The T cell receptor repertoire of intestinal intraepithelial gammadelta T lymphocytes is influenced by genes linked to the major histocompatibility complex and to the T cell receptor loci

MAITs and their mates: "Innate-like" behaviors in conventional and unconventional T cells

Most CD4 and CD8 T cells are restricted by conventional major histocompatibility complex (MHC) molecules and mount TCR-dependent adaptive immune responses. In contrast, MAIT, iNKT, and certain γδ TCR bearing cells are characterized by their abilities to recognize antigens presented by unconventional antigen-presenting molecules and to mount cytokine-mediated TCR-independent responses in an "innate-like" manner. In addition, several more diverse T-cell subsets have been described that in a similar manner are restricted by unconventional antigen-presenting molecules but mainly depend on their TCRs for activation. Vice versa, innate-like behaviour was reported in defined subpopulations of conventional T cells, particularly in barrier sites, showing that these two features are not necessarily linked. The abilities to recognize antigens presented by unconventional antigen-presenting molecules or to mount TCR-independent responses creates unique niches for these T cells and is linked to wide range of functional capabilities. This is especially exemplified by unconventional and innate-like T cells present at barrier sites where they are involved in pathogen defense, tissue homeostasis as well as in pathologic processes.

Lung-resident γδ T cells and their roles in lung diseases

γδ T cells are greatly enriched in mucosal and epithelial sites, such as the skin, respiratory, digestive and reproductive tracts, and they are defined as tissue-resident immune cells. In these tissues, the characteristics and biological roles of γδ T cells are distinguished from each other. The lungs represent the most challenging immunological dilemma for the host, and they have their own effective immune system. The abundance of γδ T cells, an estimated 8-20% of resident pulmonary lymphocytes in the lung, maintains lung tissue homeostasis. In this review, we summarize the recent research progress regarding lung-resident γδ T cells, including their development, residency and immune characteristics, and discuss the involvement of γδ T cells in infectious diseases of the lung, including bacterial, viral and fungal infections; lung allergic disease; lung inflammation and fibrosis; and lung cancer.

Epithelia Use Butyrophilin-like Molecules to Shape Organ-Specific γδ T Cell Compartments

Many body surfaces harbor organ-specific γδ T cell compartments that contribute to tissue integrity. Thus, murine dendritic epidermal T cells (DETCs) uniquely expressing T cell receptor (TCR)-Vγ5 chains protect from cutaneous carcinogens. The DETC repertoire is shaped by Skint1, a butyrophilin-like (Btnl) gene expressed specifically by thymic epithelial cells and suprabasal keratinocytes. However, the generality of this mechanism has remained opaque, since neither Skint1 nor DETCs are evolutionarily conserved. Here, Btnl1 expressed by murine enterocytes is shown to shape the local TCR-Vγ7(+) γδ compartment. Uninfluenced by microbial or food antigens, this activity evokes the developmental selection of TCRαβ(+) repertoires. Indeed, Btnl1 and Btnl6 jointly induce TCR-dependent responses specifically in intestinal Vγ7(+) cells. Likewise, human gut epithelial cells express BTNL3 and BTNL8 that jointly induce selective TCR-dependent responses of human colonic Vγ4(+) cells. Hence, a conserved mechanism emerges whereby epithelia use organ-specific BTNL/Btnl genes to shape local T cell compartments.

The ontogeny of Butyrophilin-like (Btnl) 1 and Btnl6 in murine small intestine

Murine Butyrophilin-like (Btnl) 1 and Btnl6 are primarily restricted to intestinal epithelium where they regulate the function of intraepithelial T lymphocytes. We recently demonstrated that Btnl1 and Btnl6 can form an intra-family heterocomplex and that the Btnl1-Btnl6 complex selectively expands Vγ7Vδ4 TCR IELs. To define the regulation of Btnl expression in the small intestine during ontogeny we examined the presence of Btnl1 and Btnl6 in the small bowel of newborn to 4-week-old mice. Although RNA expression of Btnl1 and Btnl6 was detected in the small intestine at day 0, Btnl1 and Btnl6 protein expression was substantially delayed and was not detectable in the intestinal epithelium until the mice reached 2–3 weeks of age. The markedly elevated Btnl protein level at week 3 coincided with a significant increase of γδ TCR IELs, particularly those bearing the Vγ7Vδ4 receptor. This was not dependent on gut microbial colonization as mice housed in germ-free conditions had normal Btnl protein levels. Taken together, our data show that the expression of Btnl1 and Btnl6 is delayed in the murine neonatal gut and that the appearance of the Btnl1 and Btnl6 proteins in the intestinal mucosa associates with the expansion of Vγ7Vδ4 TCR IELs.

The burgeoning family of unconventional T cells

While most studies of T lymphocytes have focused on T cells reactive to complexes of peptide and major histocompatibility complex (MHC) proteins, many other types of T cells do not fit this paradigm. These include CD1-restricted T cells, MR1-restricted mucosal associated invariant T cells (MAIT cells), MHC class Ib-reactive T cells, and γδ T cells. Collectively, these T cells are considered 'unconventional', in part because they can recognize lipids, small-molecule metabolites and specially modified peptides. Unlike MHC-reactive T cells, these apparently disparate T cell types generally show simplified patterns of T cell antigen receptor (TCR) expression, rapid effector responses and 'public' antigen specificities. Here we review evidence showing that unconventional T cells are an abundant component of the human immune system and discuss the immunotherapeutic potential of these cells and their antigenic targets.

Murine Butyrophilin-Like 1 and Btnl6 Form Heteromeric Complexes in Small Intestinal Epithelial Cells and Promote Proliferation of Local T Lymphocytes

To date, few molecular conduits mediating the cross-talk between intestinal epithelial cells and intraepithelial lymphocytes (IELs) have been described. We recently showed that butyrophilin-like (Btnl) 1 can attenuate the epithelial response to activated IELs, resulting in reduced production of proinflammatory mediators, such as IL-6 and CXCL1. We here report that like Btnl1, murine Btnl6 expression is primarily confined to the intestinal epithelium. Although Btnl1 can exist in a cell surface-expressed homomeric form, we found that it additionally forms heteromeric complexes with Btnl6, and that the engagement of Btnl1 is a prerequisite for surface expression of Btnl6 on intestinal epithelial cells. In an IEL-epithelial cell coculture system, enforced epithelial cell expression of Btnl1 significantly enhanced the proliferation of IELs in the absence of exogenous activation. The effect on proliferation was dependent on the presence of IL-2 or IL-15 and restricted to IELs upregulating CD25. In the γδ T-cell subset, the Btnl1-Btnl6 complex, but not Btnl1, specifically elevated the proliferation of IELs bearing the Vγ7Vδ4 receptor. Thus, our results show that murine epithelial cell-specific Btnl proteins can form intrafamily heterocomplexes and suggest that the interaction between Btnl proteins and IELs regulates the expansion of IELs in the intestinal mucosa.

Quantitative Genetic Analysis of Ventral Midbrain and Liver Iron in BXD Recombinant Inbred Mice

Male and female mice from 15 of the BXD/Ty recombinant inbred strain panel were examined for regional brain and liver iron content. Brain regions included medial prefrontal cortex, nucleus accumbens, caudate-putamen and ventral midbrain. Our focal tissue was the ventral midbrain, containing the ventral tegmentum and substantia nigra. This area contains the perikarya of the dopamine neurons that project to nucleus accumbens and caudate-putamen. Genetic correlations between ventral midbrain and liver iron content were not statistically significant, suggesting that peripheral and central iron regulatory systems are largely independent. Correlations between ventral midbrain iron and iron in the caudate-putamen and nucleus accumbens, but not the prefrontal cortex were moderately high and significant. Ventral midbrain and liver iron contents were subjected to quantitative trait loci analysis to identify associated chromosomal locations. This analysis revealed several suggestive loci for iron content in ventral midbrain but fewer loci for liver. Genetic correlations between ventral midbrain iron and published dopamine functional indices were significant, suggesting a link between ventral midbrain iron status and central dopamine neurobiology. This work shows the value of quantitative genetic analysis in the neurobiology of iron and in showing the close association between ventral midbrain iron and nigrostriatal/mesolimbic dopamine function.

Close link between development and function of gamma-delta T cells

Murine γδ T cells develop as the first T-cell lineage within the fetal thymus and disproportionately localize in mucosal tissues such as lung, skin, uterus, and intestine of adult mice. These unique developmental features and distribution patterns of γδ T cells enable rapid functioning against various insults from pathogens. γδ T cells are also able to respond to local inflammation and consequently regulate the pathogenesis of autoimmune disorders and development of tumors in mice and humans. Hence, it is clinically important to understand the mechanisms that regulate γδ T cell functions. Recent evidence has shown that generations of effector γδ T cell subsets producing IFN-γ, IL-4, and IL-17 are programmed in the murine thymus before their migration to peripheral tissues. This review outlines our current understanding of the development and function of γδ T cells as they influence both innate and acquired immunity.

CD4 T cells play important roles in maintaining IL-17-producing γδ T-cell subsets in naive animals

A proportional balance between αβ and γδ T-cell subsets in the periphery is exceedingly well maintained by a homeostatic mechanism. However, a cellular mechanism underlying the regulation remains undefined. We recently reported that a subset of developing γδ T cells spontaneously acquires interleukin (IL)-17-producing capacity even within naive animals through a transforming growth factor (TGF)β1-dependent mechanism, thus considered 'innate' IL-17-producing cells. Here, we report that γδ T cells generated within αβ T cell (or CD4 T cell)-deficient environments displayed altered cytokine profiles; particularly, 'innate' IL-17 expression was significantly impaired compared with those in wild-type mice. Impaired IL-17 production in γδ T cells was directly related to CD4 T-cell deficiency, because depletion of CD4 T cells in wild-type mice diminished and adoptive CD4 T-cell transfer into T-cell receptor β-/- mice restored IL-17 expression in γδ T cells. CD4 T cell-mediated IL-17 expression required TGFβ1. Moreover, Th17 but not Th1 or Th2 effector CD4 T cells were highly efficient in enhancing γδ T-cell IL-17 expression. Taken together, our results highlight a novel CD4 T cell-dependent mechanism that shapes the generation of IL-17+ γδ T cells in naive settings.

Butyrophilin-like 1 encodes an enterocyte protein that selectively regulates functional interactions with T lymphocytes

Although local regulation of T-cell responses by epithelial cells is increasingly viewed as important, few molecules mediating such regulation have been identified. Skint1, a recently identified member of the Ig-supergene family expressed by thymic epithelial cells and keratinocytes, specifies the murine epidermal intraepithelial lymphocyte (IEL) repertoire. Investigating whether Skint1-related molecules might regulate IEL in other compartments, this study focuses on buytrophilin-like 1 (Btnl1), which is conspicuously similar to Skint1 and primarily restricted to small intestinal epithelium. Btnl1 protein is mostly cytoplasmic, but surface expression can be induced, and in vivo Btnl1 can be detected adjacent to the IEL. In a newly developed culture system, enforced epithelial cell expression of Btnl1 attenuated the cells' response to activated IEL, as evidenced by suppression of IL-6 and other inflammatory mediators. These findings offer a unique perspective on emerging genetic data that Btnl genes may comprise novel and important local regulators of gut inflammation.

T regulatory cells maintain intestinal homeostasis by suppressing γδ T cells

Immune tolerance against enteric commensal bacteria is important for preventing intestinal inflammation. Deletion of phosphoinositide-dependent protein kinase 1 (Pdk1) in T cells via Cd4-Cre induced chronic inflammation of the intestine despite the importance of PDK1 in T cell activation. Analysis of colonic intraepithelial lymphocytes of PDK1-deficient mice revealed markedly increased CD8α(+) T cell receptor (TCR)γδ(+) T cells, including an interleukin-17 (IL-17)-expressing population. TCRγδ(+) T cells were responsible for the inflammatory colitis as shown by the fact that deletion of Tcrd abolished spontaneous colitis in the PDK1-deficient mice. This dysregulation of intestinal TCRγδ(+) T cells was attributable to a reduction in the number and functional capacity of PDK1-deficient T regulatory (Treg) cells. Adoptive transfer of wild-type Treg cells abrogated the spontaneous activation and proliferation of intestinal TCRγδ(+) T cells observed in PDK1-deficient mice and prevented the development of colitis. Therefore, suppression of intestinal TCRγδ(+) T cells by Treg cells maintains enteric immune tolerance.

Systems genetic analysis of peripheral iron parameters in the mouse

Iron homeostasis is one of the most critical functions in living systems. Too little iron can lead to anemia and tissue-specific disorders, such as splenomegaly. Excessive systemic iron is characteristic of hemochromatosis and is implicated in the brain in Parkinson's disease. With the exception of some single gene diseases like hemochromatosis, we know little about genetic-based, individual differences in iron-related parameters and their impact on biology. To model genetic control of iron homeostasis, we measured liver, spleen, and plasma iron concentrations, hematocrit and hemoglobin, transferrin saturation, and total iron-binding capacity in several BXD/Ty recombinant inbred mouse strains derived from C57BL/6 and DBA/2 progenitors. At 120 days of age, the animals were killed for iron analysis. All measures showed genetic-based variability consistent with polygenic influence. Analysis of principal components of the seven measures revealed three factors that we named availability, transport, and storage. Quantitative trait loci (QTL) analysis revealed one suggestive QTL on chromosome 5 for availability, two suggestive QTL (one on chromosome 1 and the other on chromosome 7) for transport, and one weak QTL on chromosome 2 for storage. The results show that iron homeostasis is a complex trait and is influenced by multiple genes.

Human CD8+intraepithelial lymphocytes: a unique model to study the regulation of effector cytotoxic T lymphocytes in tissue

The epithelium of the human small intestine contains a large population of intraepithelial cytolytic alphabeta T-cell receptor (TCR) CD8 alpha beta T lymphocytes (IE-CTLs), whose main role is to sustain epithelial integrity by rapidly eliminating infected and damaged cells. In mouse, the recognition of inducible/modified self-molecules, i.e. non-classical major histocompatibility complex (MHC) class I molecules, is mediated by the TCR and natural killer receptors (NKRs) co-expressed on the cell surface of a non-conventional autoreactive CD8 alpha alpha alpha beta TCR cell subset. In contrast, in humans, the recognition of non-classical MHC class I molecules induced by stress and inflammation on intestinal epithelial cells (IECs) is principally mediated by NKRs expressed on conventional CD8 alpha beta alpha beta TCR cells. By sensing microenvironmental signals of inflammation and stress through NKRs, IE-CTLs fine tune their TCR activation threshold. Furthermore, IE-CTLs under particular conditions, involving interleukin-15 upregulation, acquire the capacity to kill distressed intestinal epithelial cells in an antigen non-specific manner. Adaptive IE-CTLs appear hence to have autoreactive properties and modulate their immune response based on innate signals, reflecting the fitness of the tissue.

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.

Integrative genetic analysis of transcription modules: towards filling the gap between genetic loci and inherited traits

Genetic loci that regulate inherited traits are routinely identified using quantitative trait locus (QTL) mapping methods. However, the genotype-phenotype associations do not provide information on the gene expression program through which the genetic loci regulate the traits. Transcription modules are 'self-consistent regulatory units' and are closely related to the modular components of gene regulatory network [Ihmels, J., Friedlander, G., Bergmann, S., Sarig, O., Ziv, Y. and Barkai, N. (2002) Revealing modular organization in the yeast transcriptional network. Nat. Genet., 31, 370-377; Segal, E., Shapira, M., Regev, A., Pe'er, D., Botstein, D., Koller, D. and Friedman, N. (2003) Module networks: identifying regulatory modules and their condition-specific regulators from gene expression data. Nat. Genet., 34, 166-176]. We used genome-wide genotype and gene expression data of a genetic reference population that consists of mice of 32 recombinant inbred strains to identify the transcription modules and the genetic loci regulating them. Twenty-nine transcription modules defined by genetic variations were identified. Statistically significant associations between the transcription modules and 18 classical physiological and behavioral traits were found. Genome-wide interval mapping showed that major QTLs regulating the transcription modules are often co-localized with the QTLs regulating the associated classical traits. The association and the possible co-regulation of the classical trait and transcription module indicate that the transcription module may be involved in the gene pathways connecting the QTL and the classical trait. Our results show that a transcription module may associate with multiple seemingly unrelated classical traits and a classical trait may associate with different modules. Literature mining results provided strong independent evidences for the relations among genes of the transcription modules, genes in the regions of the QTLs regulating the transcription modules and the keywords representing the classical traits.

Interleukin 15 controls the generation of the restricted T cell receptor repertoire of gamma delta intestinal intraepithelial lymphocytes

The gammadelta T cells are prevalent in the mucosal epithelia and are postulated to act as 'sentries' for maintaining tissue integrity. What these gammadelta T cells recognize is poorly defined, but given the restricted T cell receptor (TCR) repertoire, the idea that they are selected by self antigens of low complexity has been widely disseminated. Here we present data showing that the generation of the restricted TCR variable gamma-region gene repertoire of intestinal intraepithelial lymphocytes was regulated by interleukin 15, which induced local chromatin modifications specific for the variable gamma-region gene segment and enhanced accessibility conducive to subsequent targeted gene rearrangement. This cytokine-directed tissue-specific TCR repertoire formation probably reflects distinct TCR repertoire selection criteria for gammadelta and alphabeta T cell lineages adopted for different antigen-recognition strategies.

Role of CD1d in coxsackievirus B3-induced myocarditis

The myocarditic (H3) variant of Coxsackievirus B3 (CVB3) causes severe myocarditis in BALB/c mice and BALB/c mice lacking the invariant J alpha 281 gene, but minimal disease in BALB/c CD1d(-/-) animals. This indicates that CD1d expression is important in this disease but does not involve the invariant NKT cell often associated with CD1d-restricted immunity. The H3 variant of the virus increases CD1d expression in vitro in neonatal cardiac myocytes whereas a nonmyocarditic (H310A1) variant does not. V gamma 4(+) T cells show increased activation in both H3-infected BALB/c and J alpha 281(-/-) mice compared with CD1d(-/-) animals. The activated BALB/c V gamma 4(+) T cells from H3-infected mice kill H3-infected BALB/c myocytes and cytotoxicity is blocked with anti-CD1d but not with anti-MHC class I (K(d)/D(d)) or class II (IA/IE) mAbs. In contrast, H3 virus-infected CD1d(-/-) myocytes are not killed. These studies demonstrate that CD1d expression is essential for pathogenicity of CVB3-induced myocarditis, that CD1d expression is increased early after infection in vivo in CD1d(+) mice infected with the myocarditic but not with the nonmyocarditic CVB3 variant, and that V gamma 4(+) T cells, which are known to promote myocarditis susceptibility, appear to recognize CD1d expressed by CVB3-infected myocytes.

T cell receptor-gamma allele-specific selection of V gamma 1/V delta 4 cells in the intestinal epithelium

Previous genetic analyses have shown that the relative representation of subsets of gammadelta intestinal intraepithelial lymphocytes (i-IELs) is influenced by genes linked to the TCRgamma, TCRdelta, and MHC loci. Here, we have analyzed V-gene use in gammadelta i-IELs from C57BL/6 (B6) and C57BL/10 (B10) mice and from their F(1) and F(2) progenies with a larger panel of Vgamma- and Vdelta-specific mAbs and have shown that the influence of TCRgamma-linked genes operates at two levels: one influencing the representation of Vgamma1 (or Vgamma7) i-IELs and other acting specifically on the Vgamma1/Vdelta4 i-IEL subset, which represents 3% and 15% of the gammadelta i-IELs in B6 and B10 mice, respectively. Analysis of mice transgenic for a rearranged Vgamma1Jgamma4Cgamma4 chain of B6 origin demonstrated that the TCRgamma-linked genes influencing the representation of the Vgamma1/Vdelta4 i-IEL subset are the structural genes of TCRgamma chains. This influence is allele specific and cell autonomous, as evidenced by the different behavior of Vgamma1/Vdelta4 cells bearing either parental allele in F(1) mice. The representation of Vgamma1/Vdelta4 cells among gammadelta thymocytes is similar in B6 and B10 mice, demonstrating that the Vdelta4 chain can pair well with both alleles of the Vgamma1Jgamma4Cgamma4 chain and strongly suggesting that a cellular selection mechanism is responsible for the observed differences. The Vgamma1-Jgamma4 junctional amino acid sequences of B6 Vgamma1/Vdelta4 i-IELs are diverse but display less variation in length than those found in similar cells from B10 mice, indicating that B6 Vgamma1/Vdelta4 cells are the target of this cellular selection event.

V gamma 1+ T cells suppress and V gamma 4+ T cells promote susceptibility to coxsackievirus B3-induced myocarditis in mice

Coxsackievirus B3 infections of C57BL/6 mice, which express the MHC class II IA but not IE Ag, results in virus replication in the heart but minimal myocarditis. In contrast, Bl.Tg.Ealpha mice, which are C57BL/6 mice transgenically induced to express IE Ag, develop significant myocarditis upon Coxsackievirus B3 infection. Despite this difference in inflammatory damage, cardiac virus titers are similar between C57BL/6 and Bl.Tg.Ealpha mice. Removing gammadelta T cells from either strain by genetic manipulation (gammadelta knockout(ko)) changes the disease phenotype. C57BL/6 gammadelta ko mice show increased myocarditis. In contrast, Bl.Tg.Ealpha gammadelta ko mice show decreased cardiac inflammation. Flow cytometry revealed a difference in the gammadelta cell subsets in the two strains, with Vgamma1 dominating in C57BL/6 mice, and Vgamma4 predominating Bl.Tg.Ealpha mice. This suggests that these two Vgamma-defined subsets might have different functions. To test this possibility, we used mAb injection to deplete each subset. Mice depleted of Vgamma1 cells showed enhanced myocarditis, whereas those depleted of Vgamma4 cells suppressed myocarditis. Adoptively transfusing enriched Vgamma4(+) cells to the C57BL/6 and Bl.Tg. Ealpha gammadelta ko strains confirmed that the Vgamma4 subset promoted myocarditis. Th subset analysis suggests that Vgamma1(+) cells biased the CD4(+) T cells to a dominant Th2 cell response, whereas Vgamma4(+) cells biased CD4(+) T cells toward a dominant Th1 cell response.

Genetic mapping of two murine loci that influence the development of IL-4-producing Thy-1dull gamma delta thymocytes

IL-4-producing gamma delta cells belong to a novel subset of gamma delta lymphocytes that expresses a very restricted repertoire of TCRs. To gain a deeper insight into the development and in vivo functions of these cells, we have analyzed the genetic control of their representation in the thymus. Using an intercross between C57BL/6 and DBA/2 mice we found two loci on chromosomes 13 and 17-named LadT1 and LadT2, respectively-with marked influence in their development. The LadT2 locus does not appear to be the MHC locus. The region identified on mouse chromosome 13 contains the structural genes for TCR gamma as well as the IL-9 gene, which has been suggested as a candidate gene influencing the complex pathogenesis of asthma.