Tuft cells mediate commensal remodeling of the small intestinal antimicrobial landscape

Succinate produced by the commensal protist Tritrichomonas musculis (T. mu) stimulates chemosensory tuft cells, resulting in intestinal type 2 immunity. Tuft cells express the succinate receptor SUCNR1, yet this receptor does not mediate antihelminth immunity nor alter protist colonization. Here, we report that microbial-derived succinate increases Paneth cell numbers and profoundly alters the antimicrobial peptide (AMP) landscape in the small intestine. Succinate was sufficient to drive this epithelial remodeling, but not in mice lacking tuft cell chemosensory components required to detect this metabolite. Tuft cells respond to succinate by stimulating type 2 immunity, leading to interleukin-13-mediated epithelial and AMP expression changes. Moreover, type 2 immunity decreases the total number of mucosa-associated bacteria and alters the small intestinal microbiota composition. Finally, tuft cells can detect short-term bacterial dysbiosis that leads to a spike in luminal succinate levels and modulate AMP production in response. These findings demonstrate that a single metabolite produced by commensals can markedly shift the intestinal AMP profile and suggest that tuft cells utilize SUCNR1 and succinate sensing to modulate bacterial homeostasis.

Short-term antibiotic treatment immediately after weaning prevents spontaneous ileocolitis

A spontaneous model of ileocolitis, a condition found in a majority of cases of Crohn’s Disease, allows for the study of processes that promote homeostasis within the ileum. Our system uses a “Bigenic” approach that combines TCR transgene mice with a transgenic line expressing the cognate antigen in ileal crypts. Bigenic mice exhibit decreased nTreg production that predisposes them to developing a chronic ileocolitic inflammatory disease in the weeks after weaning. Half of Bigenic mice accumulate ileal-reactive Treg cells and fail to develop colitis. In contrast, Ifng−/− Bigenic mice are unable to establish this homeostatic state, resulting in a fully penetrant and acute colitis. We aimed to identify time sensitive interventions capable of preventing colitis and restoring mucosal homeostasis in Ifng−/− Bigenic mice. Mice were started at weaning on a prophylactic treatment with ad libidum non-absorbable antibiotics in their drinking water and continued for three weeks. Alternately, mice were allowed to naturally progress into a disease state at which time the same antibiotic treatments were initiated and continued for nine weeks. We found that antibiotics prevented ileocolitic disease when initiated at weaning but were insufficient to reverse established disease measured by weight loss. Interestingly, this resistance to ileocolitis mediated by short-term early use of antibiotics was maintained, as these mice did not subsequently manifest either ileocolitis or the degree of breakthrough weight loss observed in antibiotic-rescued mice. Our findings suggest that in our model of ilieocolitis, there is a critical window of time after weaning where manipulation of the microbiome alone can facilitate resistance to spontaneous disease.

A model of TH17-associated ileal hyperplasia that requires both IL-17A and IFNγ to generate self-tolerance and prevent colitis

Homeostasis in the ileum, which is commonly disrupted in patients with Crohn's disease, involves ongoing immune responses. To study how homeostatic processes of the ileum impact CD4⁺T cell responses, we used TCR transgenic tools to breed mice that spontaneously produced CD4⁺T cells reactive to an antigen expressed in the ileum. At an early age, the ilea of these mice exhibit crypt hyperplasia and accumulate increased numbers of T_H17 cells bearing non-transgenic clonotypes. Half of these mice subsequently developed colitis linked to broad mucosal infiltration by T_H17 and T_H1 cells expressing non-transgenic clonotypes, chronic wasting disease and loss of ileal crypt hyperplasia. By contrast, adult mice with normal growth continued to exhibit T_H17-associated ileal crypt hyperplasia and additionally accumulated ileal-reactive Treg cells. Both IL-17A and IFNγ were protective, as their deficiency precluded ileal-reactive Treg accumulation and exacerbated colitic disease. IL-23R blockade prevented progression to colitis, whereas nTreg cell transfers prevented colitic disease, ileal crypt hyperplasia and ileal-reactive Treg accumulation. Thus, our studies identify an IL-17A and IFNγ-dependent homeostatic process that mobilizes ileal-reactive Treg cells and is disrupted by IL-23.

In TH17-associated ileal inflammation, IFNγ and IL-17A support mucosal homeostasis and modulate self-tolerance

Crohn’s Disease is characterized by dysregulation of the immune response in the ileum. To study the perturbation of homeostasis within the ileum we utilize a “Bigenic” mouse model combining a TCR transgene and matching transgenic target antigen restricted to ileal crypt expression. Juvenile Bigenic mice experience TH17-associated ileitis and ileal hyperplasia. Antigen transcript is demonstrated to be localized exclusively in ileal crypts, as visualized with in situ hybridization of mRNA. Roughly 50% of Bigenic mice mature and acquire symptomatic phenotype (BigenicS) including colitis and suboptimal growth, while losing ileal hyperplasia. The remaining non-symptomatic Bigenic mice (BigenicNS) retain ileal hyperplasia and successfully build a supply of TH17 and ileal-reactive Treg cells.

Previous studies in our lab indicate that IL-17A and INFγ are protective in this Bigenic model – their depletion by antibody binding accelerates disease and hinders accumulation of ileal-reactive Tregs. Furthermore, INFγKO Bigenic mice demonstrate almost complete penetrance of the symptomatic phenotype, with near 100% fatality by 50 days of life. Manipulation of microbiota in juvenile INFγKO Bigenic mice can protect them from this dysregulated state. Administration of a streptomycin/bacitracin antibiotic cocktail ad libitum in drinking water upon initial manifestation of weight loss symptoms or initiated preemptively upon weaning rescues and prevents, respectively, the weight loss phenotype and colitis.

TGF-β1 produced by alternatively activated macrophages synchronizes a tolerogenic iTreg-Th17 cell axis

Induced regulatory T (iTreg) cells and T helper 17 (Th17) cells develop at mucosal interfaces. While iTreg cells are required for the maintenance of mucosal tolerance, Th17 cells can play divergent roles. Both iTreg and Th17 cells require TGF-β1 for their differentiation. Importantly, the primary source of TGF-β1 in vivo is unknown. Here, we investigate the mechanisms that control iTreg and Th17 cell development. We hypothesize that alternatively activated M2a macrophages support mucosal tolerance through TGF-β1 mediated expansion of both iTreg and Th17 cells.

We pretreat Rag1­­−/− C57BL/6 mice prior to colitis induction or treat mice with established disease with polarized macrophages. In some experiments the macrophages lacked the capacity to produce TGF-β1. Weight loss, colitis disease scores, and FACS analysis of mesenteric lymph nodes and gut infiltrates were used to evaluate the impact of macrophage therapy on the expansion and function of the iTreg-Th17 cell axis.

Pre-conditioning mice with M2a but not M1 macrophages resulted in a 16-fold increase in the number of iTreg cells and a 8-fold increase in the number of Th17 cells when compared to control mice. This expansion required TGF-β1 produced by both the transferred M2a cells and by host myeloid cells. Established colitis was successfully treated by concomitant M2a and nTreg cell transfers, but not by nTreg cell transfers alone.

In conclusion, M2a macrophages that produce TGF-β1 regulate development of a tolerogenic iTreg-Th17 cell axis. The change in iTreg and Th17 cells numbers linked to M2a macrophage polarization is directionally concordant and dependent on T cell produced IL17A, confirming that both iTreg and Th17 cells are required to mediate intestinal homeostasis.

Alternatively Activated Macrophages Boost Induced Regulatory T and Th17 Cell Responses during Immunotherapy for Colitis

Induced regulatory T (iTreg) and Th17 cells promote mucosal homeostasis. We used a T cell transfer model of colitis to compare the capacity of iTreg and Th17 cells to develop in situ following the transfer of naive CD4(+)CD45RB(hi)T cells intoRag1(-/-)C57BL/6 or BALB/c mice, the prototypical Th1/M1- and Th2/M2-prone strains. We found that the frequency and number of Foxp3(+)iTreg cells and Th17 cells were significantly reduced in C57BL/6 mice compared with the BALB/c strain. C57BL/6 mice with colitis were also resistant to natural Treg cell immunotherapy. Pretreatment of C57BL/6Rag1(-/-)mice with IL-4 plus IL-13, or with M2a but not M1 macrophages, dramatically increased the generation of iTreg and Th17 cells. Importantly, M2a transfers, either as a pretreatment or in mice with established colitis, allowed successful immunotherapy with natural Treg cells. M2a macrophages also reduced the generation of pathogenic iTreg cells that lost Foxp3 expression, suggesting that they stabilize the expression of Foxp3. Thus, polarized M2a macrophages drive a directionally concordant expansion of the iTreg-Th17 cell axis and can be exploited as a therapeutic adjuvant in cell-transfer immunotherapy to re-establish mucosal tolerance.

Chronic follicular bronchiolitis requires antigen-specific regulatory T cell control to prevent fatal disease progression

To study regulatory T (Treg) cell control of chronic autoimmunity in a lymphoreplete host, we created and characterized a new model of autoimmune lung inflammation that targets the medium and small airways. We generated transgenic mice that express a chimeric membrane protein consisting of hen egg lysozyme and a hemoglobin epitope tag under the control of the Clara cell secretory protein promoter, which largely limited transgene expression to the respiratory bronchioles. When Clara cell secretory protein-membrane hen egg lysozyme/hemoglobin transgenic mice were crossed to N3.L2 TCR transgenic mice that recognize the hemoglobin epitope, the bigenic progeny developed dense, pseudo-follicular lymphocytic peribronchiolar infiltrates that resembled the histological pattern of follicular bronchiolitis. Aggregates of activated IFN-γ- and IL-17A-secreting CD4(+) T cells as well as B cells surrounded the airways. Lung pathology was similar in Ifng(-/-) and Il17a(-/-) mice, indicating that either cytokine is sufficient to establish chronic disease. A large number of Ag-specific Treg cells accumulated in the lesions, and Treg cell depletion in the affected mice led to an interstitial spread of the disease that ultimately proved fatal. Thus, Treg cells act to restrain autoimmune responses, resulting in an organized and controlled chronic pathological process rather than a progressive disease.

IL-10 produced by induced regulatory T cells (iTregs) controls colitis and pathogenic ex-iTregs during immunotherapy

"Natural" regulatory T cells (nTregs) that express the transcription factor Foxp3 and produce IL-10 are required for systemic immunological tolerance. "Induced" regulatory T cells (iTregs) are nonredundant and essential for tolerance at mucosal surfaces, yet their mechanisms of suppression and stability are unknown. We investigated the role of iTreg-produced IL-10 and iTreg fate in a treatment model of inflammatory bowel disease. Colitis was induced in Rag1(-/-) mice by the adoptive transfer of naive CD4(+) T cells carrying a nonfunctional Foxp3 allele. At the onset of weight loss, mice were treated with both iTregs and nTregs where one marked subset was selectively IL-10 deficient. Body weight assessment, histological scoring, cytokine analysis, and flow cytometry were used to monitor disease activity. Transcriptional profiling and TCR repertoire analysis were used to track cell fate. When nTregs were present but IL-10 deficient, iTreg-produced IL-10 was necessary and sufficient for the treatment of disease, and vice versa. Invariably, ∼85% of the transferred iTregs lost Foxp3 expression (ex-iTregs) but retained a portion of the iTreg transcriptome, which failed to limit their pathogenic potential upon retransfer. TCR repertoire analysis revealed no clonal relationships between iTregs and ex-iTregs, either within mice or between mice treated with the same cells. These data identify a dynamic IL-10-dependent functional reciprocity between regulatory T cell subsets that maintains mucosal tolerance. The niche supporting stable iTregs is limited and readily saturated, which promotes a large population of ex-iTregs with pathogenic potential during immunotherapy.

The TCR repertoires of regulatory and conventional T cells specific for the same foreign antigen are distinct

The relationship between the TCR repertoires of natural regulatory T cells (nTregs) and conventional CD4(+) T cells (Tconv) capable of responding to the same antigenic epitope is unknown. In this study, we used TCRβ-chain transgenic mice to generate polyclonal nTreg and Tconv populations specific for a foreign Ag. CD4(+) T cells from immunized 3.L2β(+/-) TCRα(+/-) Foxp3(EGFP) mice were restimulated in culture to yield nTregs (EGFP(+)) and Tconv (EGFP(-)) defined by their antigenic reactivity. Relative to Tconv, nTreg expansion was delayed, although a higher proportion of viable nTregs had divided after 72 h. Spectratype analysis revealed that both the nTreg and Tconv responses were different and characterized by skewed distributions of CDR3 lengths. CDR3 sequences from nTregs displayed a divergent pattern of Jα usage, minimal CDR3 overlap (3.4%), and less diversity than did CDR3 sequences derived from Tconv. These data indicate that foreign Ag-specific nTregs and Tconv are clonally distinct and that foreign Ag-specific nTreg populations are constrained by a limited TCR repertoire.

Interleukin 4 increases the production of indduced regulatory T cells in C57BL/6 mice (120.12)

Naïve peripheral T cells can express the transcription factor Foxp3 and develop regulatory function. These “induced” regulatory T (iTreg) cells are essential for tolerance and complement the function of other regulatory populations. In this study we used the lymphopenia-associated model of colitis to compare the kinetics of iTreg cell production in both C57BL/6 and BALB/c mice, the prototypical Th1- and Th2-type strains. Following the adoptive transfer of naïve CD4+ T cells into Rag-/- recipients, we tracked the development of iTreg cells over a period of 40 days. Whereas iTreg cell production in C57BL/6 was minimal, by 10 days there was a relative five-fold increase in the number of iTreg cells in BALB/c mice that grew to 35 fold by 40 days. This dramatic increase in iTreg cells correlated with a similar increase in IL-17A producing cells in the mesenteric lymph nodes and gut of BALB/c mice. In contrast, the proportion of CD4+ T cells that produced IFN-γ was comparable between the two strains. Pretreatment of C57BL/6 Rag-/- recipient mice with IL-4 resulted in a six-fold increase in the number of iTreg cells compared to untreated mice. Pretreatment with in vitro derived M2 macrophages also resulted in a five to fifteen fold increase in the number of iTreg cells compared to pretreated mice with M0 or M1 macrophages, respectively. These results indicate that a Th2 milieu supports iTreg cell induction.

A requisite role for induced regulatory T cells in tolerance based on expanding antigen receptor diversity

Although both natural and induced regulatory T (nTreg and iTreg) cells can enforce tolerance, the mechanisms underlying their synergistic actions have not been established. We examined the functions of nTreg and iTreg cells by adoptive transfer immunotherapy of newborn Foxp3-deficient mice. As monotherapy, only nTreg cells prevented disease lethality, but did not suppress chronic inflammation and autoimmunity. Provision of Foxp3-sufficient conventional T cells with nTreg cells reconstituted the iTreg pool and established tolerance. In turn, acute depletion of iTreg cells in rescued mice resulted in weight loss and inflammation. Whereas the transcriptional signatures of nTreg and in vivo-derived iTreg cells were closely matched, there was minimal overlap in their T cell receptor (TCR) repertoires. Thus, iTreg cells are an essential nonredundant regulatory subset that supplements nTreg cells, in part by expanding TCR diversity within regulatory responses.

A central role for induced regulatory T cells in tolerance induction in experimental colitis

In addition to thymus-derived or natural T regulatory (nT(reg)) cells, a second subset of induced T regulatory (iT(reg)) cells arises de novo from conventional CD4(+) T cells in the periphery. The function of iT(reg) cells in tolerance was examined in a CD45RB(high)CD4(+) T cell transfer model of colitis. In situ-generated iT(reg) cells were similar to nT(reg) cells in their capacity to suppress T cell proliferation in vitro and their absence in vivo accelerated bowel disease. Treatment with nT(reg) cells resolved the colitis, but only when iT(reg) cells were also present. Although iT(reg) cells required Foxp3 for suppressive activity and phenotypic stability, their gene expression profile was distinct from the established nT(reg) "genetic signature," indicative of developmental and possibly mechanistic differences. These results identified a functional role for iT(reg) cells in vivo and demonstrated that both iT(reg) and nT(reg) cells can act in concert to maintain tolerance.

Affinity-based selection of regulatory T cells occurs independent of agonist-mediated induction of Foxp3 expression

Natural regulatory T (nT(reg)) cells recognize self-peptides with high affinity, yet the understanding of how affinity influences their selection in the thymus is incomplete. We use altered peptide ligands in transgenic mice and in organ culture to create thymic environments spanning a broad range of ligand affinity. We demonstrate that the nT(reg) TCR repertoire is shaped by affinity-based selection, similar to conventional T cells. The effect of each ligand on the two populations is distinct, consistent with early nT(reg) cell lineage specification. Foxp3 expression is an independent process that does not rely on "high affinity" binding per se, but requires a high-potency agonistic interaction for its induction. The timing of ligand exposure, TGFbeta signaling, and the organization of the thymic architecture are also important. The development of nT(reg) cells is therefore a multistep process in which ligand affinity, potency, and timing of presentation all play a role in determining cell fate.

Transcription factor MIZ-1 is regulated via microtubule association

A synthetic drug, T113242, activates low-density lipoprotein receptor (LDLR) transcription in the presence of sterols. T113242 also covalently binds to beta-tubulin and induces microtubule depolymerization. The myc-interacting zinc finger protein (MIZ-1) associates with microtubules, can bind directly to the LDLR promoter, and can activate LDLR transcription. MIZ-1 also binds to the promoter and activates transcription of other T113242-induced genes such as alpha(2) integrin. Soft X-ray, indirect immunofluorescence, and green fluorescent protein time-lapse microscopy reveal that MIZ-1 is largely cytoplasmic but accumulates in the nuclei of HepG2 cells upon treatment with T113242. Thus, MIZ-1 appears to be regulated by association with microtubules and may activate gene transcription in response to changes in the cytoskeleton.