potentially excessive inflammation leading to intestinal disorders. It has been suggested that toll-like receptor (TLR) 2 ligands and some bacteria enhance epithelial barrier function in humans and mice. However, no such study has yet to be claimed in swine. The aim of the present study was to examine whether Bacillus subtilis could improve barrier integrity and protection against deoxynivalenol (DON)-induced barrier disruption in porcine intestinal epithelial cell line (IPEC-J2). We found that B. subtilis decreased permeability of TJ and improved the expression of zonula occludens (ZO)-1 and occludin during the process of forming TJ. In addition, ZO-1 expression of IPEC-J2 cells treated with B. subtilis was up-regulated against DON-induced damage. In conclusion, B. subtilis may have potential to enhance epithelial barrier function and to prevent the cells from DON-induced barrier dysfunction.

In vitro evaluation of intestinal epithelial TLR activation in preventing food allergic responses

Alterations in the gut microbiota composition are associated with food allergy. Toll-like receptors (TLRs) respond to microbial stimuli. We studied the effects of the ligation of TLRs on intestinal epithelial cells (IECs) in preventing an allergic effector response. IEC monolayers (T84 cells) were co-cultured with CD3/28-activated PBMCs from healthy controls or atopic patients and simultaneously apically exposed to TLR2, TLR4 or TLR9 ligands. The barrier integrity of T84 cell monolayers was significantly reduced upon co-culture with PBMCs of food allergic subjects compared to healthy subjects. Apical exposure of IECs to a TLR9 ligand prevented PBMC-induced epithelial barrier disruption. Using PBMCs from food allergic subjects, apical TLR9 activation on IECs increased the IFN-γ/IL-13 and IL-10/IL-13 ratio, while suppressing pro-inflammatory IL-6, IL-8 and TNF-α production in an IEC-dependent manner. Hence, the activation of apical TLR9 on IECs, potentially by microbiota-derived signals, may play an important role in the prevention of allergic inflammation.

NOD-like receptors in intestinal homeostasis and epithelial tissue repair

The intestinal epithelium constitutes a dynamic physical barrier segregating the luminal content from the underlying mucosal tissue. Following injury, the epithelial integrity is restored by rapid migration of intestinal epithelial cells (IECs) across the denuded area in a process known as wound healing. Hence, through a sequence of events involving restitution, proliferation and differentiation of IECs the gap is resealed and homeostasis reestablished. Relapsing damage followed by healing of the inflamed mucosa is a hallmark of several intestinal disorders including inflammatory bowel diseases (IBD). While several regulatory peptides, growth factors and cytokines stimulate restitution of the epithelial layer after injury, recent evidence in the field underscores the contribution of innate immunity in controlling this process. In particular, nucleotide-binding and oligomerization domain-like receptors (NLRs) play critical roles in sensing the commensal microbiota, maintaining homeostasis, and regulating intestinal inflammation. Here, we review the process of intestinal epithelial tissue repair and we specifically focus on the impact of NLR-mediated signaling mechanisms involved in governing epithelial wound healing during disease.

Regulation of Toll-like receptor expression in human conjunctival epithelial cells

Previous studies showed marked decrease of multiple Toll-like receptor (TLR) expression in corneal and conjunctival epithelial cells upon culture in vitro. The aim of this study was to identify factor(s) which regulate TLR expression. Primary human conjunctival epithelial cells and immortal conjunctival (IOBA-NHC) and corneal epithelial cell lines (HCET) were used. The effect of various cytokines, hypoxia, mechanical wounding, and airlifting culture on TLR expression was examined by quantitative PCR and western blot analysis. Ligand stimulated TLR activation was analyzed. TLR mRNA expression increased modestly when cultured monolayered cells were stimulated by TNF-α, IL-1α, IL-1β, IL-6, IL-8, IFN-γ (about 2-fold), hypoxia (2.1- to 4.8-fold selectively), and wounding (3.1- to 9.3-fold). In airlifted multilayered cells, TLR expression increased 7.8- to 25.9-fold compared to monolayered cells. Airlifted cells showed increased response to low concentrations of lipopolysaccharide (LPS) and peptidoglycan (PGN) stimulation. NF κ B inhibition prevented the formation of cell sheets and led to the collapse of already-formed multilayered structure and the simultaneous reduction of TLR mRNA level. In conclusion, our study showed that the conjunctival epithelial cell expressed TLR was sensitive to various stimulants, and a multilayered epithelium-like structure was needed to maintain TLR expression.

Toll-like receptor 2 regulates the barrier function of human bronchial epithelial monolayers through atypical protein kinase C zeta, and an increase in expression of claudin-1

We investigated the role of Toll-like receptor (TLR) 2 in maintaining the integrity of the airway epithelial barrier using the human bronchial epithelial cell line Calu-3. Activation of TLR2 by its ligands, Pam3CysSK4 and Peptidoglycan showed a concentration dependent increase in epithelial barrier function, as measured by transepithelial electrical resistance (TEER). This was confirmed by a decrease in paracellular flux of fluorescein sodium. This TLR2 induced increase in TEER was significantly reduced by pretreatment with polyclonal anti-human TLR2-neutralizing antibody. TLR2 stimulation in Calu-3 cell monolayers resulted in an increased expression of the tight junction proteins claudin-1 and ZO-1, and a decreased expression of occludin, at both the mRNA and protein levels. A pseudosubstrate inhibitor to PKCζ significantly prevented the TLR2 mediated increase in barrier function. It also prevented the increase in claudin-1 in a concentration dependent manner up to 1 µM. TLR2 stimulation led to an increase in phosphorylation of atypical PKC ζ, which was prevented by the pseudosubstrate inhibitor in a concentration dependent manner. Taken together, our observations support a model whereby increased tight junction barrier function induced by activation of TLR2 occurs through increased expression of claudin-1, and through modulation of PKC ζ activity.

In vitro exposure to Escherichia coli decreases ion conductance in the jejunal epithelium of broiler chickens

Escherichia coli (E. coli) infections are very widespread in poultry. However, little is known about the interaction between the intestinal epithelium and E. coli in chickens. Therefore, the effects of avian non-pathogenic and avian pathogenic Escherichia coli (APEC) on the intestinal function of broiler chickens were investigated by measuring the electrogenic ion transport across the isolated jejunal mucosa. In addition, the intestinal epithelial responses to cholera toxin, histamine and carbamoylcholine (carbachol) were evaluated following an E. coli exposure. Jejunal tissues from 5-week-old broilers were exposed to 6×10(8) CFU/mL of either avian non-pathogenic E. coli IMT11322 (Ont:H16) or avian pathogenic E. coli IMT4529 (O24:H4) in Ussing chambers and electrophysiological variables were monitored for 1 h. After incubation with E. coli for 1 h, either cholera toxin (1 mg/L), histamine (100 μM) or carbachol (100 μM) were added to the incubation medium. Both strains of avian E. coli (non-pathogenic and pathogenic) reduced epithelial ion conductance (Gt) and short-circuit current (Isc). The decrease in ion conductance after exposure to avian pathogenic E. coli was, at least, partly reversed by the histamine or carbachol treatment. Serosal histamine application produced no significant changes in the Isc in any tissues. Only the uninfected control tissues responded significantly to carbachol with an increase of Isc, while the response to carbachol was blunted to non-significant values in infected tissues. Together, these data may explain why chickens rarely respond to intestinal infections with overt secretory diarrhea. Instead, the immediate response to intestinal E. coli infections appears to be a tightening of the epithelial barrier.

Intestinal epithelial cells: regulators of barrier function and immune homeostasis

The abundance of innate and adaptive immune cells that reside together with trillions of beneficial commensal microorganisms in the mammalian gastrointestinal tract requires barrier and regulatory mechanisms that conserve host-microbial interactions and tissue homeostasis. This homeostasis depends on the diverse functions of intestinal epithelial cells (IECs), which include the physical segregation of commensal bacteria and the integration of microbial signals. Hence, IECs are crucial mediators of intestinal homeostasis that enable the establishment of an immunological environment permissive to colonization by commensal bacteria. In this Review, we provide a comprehensive overview of how IECs maintain host-commensal microbial relationships and immune cell homeostasis in the intestine.

Heat Shock Proteins: Intestinal Gatekeepers that Are Influenced by Dietary Components and the Gut Microbiota

Trillions of microorganisms that inhabit the intestinal tract form a diverse and intricate ecosystem with a deeply embedded symbiotic relationship with their hosts. As more detailed information on gut microbiota complexity and functional diversity accumulates, we are learning more about how diet-microbiota interactions can influence the immune system within and outside the gut and host health in general. Heat shock proteins are a set of highly conserved proteins that are present in all types of cells, from microbes to mammals. These proteins carry out crucial intracellular housekeeping functions and unexpected extracellular immuno-regulatory features in order to maintain the mucosal barrier integrity and gut homeostasis. It is becoming evident that the enteric microbiota is one of the major determinants of heat shock protein production in intestinal epithelial cells. This review will focus on the interactions between diet, gut microbiota and their role for regulating heat shock protein production and, furthermore, how these interactions influence the immune system and the integrity of the mucosal barrier.

Regulation of Intestinal Immune Responses through TLR Activation: Implications for Pro- and Prebiotics

The intestinal mucosa is constantly facing a high load of antigens including bacterial antigens derived from the microbiota and food. Despite this, the immune cells present in the gastrointestinal tract do not initiate a pro-inflammatory immune response. Toll-like receptors (TLRs) are pattern recognition receptors expressed by various cells in the gastrointestinal tract, including intestinal epithelial cells (IEC) and resident immune cells in the lamina propria. Many diseases, including chronic intestinal inflammation (e.g., inflammatory bowel disease), irritable bowel syndrome (IBS), allergic gastroenteritis (e.g., eosinophilic gastroenteritis and allergic IBS), and infections are nowadays associated with a deregulated microbiota. The microbiota may directly interact with TLR. In addition, differences in intestinal TLR expression in health and disease may suggest that TLRs play an essential role in disease pathogenesis and may be novel targets for therapy. TLR signaling in the gut is involved in either maintaining intestinal homeostasis or the induction of an inflammatory response. This mini review provides an overview of the current knowledge regarding the contribution of intestinal epithelial TLR signaling in both tolerance induction or promoting intestinal inflammation, with a focus on food allergy. We will also highlight a potential role of the microbiota in regulating gut immune responses, especially through TLR activation.

Pleiotropic effects of Blastocystis spp. Subtypes 4 and 7 on ligand-specific toll-like receptor signaling and NF-κB activation in a human monocyte cell line

Blastocystis spp. is a common enteric stramenopile parasite that colonizes the colon of hosts of a diverse array of species, including humans. It has been shown to compromise intestinal epithelial cell barrier integrity and mediate the production of pro-inflammatory cytokines and chemokines. Mucosal epithelial surfaces, including the intestinal epithelium, are increasingly recognized to perform a vital surveillance role in the context of innate immunity, through the expression of pathogen recognition receptors, such as Toll-like receptors (TLRs). In this study, we use the human TLR reporter monocytic cell line, THP1-Blue, which expresses all human TLRs, to investigate effects of Blastocystis on TLR activation, more specifically the activation of TLR-2, -4 and -5. We have observed that live Blastocystis spp. parasites and whole cell lysate (WCL) alone do not activate TLRs in THP1-Blue. Live ST4-WR1 parasites inhibited LPS-mediated NF-κB activation in THP1-Blue. In contrast, ST7-B WCL and ST4-WR1 WCL induced pleiotropic modulation of ligand-specific TLR-2 and TLR-4 activation, with no significant effects on flagellin-mediated TLR-5 activation. Real time-qPCR analysis on SEAP reporter gene confirmed the augmenting effect of ST7-B on LPS-mediated NF-κB activation in THP1-Blue. Taken together, this is the first study to characterize interactions between Blastocystis spp. and host TLR activation using an in vitro reporter model.

Mucosal immunology of HIV infection

Recent advances in the immunology, pathogenesis, and prevention of human immunodeficiency virus (HIV) infection continue to reveal clues to the mechanisms involved in the progressive immunodeficiency attributed to infection, but more importantly have shed light on the correlates of immunity to infection and disease progression. HIV selectively infects, eliminates, and/or dysregulates several key cells of the human immune system, thwarting multiple arms of the host immune response, and inflicting severe damage to mucosal barriers, resulting in tissue infiltration of 'symbiotic' intestinal bacteria and viruses that essentially become opportunistic infections promoting systemic immune activation. This leads to activation and recruitment or more target cells for perpetuating HIV infection, resulting in persistent, high-level viral replication in lymphoid tissues, rapid evolution of resistant strains, and continued evasion of immune responses. However, vaccine studies and studies of spontaneous controllers are finally providing correlates of immunity from protection and disease progression, including virus-specific CD4(+) T-cell responses, binding anti-bodies, innate immune responses, and generation of antibodies with potent antibody-dependent cell-mediated cytotoxicity activity. Emerging correlates of immunity indicate that prevention of HIV infection may be possible through effective vaccine strategies that protect and stimulate key regulatory cells and immune responses in susceptible hosts. Furthermore, immune therapies specifically directed toward boosting specific aspects of the immune system may eventually lead to a cure for HIV-infected patients.