The spleen and organized lymph nodes are not essential for the development of gut-induced mucosal immune responses in lymphotoxin-alpha deficient mice

Splenic immune cells in experimental neonatal hypoxia-ischemia

Neuroimmune processes contribute to hypoxic-ischemic damage in the immature brain and may play a role in the progression of particular variants of neonatal encephalopathy. The present study was designed to elucidate molecular mediators of interactions between astrocytes, neurons and infiltrating peripheral immune cells after experimental neonatal hypoxia-ischemia (HI). Splenectomy was performed on postnatal day-7 Sprague-Dawley rats 3 days prior to HI surgery; in which the right common carotid artery was permanently ligated followed by 2 hours of hypoxia (8% O2). Quantitative analysis showed that natural killer (NK) and T cell expression was reduced in spleen but increased in the brain following HI. Elevations in cyclooxygenase-2 (COX-2) expression after HI by immune cells promoted interleukin-15 expression in astrocytes and infiltration of inflammatory cells to site of injury; additionally, down-regulated the pro-survival protein, phosphoinositide-3-kinase, resulting in caspase-3 mediated neuronal death. The removal of the largest pool of peripheral immune cells in the body by splenectomy, COX-2 inhibitors, as well as rendering NK cells inactive by CD161 knockdown, significantly ameliorated cerebral infarct volume at 72 hours, diminished body weight loss and brain and systemic organ atrophy, and reduced neurobehavioral deficits at 3 weeks. Herein we demonstrate with the use of surgical approach (splenectomy), with pharmacological loss-gain function approach using COX-2 inhibitors/agonists, as well as with NK cell-type specific siRNA that after neonatal HI, the infiltrating peripheral immune cells may modulate downstream targets of cell death and neuroinflammation by COX-2 regulated signals.

Bacterial β-(1,3)-glucan prevents DSS-induced IBD by restoring the reduced population of regulatory T cells

Bacterial β-(1,3)-glucan has more advantages in terms of cost, yield and efficiency than that derived from mushrooms, plants, yeasts and fungi. We have previously developed a novel and high-yield β-(1,3)-glucan produced by Agrobacterium sp. R259. This study aimed to elucidate the functional mechanism and therapeutic efficacy of bacterial β-(1,3)-glucan in dextran sulfate sodium (DSS)-induced inflammatory bowel disease (IBD).Mice were orally pretreated with bacterial β-(1,3)-glucan at daily doses of 2.5 or 5mg/kg for 2 weeks. After 6 days of DSS treatment, clinical assessment of IBD severity and expression of pro-inflammatory cytokines were evaluated. In vivo cell proliferation was examined by immunohistochemistry using Ki-67 and ER-TR7 antibodies. The frequency of regulatory T cells (Tregs) was analyzed by flow cytometry. Natural killer (NK) activity and IgA level were evaluated using NK cytotoxicity assay and ELISA.The deterioration of body weight gain, colonic architecture, disease score and histological score was recovered in DSS-induced IBD mice when pretreated with bacterial β-(1,3)-glucan. The recruitment of macrophages and the gene expression of proinflammatory cytokines, such as IL-1β, IL-6 and IL-17A/F, were markedly decreased in the colon of β-(1,3)-glucan-pretreated mice. β-(1,3)-Glucan induced the recovery of Tregs in terms of their frequency in DSS-induced IBD mice. Intriguingly, β-(1,3)-glucan reversed the functional defects of NK cells and excessive IgA production in DSS-induced IBD mice.We conclude that bacterial β-(1,3)-glucan prevented the progression of DSS-induced IBD by recovering the reduction of Tregs, functional defect of NK cells and excessive IgA production.

Immunological tumor destruction in a murine melanoma model by targeted LTalpha independent of secondary lymphoid tissue

BACKGROUND

We previously demonstrated that targeting lymphotoxin alpha (LTalpha) to the tumor evokes its immunological destruction in a syngeneic B16 melanoma model. Since treatment was associated with the induction of peritumoral tertiary lymphoid tissue, we speculated that the induced immune response was initiated at the tumor site.

METHODS AND RESULTS

In order to directly test this notion, we analyzed the efficacy of tumor targeted LTalpha in LTalpha knock-out (LTalpha(-/-)) mice which lack peripheral lymph nodes. To this end, we demonstrate that tumor-targeted LTalpha mediates the induction of specific T-cell responses even in the absence of secondary lymphoid organs. In addition, this effect is accompanied by the initiation of tertiary lymphoid tissue at the tumor site in which B and T lymphocytes are compartmentalized in defined areas and which harbor expanded numbers of tumor specific T cells as demonstrated by in situ TRP-2/K(b) tetramer staining. Mechanistically, targeted LTalpha therapy seems to induce changes at the tumor site which allows a coordinated interaction of immune competent cells triggering the induction of tertiary lymphoid tissue.

CONCLUSION

Thus, our data demonstrate that targeted LTalpha promotes an accelerated immune response by enabling the priming of T cells at the tumor site.

CD70+ antigen-presenting cells control the proliferation and differentiation of T cells in the intestinal mucosa

One unresolved issue in gut immunity is how mucosal T lymphocytes are activated and which antigen-presenting cell (APC) is critical for the regulation of this process. We have identified a unique population of APCs that is exclusively localized in the lamina propria. These APCs constitutively expressed the costimulatory molecule CD70 and had antigen-presenting functions. After oral infection of mice with Listeria monocytogenes, proliferation and differentiation of antigen-specific T cells occurred in the gut mucosa in situ and blockade of CD70 costimulation abrogated the mucosal T cell proliferation and effector functions. Thus, a potent CD70-dependent stimulation via specialized tissue-specific APCs is required for the proliferation and differentiation of gut mucosal T cells after oral infection.

NALT- versus Peyer's-patch-mediated mucosal immunity

Recent studies indicate that the mechanism of nasopharynx-associated lymphoid tissue (NALT) organogenesis is different from that of other lymphoid tissues. NALT has an important role in the induction of mucosal immune responses, including the generation of T helper 1 and T helper 2 cells, and IgA-committed B cells. Moreover, intranasal immunization can lead to the induction of antigen-specific protective immunity in both the mucosal and systemic immune compartments. Therefore, a greater understanding of the differences between NALT and other organized lymphoid tissues, such as Peyer's patches, should facilitate the development of nasal vaccines.

Lymphoid microenvironment in the gut for immunoglobulin A and inflammation

Signaling through lymphotoxin beta receptor (LTbetaR) initiates the unfolding of a host of developmental programs ranging from the organogenesis of lymph nodes and Peyer's patches (PPs) to the coordination of splenic microarchitecture. While investigating an alternative pathway to immunoglobulin A (IgA) production, it was uncovered that LTbetaR signaling in the lamina propria (LP) stroma orchestrates the coordinated expression of key chemokines and adhesion molecules, creation of a cytokine milieu, and stroma development that facilitates robust IgA production independent of secondary lymphoid structures. Simultaneously, this same infrastructure can be commandeered by autoreactive T cells to organize both the acute destruction of the intestinal mucosa and chronic intestinal inflammation via the ligands for LTbetaR. The ability to modulate LTbetaR signaling may alternatively permit the suppression of autoimmune responses and augmentation of gut defenses.

Signaling via LTbetaR on the lamina propria stromal cells of the gut is required for IgA production

Peyer's patches (PPs) and/or mesenteric lymph nodes (MLNs) are thought to be essential for immunoglobulin A (IgA) production. We found that the severe IgA deficiency in lymphotoxin-deficient (LT(-/-)) mice could be fully reversed by reconstitution with LT-expressing bone marrow, despite the absence of both LNs and PPs. The number of IgA precursors from LT(-/-) mice was not reduced, and they were able to migrate into the lamina propria (LP) of wild-type mice but not of LTbetaR(-/-) mice. Consistently, lymphoid tissue chemokines and adhesion molecules were reduced within the LP of LTalpha(-/-) and LTbetaR(-/-) mice. IgA deficiency in LTalpha(-/-) mice was reversed by the transplantation of a segment of RAG-1 (recombination-activating gene 1) deficient intestine, which confirmed the dispensability of the MLNs and PPs and the sufficiency of the LT-mediated gut microenvironment for IgA production.

Generation of experimental allergic airways inflammation in the absence of draining lymph nodes

The objective of this study was to investigate the contribution of secondary lymphoid organs in the generation and maintenance of experimental allergic airway inflammation. We employed a previously reported murine model of respiratory mucosal allergic sensitization, induced by repeated aerosolizations of ovalbumin in the context of a GM-CSF airway environment. We executed this protocol in wild-type (WT) and lymphotoxin-alpha-deficient mice (LTalpha-KO) mice, which are devoid of lymph nodes (LNs) and possess rudimentary spleen structures. Despite the lack of pulmonary LNs draining the airway compartment, LTalpha-KO mice were fully capable of mounting a robust inflammatory response in the airways, consisting of Th2 polarized CD4+ T cells and eosinophils. This was accompanied by IL-5, IL-13, and IFN-gamma production by splenocytes and generation of ovalbumin-specific serum IgE. Exposure to the same antigen 7 weeks after complete resolution of airway inflammation once again induced a Th2 polarized infiltrate, demonstrating intact immunological memory. To investigate inherent plasticity in establishing antigen-specific immunity, mice were splenectomized before sensitization. Allergic sensitization was completely abrogated in splenectomized LTalpha-KO mice, compared with eusplenic LTalpha-KO controls. These data demonstrate that secondary lymphoid organs, either LN or spleen, are essential for the generation of allergic airway responses.

Induction of oral tolerance to cellular immune responses in the absence of Peyer's patches

Systemic hyporesponsiveness occurs following oral administration of antigen (oral tolerance) and involves the uptake and processing of antigen by the gut-associated lymphoid tissue (GALT), which includes Peyer's patches (PP) lamina propria lymphocytes and mesenteric lymph nodes (MLN). Animals with targeted mutations of genes in the tumor necrosis factor (TNF) family have differential defects in the development of peripheral lymphoid organs including PP and MLN, and provide a unique opportunity to investigate the role of GALT structures in the induction of oral tolerance. Oral tolerance could not be induced in TNF/lymphotoxin (LT) alpha-/- mice, which are devoid of both PP and MLN, although these animals could be tolerized by intraperitoneal administration of antigen, demonstrating the requirement for GALT for oral tolerance induction. LTbeta-/- mice and LTalpha/LTbeta+/- animals do not have PP but could be orally tolerized, as measured by IFN-gamma production and delayed-type hypersensitivity responses by administration of both low or high doses of ovalbumin. To further investigate the requirement for PP, we tested the progeny of LTbeta-receptor-IgG-fusion-protein (LTbetaRigG)-treated mice, which do not form PP but have an otherwise intact immune system. Although these animals had decreased fecal IgA production, they could be orally tolerized. Our results demonstrate that PP are not an absolute requirement for the induction of either high- or low-dose oral tolerance, although oral tolerance could not be induced in animals devoid of both PP and MLN.

Lymphotoxin-alpha-deficient mice can clear a productive infection with murine gammaherpesvirus 68 but fail to develop splenomegaly or lymphocytosis

Respiratory challenge with murine gammaherpesvirus 68 (MHV-68) leads to an acute productive infection of the lung and a persistent latent infection in B lymphocytes, epithelia, and macrophages. The virus also induces splenomegaly and an increase in the number of activated CD8 T cells in the circulation. Lymphotoxin- alpha-deficient (LTalpha(-/-)) mice have no lymph nodes and have disrupted splenic architecture. Surprisingly, in spite of the severe defect in secondary lymphoid tissue, LTalpha(-/-) mice could clear a productive MHV-68 infection, although with delayed kinetics compared to wild-type mice, and could control latent infection. Cytotoxic T-cell activity was comparable in the lungs and spleens of LTalpha(-/-) and wild-type mice. However, splenic gamma interferon responses were substantially reduced in LTalpha(-/-) mice. Furthermore, LTalpha(-/-) mice failed to develop splenomegaly or lymphocytosis. Although germinal centers were absent, LTalpha(-/-) mice were able to class switch and showed significant virus-specific antibody titers. This work demonstrates that organized secondary lymphoid tissue is not an absolute requirement for the generation of immune responses to viral infections.