Identification and characterization of lymphoid precursors in the murine intestinal epithelium

Increased uNK Progenitor Cells in Women With Endometriosis and Infertility are Associated With Low Levels of Endometrial Stem Cell Factor

PROBLEM

Uterine natural killer (uNK) cells play a significant role in successful human pregnancy. Having previously demonstrated uNK cell progenitors in human endometrium, we hypothesized that abnormal uNK cell maturation contributes to infertility in women with endometriosis. We aimed to characterize uNK cells at different developmental stages in women with and without endometriosis and to investigate possible mechanisms to explain any differences.

METHOD OF STUDY

We characterized uNK cell development in women with and without endometriosis using flow cytometry, protein array and in vitro experiments.

RESULTS

We found increased proportions of uNK cells at developmental stages 1 and 2 in endometrium from women with endometriosis (n = 36; mean = 21.2%) when compared with healthy fertile women (n = 9; mean = 7.0%). Protein array analysis revealed significantly lower levels of stem cell factor (SCF) in the eutopic endometrium of women with endometriosis when compared to healthy women. Addition of SCF to endometrial progenitor cells in vitro restored uNK cell maturation.

CONCLUSION

We have shown that women with endometriosis have low levels of endometrial SCF, which we hypothesize contributes to abnormal maturation of local uNK cell populations. This defect may also compromise embryo implantation and hence contribute to endometriosis-associated infertility. SCF replacement may be a new therapeutic approach.

Fc Receptors and Fc Receptor-Like Molecules within the Immunoreceptor Family

Receptors for the Fc portion of immunoglobulins (FcRs) account for most cell-mediated biological activities of antibodies. The majority of FcRs are encoded by a set of genes, clustered in the fcr locus, on chromosome 1 in humans and on chromosome 1 and 3 in mice. Eight (in humans) and six (in mice) new genes were found, intermixed with FcR genes in corresponding fcr loci, which encode FcR-like molecules (FcRLs). FcRs and FcRLs are genetically, phylogenetically, structurally, and functionally related. FcRs and FcRLs, however, markedly differ by their ligands, their tissue distribution, and, therefore, by the biological functions they control. A systematic comparison of their biological properties leads to the conclusion that FcRLs are not like FcRs. They altogether form a single family within the immunoreceptor family, whose members fulfill distinct but complementary roles in immunity by differentially controlling innate and adaptive responses.

CD8αα⁺ innate-type lymphocytes in the intestinal epithelium mediate mucosal immunity

Innate immune responses are critical for mucosal immunity. Here we describe an innate lymphocyte population, iCD8α cells, characterized by expression of CD8α homodimers. iCD8α cells exhibit innate functional characteristics such as the capacity to engulf and kill bacteria. Development of iCD8α cells depends on expression of interleukin-2 receptor γ chain (IL-2Rγc), IL-15, and the major histocompatibility complex (MHC) class Ib protein H2-T3, also known as the thymus leukemia antigen or TL. While lineage tracking experiments indicated that iCD8α cells have a lymphoid origin, their development was independent of the transcriptional suppressor Id2, suggesting that these cells do not belong to the family of innate lymphoid cells. Finally, we identified cells with a similar phenotype in humans, which were profoundly depleted in newborns with necrotizing enterocolitis. These findings suggest a critical role of iCD8α cells in immune responses associated with the intestinal epithelium.

Fc Receptors in Immune Responses

Antibodies are major molecular effectors of adaptive immune responses. Most, if not all, biological activities of antibodies, however, depend on the functional properties of cells that express receptors for the Fc portion of antibodies (FcR). Most FcR are activating receptors; some are inhibitory. When engaged by antibodies and antigen, the various FcR expressed by a given cell trigger a mixture of positive and negative signals whose integration determines cellular responses. Responses of cell populations can be either protective or pathogenic. As a consequence, FcR are potential target/tools in a variety of diseases including infection, allergy, autoimmune diseases, and cancer.

Fc receptors as adaptive immunoreceptors

Most biological activities of antibodies depend on their ability to engage Receptors for the Fc portion of immunoglobulins (FcRs) on a variety of cell types. As FcRs can trigger positive and negative signals, as these signals control several biological activities in individual cells, as FcRs are expressed by many cells of hematopoietic origin, mostly of the myeloid lineage, as these cells express various combinations of FcRs, and as FcR-expressing cells have different functional repertoires, antibodies can exert a wide spectrum of biological activities. Like B and T Cell Receptors (BCRs and TCRs), FcRs are bona fide immunoreceptors. Unlike BCRs and TCRs, however, FcRs are immunoreceptors with an adaptive specificity for antigen, with an adaptive affinity for antibodies, with an adaptive structure and with an adaptive signaling. They induce adaptive biological responses that depend on their tissue distribution and on FcR-expressing cells that are selected locally by antibodies. They critically determine health and disease. They are thus exquisitely adaptive therapeutic tools.

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.

Detection and characterization of hemopoietic stem cells in the adult human small intestine

The concept of lymphoid differentiation in the human gastrointestinal tract is controversial but is the focus of this study, which examined adult human small intestinal tissue for the presence of CD34(+)CD45(+) hemopoietic stem cells (HSCs) and lymphoid progenitors. Flow cytometry demonstrated that over 5% of leukocytes (CD45(+) cells) isolated from human gut were HSCs coexpressing CD34, a significantly higher incidence than in matched peripheral blood or control bone marrow. HSCs were detected in cell preparations from both the epithelium and lamina propria of all samples tested and localized to the intestinal villous and crypt regions using immunofluorescence. A high proportion of gut HSCs expressed the activation marker CD45RA, and few expressed c-kit, indicating ongoing differentiation. The vast majority of intestinal HSCs coexpressed the T cell Ag, CD7 (92% in the epithelium, 80% in the lamina propria) whereas <10% coexpressed the myeloid Ag CD33, suggesting that gut HSCs are a relatively mature population committed to the lymphoid lineage. Interestingly, almost 50% of epithelial layer HSCs coexpressed CD56, the NK cell Ag, compared with only 10% of the lamina propria HSC population, suggesting that the epithelium may be a preferential site of NKR(+) lymphoid differentiation. In contrast, bone marrow HSCs displayed low coexpression of CD56 and CD7 but high coexpression of CD33. The phenotype of intestinal HSCs, which differs significantly from circulating or bone marrow HSCs, is consistent with a role in local lymphoid development.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Isolation and characterization of a putative intestinal stem cell fraction from mouse jejunum

BACKGROUND & AIMS

Although there have been many recent advances regarding the biology of intestinal stem cells, the field has been hampered significantly by the lack of a method to isolate these cells. Therefore, the aim of this study was to explore the hypothesis that viable intestinal stem cells can be isolated as a side population (SP) by fluorescence-activated cell sorting after staining with the DNA-binding dye Hoechst 33342.

METHODS

Preparations of individual cells from either whole mucosa or epithelium of mouse jejunum were stained with Hoechst 33342 and propidium iodide and then sorted using fluorescence-activated cell sorting. Cells were characterized using fluorochrome-labeled antibodies to surface markers, intracellular markers, and annexin V to detect early apoptosis. Total RNA was isolated from sorted fractions and used for quantitative real-time reverse-transcription polymerase chain reaction to evaluate the expression of cell lineage markers and the intestinal stem-cell marker, Musashi-1.

RESULTS

Adult and neonatal jejunum contain a viable population of cells that shows the SP phenotype and is sensitive to verapamil. This population of cells (from both mucosal and epithelial preparations) includes a CD45-negative fraction corresponding to nonhematopoietic cells, which shows minimal expression of surface markers typically found on stem cells from other tissues and of intracellular markers found in mesenchymal cells. Additionally, these cells were enriched for Musashi-1 and beta1-integrin, were cytokeratin positive, and survived in culture for up to 14 days.

CONCLUSIONS

The CD45-negative SP fraction, although not pure, represents the successful isolation of a viable population significantly enriched in small intestinal epithelial stem cells.

IELs: enforcing law and order in the court of the intestinal epithelium

The intestinal intraepithelial lymphocytes (IELs) are mostly T cells dispersed as single cells within the epithelial cell layer that surrounds the intestinal lumen. IELs are, therefore, strategically located at the interface between the antigen-rich outside world and the sterile core of the body. The intestine of higher vertebrates has further evolved to harbor numerous commensal bacteria that carry out important functions for the host, and while defensive immunity can effectively protect against the invasion of pathogens, similar immune reactions against food-derived antigens or harmless colonizing bacteria can result in unnecessary and sometimes damaging immune responses. Probably as a result of this unique dilemma imposed by the gut environment, multiple subsets of IEL have differentiated, which all display characteristics of 'activated yet resting' immune cells. Despite this common feature, IELs are heterogeneous with regard to their phenotype, ontogeny, and function. In this review, we discuss the different subtypes of IELs and highlight the distinct pathways they took that led to their unique differentiation into highly specialized effector memory T cells, which provide the most effective immune protection yet in a strictly regulated fashion to preserve the integrity and vital functions of the intestinal mucosal epithelium.

Intestinal γδ T Cells Develop in Mice Lacking Thymus, All Lymph Nodes, Peyer’s Patches, and Isolated Lymphoid Follicles

Through analysis of athymic (nu/nu) mice carrying a transgenic gene encoding GFP instead of RAG-2 product, it has recently been reported that, in the absence of thymopoiesis, mesenteric lymph nodes and Peyer's patches (PP) but not gut cryptopatches are pivotal birthplace of mature T cells such as the thymus-independent intestinal intraepithelial T cells (IEL). To explore and evaluate this important issue, we generated nu/nu mice lacking all lymph nodes (LN) and PP by administration of lymphotoxin-beta receptor-Ig and TNF receptor 55-Ig fusion proteins into the timed pregnant nu/+ mice that had been mated with male nu/nu mice (nu/nu LNP- mice). We also generated nu/nu aly/aly (aly, alymphoplasia) double-mutant mice that inherently lacked all LN, PP, and isolated lymphoid follicles. Although gammadelta-IEL were slightly smaller in number than those in nu/nu mice, substantial colonization of gammadelta-IEL was found to take place in the intestinal epithelia of nu/nu LNP- and nu/nu aly/aly mice. Notably, the population size of a major CD8alphaalpha+ gammadelta-IEL subset was maintained, the use of TCR-gamma-chain variable gene segments by these gammadelta-IEL was unaltered, and the development of cryptopatches remained intact in these nu/nu LNP- and nu/nu aly/aly mice. These findings indicate that all LN, including mesenteric LN, PP, and isolated lymphoid follicles, are not an absolute requirement for the development of gammadelta-IEL in athymic nu/nu mice.

Starting at the beginning: new perspectives on the biology of mucosal T cells

The gastrointestinal tract is the central organ for uptake of fluids and nutrients, and at the same time it forms the main protective barrier between the sterile environment of the body and the outside world. In mammals, the intestine has further evolved to harbor a vast load of commensal bacteria that have important functions for the host. Discrimination by the host defense system of nonself from self can prevent invasion of pathogens, but equivalent responses to dietary or colonizing bacteria can lead to devastating consequences for the organism. This dilemma imposed by the gut environment has probably contributed significantly to the evolutionary drive that has led to sophisticated mechanisms and diversification of the immune system to allow for protection while maintaining the integrity of the mucosal barrier. The immense expansion and specialization of the immune system is particularly mirrored in the phylogeny, ontogeny, organization, and regulation of the adaptive intraepithelial lymphocytes, or IEL, which are key players in the unique intestinal defense mechanisms that have evolved in mammals.

gammadelta T-cell clones from intestinal intraepithelial lymphocytes inhibit development of CTL responses ex vivo

Oral administration of antigen induces a state of tolerance that is associated with activation of CD8+ T cells that can transfer unresponsiveness to naïve syngeneic hosts. These T cells are not lytic, but they inhibit development of antibody, CD4+ T helper cell, and CD8+ cytotoxic T lymphocyte (CTL) responses upon adoptive transfer into naïve, syngeneic mice. In addition, we have shown that depletion of gammadelta T cells by injection of the anti-delta chain antibody (GL3) down modulates the expression of gammadelta T-cell receptor (TCR) and inhibits the induction of oral tolerance to ovalbumin. Oral administration of antigen also fails to induce tolerance in TCR delta-chain knockout mice suggesting that gammadelta T cells play a critical, active role in tolerance induced by orally administered antigen. To further study the contribution of gammadelta T cells to tolerance, murine gammadelta T cells were isolated from intraepithelial lymphocytes (IEL) of the small intestine by stimulation with splenic filler cells, concanavalin A and growth factors. gammadelta IEL lines demonstrated lytic activity in a redirected lysis assay. gammadelta T-cell clones express different gammadelta TCR genes and secrete large amounts of interleukin (IL)-10, but little or no IL-2, IL-4, or interferon-gamma. gammadelta IEL clones expressed transforming growth factor-beta1 and macrophage migration inhibitory factor, as well as IL-10, mRNA. Moreover, gammadelta T-cell clones potently inhibited the generation of CTL responses by secreted molecules rather than by direct cell-to-cell contact.

Extrathymic TCR Gene Rearrangement in Human Small Intestine: Identification of New Splice Forms of Recombination Activating Gene-1 mRNA with Selective Tissue Expression

Two new 5'-untranslated region (5'UTR) exons were identified in the human gene for the lymphocyte-specific endonuclease recombination activating gene-1 (RAG1) required for the somatic recombination yielding functional Ag receptors. These 5'UTR exons were used in three different splice forms by jejunal lymphocytes of the T cell lineage. RAG1 mRNA containing the previously described 5'UTR exon was not expressed in these cells. Conversely, one of the new 5'UTR exons was not expressed in thymus. The new RAG1 mRNA splice forms were all expressed in immature T cells (CD2(+)CD7(+)CD3(-)). This cell population also expressed high levels of mRNA for the pre-T alpha-chain. In situ hybridization demonstrated jejunal cells expressing the new splice forms of RAG1 mRNA, both intraepithelially and in lamina propria. Pre-T alpha-chain mRNA-expressing cells were detected at the same sites. These results strongly suggest ongoing TCR gene rearrangement in human small intestinal mucosa, yielding T cells specially adapted for this environment. This seems to be achieved by two parallel processes, extrathymic T cell development and peripheral Ag-driven TCR editing.

Involvement of CCL25 (TECK) in the generation of the murine small-intestinal CD8alpha alpha+CD3+ intraepithelial lymphocyte compartment

The CC chemokine CCL25 (TECK) is selectively expressed in the thymus and small intestine, indicating a potential role in T lymphocyte development. In the present study we examined the role of CCL25 in the generation of the small-intestinal CD8alpha alpha(+)CD3(+) intraepithelial lymphocyte (IEL) compartment. CCL25 mRNA expression in the murine small intestine increased at three weeks of age and corresponded with the appearance of CD8alpha alpha(+)CD3(+) lymphocytes in the small-intestinal epithelium. Administration of monoclonal neutralizing anti-CCL25 antibody to two-week-old mice led to a approximately 50% reduction in the total number of CD8alpha alpha(+)TCRgamma delta(+) and CD8alpha alpha(+)TCRalpha beta(+) IEL at four weeks of age. Freshly isolated murine CD8alpha alpha(+)CD3(+) IEL migrated in response to CCL25 and expressed the CCL25 receptor, CCR9. Analysis of CCR9 expression on putative IEL precursor populations demonstrated the presence of both CCR9(-) and CCR9(+) cells and indicated that up-regulation of this receptor occurred during IEL precursor differentiation. Finally, data from wild-type and RAG(-/-) mice suggested that the reduction in CD8alpha alpha(+)CD3(+) IEL in anti-CCL25 antibody treated mice resulted primarily from defective maintenance and/or development of IEL precursors rather than a direct effect on mature CD8alpha alpha(+)CD3(+) IEL.