Replication and distribution of Toxoplasma gondii in the small intestine after oral infection with tissue cysts.

Natural infection by Toxoplasma gondii occurs via oral ingestion of tissue cysts that rupture in the small intestine, releasing zoites that infect locally before disseminating throughout the host. The studies presented here used fluorescent parasites combined with flow cytometry and multiphoton microscopy techniques to understand the events associated with parasite replication in the mucosa. At 3 days postinfection with tissue cysts, parasites were localized in small foci and flow cytometry revealed parasites present in macrophages, neutrophils, and monocytes in the lamina propria. By day 6 postinfection, there were large foci of replicating parasites; however, foci unexpectedly varied in the number of villi involved and were associated with the presence of viable tachyzoites within the intestinal lumen. Consistent with the flow cytometry data, neutrophils and monocytes in the lamina propria were preferentially associated with parasite plaques. In contrast, dendritic cells comprised a small fraction of the infected immune cell population and were localized at the periphery of parasite plaques. Together, these findings reveal the formation of localized sites of parasite replication and inflammation early during infection and suggest that sustained replication of T. gondii in the gut may be a function of pathogen luminal spread.

Nucleotide oligomerization domain-containing proteins instruct T cell helper type 2 immunity through stromal activation.

Although a number of studies have examined the development of T-helper cell type 2 (Th2) immunity in different settings, the mechanisms underlying the initiation of this arm of adaptive immunity are not well understood. We exploited the fact that immunization with antigen plus either nucleotide-binding oligomerization domain-containing proteins 1 (Nod1) or 2 (Nod2) agonists drives Th2 induction to understand how these pattern-recognition receptors mediate the development of systemic Th2 immune responses. Here, we show in bone-marrow chimeric mice that Nod1 and Nod2 expression within the stromal compartment is necessary for priming of effector CD4(+) Th2 responses and specific IgG1 antibodies. In contrast, sensing of these ligands by dendritic cells was not sufficient to induce Th2 immunity, although these cells contribute to the response. Moreover, we determined that CD11c(+) cells were the critical antigen-presenting cells, whereas basophils and B cells did not affect the capacity of Nod ligands to induce CD4(+) Th2 effector function. Finally, we found that full Th2 induction upon Nod1 and Nod2 activation was dependent on both thymic stromal lymphopoietin production by the stromal cells and the up-regulation of the costimulatory molecule, OX40 ligand, on dendritic cells. This study provides in vivo evidence of how systemic Th2 immunity is induced in the context of Nod stimulation. Such understanding will influence the rational design of therapeutics that could reprogram the immune system during an active Th1-mediated disease, such as Crohn's disease.

IL-27 regulates homeostasis of the intestinal CD4+ effector T cell pool and limits intestinal inflammation in a murine model of colitis.

IL-27 limits CD4(+) T(H)17 cell development in vitro and during inflammatory responses in the CNS. However, whether IL-27-IL-27R interactions regulate the homeostasis or function of CD4(+) T cell populations in the intestine is unknown. To test this, we examined CD4(+) T cell populations in the intestine of wild-type and IL-27R(-/-) mice. Naive IL-27R(-/-) mice exhibited a selective decrease in the frequency of IFN-gamma producing CD4(+) T(H)1 cells and an increase in the frequency of T(H)17 cells in gut-associated lymphoid tissues. Associated with elevated expression of IL-17A, IL-27R(-/-) mice exhibited earlier onset and significantly increased severity of clinical disease compared with wild-type controls in a murine model of intestinal inflammation. Rag(-/-)/IL-27R(-/-) mice were also more susceptible than Rag(-/-) mice to development of dextran sodium sulfate-induced intestinal inflammation, indicating an additional role for IL-27-IL-27R in the regulation of innate immune cell function. Consistent with this, IL-27 inhibited proinflammatory cytokine production by activated neutrophils. Collectively, these data identify a role for IL-27-IL-27R interaction in controlling the homeostasis of the intestinal T cell pool and in limiting intestinal inflammation through regulation of innate and adaptive immune cell function.

MHC class II-dependent basophil-CD4+ T cell interactions promote T(H)2 cytokine-dependent immunity.

Dendritic cells can prime naive CD4+ T cells; however, here we demonstrate that dendritic cell-mediated priming was insufficient for the development of T helper type 2 cell-dependent immunity. We identify basophils as a dominant cell population that coexpressed major histocompatibility complex class II and interleukin 4 message after helminth infection. Basophilia was promoted by thymic stromal lymphopoietin, and depletion of basophils impaired immunity to helminth infection. Basophils promoted antigen-specific CD4+ T cell proliferation and interleukin 4 production in vitro, and transfer of basophils augmented the population expansion of helminth-responsive CD4+ T cells in vivo. Collectively, our studies suggest that major histocompatibility complex class II-dependent interactions between basophils and CD4+ T cells promote T helper type 2 cytokine responses and immunity to helminth infection.

TSLP regulates intestinal immunity and inflammation in mouse models of helminth infection and colitis.

Intestinal epithelial cells (IECs) produce thymic stromal lymphopoietin (TSLP); however, the in vivo influence of TSLP-TSLP receptor (TSLPR) interactions on immunity and inflammation in the intestine remains unclear. We show that TSLP-TSLPR interactions are critical for immunity to the intestinal pathogen Trichuris. Monoclonal antibody-mediated neutralization of TSLP or deletion of the TSLPR in normally resistant mice resulted in defective expression of Th2 cytokines and persistent infection. Susceptibility was accompanied by elevated expression of interleukin (IL) 12/23p40, interferon (IFN) gamma, and IL-17A, and development of severe intestinal inflammation. Critically, neutralization of IFN-gamma in Trichuris-infected TSLPR(-/-) mice restored Th2 cytokine responses and resulted in worm expulsion, providing the first demonstration of TSLPR-independent pathways for Th2 cytokine production. Additionally, TSLPR(-/-) mice displayed elevated production of IL-12/23p40 and IFN-gamma, and developed heightened intestinal inflammation upon exposure to dextran sodium sulfate, demonstrating a previously unrecognized immunoregulatory role for TSLP in a mouse model of inflammatory bowel disease.

Commensal-dependent expression of IL-25 regulates the IL-23-IL-17 axis in the intestine.

Alterations in the composition of intestinal commensal bacteria are associated with enhanced susceptibility to multiple inflammatory diseases, including those conditions associated with interleukin (IL)-17-producing CD4(+) T helper (Th17) cells. However, the relationship between commensal bacteria and the expression of proinflammatory cytokines remains unclear. Using germ-free mice, we show that the frequency of Th17 cells in the large intestine is significantly elevated in the absence of commensal bacteria. Commensal-dependent expression of the IL-17 family member IL-25 (IL-17E) by intestinal epithelial cells limits the expansion of Th17 cells in the intestine by inhibiting expression of macrophage-derived IL-23. We propose that acquisition of, or alterations in, commensal bacteria influences intestinal immune homeostasis via direct regulation of the IL-25-IL-23-IL-17 axis.

Diverse myeloid and lymphoid cell subpopulations produce gamma interferon during early innate immune responses to Francisella tularensis live vaccine strain.

Francisella tularensis, a small gram-negative intracellular bacterium responsible for causing tularemia, is highly pathogenic and classified as a category A agent of bioterrorism. As for other intracellular pathogens, successful protective immune responses to Francisella tularensis require rapid and efficient induction of gamma interferon (IFN-gamma) production. Studies using intracellular bacteria such as Listeria monocytogenes as well as Francisella suggest that natural killer (NK) and T cells are important sources of IFN-gamma. However, comprehensive characterization of specific sources of IFN-gamma produced during Francisella infection in vivo remains incomplete, and depletion of NK cells before infection of mice with the F. tularensis live vaccine strain (LVS) has little impact on the course or outcome of infection. In this study, we determined the cell subpopulations that respond quickly to intradermal F. tularensis LVS infection of mice by producing IFN-gamma within hours to a few days. Splenic and liver lymphocytes were obtained from LVS-infected mice and analyzed for IFN-gamma mRNA by reverse transcription-PCR, for intracellular cytokine expression by multiparameter flow cytometry, and for ex vivo production of IFN-gamma protein by enzyme-linked immunosorbent assay. Cells producing IFN-gamma were readily detectable by day 3 after infection, and numbers progressively increased through days 5 to 7. Importantly, the cell types responsible for IFN-gamma production were much more varied than expected: these included not only NK cells and T cells, which might be predicted, but also other cells, including dendritic cells (DCs), "NK DCs," NK T cells, and neutrophils. Most importantly, since RAG-1 knockout mice appeared to exhibit a frequency of IFN-gamma-producing cells comparable to that of intact wild-type mice, early IFN-gamma production by innate immune cells does not depend on the presence of T or B cells.

Welcome to the neighborhood: epithelial cell-derived cytokines license innate and adaptive immune responses at mucosal sites.

There is compelling evidence that epithelial cells (ECs) at mucosal surfaces, beyond their role in creating a physical barrier, are integral components of innate and adaptive immunity. The capacity of these cells to license the functions of specific immune cell populations in the airway and gastrointestinal tract offers the prospect of novel therapeutic strategies to target multiple inflammatory diseases in which barrier immunity is dysregulated. In this review, we discuss the critical functions of EC-derived thymic stromal lymphopoietin (TSLP), interleukin-25 (IL-25), and IL-33 in the development and regulation of T-helper 2 (Th2) cytokine-dependent immune responses. We first highlight recent data that have provided new insights into the factors that control expression of this triad of cytokines and their receptors. In addition, we review their proinflammatory and immunoregulatory functions in models of mucosal infection and inflammation. Lastly, we discuss new findings indicating that despite their diverse structural features and differential expression of their receptors, TSLP, IL-25, and IL-33 cross-regulate one another and share overlapping properties that influence Th2 cytokine-dependent responses at mucosal sites.

Epithelial-cell-intrinsic IKK-beta expression regulates intestinal immune homeostasis.

Intestinal epithelial cells (IECs) provide a primary physical barrier against commensal and pathogenic microorganisms in the gastrointestinal (GI) tract, but the influence of IECs on the development and regulation of immunity to infection is unknown. Here we show that IEC-intrinsic IkappaB kinase (IKK)-beta-dependent gene expression is a critical regulator of responses of dendritic cells and CD4+ T cells in the GI tract. Mice with an IEC-specific deletion of IKK-beta show a reduced expression of the epithelial-cell-restricted cytokine thymic stromal lymphopoietin in the intestine and, after infection with the gut-dwelling parasite Trichuris, fail to develop a pathogen-specific CD4+ T helper type 2 (T(H)2) response and are unable to eradicate infection. Further, these animals show exacerbated production of dendritic-cell-derived interleukin-12/23p40 and tumour necrosis factor-alpha, increased levels of CD4+ T-cell-derived interferon-gamma and interleukin-17, and develop severe intestinal inflammation. Blockade of proinflammatory cytokines during Trichuris infection ablates the requirement for IKK-beta in IECs to promote CD4+ T(H)2 cell-dependent immunity, identifying an essential function for IECs in tissue-specific conditioning of dendritic cells and limiting type 1 cytokine production in the GI tract. These results indicate that the balance of IKK-beta-dependent gene expression in the intestinal epithelium is crucial in intestinal immune homeostasis by promoting mucosal immunity and limiting chronic inflammation.