Thymus influences the development of extrathymically derived intestinal intraepithelial lymphocytes

Development and function of natural TCR+ CD8αα+ intraepithelial lymphocytes

The complexity of intestinal homeostasis results from the ability of the intestinal epithelium to absorb nutrients, harbor multiple external and internal antigens, and accommodate diverse immune cells. Intestinal intraepithelial lymphocytes (IELs) are a unique cell population embedded within the intestinal epithelial layer, contributing to the formation of the mucosal epithelial barrier and serving as a first-line defense against microbial invasion. TCRαβ⁺ CD4^- CD8αα⁺ CD8αβ^- and TCRγδ⁺ CD4^- CD8αα⁺ CD8αβ^- IELs are the two predominant subsets of natural IELs. These cells play an essential role in various intestinal diseases, such as infections and inflammatory diseases, and act as immune regulators in the gut. However, their developmental and functional patterns are extremely distinct, and the mechanisms underlying their development and migration to the intestine are not fully understood. One example is that Bcl-2 promotes the survival of thymic precursors of IELs. Mature TCRαβ⁺ CD4^- CD8αα⁺ CD8αβ^- IELs seem to be involved in immune regulation, while TCRγδ⁺ CD4^- CD8αα⁺ CD8αβ^- IELs might be involved in immune surveillance by promoting homeostasis of host microbiota, protecting and restoring the integrity of mucosal epithelium, inhibiting microbiota invasion, and limiting excessive inflammation. In this review, we elucidated and organized effectively the functions and development of these cells to guide future studies in this field. We also discussed key scientific questions that need to be addressed in this area.

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.

CD4CD8αα IELs: They Have Something to Say

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine play a critical role in maintaining the immune balance of the gut. CD4CD8αα IELs are one of the most important types of IELs, and they play an irreplaceable role in maintaining the balance of the intestinal immune system. CD4CD8αα IELs are often regarded as a special subtype of CD4⁺ IELs that can express CD8αα on their cytomembrane. Hence, CD4CD8αα IELs not only have the ability to modulate the functions of immune cells but also are regarded as cytotoxic T lymphocytes (CTLs). Transcription factors, microbes, and dietary factors have a substantial effect on the development of CD4CD8αα IELs, which make them exert immunosuppression and cytotoxicity activities. In addition, there is an intimate relationship between CD4CD8αα IELs and inflammatory bowel disease (IBD), whereas it is still unclear how CD4CD8αα IELs influence IBD. As such, this review will focus on the unparalleled differentiation of CD4CD8αα IELs and discuss how these cells might be devoted to tolerance and immunopathological responses in the intestinal tract. In addition, the role of CD4CD8αα IELs in IBD would also be discussed.

Development, Homeostasis, and Functions of Intestinal Intraepithelial Lymphocytes

The intestine is continuously exposed to commensal microorganisms, food, and environmental agents and also serves as a major portal of entry for many pathogens. A critical defense mechanism against microbial invasion in the intestine is the single layer of epithelial cells that separates the gut lumen from the underlying tissues. The barrier function of the intestinal epithelium is supported by cells and soluble factors of the intestinal immune system. Chief among them are intestinal intraepithelial lymphocytes (iIELs), which are embedded in the intestinal epithelium and represent one of the single largest populations of lymphocytes in the body. Compared with lymphocytes in other parts of the body, iIELs exhibit unique phenotypic, developmental, and functional properties that reflect their key roles in maintaining the intestinal epithelial barrier. In this article, we review the biology of iIELs in supporting normal health and how their dysregulation can contribute to disease.

Intestinal Intraepithelial Lymphocytes: Sentinels of the Mucosal Barrier

Intestinal intraepithelial lymphocytes (IELs) are a large and diverse population of lymphoid cells that reside between the intestinal epithelial cells (IECs) that form the intestinal mucosal barrier. Although IEL biology has traditionally focused on T cells, recent studies have identified several subsets of T cell receptor (TCR)-negative IELs with intriguing properties. New insight into the development, homeostasis, and functions of distinct IEL subsets has recently been provided. Additional studies have revealed intricate interactions between different IEL subsets, reciprocal interactions between IELs and IECs, and communication of IELs with immune cells that reside outside the intestinal epithelium. We review here sentinel functions of IELs in the maintenance of the mucosal barrier integrity, as well as how dysregulated IEL responses can contribute to pathology.

CD8αα TCRαβ Intraepithelial Lymphocytes in the Mouse Gut

The epithelium of the mouse small intestine harbors an abundant CD8αα(+)TCRαβ(+) intraepithelial lymphocyte (IEL) population. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similarly to other regulatory T cell populations. After leaving the thymus, these cells directly seed the intestinal epithelium, which provides a unique combination of cellular interactions together with cytokines, nutrients, and antigens that guide the lineage-specific differentiation and function of these IELs. For instance, epithelial cells and nearby immune cells secrete a number of cytokines, including interleukin-15 (IL-15), IL-7, and transforming growth factor-β, resulting in an assortment of cellular responses, including activation of master transcription factors, cell proliferation, and cytokine secretion. Recent advances have also highlighted the importance of diet-derived substances and commensal metabolites, such as the aryl hydrocarbon receptor ligands and vitamin D, in controlling the survival and gene expression of CD8αα(+)TCRαβ(+) IELs. Furthermore, these cells function in the epithelium and require constant communication between cells in the form of cell-to-cell contacts. These interactions tune the antigen sensitivity of the TCR and maintain the quiescence of the CD8αα(+)TCRαβ(+) IELs. Finally, we discuss how these cells might contribute to tolerance and immunopathological responses in the gut. Therefore, an increased understanding of CD8αα(+)TCRαβ(+) IELs in the gut will help us understand how these cells participate in immune regulation and protection.

Role of the intestinal cytokine microenvironment in shaping the intraepithelial lymphocyte repertoire

Local resident IELs are composed of distinct subsets of T cells with potent cytolytic and immunoregulatory capacities. As IELs are located within this unique interface between the core of the body and the outside environment, the specific development and function of intestinal IELs must be tightly regulated. To accomplish this, the cytokine microenvironment of the intestine has evolved sophisticated mechanisms that modulate the phenotype, ontogeny, and function of these cells. In this review, we summarize the evidence demonstrating the origin of certain intestinal cytokines, including IL-7, IL-15, IL-2, TGF-β, and SCF and discuss what influence such cytokines may have on IELs. Moreover, we review data suggesting that the abnormal expression of cytokines that leads to the heightened activation of IELs may also contribute to immunopathological responses or exacerbate inflammatory diseases, such as IBD and celiac disease, or promote cancer development and progression.

Immune mediators of rotavirus antigenemia clearance in mice

The immunological mediators that clear rotavirus antigenemia or viremia remain undefined. Immunodeficient mice and antibody transfer were used to test whether lymphocytes or rotavirus-specific serum antibodies are essential for resolving antigenemia. Clearance of antigenemia required lymphocytes, but neither T nor B lymphocytes were absolutely required. Transfer of convalescent-phase or nonneutralizing rotavirus-specific serum antibodies to the systemic compartment of severe-combined-immunodeficient (SCID) mice temporarily suppressed the onset or level of chronic rotavirus antigenemia. Our findings provide the first report demonstrating that clearance of rotavirus antigenemia and possibly viremia are mediated by multiple effector lymphocyte subsets and serum antibodies.

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.

Expression of the chemokine binding protein M3 promotes marked changes in the accumulation of specific leukocytes subsets within the intestine

BACKGROUND & AIMS

Chemokines are small proteins that direct leukocyte trafficking under homeostatic and inflammatory conditions. We analyzed the differential expression of chemokines in distinct segments of the intestine and investigated the importance of chemokines for the distribution of leukocytes in the intestine during homeostatic and inflammatory conditions.

METHODS

We analyzed messenger RNA for all known chemokines in different segments of the gut by quantitative polymerase chain reaction. To study the effect of multiple-chemokine blockade in the gut, we generated transgenic mice that expressed the chemokine binding protein M3 in the intestine (V-M3 mice). We used flow cytometry to evaluate the changes in the numbers of leukocytes.

RESULTS

We observed distinct chemokine expression profiles in the 6 segments of the gut. Some chemokines were expressed throughout the intestine (CCL28, CCL6, CXCL16, and CX3CL1), whereas others were expressed preferentially in the small (CCL25 and CCL5) or large intestine (CCL19, CCL21, and CXCL5). Expression of the chemokine blocker M3 in intestinal epithelial cells resulted in reduced numbers of B and T cells in Peyer's patches, reduced numbers of intraepithelial CD8alphabeta(+)/TCRalphabeta(+) and CD8alphaalpha(+)/TCRalphabeta(+) T cells, and reduced numbers of lamina propria CD8(+) T cells. Strikingly, M3 expression markedly reduced the number of eosinophils and macrophages in the small and large intestines. Dextran sulfate sodium treatment of control mice led to marked changes in the expression of chemokines and in the number of myeloid cells in the colon. These cellular changes were significantly attenuated in the presence of M3.

CONCLUSIONS

Our study reveals a complex pattern of chemokine expression in the intestine and indicates that chemokines are critical for leukocyte accumulation in the intestine during homeostasis and inflammation.

IL-15 does not affect IEL development in the thymus but regulates homeostasis of putative precursors and mature CD8 alpha alpha+ IELs in the intestine

Mice devoid of the IL-15 system lose over 90% of CD8alphaalpha(+) TCRalphabeta and TCRgammadelta intestinal intraepithelial lymphocytes (iIELs). Previous work revealed that IL-15Ralpha and IL-15 expressed by parenchymal cells, but not by bone marrow-derived cells, are required for normal CD8alphaalpha(+) iIEL homeostasis. However, it remains unclear when and how the IL-15 system affects CD8alphaalpha(+) iIELs through their development. This study found that IL-15Ralpha is dispensable for the thymic stage of CD8alphaalpha(+) TCRalphabeta and TCRgammadelta iIEL development but is required for the maintenance and/or differentiation of the putative lineage marker negative precursors in the intestinal epithelium, especially for the most mature CD8 single positive subset. Moreover, the IL-15 system directly supports the survival of mature CD8alphaalpha(+) iIEL in vivo. Taken together, this study suggests that regulation of CD8alphaalpha(+) iIEL homeostasis by the IL-15 system does not occur in the thymus but involves mature cells and putative precursors in the intestine.

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.

Thymic differentiation of TCRαβ+CD8αα+IELs

Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)alphabeta(+) CD8 alpha alpha(+) IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCR alpha beta(+) CD8 alpha alpha(+) IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCR alpha beta(+) CD8 alpha alpha(+) IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8 alpha alpha homodimers. The agonist-selected cells in the thymus are TCRbeta(+) but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8 alpha alpha and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCR alpha beta(+) CD8 alpha alpha(+) IELs may help us to understand how they participate in immune regulation and protection in the intestine.

TCR gamma delta intraepithelial lymphocytes are required for self-tolerance

Neonatal thymectomy (NTX) impairs T cell regulation and leads to organ-specific autoimmune disease in susceptible mouse strains. In the NOD mouse model of spontaneous type 1 diabetes, we observed that NTX dramatically accelerated autoimmune pancreatic beta cell destruction and diabetes. NTX had only a minor effect in NOD mice protected from diabetes by transgenic expression of the beta cell autoantigen proinsulin in APCs, inferring that accelerated diabetes after NTX is largely due to failure to regulate proinsulin-specific T cells. NTX markedly impaired the development of intraepithelial lymphocytes (IEL), the number of which was already reduced in euthymic NOD mice compared with control strains. IEL purified from euthymic NOD mice, specifically CD8alphaalpha TCRgammadelta IEL, when transferred into NTX-NOD mice, trafficked to the small intestinal epithelium and prevented diabetes. Transfer of prototypic CD4+CD25+ regulatory T cells also prevented diabetes in NTX-NOD mice; however, the induction of these cells by oral insulin in euthymic mice depended on the integrity of TCRgammadelta IEL. We conclude that TCRgammadelta IEL at the mucosal interface between self and nonself play a key role in maintaining peripheral tolerance both physiologically and during oral tolerance induction.

Late postnatal expansion of self-reactive CD8alphaalpha+ intestinal intraepithelial lymphocytes in mice

The intestinal epithelium is unique in that it harbors auto-reactive T cells largely absent from the peripheral TCR repertoire in normal mice. Intestinal intraepithelial lymphocytes (IEL) expressing self-reactive TCR are mostly CD8alphaalpha+ cells in adult H-Y TCR RAG(-/-) male mice homozygous for the restricting MHC I allele, H-2D(b). By contrast, in male mice heterozygous for the restricting and non-restricting MHC I allele, H-2D(d) (MHC F(1), H-2D(b/d)), IEL are composed of CD8alphabeta and CD8alphaalpha+ T cells. Here we demonstrate that IEL in the immediate postnatal period of MHC homozygous male mice were mostly CD8(-) T cells, while IEL in MHC F(1) male mice were CD8(-) and CD8alphabeta+ T cells. Regardless of the MHC I configuration and the ability to support positive selection of CD8alphabeta+ cells in the thymus, the expansion of CD8alphaalpha+ IEL was a late postnatal event that followed a reduction in CD8(-) IEL. Furthermore, although in vivo treatment with the specific peptide antigen resulted in an earlier accumulation of activated IEL, the expansion of CD8alphaalpha+ IEL remained inefficient until late in postnatal life. Finally, as CD8(-) IEL stimulated with TCR agonists in vitro, acquired expression of CD8alphaalpha, we propose that CD8alphaalpha+ IEL derive from CD8(-) IEL intermediates. Whether CD8(-) IEL are CD8alphabeta-lineage cells that escape deletion in the thymus or are T cells targeted to the intestine from the thymus because of the early and high level TCR transgene expression in this model, is not clear. The signals required for the expansion of CD8alphaalpha+ IEL are however, incomplete in the immediate postnatal intestine. Determining the factors required for the expansion or retention of CD8alphaalpha+ IEL bearing high affinity, self-specific TCR will further elucidate the in vivo role of these T cells in intestinal homeostasis and perhaps, autoimmunity.

Inducible lymphoid tissues in the adult gut: recapitulation of a fetal developmental pathway?

The intestinal immune system faces an extraordinary challenge from the large numbers of commensal bacteria and potential pathogens that are restrained by only a single layer of epithelial cells. Here, I discuss evidence that the intestinal immune system develops an extensive network of inducible, reversible lymphoid tissues that contributes to the vital equilibrium between the gut and the bacterial flora. I propose that this network is induced by cryptopatches, which are small clusters of dendritic cells and lymphoid cells that are identical to fetal inducers of lymph-node and Peyer's-patch development.

Cryptopatches and isolated lymphoid follicles: dynamic lymphoid tissues dispensable for the generation of intraepithelial lymphocytes

In comparison to secondary lymphoid organs, gut-associated lymphoid tissues such as isolated lymphoid follicles (ILF) and cryptopatches (CP) have been less intensively studied. To gain a better insight into processes regulating organization and function of these structures, which are believed to participate in immune responses and extrathymic T cell development, we characterized the lymphoid structures of the murine small intestine in more detail. The size and cellular composition of small intestinal lymphoid aggregations were analyzed in C57BL/6 and BALB/c wild-type and lymphotoxin (LT)-deficient mice, by flow cytometry, histology and automated multi-color immunofluorescence microscopy evaluating large coherent areas of the intestine. These evaluations demonstrate that aggregated lymphoid structures in the small intestine vary in size and cellular composition, with a majority of structures not matching the current definitions of CP or ILF. Accordingly, significant variations depending on species, age and mouse strain were observed. Furthermore, small bowel transplantation revealed a rapid exchange of B but not T cells between host and grafted tissue. Moreover, LT-deficient animals lack any intestinal lymphoid aggregations yet possess the complete panel of intraepithelial lymphocytes (IEL). In summary, our observations disclose intestinal lymphoid aggregations as dynamic structures with a great deal of inborn plasticity and demonstrate their dispensability for the generation of IEL.

The Role of p53 and Fas in a Model of Acute Murine Graft-versus-Host Disease

Graft-vs-host disease (GVHD) is a devastating, frequently fatal, pathological condition associated with lesions in specific target organs, including the intestine, liver, lung, and skin, as well as pancytopenia and alopecia. Bone marrow (BM) atrophy is observed in acutely diseased animals, but the underlying mechanisms of hemopoietic stem cell depletion remained to be established. We used an experimental mouse model of acute GVHD in which parental cells were injected into F(1) hosts preconditioned by sublethal irradiation. The resulting graft-vs-host response was kinetically consistent, resulting in lethality within 3 wk. We observed disease pathology in the liver and small intestine, and consistent with previous observations, we found BM atrophy to be a factor in the onset of acute disease. The product of the protooncogene, p53, is known to be a key player in many physiological examples of apoptosis. We investigated the role of p53 in the apoptosis of BM cells (BMC) during the development of acute disease and found that at least one copy of the p53 gene is necessary for depletion of BM and subsequent lethality in host animals. BM depletion was preceded by induction of the death receptor, Fas, on the surface of host stem cells, and induction of Fas was coincidental with the sensitization of BMC to Fas-mediated apoptosis. Our data indicate that BM depletion in acute GVHD is mediated by p53-dependent up-regulation of Fas on BMC, which leads to Fas-dependent depletion and subsequent disease.

The state of CD4+ T cell activation is a major factor for determining the kinetics and location of T cell responses to oral antigen

Current models suggest that inductive immune responses to enteric Ag are initiated in Peyer's patches (PP) and mesenteric lymph nodes (MLN) followed by migration of activated, memory-like CD4(+) T cells to extralymphoid sites in the intestinal lamina propria (LP). The resultant immune system contains both naive and activated T cells. To examine the differential responses of naive and memory-like T cells to oral Ag, bone marrow chimeras (BMC) were generated. Irradiated BALB/c hosts were reconstituted with a mix of DO11.10 x RAG-1(-/-) and BALB/c bone marrow. In unprimed DO11.10 and BMC models, LP and PP DO11.10 T cells responded to oral Ag with similar kinetics. Responses of activated, memory-like T cells to oral Ag were examined in thymectomized BMC 60 days after i.p. immunization with OVA peptide in Freund's adjuvant (OVA(323-339)/CFA). Results indicate that i.p. OVA(323-339)/CFA generated a high proportion of memory-like CD45RB(low) DO11.10 T cells in peripheral lymphoid (40%) and intestinal LP (70%) tissue. Previously activated DO11.10 T cells in the LP responded to oral Ag earlier and at 50% higher levels compared with memory CD4(+) T cells localized to PP tissue. These data indicate that responses to oral Ag in antigenically naive animals are initiated in PP whereas in Ag-experienced animals LP T cells respond earlier and more vigorously than cells in PP. Taken together, these data suggest that previous activation alters the hierarchy of T cell responses to oral Ag by enhancing the efficiency of LP T cell activation.

Precursors of functional MHC class I- or class II-restricted CD8alphaalpha(+) T cells are positively selected in the thymus by agonist self-peptides

The origin and specificity of alphabeta TCR(+) T cells that express CD8alphaalpha have been controversial issues. Here we provide direct evidence that precursors of functional CD8alphaalpha T cells are positively selected in the thymus in the presence of agonist self-peptides. Like conventional positive selection, this agonist selection process requires functional TCR alpha-CPM, whereas it is independent of CD8beta expression. Furthermore, CD8alphaalpha expression on mature, agonist-selected T cells does not imply selection by MHC class I, and CD8alphaalpha(+) T cells can be either class I or class II restricted. Our data define a distinct agonist-dependent, positive selection process in the thymus, and they suggest a function for CD8alphaalpha distinct from the conventional TCR coreceptor function of CD8alphabeta or CD4.

Evidence for the extrathymic origin of intestinal TCRgammadelta(+) T cells in normal rats and for an impairment of this differentiation pathway in BB rats

The BB rat lyp mutation, one of its diabetes susceptibility genes, is responsible for a 5-fold decrease in the number of peripheral TCRalphabeta(+) T cells. In this study we show that TCRgammadelta(+) T cells are virtually undetectable among splenic T cells and intestinal intraepithelial T lymphocytes (IEL) of BB rats, while they account for 3 and 30% of these two T cell populations, respectively, in normal animals. It has been shown that murine IEL expressing TCRgammadelta develop extrathymically. We determined whether this is the case in rats. Athymic radiation chimeras reconstituted with normal hemopoietic precursors were devoid of donor-derived TCRalphabeta(+) T cells and TCRgammadelta(+) splenocytes but contained a normal number of TCRgammadelta(+) IEL, suggesting that in unmanipulated rats some of the TCRgammadelta(+) IEL may have an extrathymic origin. This was further supported by the observation that RAG1 transcripts are present in IEL of unmanipulated animals. No T cells developed in chimeras reconstituted with BB hemopoietic precursors, demonstrating that the BB rat lyp mutation inhibits both intrathymic and extrathymic development of TCRgammadelta(+) T cells.

Role for T cell-independent B cell activity in the resolution of primary rotavirus infection in mice

We examined the importance of T cell-independent B cell activity in the resolution of primary murine (EDIM) rotavirus infection in adult mice. We showed that Rag 1 (C57BL / 6 background) and Rag 2 (BALB / c background) knockout mice, which lack both T and B cells, chronically shed high levels of rotavirus Ag in stool samples following oral inoculation. However, nude mice (BALB / c and C57BL / 6 backgrounds) and alpha beta TCR knockout mice (C57BL / 6 background) chronically shed 100-fold lower levels of virus in stool samples. Thus, B cells appeared to sharply reduce the level of chronic rotavirus shedding by a T cell-independent mechanism. C57BL / 6 mice depleted of CD4(+) cells or both CD4(+) and CD8(+) cells were also unable to resolve primary rotavirus infection but chronically shed equally low levels of rotavirus Ag in stool samples, whereas mice depleted of only CD8(+) cells resolved infection. Similar results were obtained with a second rotavirus strain (EC(w)) in which virus was shed chronically in stool samples at low levels in alpha beta TCR knockout mice and at high levels in Rag 1 knockout mice. Virus-specific intestinal IgA was readily detected in mice lacking thymic T cells and alpha beta T cells and in mice depleted of CD4(+) cells but levels were 95 % reduced in comparison to immunocompetent control mice. Together, these results show that B cells lacking CD4(+) T cell help have the capacity to substantially reduce rotavirus shedding, possibly through the production of T cell-independent IgA to rotavirus, but full resolution requires alpha beta T cells.

Identification and characterization of lymphoid precursors in the murine intestinal epithelium

Although in vivo evidence supports a role for the murine intestinal epithelium in the extrathymic generation of certain intraepithelial T lymphocytes (IEL), no intraepithelial cells with in vitro lymphoid progenitor potential have yet been demonstrated. Using reaggregate fetal thymic organ culture techniques, we show that a subset of CD3(-) cells isolated from the intestinal epithelium of young mice is capable of generating T cells (alpha beta and gamma delta) and NK1.1(+) cells in vitro. A novel IEL subset bearing a low level of CD45 was identified and found to comprise cells expressing highly immature lymphoid markers including CD34, c-kit, CD122, CD127 and high levels of CD16 and CD44. This subset represents 20-30% of intraepithelial CD45(+) cells from 4-week-old wild-type and nude mouse strains and contains cells with in vitro T cell differentiation capacity. The identification of such an early pluripotent precursor phenotype within the intestinal epithelium implies that the potential for T cell generation exists at this site, and suggests that extrathymic T cell generation may occur within the epithelium itself.

IL-7 prevents both caspase-dependent and -independent pathways that lead to the spontaneous apoptosis of i-IEL

Intestinal intraepithelial lymphocytes (i-IEL) readily undergo spontaneous apoptosis in vitro through an unclear mechanism. Here we examined the relationship between caspases, which plays a major role in apoptosis, and IL-7 in the spontaneous apoptosis of i-IEL in vitro. We demonstrated that IL-7 and zVAD prevented the spontaneous apoptosis of i-IEL by approximately 50% and 25% respectively with no additive protection seen when both are used. IL-7 preferentially prevented the apoptosis of gammadelta i-IEL, while zVAD equally prevented the apoptosis of gammadelta and alphabeta i-IEL. Lastly, we demonstrated that the spontaneous apoptosis of i-IEL is associated with a marked increase in caspase activity. Caspase activity was completely inhibited by zVAD, but only slightly by IL-7. Overall these results suggest that two pathways lead to the spontaneous apoptosis of i-IEL, one which is caspase dependent and the other which is caspase independent. IL-7 appears to exert its effect on i-IEL undergoing spontaneous by partially inhibiting both apoptotic pathways.

Gut cryptopatches: direct evidence of extrathymic anatomical sites for intestinal T lymphopoiesis

Athymic cytokine receptor gamma chain mutant mice that lack the thymus, Peyer's patches, cryptopatches (CP), and intestinal T cells were reconstituted with wild-type bone marrow cells. Bone marrow-derived TCR(-) intraepithelial lymphocytes (IEL) first appeared within villous epithelia of small intestine overlying the regenerated CP, and these TCR(-) IEL subsequently emerged throughout the epithelia. Thereafter, TCR(+) IEL increased to a comparable number to that in athymic mice and consisted of TCRgammadelta and TCRalphabeta IEL. In gut-associated lymphoid tissues of wild-type mice, only CP harbored a large population of c-kit(high)IL-7R(+)CD44(+)Thy-1(+/-)CD4(+/-)CD25(low/-)alpha(E) beta(7)(-)Lin(-) (Lin, lineage markers) lymphocytes that included cells expressing germline but not rearranged TCRgamma and TCRbeta gene transcripts. These findings provide direct evidence that gut CP develop progenitor T cells for extrathymic IEL descendants.

Intestinal immune responses to coccidiosis

Intestinal parasitism is a major stress factor leading to malnutrition and lowered performance and production efficiency of livestock and poultry. Coccidiosis is an intestinal infection caused by intracellular protozoan parasites belonging to several different species of Eimeria. Infection with coccidia parasites seriously impairs the growth and feed utilization of chickens and costs the US poultry industry more than $1.5 billion in annual losses. Although acquired immunity to Eimeria develops following natural infection, due to the complex life cycle and intricate host immune response to Eimeria, vaccine development has been difficult and a better understanding of the basic immunobiology of pertinent host-parasite interactions is necessary for developing effective immunological control strategies against coccidiosis. Chickens infected with Eimeria produce parasite specific antibodies in both the circulation and mucosal secretions but humoral immunity plays only a minor role in protection against this disease. Rather, recent evidence implicates cell-mediated immunity as the major factor conferring resistance to coccidiosis. This review will summarize current understanding of the avian intestinal immune system and its response to Eimeria as well as provide a conceptual overview of the complex molecular and cellular events involved in intestinal immunity to coccidiosis. It is anticipated that increased knowledge of the interaction between parasites and host immunity will stimulate the birth of novel immunological and molecular biological concepts in the control of intestinal parasitism.

Autospecific gammadelta thymocytes that escape negative selection find sanctuary in the intestine

alphabeta or gammadelta thymocytes whose T-cell receptors (TCRs) recognize endogenously expressed antigens (Ag) are autospecific and, thus, potentially self-reactive. In the thymus, such T cells are eliminated during T-cell development through a process known as negative selection. As a model of negative selection of gammadelta T cells, we have used G8 gammadelta-T cell transgenic mice, which express a gammadelta TCR that recognizes the nonpolymorphic MHC class I TL(b) molecule. Here, we demonstrate that negative selection of autospecific gammadelta T cells is almost complete in the adult thymus but is markedly attenuated in the neonatal thymus. A consequence of this attenuated negative selection is that potentially self-reactive gammadelta thymocytes are allowed to escape negative selection, undergo extrathymic differentiation, and find sanctuary in the intestinal epithelium. Interestingly, the ability of these potentially self-reactive gammadelta T cells to find sanctuary requires both the intestinal epithelial environment and the extrathymic presence of the self-Ag. The implications of these findings on the development and persistence of autoreactive T cells in autoimmune disease are discussed.

Development of intraepithelial T lymphocytes in the intestine of irradiated SCID mice by adult liver hematopoietic stem cells from normal mice

BACKGROUND/AIMS

We recently reported the adult mouse liver to contain c-kit+ stem cells that can give rise to multilineage leukocytes. This study was designed to determine whether or not adult mouse liver stem cells can generate intraepithelial T cells in the intestine as well as to examine the possibility that adult liver c-kit+ stem cells originate from the fetal liver.

METHODS

Adult liver mononuclear cells, bone marrow (BM) cells, liver c-kit+ cells or bone BM c-kit+ cells of BALB/c mice were i.v. transferred into 4 Gy irradiated CB17/-SCID mice. In other experiments, fetal liver cells from Ly5.1 C57BL/6 mice and T cell depleted adult BM cells from Ly5.2 C57BL/6 mice were simultaneously transferred into irradiated C57BL/6 SCID mice (Ly5.2). At 1 to 8 weeks after cell transfer, the SCID mice were examined.

RESULTS

Not only BM cells and BM c-kit+ cells but also liver mononuclear cells and liver c-kit+ cells reconstituted gamma delta T cells, CD4+ CD8+ double-positive T cells and CD8 alpha+beta- T cells of intestinal intraepithelial lymphocytes of SCID mice. Injection of a mixture of fetal liver cells from Ly5.1 C57BL/6 mice and adult BM cells from Ly5.2 C57BL/6 mice into Ly5.2 C57BL/6 SCID mice induced both Ly5.1 and Ly5.2 T cells, while also generating c-kit+ cells of both Ly5.1 and Ly5.2 origins in the liver.

CONCLUSIONS

Adult mouse liver stem cells were able to generate intestinal intraepithelial T cells of the SCID mice, and it is thus suggested that some adult liver stem cells may indeed be derived from the fetal liver.

Differential effects of manipulating signaling in early T cell development in intestinal intraepithelial lymphocytes and thymocytes

A pre-TCR-CD3 signal is required for the efficient maturation of CD4- CD8- thymocytes to the CD4+ CD8+ stage. This study addressed whether a similar signal is required for maturation of intestinal intraepithelial lymphocytes (IEL) that may develop extrathymically. We have shown previously that IEL from mice deficient for CD3- associated zeta chains include an immature population of CD3- CD8alphaalpha+ cells expressing cytoplasmic TCR beta chains but lacking detectable surface TCRalphabeta, CD16 and B220. Here we stimulated the appearance of such IEL in epsilon+/- zeta-/- mice by expression of an activated Lck transgene or in vivo treatment with anti-CD3epsilon. Anti-CD3epsilon treatment of RAG-deficient animals also yielded CD16- B220- IEL. In contrast, expression of a TCRbeta transgene in rag-1(-/-) mice did not stimulate the appearance of CD3- CD8alphaalpha+ CD16- B220- cells. Taken together these data indicate that although anti-CD3epsilon treatment and LckF505 assist in catalyzing a CD16+ B220+ --> CD16- B220- transition, these manipulations are not equivalent to a pre-TCR signal in IEL lymphocytes.

T-cell and natural killer cell development in thymectomized Xenopus

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell development in minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.

Differential Requirements of CD45 Protein Tyrosine Phosphatase for Cytolytic Activities and Intrathymic and Extrathymic Development of Intestinal Intraepithelial Lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase essential for Ag receptor-mediated signaling in both T and B cells. In this study we investigated roles of CD45 in development and cytolytic activities of murine intestinal intraepithelial lymphocytes (i-IEL) using CD45 exon 6 knockout (CD45−/−) mice. Interestingly, the total cell number of i-IEL was significantly reduced in CD45−/− mice during aging (10–20 wk of age), whereas the i-IEL number was normally increased in the wild-type littermates. Especially, the number of γδTCR+ i-IEL decreased markedly in CD45−/− mice during aging. The i-IEL in CD45−/− mice were more susceptible to in vitro spontaneous apoptosis than the normal i-IEL, implying that CD45 is required for maintenance of the cellularity of i-IEL. Results from in vivo analyses of the extrathymic and intrathymic development of i-IEL suggested that CD45-mediated signaling is required for the intrathymic, but not the extrathymic, development of i-IEL. Moreover, the whole i-IEL from CD45−/− mice showed a significantly reduced cytolytic activity, and the residual cytolytic activity was completely diminished by depleting CD45+ i-IEL, suggesting that CD45 is indispensable for the TCR-mediated cytolytic activity of i-IEL. Furthermore, we found differential contributions of CD45 and p56lck to development and induction of cytolytic activities of i-IEL.

Positive selection of extrathymically developed T cells by self-antigens

Most T cells develop through the thymus, where they undergo positive and negative selection. Some peripheral T cells are known to develop in the absence of thymus, but there is insufficient information about their selection. To analyze the selection of extrathymically developed T cells, we reconstituted thymectomized male or female recipient mice with bone marrow cells of mice transgenic for male H-Y antigen-specific T cell receptor (TCR). It was revealed that the T cells bearing self-antigen-specific TCR were not deleted in thymectomized male recipients. More importantly, the absence of H-Y antigen-specific T cells in thymectomized female recipients suggests positive selection of extrathymically developed T cells by the self-antigen. The extrathymically developed T cells in male mice expressed interleukin (IL)-2 receptor beta chain (IL-2Rbeta) and intermediate levels of CD3 (CD3(int)) but were natural killer cell (NK)1.1(-). They rapidly produced interferon gamma but not IL-4 after TCR cross-linking. Furthermore, a similar pattern of cytokine production was observed in CD3(int)IL-2Rbeta+NK1.1(-) cells in normal mice which have been shown to develop extrathymically. These results suggest that extrathymically developed CD3(int)IL-2Rbeta+NK1. 1(-) cells in normal mice are also positively selected by self-antigens.

The Role of IL-7 in Thymic and Extrathymic Development of TCRγδ Cells

IL-7-deficient (IL-7−/−) mice have reduced numbers of B and TCRαβ cells, but lack mature TCRγδ cells. Although most T cell development occurs in the thymus, some intestinal intraepithelial lymphocytes (IEL), including TCRγδ cells, can develop extrathymically. Epithelial cells in both thymus and intestine synthesize IL-7, suggesting that TCRγδ cell development could occur in either site. To evaluate the role of thymic IL-7 in development of TCRγδ cells, newborn TCRβ-deficient (TCRβ−/−) thymi were grafted to IL-7−/− mice. Donor- and host-derived TCRγδ cells were recovered from thymus grafts, spleen, and IEL. However, when IL-7−/− thymi were grafted to TCRβ−/− mice, no development of graft-derived TCRγδ cells occurred, indicating that extrathymic IL-7 did not support TCRγδ IEL generation from newborn thymic precursors. In contrast, TCRγδ IEL development occurred efficiently in adult, thymectomized, irradiated C57BL/6J mice reconstituted with IL-7−/− bone marrow. This demonstrated that extrathymic development of TCRγδ IEL required extrathymic IL-7 production. Thus, intrathymic IL-7 was required for development of thymic TCRγδ cells, while peripheral IL-7 was sufficient for development of extrathymic TCRγδ IEL.

Expansion of a novel pulmonary CD3(-) CD4(+) CD8(+) cell population in mice during Chlamydia pneumoniae infection

A new pulmonary T-cell-like lymphocyte population with the phenotype CD3(-) CD4(+) CD8(+) was discovered in mice. CD4(+) CD8(+) but CD3(+) cells among murine intestinal intraepithelial lymphocytes have previously been described. We describe herein a dramatic expansion of the CD3(-) CD4(+) CD8(+) cell population in response to experimental respiratory infection. After intranasal Chlamydia pneumoniae infection, CD4(+) CD8(+) cells became transiently the dominant lymphocyte type (maximum of 87% of all lymphocytes) in the lungs of NIH/S mice but remained virtually undetectable in spleen and blood. The enrichment of these cells was not a C. pneumoniae-specific event, since infection of NIH/S mice with influenza A virus also resulted in an increase in the number of CD4(+) CD8(+) cells (maximum of 42% of all lymphocytes). In addition to outbred NIH/S mice, two other mouse strains were studied: BALB/c (H-2(d)) and C57BL/6 (H-2(b)). C. pneumoniae-infected BALB/c mice responded with an intermediate increase in the number of CD4(+) CD8(+) cells in lungs, whereas C57BL/6 mice did not respond. The double-positive CD4(+) CD8(+) cells lacked a major part of the T-cell receptor complex, being both CD3(-) and TCR alpha beta-. However, when they were stimulated in vitro with a T-cell mitogen, they responded by proliferation but did not secrete gamma interferon. The dramatic expansion of this cell population at the infection site suggests an active role for them in respiratory infection, but the specification of this requires further study.

Hormone regulation of immune homeostasis: local or long distance?

The endocrine system and the immune system consist of dynamic biological processes involved on the one hand in the regulation of a complex array of metabolic and physiologic activities, and on the other hand in protection against infection and disease. Evidence for bidirectional functional involvement of immune-endocrine interactions can be seen at many levels, including codependence during critical stages of development, the complementary use of shared molecular mediators and receptors by both systems, and the integrated participation of the immune system and the endocrine system in resistance or susceptibility to disease. Moreover, recent findings-principally derived from studies of intestinal intraepithelial lymphocytes in mice--suggest that immune-endocrine interactions are essential for the proper development of intestinal T cells, and indicate that this most likely occurs via a local network of hormone synthesis and utilization. In the present article, these findings will be discussed in the context of immune-endocrine collaboration, with particular attention given to the involvement of the thymus in this process, and a hypothesis will be proposed which suggests that the homeostatic balance between health and disease is largely driven by local rather than systemic hormonal regulatory events.

Altered Intestinal Immune System but Normal Antibacterial Resistance in the Absence of P-Selectin and ICAM-1

ICAM-1 and P-selectin are adhesion molecules that regulate leukocyte migration, extravasation to inflammatory sites, and other immune cell interactions. T cell-mediated resistance against acute infection with Listeria monocytogenes and chronic infection with Mycobacterium bovis Calmette-Guérin bacillus was investigated in mutant mice lacking P-selectin and/or ICAM-1. Mice deficient in P-selectin (Psel−/−), ICAM-1 (ICAM−/−), or the combination of both (Psel−/− × ICAM−/−) showed normal bacterial clearance, comparable delayed-type hypersensitivity reactions, and equivalent memory T cell responses. Additionally, the distribution of αβ vs γδ T lymphocyte populations was examined. Normal lymphocyte distributions were noted in thymus, spleen, and blood, whereas mutant mice showed marked alterations in the intestinal intraepithelial (i-IEL) and lamina propria lymphocytes. Differences in i-IEL populations were reflected functionally by differential lytic activities and cytokine productions of i-IEL populations from mutant mice. Despite these changes within the mucosal immune system of mutant mice, their resistance against oral infection with L. monocytogenes was apparently unimpaired. These findings demonstrate that P-selectin and ICAM-1 are critically involved in the shaping of lymphocyte populations of the gut but have only a minor influence on systemic and regional host defense against intracellular bacteria.

Generation of Intestinal Mucosal Lymphocytes in SCID Mice Reconstituted with Mature, Thymus-Derived T Cells

Transfer of peripheral lymph node lymphocytes to SCID mice leads to the long term establishment of mucosal T lymphocytes within the epithelium and lamina propria of the small and large intestines. Analysis of engrafted intraepithelial lymphocytes (IEL) showed that they had acquired a surface phenotype that in several respects is typical of IEL. In addition, the functional profile of engrafted IEL derived from lymph node T cells was similar to that of normal IEL; as the donor-derived T cells exhibited a strong cytolytic activity, a poor proliferative response to mitogenic stimuli, and a tendency to home and expand specifically in the intestine upon transfer to secondary SCID recipients. Optimal engraftment of intestinal T cells required bacterial flora, as the number of lymphocytes was greatly reduced in SCID recipients with a reduced flora. These results demonstrate that mature, thymus-derived T cells can migrate to the intestine and become functionally specialized to the intestinal milieu. The acquisition of phenotypic markers characteristic of the intestinal microenvironment by engrafted cells suggests that T cell migration of lymphocytes to the SCID intestine is not aberrant, but it may reflect processes that are ongoing in immunocompetent mice. Furthermore, these data suggest that the homing and/or expansion of typical, thymus-derived T cells in the intestine may be driven by luminal Ags such as those derived from bacterial flora.

Fas ligand- mediated killing by intestinal intraepithelial lymphocytes. Participation in intestinal graft-versus-host disease

In vitro studies have demonstrated that intestinal intraepithelial lymphocytes (IEL) are constitutively cytotoxic; however, the mechanism and target of their cytotoxicity are unknown. Apoptosis of intestinal epithelial cells (IEC) and an increase in IEL numbers are classical signs of intestinal graft-versus-host disease (GVHD), although whether IEL can mediate IEC apoptosis directly in GVHD is unclear. Recent evidence suggests that target epithelial organ injury observed in GVHD is predominantly Fas-mediated; therefore, we investigated the possibility that IEL induce apoptosis of IEC through a Fas-mediated mechanism. Here, we demonstrate that the IEL isolated from normal mice readily display potent Fas ligand (FasL)-mediated killing activity after CD3 stimulation, and that IEC express Fas, suggesting that IEC are potential targets for FasL-mediated killing by IEL. In vitro, IEL isolated from GVHD mice have markedly increased FasL-mediated killing potential and are spontaneously cytolytic toward host-derived tumor cells predominantly through a Fas-mediated pathway. In vivo transfer of IEL isolated from GVHD mice induced significantly more IEC apoptosis in F1 wild-type mice than in Fas-defective F1lpr mice. Thus, these results demonstrate that FasL-mediated death of IEC by IEL is a major mechanism of IEC apoptosis seen in GVHD.

Immunity to rotavirus in T cell deficient mice

Rotavirus infection was studied in adult nude mice (BALB/c background), alpha beta or gamma delta and alpha beta/gamma delta T cell receptor (TCR) knockout (-/-) mice (C57BL/6 and C57BL/6 x 129 backgrounds), and SCID mice (C57BL/6 background). The gamma delta TCR -/- mice cleared infection just like control mice. All of the nude mice, alpha beta, and alpha beta/gamma delta TCR -/- mice cleared primary rotavirus infection, with a short delay, compared to immunocompetent control mice and developed a rotavirus-specific intestinal IgA measured by ELISA. Elispot analysis with spleen and lamina propia cells showed that the virus-specific intestinal IgA response in immunocompetent C57BL/6 mice was similar to the gamma delta TCR -/- mice and 7- to 60-fold higher than in the alpha beta TCR -/- and alpha beta/gamma delta TCR -/- mice. Likewise, the response of nude +/- mice was 20 times greater than that of nude -/- littermates. While the intestinal IgA antibodies of C57BL/6 mice, gamma delta TCR -/- mice, and nude +/- mice recognized insect cells infected with recombinant baculovirus expressing rotavirus VP6 and VP4 proteins, those of the alpha beta TCR -/-, alpha beta/gamma delta TCR -/-, and nude -/- mice recognized only VP6. Immunocompetent C57BL/6 mice depleted of CD4+ T cell developed similar levels of rotavirus-specific intestinal IgA as the alpha beta TCR -/- mice, suggesting that this T cell-independent IgA response is present in normal mice. In contrast to previously published results with BALB/c SCID and RAG 2 -/- (C57BL/6 x 129 background) mice, all of which become chronically infected with murine rotavirus, 40% of the C57BL/6 SCID mice cleared primary rotavirus infection. These results suggest that both a T cell-independent antibody response and innate mechanisms can contribute to immunity to murine rotavirus and show that gamma delta T cells are not necessary for efficient clearance of primary rotavirus infection in mice.

The anatomical basis of intestinal immunity

The lymphoid tissues associated with the intestine are exposed continuously to antigen and are the largest part of the immune system. Many lymphocytes are found in organised tissues such as the Peyer's patches and mesenteric lymph nodes, as well as scattered throughout the lamina propria and epithelium of the mucosa itself. These lymphocyte populations have several unusual characteristics and the intestinal immune system is functionally and anatomically distinct from other, peripheral compartments of the immune system. This review explores the anatomical and molecular basis of these differences, with particular emphasis on the factors which determine how the intestinal lymphoid tissues discriminate between harmful pathogens and antigens which are beneficial, such as food proteins or commensal bacteria. These latter antigens normally provoke immunological tolerance, and inappropriate responses to them are responsible for immunopathologies such as food hypersensitivity and inflammatory bowel disease. We describe how interactions between local immune cells, epithelial tissues and antigen-presenting cells may be critical for the induction of tolerance and the expression of active mucosal immunity. In addition, the possibility that the intestine may act as an extrathymic site for T-cell differentiation is discussed. Finally, we propose that, under physiological conditions, immune responses to food antigens and commensal bacteria are prevented by common regulatory mechanisms, in which transforming growth factor beta plays a critical role.

Absolute dependence of T cell receptor(hi) cell generation and relative dependence of T cell receptor(int) cell generation on the thymus

Recent evidence indicates that conventional T cells are generated by the mainstream of T cell differentiation in the thymus and acquire a high density of T cell receptor expression (i.e. TCRhi). In contrast, primordial T cells (or NK1.1+ T cells) are generated by the extrathymic pathways or an alternative intrathymic pathway and express an intermediate density of TCR (i.e. TCRint). To obtain further evidence, it was examined how thymus grafting influenced the distribution of T cell populations in athymic nude mice. When BALB/c nu/nu mice were engrafted with thymocyte-depleted BALB/c+/+ fetal thymi, two changes emerged after grafting: nude mice generated TCRhi cells de novo in the periphery as well as in the grafted thymi, and the absolute number of interleukin-2 receptor beta chain+ TCRint cells increased prominently in number in the periphery. Among thymic hormones tested, the administration of thymosin alpha induced a slight expansion of CD3int cells in nude mice. To examine a possible interaction of TCRint cells with TCRhi cells in the periphery, B6 nu/nu mice (Ly5.2+) were injected with TCRhi cells purified from the spleen of B6 Ly5.1 congenic mice. In this case, TCRint (Ly5.2+) cells expanded well in all tested organs of nude mice. These results suggest that the generation of TCRhi cells is absolutely dependent on the thymus and that TCRint cells expand under the influence of the thymus (humoral) and due to interaction with thymus-derived conventional T cells.

Development of CD8 alpha/beta + TCR alpha beta intestinal intraepithelial lymphocytes in athymic nu/nu mice and participation in regional immune responses

On the basis of the CD8 coreceptor expression, T-cell receptor (TCR)alpha beta-bearing intestinal intraepithelial lymphocytes (i-IEL) segregate into two populations. The CD8 alpha alpha + TCR alpha beta i-IEL develop thymus independently, whereas the CD8 alpha beta + TCR alpha beta i-IEL are generally considered to be thymus dependent. Flow cytometry analysis revealed a distinct population of CD8 alpha beta + TCR alpha beta i-IEL in individual athymic nu/nu mice. The i-IEL encompassing CD8 alpha beta + TCR alpha beta cells expressed potent cytolytic and interferon-gamma-producing activities. These findings demonstrate that CD8 alpha beta + TCR alpha beta i-IEL can develop in nu/nu mice independently from a functional thymus and suggest that these cells, directly or indirectly, perform biological functions in the gut.