Peyer's patch dendritic cells and the induction of mucosal immune responses

The immune system of the gut and potential adverse effects of oral nanocarriers on its function

There is substantial effort in modern pharmacotherapy to use nanoparticle-based drug delivery systems (nDDS) for improving the oral absorption of drugs. An often neglected circumstance regarding this approach is that the gut is a major part of the immune system that may be vulnerable for immune-cell toxicity, or mediate humoral immune response against various components of nDDS, recognized as foreign. This review recapitulates the structure and function of gut-associated lymphoid tissue (GALT), i.e., the enteral section of mucosa-associated lymphoid tissue (MALT) and reminds how virus-like nDDS may potentially induce immunogenicity just as attenuated or killed viruses do in oral vaccines. Furthermore, we present examples for immune toxicities of emulsifiers and polymer-containing micelles, manifested in complement activation-related pseudoallergy (CARPA). A major message of the review is that early testing of immunogenicity or other adverse immune effects of nDDS in appropriate test systems or models may be prudent to recognize the risk of rare immune problems that may surface in late-stage clinical trials or after marketing of nDDS.

Specific Lactobacillus species differentially activate Toll-like receptors and downstream signals in dendritic cells

BACKGROUND

Dendritic cells (DCs) regulate mucosal T-cell immunity and encounter several distinct bacteria of the gut flora, including lactobacilli. Gram-positive lactobacilli have been suggested to play an important role in exerting adjuvanticity effects on innate immune cells at mucosal sites.

AIMS & METHODS

In the present report, we studied the effects of specific Lactobacillus species on human monocyte derived DCs.

RESULTS

We show that lactobacilli activate DCs by differentially inducing the expression of Toll-like receptors and bioactive IL-12 in Lactobacillus-treated DCs. Further, these specific Lactobacillus spp. did not activate the phosphorylation of p38 MAPK, which might be a downstream effect of the remarkable capacity of lactobacilli to induce IL-12 in DCs that skew T cells significantly toward an IFN-gamma-secreting Th1 response.

CONCLUSION

These results highlight an important role of specific Lactobacillus spp. as adjuvants in triggering DC function, which in turn may determine the immunological outcome in an environment wherein innate cells reside.

Regulatory role of Peyer’s patches for the inhibition of OVA-induced allergic diarrhea

Intestinal allergic diseases are initiated by aberrant Th2-type immune responses, including elevation of IgE antibodies (Abs) and infiltration of eosinophils. However, little is known about the role of Peyer's patches (PP) in the control of allergic diseases. Using a mouse model for food allergy, we here show that mice lacking PP are more susceptible to disease development and show higher levels of antigen-specific IgE Abs than do PP-intact mice. In our study, we noted that high numbers of eosinophils infiltrated into the small intestine of PP-null mice. In contrast, the PP of intact mice contained regulatory CD4+CD25+ Foxp3+ T cells (Treg) that are known to produce high levels of IL-10, and inhibited the development of allergic diarrhea. PP-intact mice thus developed allergic diarrhea when treated with anti-CD25 or anti-IL-10 monoclonal antibody (mAb) in vivo. These studies demonstrate that PP, as the site where IL-10-producing Treg cells are created, mediate the mucosal regulatory network for the control of undesired allergic responses in the intestine.

Cross-talk between probiotic bacteria and the host immune system

Among the numerous purported health benefits attributed to probiotic bacteria, their capacity to interact with the immune system of the host is now supported by an increasing number of in vitro and in vivo experiments. In addition to these, a few well-controlled human intervention trials aimed at preventing chronic immune dysregulation have been reported. Even though the precise molecular mechanisms governing the cross-talk between these beneficial bacteria and the intestinal ecosystem remain to be discovered, a new and fascinating phase of research has been initiated in this area as demonstrated by a series of recent articles. This article summarizes the status and latest progress of the field in selected areas and aims at identifying key questions that remain to be addressed, especially concerning the translocation of ingested bacteria, the identification of major immunomodulatory compounds of probiotics, and specific aspects of the host-microbe cross-talk. The interaction with immunocompetent cells and the role of secretory IgA in gut homeostasis are also evoked. Finally, a brief overview is provided on the potential use of recombinant DNA technology to enhance the health benefits of probiotic strains and to unravel specific mechanisms of the host-microbe interaction.

Secretory IgA possesses intrinsic modulatory properties stimulating mucosal and systemic immune responses

Secretory IgA (SIgA) is essential in protecting mucosal surfaces by ensuring immune exclusion. In addition, SIgA binds selectively to M cells in Peyer's patches (PP), resulting in transport across the epithelium and targeting of dendritic cells (DC) in the dome region. The immunological consequences of such an interaction are unknown. In this study, we find that oral delivery of SIgA comprising human secretory component and mouse IgA induces human secretory component-specific Ab and cellular responses in mucosal and peripheral tissues in mice. This takes place in the absence of co-addition of cholera toxin, identifying so far unraveled properties in SIgA. Specific immune responses are accompanied by sustained IL-10 and TGF-beta expression in draining mesenteric lymph nodes and spleen. SIgA also triggers migration of DC to the T cell-rich regions of PP, and regulates expression of CD80 and CD86 on DC in PP, mesenteric lymph nodes, and spleen. These results provide evidence that mucosal SIgA re-entering the body exerts a function of Ag delivery that contributes to effector and/or regulatory pathways characteristic of the intestinal mucosal compartment.

Migration and maturation of human dendritic cells infected with Toxoplasma gondii depend on parasite strain type

Migration and maturation of human dendritic cells derived from CD34+ progenitor cells (DC) infected by Toxoplasma gondii were studied in an in vitro model. We demonstrated that infection with virulent type I strains RH and ENT or type II low virulent strains PRU and CAL induced DC migration towards MIP-3beta. However, type II strains induced a higher percentage of migrating cells than that induced by type I strains or positive controls (chemical allergen or lipopolysaccharides). Type II strains produced soluble factors responsible of the high migration whereas heat killed tachyzoites did not induced a migration higher than positive controls. We also demonstrated that infection by virulent strains and not by type II stains or heat killed tachyzoites triggers DC maturation. A soluble factor released by type II strains was responsible of the absence of DC maturation. Taken together, these results demonstrated that the interference of T. gondii in the behaviour of DC functions is related to the strain types and can be supported by secretion of soluble factors by the parasite.

Use of probiotics in the treatment of inflammatory bowel disease

The gut flora plays a fundamental role in maintaining normal intestinal function. A disturbance of this flora, or the host response to this flora, has been clearly demonstrated to play a role in the pathogenesis of inflammatory bowel disease (IBD). This has led to attempts to modify the bacterial flora with "live non-pathogenic organisms that confer health benefits by improving the microbial balance," otherwise known as probiotics. Recent attention has focused on this potential strategy to treat or prevent IBD. The potential therapeutic benefit is enhanced by the natural and apparently safe approach that probiotics offer. Animal models of colitis have provided the proof of principle that probiotics can prevent and treat established intestinal inflammation. Controlled clinical studies have demonstrated the efficacy of probiotics in the maintenance of remission of pouchitis, prophylaxis of pouchitis after the formation of an ileoanal reservoir, maintenance of remission of ulcerative colitis, and treatment of Crohn's disease. However, large controlled trials are needed to definitively establish the place for probiotics in the treatment of IBD and resolve issues such as the dose, duration, frequency of treatment, and use of single or multiple strains. Research is focusing on establishing the mechanism of action, so that treatments with individually tailored properties are developed and innovative approaches are explored.

Murine Peyer's patches favor development of an IL-10-secreting, regulatory T cell population

Peyer's patches (PP) are believed to be the principal sites for induction of tolerance to Ags from food and commensal flora, yet the phenotype of T cells activated within the PP is largely unexplored. We hypothesize that exposure to Ags within the PP promotes differentiation of T cells with immunoregulatory functions. Cytokine production and cell surface marker expression of murine PP mononuclear cells (MC) are compared with those from mesenteric lymph nodes and peripheral lymph nodes (PLN). In response to stimulation through the TCR/CD3 complex, PP MC exhibit vigorous proliferation, modest production of IL-2, and significantly elevated synthesis of IL-10. Exogenous IL-12 enhances both IL-10 and IFN-gamma secretion by activated PP MC. Cell surface marker analysis reveals that PP T cells consist of activated and memory subpopulations compared with the predominantly naive T cells identified in the PLN and mesenteric lymph nodes. Upon stimulation, only CD45RB(low)CD4(+) PP T cells produce IL-10, whereas secretion of IL-2, IL-4, and IFN-gamma was not detected. Furthermore, PP MC, but not PLN MC, stimulated through the TCR/CD3 complex suppress proliferation of purified PLN T cells in vitro, evidence for a regulatory function among PP lymphocytes. We conclude that PP favor differentiation of an IL-10-producing, regulatory CD45RB(low)CD4(+) T cell population and that inhibition of T cell proliferation by activated PP MC may reflect regulatory activity consistent with T regulatory cells.

Lactobacillus bulgaricus proteinase expressed in Lactococcus lactis is a powerful carrier for cell wall-associated and secreted bovine beta-lactoglobulin fusion proteins

Lactic acid bacteria have a good potential as agents for the delivery of heterologous proteins to the gastrointestinal mucosa and thus for the reequilibration of inappropriate immune responses to food antigens. Bovine beta-lactoglobulin (BLG) is considered a major allergen in cow's milk allergy. We have designed recombinant Lactococcus lactis expressing either full-length BLG or BLG-derived octapeptide T6 (IDALNENK) as fusions with Lactobacillus bulgaricus extracellular proteinase (PrtB). In addition to constructs encoding full-length PrtB for the targeting of heterologous proteins to the cell surface, we generated vectors aiming at the release into the medium of truncated PrtB derivatives lacking 100 (PrtB partial differential, PrtB partial differential-BLG, and PrtB partial differential-T6) or 807 (PrtBdelta) C-terminal amino acids. Expression of recombinant products was confirmed using either anti-PrtB, anti-BLG, or anti-peptide T6 antiserum. All forms of the full-length and truncated recombinant products were efficiently translocated, irrespective of the presence of eucaryotic BLG sequences in the fusion proteins. L. lactis expressing PrtB partial differential-BLG yielded up to 170 microg per 10(9) CFU in the culture supernatant and 9 microg per 10(9) CFU at the bacterial cell surface within 14 h. Therefore, protein fusions relying on the use of PrtB gene products are adequate for concomitant cell surface display and secretion by recombinant L. lactis and thus may ensure maximal bioavailability of the eucaryotic antigen in the gut-associated lymphoid tissue.

Approaches to improve engineered vaccines for human immunodeficiency virus and other viruses that cause chronic infections

We used several approaches to develop enhanced vaccines for chronic viral infections such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV). 1) Selected epitopes were used to avoid potentially harmful immune responses. 2) Linkage between helper and cytotoxic T-lymphocyte (CTL) epitopes was found to be important. 3) We developed an "epitope enhancement" approach modifying the sequences of epitopes to make more potent vaccines, including examples for HIV and HCV epitopes presented by murine class II and human class I major histocompatibility complex (MHC) molecules. 4) CTL avidity was found to be important for clearing viral infections in vivo, and the mechanism was examined. High-avidity CTLs, however, were found to undergo apoptosis when confronted with high-density antigen, through a mechanism involving tumor necrosis factor (TNF), TNF-RII, and a permissive state induced through the T-cell receptor. 5) We employed cytokines in the adjuvant to steer immune responses toward desired phenotypes, and showed synergy between cytokines. 6) Intrarectal immunization with peptide vaccine induced mucosal and systemic CTL. Local mucosal CTL were found to be critical for resistance to mucosal viral transmission and this resistance was enhanced with mucosally delivered interleukin-12. 7) We used an asymmetry in induction of mucosal and systemic immune responses to circumvent pre-existing vaccinia immunity for use of recombinant vaccinia vaccines.