Extrathymic derivation of gut lymphocytes in parabiotic mice

Intraepithelial Lymphocytes of the Intestine

The intestinal epithelium, which segregates the highly stimulatory lumen from the underlying tissue, harbors one of the largest lymphocyte populations in the body, intestinal intraepithelial lymphocytes (IELs). IELs must balance tolerance, resistance, and tissue protection to maintain epithelial homeostasis and barrier integrity. This review discusses the ontogeny, environmental imprinting, T cell receptor (TCR) repertoire, and function of intestinal IELs. Despite distinct developmental pathways, IEL subsets share core traits including an epithelium-adapted profile, innate-like properties, cytotoxic potential, and limited TCR diversity. IELs also receive important developmental and functional cues through interactions with epithelial cells, microbiota, and dietary components. The restricted TCR diversity of IELs suggests that a limited set of intestinal antigens drives IEL responses, with potential functional consequences. Finally, IELs play a key role in promoting homeostatic immunity and epithelial barrier integrity but can become pathogenic upon dysregulation. Therefore, IELs represent intriguing but underexamined therapeutic targets for inflammatory diseases and cancer.

Multimodal profiling reveals site-specific adaptation and tissue residency hallmarks of γδ T cells across organs in mice

γδ T cells perform heterogeneous functions in homeostasis and disease across tissues. However, it is unclear whether these roles correspond to distinct γδ subsets or to a homogeneous population of cells exerting context-dependent functions. Here, by cross-organ multimodal single-cell profiling, we reveal that various mouse tissues harbor unique site-adapted γδ subsets. Epidermal and intestinal intraepithelial γδ T cells are transcriptionally homogeneous and exhibit epigenetic hallmarks of functional diversity. Through parabiosis experiments, we uncovered cellular states associated with cytotoxicity, innate-like rapid interferon-γ production and tissue repair functions displaying tissue residency hallmarks. Notably, our observations add nuance to the link between interleukin-17-producing γδ T cells and tissue residency. Moreover, transcriptional programs associated with tissue-resident γδ T cells are analogous to those of CD8+ tissue-resident memory T cells. Altogether, this study provides a multimodal landscape of tissue-adapted γδ T cells, revealing heterogeneity, lineage relationships and their tissue residency program.

Gut γδ T cells as guardians, disruptors, and instigators of cancer

Colorectal cancer is the third most common cancer worldwide with nearly 2 million cases per year. Immune cells and inflammation are a critical component of colorectal cancer progression, and they are used as reliable prognostic indicators of patient outcome. With the growing appreciation for immunology in colorectal cancer, interest is growing on the role γδ T cells have to play, as they represent one of the most prominent immune cell populations in gut tissue. This group of cells consists of both resident populations-γδ intraepithelial lymphocytes (γδ IELs)-and transient populations that each has unique functions. The homeostatic role of these γδ T cell subsets is to maintain barrier integrity and prevent microorganisms from breaching the mucosal layer, which is accomplished through crosstalk with enterocytes and other immune cells. Recent years have seen a surge in discoveries regarding the regulation of γδ IELs in the intestine and the colon with particular new insights into the butyrophilin family. In this review, we discuss the development, specialities, and functions of γδ T cell subsets during cancer progression. We discuss how these cells may be used to predict patient outcome, as well as how to exploit their behavior for cancer immunotherapy.

The Role of Tissue-resident T Cells in Stress Surveillance and Tissue Maintenance

While forming a minor population in the blood and lymphoid compartments, T cells are significantly enriched within barrier tissues. In addition to providing protection against infection, these tissue-resident T cells play critical roles in tissue homeostasis and repair. T cells in the epidermis and intestinal epithelium produce growth factors and cytokines that are important for the normal turnover and maintenance of surrounding epithelial cells and are additionally required for the efficient recognition of, and response to, tissue damage. A role for tissue-resident T cells is emerging outside of the traditional barrier tissues as well, with recent research indicating that adipose tissue-resident T cells are required for the normal maintenance and function of the adipose tissue compartment. Here we review the functions of tissue-resident T cells in the epidermis, intestinal epithelium, and adipose tissue, and compare the mechanisms of their activation between these sites.

Chronic Inflammation Permanently Reshapes Tissue-Resident Immunity in Celiac Disease

Tissue-resident lymphocytes play a key role in immune surveillance, but it remains unclear how these inherently stable cell populations respond to chronic inflammation. In the setting of celiac disease (CeD), where exposure to dietary antigen can be controlled, gluten-induced inflammation triggered a profound depletion of naturally occurring Vγ4⁺/Vδ1⁺ intraepithelial lymphocytes (IELs) with innate cytolytic properties and specificity for the butyrophilin-like (BTNL) molecules BTNL3/BTNL8. Creation of a new niche with reduced expression of BTNL8 and loss of Vγ4⁺/Vδ1⁺ IELs was accompanied by the expansion of gluten-sensitive, interferon-γ-producing Vδ1⁺ IELs bearing T cell receptors (TCRs) with a shared non-germline-encoded motif that failed to recognize BTNL3/BTNL8. Exclusion of dietary gluten restored BTNL8 expression but was insufficient to reconstitute the physiological Vγ4⁺/Vδ1⁺ subset among TCRγδ⁺ IELs. Collectively, these data show that chronic inflammation permanently reconfigures the tissue-resident TCRγδ⁺ IEL compartment in CeD. VIDEO ABSTRACT.

Multiple receptors converge on H2-Q10 to regulate NK and γδT-cell development

Class Ib major histocompatibility complex (MHC) is an extended family of molecules, which demonstrate tissue-specific expression and presentation of monomorphic antigens. These characteristics tend to imbue class Ib MHC with unique functions. H2-Q10 is potentially one such molecule that is overexpressed in the liver but its immunological function is not known. We have previously shown that H2-Q10 is a ligand for the natural killer cell receptor Ly49C and now, using H2-Q10-deficient mice, we demonstrate that H2-Q10 can also stabilize the expression of Qa-1b. In the absence of H2-Q10, the development and maturation of conventional hepatic natural killer cells is disrupted. We also provide evidence that H2-Q10 is a new high affinity ligand for CD8αα and controls the development of liver-resident CD8αα γδT cells. These data demonstrate that H2-Q10 has multiple roles in the development of immune subsets and identify an overlap of recognition within the class Ib MHC that is likely to be relevant to the regulation of immunity.

Vedolizumab is associated with changes in innate rather than adaptive immunity in patients with inflammatory bowel disease

OBJECTIVE

Vedolizumab, a monoclonal antibody directed against the integrin heterodimer α4β7, is approved for the treatment of Crohn's disease and ulcerative colitis. The efficacy of vedolizumab has been suggested to result from inhibition of intestinal T cell trafficking although human data to support this conclusion are scarce. We therefore performed a comprehensive analysis of vedolizumab-induced alterations in mucosal and systemic immunity in patients with inflammatory bowel disease (IBD), using anti-inflammatory therapy with the TNFα antibody infliximab as control.

DESIGN

Immunophenotyping, immunohistochemistry, T cell receptor profiling and RNA sequencing were performed using blood and colonic biopsies from patients with IBD before and during treatment with vedolizumab (n=18) or, as control, the anti-TNFα antibody infliximab (n=20). Leucocyte trafficking in vivo was assessed using single photon emission computed tomography and endomicroscopy.

RESULTS

Vedolizumab was not associated with alterations in the abundance or phenotype of lamina propria T cells and did not affect the mucosal T cell repertoire or leucocyte trafficking in vivo. Surprisingly, however, α4β7 antibody treatment was associated with substantial effects on innate immunity including changes in macrophage populations and pronounced alterations in the expression of molecules involved in microbial sensing, chemoattraction and regulation of the innate effector response. These effects were specific to vedolizumab, not observed in response to the TNFα antibody infliximab, and associated with inhibition of intestinal inflammation.

CONCLUSION

Our findings suggest that modulation of innate immunity contributes to the therapeutic efficacy of vedolizumab in IBD.

TRIAL REGISTRATION NUMBER

NCT02694588.

Human intraepithelial lymphocytes

The location of intraepithelial lymphocytes (IEL) between epithelial cells, their effector memory, cytolytic and inflammatory phenotype positions them to kill infected epithelial cells and protect the intestine against pathogens. Human TCRαβ+CD8αβ+ IEL have the dual capacity to recognize modified self via natural killer (NK) receptors (autoreactivity) as well as foreign antigen via the T cell receptor (TCR), which is accomplished in mouse by two cell subsets, the naturally occurring TCRαβ+CD8αα+ and adaptively induced TCRαβ+CD8αβ+ IEL subsets, respectively. The private/oligoclonal nature of the TCR repertoire of both human and mouse IEL suggests local environmental factors dictate the specificity of IEL responses. The line between sensing of foreign antigens and autoreactivity is blurred for IEL in celiac disease, where recognition of stress ligands by induced activating NK receptors in conjunction with inflammatory signals such as IL-15 can result in low-affinity TCR/non-cognate antigen and NK receptor/stress ligand interactions triggering destruction of intestinal epithelial cells.

The Innate Immune System in the Gastrointestinal Tract: Role of Intraepithelial Lymphocytes and Lamina Propria Innate Lymphoid Cells in Intestinal Inflammation

BACKGROUND

The gastrointestinal tract harbors the largest microbiota load in the human body, hence maintaining a delicate balance between immunity against invading pathogens and tolerance toward commensal. Such immune equilibrium, or intestinal homeostasis, is conducted by a tight regulation and cooperation of the different branches of the immune system, including the innate and the adaptive immune system. However, several factors affect this delicate equilibrium, ultimately leading to gastrointestinal disorders including inflammatory bowel disease. Therefore, here we decided to review the currently available information about innate immunity lymphocyte subsets playing a role in intestinal inflammation.

RESULTS

Intestinal innate lymphocytes are composed of intraepithelial lymphocytes (IELs) and lamina propria innate lymphoid cells (ILCs). While IELs can be divided into natural or induced, ILCs can be classified into type 1, 2, or 3, resembling, respectively, the properties of TH1, TH2, or TH17 adaptive lymphocytes. Noteworthy, the phenotype and function of both IELs and ILCs are disrupted under inflammatory conditions, where they help to exacerbate intestinal immune responses.

CONCLUSIONS

The modulation of both IELs and ILCs to control intestinal inflammatory responses represents a major challenge, as they provide tight regulation among the epithelium, the microbiota, and the adaptive immune system. An improved understanding of the innate immunity mechanisms involved in gastrointestinal inflammation would therefore aid in the diagnosis and further treatment of gastrointestinal inflammatory disorders.

Human γδ T-Cell Control of Mucosal Immunity and Inflammation

Human γδ T-cells include some of the most common "antigen-specific" cell types in peripheral blood and are enriched yet further at mucosal barrier sites where microbial infection and tumors often originate. While the γδ T-cell compartment includes multiple subsets with highly flexible effector functions, human mucosal tissues are dominated by host stress-responsive Vδ1+ T-cells and microbe-responsive Vδ2+ T-cells. Widely recognized for their potent cytotoxicity, emerging data suggest that γδ T-cells also exert strong influences on downstream adaptive immunity to pathogens and tumors, in particular via activation of antigen-presenting cells and/or direct stimulation of other mucosal leukocytes. These unique functional attributes and lack of MHC restriction have prompted considerable interest in therapeutic targeting of γδ T-cells. Indeed, several drugs already in clinical use, including vedolizumab, infliximab, and azathioprine, likely owe their efficacy in part to modulation of γδ T-cell function. Recent clinical trials of Vδ2+ T-cell-selective treatments indicate a good safety profile in human patients, and efficacy is set to increase as more potent/targeted drugs continue to be developed. Key advances will include identifying methods of directing γδ T-cell recruitment to specific tissues to enhance host protection against invading pathogens, or alternatively, retaining these cells in the circulation to limit peripheral inflammation and/or improve responses to blood malignancies. Human γδ T-cell control of mucosal immunity is likely exerted via multiple mechanisms that induce diverse responses in other types of tissue-resident leukocytes. Understanding the microenvironmental signals that regulate these functions will be critical to the development of new γδ T-cell-based therapies.

Sentinels at the frontline: the role of intraepithelial lymphocytes in inflammatory bowel disease

Purpose of review

Intestinal mucosal immunity is tightly regulated to ensure effective host defense against invasive microorganisms while limiting the potential for aberrant damage. In inflammatory bowel disease (IBD), an imbalance between effector and regulatory T cell populations results in an uncontrolled inflammatory response to commensal bacteria. Intraepithelial lymphocytes (IEL) are perfectly positioned within the intestinal epithelium to provide the first line of mucosal defense against luminal microbes or rapidly respond to epithelial injury. This review will highlight how IELs promote protective intestinal immunity and discuss the evidence indicating that altered IEL responses contribute to the pathogenesis of IBD.

Recent findings

Although the role of IELs in mucosal homeostasis has been largely underappreciated, many of the same factors that contribute to the dysregulation of host defense in IBD also adversely affect IELs. For example, IL-23 and the endoplasmic reticulum stress response can enhance IEL lytic activity toward enterocytes. Microbial dysbiosis or defective microbial recognition results in the loss of regulatory IELs, further amplifying these pro-inflammatory effects. Migration of T cells into or within the intraepithelial compartment has a profound effect on their differentiation or effector function demonstrating that IELs are exquisitely sensitive to changes in the local intestinal microenvironment.

Summary

Enhanced mechanistic insight into the regulation of IEL survival, differentiation and effector function may provide useful tools to modulate IEL surveillance or enhance IEL regulatory function. Elucidation of these processes may result in the development of novel therapeutics to reduce intestinal inflammation and reinforce the mucosal barrier in IBD.

Hallmarks of Tissue-Resident Lymphocytes

Although they are classically viewed as continuously recirculating through the lymphoid organs and blood, lymphocytes also establish residency in non-lymphoid tissues, most prominently at barrier sites, including the mucosal surfaces and skin. These specialized tissue-resident lymphocyte subsets span the innate-adaptive continuum and include innate lymphoid cells (ILCs), unconventional T cells (e.g., NKT, MAIT, γδ T cells, and CD8αα(+) IELs), and tissue-resident memory T (T(RM)) cells. Although these diverse cell types differ in the particulars of their biology, they nonetheless exhibit important shared features, including a role in the preservation of tissue integrity and function during homeostasis, infection, and non-infectious perturbations. In this Review, we discuss the hallmarks of tissue-resident innate, innate-like, and adaptive lymphocytes, as well as their potential functions in non-lymphoid organs.

Gamma/delta intraepithelial lymphocytes in the mouse small intestine

Although many studies of intraepithelial lymphocytes (IELs) have been reported, most of them have focused on αβ-IELs; little attention has been paid to γδ-IELs. The function of γδ-IELs remains largely unclear. In this article, we briefly review a number of reports on γδ-IELs, especially those in the small intestine, along with our recent studies. We found that γδ-IELs are the most abundant (comprising >70 % of the) IELs in the duodenum and the jejunum, implying that it is absolutely necessary to investigate the function(s) of γδ-IELs when attempting to delineate the in vivo defense system of the small intestine. Intraperitoneal injection of anti-CD3 mAb stimulated the γδ-IELs and caused rapid degranulation of them. Granzyme B released from their granules induced DNA fragmentation of duodenal and jejunal epithelial cells (paracrine) and of the IELs themselves (autocrine). However, perforin (Pfn) was not detected, and DNA fragmentation was induced even in Pfn-knockout mice; our system was therefore found to present a novel type of in vivo Pfn-independent DNA fragmentation. We can therefore consider γδ-IELs to be a novel type of large granular lymphocyte without Pfn. Fragmented DNA was repaired in the cells, indicating that DNA fragmentation alone cannot be regarded as an unambiguous marker of cell death or apoptosis. Finally, since the response was so rapid and achieved without the need for accessory cells, it seems that γδ-IELs respond readily to various stimuli, are activated only once, and die 2-3 days after activation in situ without leaving their site. Taken together, these results suggest that γδ-IELs are not involved in the recognition of specific antigen(s) and are not involved in the resulting specific killing or exclusion of the relevant antigen(s).

Molecular evidence for a thymus-independent partial T cell development in a FOXN1-/- athymic human fetus

The thymus is the primary organ able to support T cell ontogeny, abrogated in FOXN1(-/-) human athymia. Although evidence indicates that in animal models T lymphocytes may differentiate at extrathymic sites, whether this process is really thymus-independent has still to be clarified. In an athymic FOXN1(-/-) fetus, in which we previously described a total blockage of CD4(+) and partial blockage of CD8(+) cell development, we investigated whether intestine could play a role as extrathymic site of T-lymphopoiesis in humans. We document the presence of few extrathymically developed T lymphocytes and the presence in the intestine of CD3(+) and CD8(+), but not of CD4(+) cells, a few of them exhibiting a CD45RA(+) naïve phenotype. The expression of CD3εεpTα, RAG1 and RAG2 transcripts in the intestine and TCR gene rearrangement was also documented, thus indicating that in humans the partial T cell ontogeny occurring at extrathymic sites is a thymus- and FOXN1-independent process.

Biology of autoreactive extrathymic T cells and B-1 cells of the innate immune system

Cumulative evidence has shown that extrathymic T cells can be autoreactive and that B-1 cells may produce autoantibodies. These T and B-1 cells, which form part of the innate immune system, tend to be activated simultaneously when conventional T and B cells are in a suppressive state, for example, when thymic atrophy occurs by stress or involution with aging. In other words, autoreactive T cells and autoantibody-producing B cells are different from thymus-derived T cells and bone marrow-derived B cells. Activated extrathymic T cells and B-1 cells are often observed in numerous autoimmune diseases, aging, malarial infection and chronic graft-versus-host disease. It is thought that the autoreactivity of extrathymic T cells and B-1 cells may be important for the elimination of "abnormal self" tissues or cells. However, over-activation of innate lymphocytes may be related to the onset of disease or self-tissue destruction. However, it must be emphasized that the autoreactivity of innate lymphocytes is not generated by failure of the thymic pathway of T-cell differentiation or the conventional pathway of B-2 cells.

The light and dark sides of intestinal intraepithelial lymphocytes

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine form one of the main branches of the immune system. As IELs are located at this critical interface between the core of the body and the outside environment, they must balance protective immunity with an ability to safeguard the integrity of the epithelial barrier: failure to do so would compromise homeostasis of the organism. In this Review, we address how the unique development and functions of intestinal IELs allow them to achieve this balance.

The role of the gut as a primary lymphoid organ: CD8αα intraepithelial T lymphocytes in euthymic mice derive from very immature CD44+ thymocyte precursors

Intestinal CD8αα intraepithelial T lymphocytes (T-IELs) have a key role in mucosal immunity and, unlike other T cells, were proposed to differentiate locally. In apparent contradiction, these cells were also shown to originate from a wave of thymus migrants colonizing the gut in the first 3 weeks after birth. We here identify previously uncharacterized very immature CD4(-)CD8(-)CD3(-)CD44(+)CD25(int) thymocytes, which have not yet rearranged their T-cell antigen receptor (TCR), as having the capacity to leave the thymus, migrate to the blood, colonize the gut, and reconstitute CD8αα T-IEL, and show that this cell set is fully responsible for the generation of the CD8αα T-IEL pool. Thus, although the thymus may be fundamental for efficient T-cell commitment, CD8αα T-IEL' complete TCR rearrangements and TCR-αβ/γδ lineage commitment must occur in the gut. These results demonstrate a major role of the gut environment as a primary lymphoid organ.

The extrathymic T-cell differentiation in the murine gut

The gut epithelial border is in continuous contact with exogenous antigens and harbors a distinctive and very abundant CD8 alpha alpha intraepithelial T-lymphocyte effector population. We describe here the characteristics of these cells that distinguish them from all other T-cell types in the body as well as their functions in local protection. We also describe how these cells differentiate from local precursors present in the gut cryptopatches (CPs) following a pathway of T-cell differentiation unique to the gut wall. Finally, we describe the origin of the precursors of CD8 alpha alpha T cells, which come from the bone marrow in athymic mice but are first imprinted in the thymus in euthymic mice. Indeed, CD3(-)CD4(-)CD8(-) T-cell-committed precursors can leave the thymus before T-cell receptor rearrangements and then colonize the gut CPs, proceeding with their differentiation within the gut wall.

Isolation of mesenchymal stem cells from human vermiform appendix

BACKGROUND

Recent findings have shown that pluripotent stem cells exist in areas outside the bone marrow (BM). Moreover, it has been demonstrated that the appendix is important for the development of mucosal gut immunity, and hematopoietic progenitors have been isolated from animal and human appendices.

MATERIALS AND METHODS

Non-inflamed appendices removed during laparotomy were processed and cultured until the appearance of adherent cells. Differentiations (performed under osteogenic, adipogenic, and myogenic conditions) were confirmed by immunohistochemistry and cytochemistry. Polymerase chain reaction and cytofluorimetric analyses were performed to evidence the presence of genes and protein specific lineages in appendix-derived mesenchymal stem cells (ADMCs).

RESULTS

ADMCs were present in non-inflamed appendices. ADMCs under osteogenic conditions differentiated in osteoblasts and showed increased alkaline phosphatase expression; at the gene level, we observed the expression of Core binding factor alpha 1 (Cbfa1) and osteocalcin in osteogenic induced ADMCs. Under adipogenic conditions, lipidic drops in the cytoplasm, expression of lipoprotein lipase (LpL), and peroxisome proliferator-activated receptor gamma were observed; under myogenic conditions, myotubes expressing muscle specific proteins like desmin were formed. Myogenic regulatory factor 4 and MyoD were selectively induced in the ADMCs under myogenic conditions.

CONCLUSIONS

This study shows for the first time that mesenchymal stem cells can be isolated from normal appendices obtained from a pediatric and adult age group (0-18 years of age). This finding not only may further knowledge of the maturation of the intestinal immunesystem but also could indicate a new physiological role of the human vermiform appendix.

Reasons why DBA/2 mice are resistant to malarial infection: expansion of CD3int B220+ gammadelta T cells with double-negative CD4- CD8- phenotype in the liver

DBA/2 (H-2(d)) mice are known to be more resistant than C57BL/6 (B6, H-2(b)) mice to the non-lethal 17XNL strain of Plasmodium yoelii. This is a very strange phenomenon because the functions of conventional T cells, especially CD8(+) T cells, are known to be somewhat lower in DBA/2 mice than in other strains of mice. We examined herein how immune responses differed between DBA/2 mice and B6 mice during malarial infection. DBA/2 mice and (DBA/2 x B6)F(1) (BDF(1), H-2(b/d)) mice were found to have milder parasitaemia and to recover more quickly from malarial infection than B6 mice. These DBA/2 and BDF(1) mice were also found to experience a marked expansion of interleukin (IL)-2Rbeta(+) CD3(int) cells and gammadelta T cells in the liver, especially in the recovery phase. The expansion of unconventional T cells (i.e. B220(+) T cells) was also marked in DBA/2 and BDF(1) mice. The majority of B220(+) T cells were gammadelta T cells and these T cells were double-negative CD4(-) CD8(-). More importantly, the production of immunoglobulin M (IgM)-type anti-DNA autoantibody was also higher in DBA/2 and BDF(1) mice than in B6 mice. In conjunction with data on cytokine production, these results indicate that primitive T and B cells, namely autoreactive extrathymic T cells and autoantibody-producing B cells, may be much more activated in DBA/2 mice and therefore resistant to the non-lethal 17XNL strain of P. yoelii.

Liver immunobiology

Tle liver has a number of important functions in innate and adaptive immunity. Contributions to the innate (nonspecific) immune system include production of acute phase proteins, nonspecific phagocytosis of particles, nonspecific pinocytosis of molecules, and nonspecific cell killing. Hepatic involvement in innate immunity contributes to the systemic response to local inflammation, clearance of particles and soluble molecules from the circulation, and killing of invading cells such as neoplastic cells. Liver involvement in the adaptive (specific) immune system includes deletion of activated T cells, induction of tolerance to ingested and self-antigens, extrathymic proliferation of T cells, and deletion of many of the signaling and effector molecules. Hepatic involvement in adaptive immunity allows clearance of activated T cells and signaling molecules following inflammatory reactions, and promotes immunologic tolerance toward potentially antigenic proteins that are absorbed from the intestinal tract. The liver is a major site of extrathymic T cell development, which assumes increasing significance with aging in mammals. Perturbations in hepatic structure or function can result in significant ramifications in both the innate and adaptive immune systems.