Yueh-hsiu Chien

A DEVELOPMENTAL SWITCH IN THYMIC LYMPHOCYTE MATURATION POTENTIAL OCCURS AT THE LEVEL OF HEMATOPOIETIC STEM-CELLS

Hematopoietic stem cells (HSCs) isolated from mouse fetal liver, like adult HSCs, are Thy-1lo Lin- Sca-1+. Donor-derived V gamma 3+ T cells were detected in fetal thymic lobes repopulated in vitro with fetal liver HSCs, but not in those with adult bone marrow HSCs. Single clonogenic fetal HSCs gave rise to thymic progeny that include V gamma 3+, other gamma delta+, and alpha beta+ T cells. No V gamma 3+ T cells were detected in adult thymus injected intrathymically with either fetal or adult HSCs. These results support the hypothesis that only fetal HSCs have the capacity to differentiate into V gamma 3+ T cells in the fetal thymic microenvironment and that the developmental potential of HSCs may change during ontogeny.

A TCR Binds to Antagonist Ligands with Lower Affinities and Faster Dissociation Rates Than to Agonists

T lymphocyte activation is mediated by the interaction of specific TCR with antigenic peptides bound to MHC molecules. Single amino acid substitutions are often capable of changing the effect of a peptide from stimulatory to antagonistic. Using surface plasmon resonance, we have analyzed the interaction between a complex consisting of variants of the MCC peptide bound to a mouse class II MHC (Ek) and a specific TCR. Using both an improved direct binding method as well as a novel inhibition assay, we show that the affinities of three different antagonist peptide-Ek complexes are approximately 10-50 times lower than that of the wildtype MCC-Ek complex for the TCR, largely due to an increased off-rate. These results suggest that the biological effects of peptide antagonists and partial agonists may be largely based on kinetic parameters.

The nature of major histocompatibility complex recognition by γδ T cells

Abstract Despite intensive efforts, the general rules for γδ T cell recognition remain undefined. Here, we take advantage of the detailed knowledge of the molecular structure and biosynthetic pathways of major histocompatibility complex (MHC) molecules to analyze the recognition properties of the γδ T cell clones LBK5 (specific for the class II MHC, IE k ) and G8 (specific for the nonclassical class I MHC, TL 10 b ). We find that the activation of these clones requires neither class I nor class II antigen-processing and that peptides do not confer specificity. Epitope mapping also shows that the topology of γδ T cell receptor interaction with the MHC is distinct from that of αβ T cells. These results suggest that the molecular nature of γδ T cell recognition is fundamentally different than that of αβ T cells.

Thymic Selection Determines γδ T Cell Effector Fate: Antigen-Naive Cells Make Interleukin-17 and Antigen-Experienced Cells Make Interferon γ

gammadelta T cells uniquely contribute to host immune defense, but how this is accomplished remains unclear. Here, we analyzed the nonclassical major histocompatibility complex class I T10 and T22-specific gammadelta T cells in mice and found that encountering antigen in the thymus was neither required nor inhibitory for their development. But when triggered through the T cell receptor, ligand-naive lymphoid-gammadelta T cells produced IL-17, whereas ligand-experienced cells made IFN-gamma. Immediately after immunization, a large fraction of IL-17(+) gammadelta T cells were found in the draining lymph nodes days before the appearance of antigen-specific IL-17(+) *beta T cells. Thus, thymic selection determines the effector fate of gammadelta T cells rather than constrains their antigen specificities. The swift IL-17 response mounted by antigen-naive gammadelta T cells suggests a critical role for these cells at the onset of an acute inflammatory response to novel antigens.

RECOGNITION BY γ/δ T CELLS

In contrast with the study of T cells, that of T cells is relatively recent and stems from the discovery of their rearranged genes, rather than from any knowledge of their biological function. Thus, experiments designed to characterize their specificity and function have drawn heavily on our knowledge of T cells. During the past few years, many studies, especially with mice lacking either or T cells, have demonstrated that T cells can contribute to immune competence, but they do so in a way that is distinct from T cells. It is also evident that T cells may not recognize antigen the same way as do T cells. Analysis of three protein antigens—the murine MHC class II IE k , the nonclassical MHC T10/T22, and the Herpes virus glycoprotein gI—indicates that T cell recognition does not require antigen processing and that the proteins are recognized directly. In all three cases, recognition by these T cell clones involves neither peptides bound to these proteins nor peptides derived from them. Moreover, a group of small phosphate-containing nonpeptide compounds derived from mycobacterial extracts has been found to stimulate a major population of human peripheral T cells in a T cell receptor (TCR)-dependent manner. This indicates that T cells can respond to ligands that are different from those of T cells. Analysis of complementarity determining region (CDR3) length distributions of and chains indicates that they are more similar to those of immunoglobulins than to TCR and . This further supports the idea that and T cells recognize antigens differently and suggests that T cells may be more like immunoglobulins in their recognition properties. T cells share many cell surface proteins with T cells and are able to secrete lymphokines and express cytolytic activities in response to antigenic stimulation. These, together with the results cited above, indicate that T cells can mediate cellular immune functions without a requirement for antigen processing. Thus, pathogens, damaged tissues, or even B and T cells can be recognized directly, and cellular immune responses

INDUCTION OF RAPID T CELL ACTIVATION AND TOLERANCE BY SYSTEMIC PRESENTATION OF AN ORALLY ADMINISTERED ANTIGEN

To understand how orally introduced antigen regulates peripheral immune responses, we fed cytochrome c protein to mice transgenic for the beta chain of a cytochrome c-specific TCR and followed the antigen-specific T cell responses with a cyt c/I-Ek tetramer staining reagent. We find that within 6 hr of cytochrome c administration, antigen-specific systemic T cell activation is induced, and spleen cells gain the ability to stimulate cytochrome c-specific T cell responses. Feeding multiple low doses of cytochrome c down-regulates the systemic immune response, which can be correlated with a reduction of antigen-specific T cells and not with immune deviation. These results suggest that systemic distribution of antigen contributes significantly to oral tolerance induction.

γδ T Cells: First Line of Defense and Beyond

γδ T cells, αβ T cells, and B cells are present together in all but the most primitive vertebrates, suggesting that each population contributes to host immune competence uniquely and that all three are necessary for maintaining immune competence. Functional and molecular analyses indicate that in infections, γδ T cells respond earlier than αβ T cells do and that they emerge late after pathogen numbers start to decline. Thus, these cells may be involved in both establishing and regulating the inflammatory response. Moreover, γδ T cells and αβ T cells are clearly distinct in their antigen recognition and activation requirements as well as in the development of their antigen-specific repertoire and effector function. These aspects allow γδ T cells to occupy unique temporal and functional niches in host immune defense. We review these and other advances in γδ T cell biology in the context of their being the major initial IL-17 producers in acute infection.

Attributes of gammadelta intraepithelial lymphocytes as suggested by their transcriptional profile.

γδ T lymphocytes in the intestinal intraepithelial layer (γδ IELs) are thought to contribute to immune competence, but their actual function remains poorly understood. Here we used DNA microarrays to study the gene expression profile of γδ IELs in a Yersinia infection system to better define their roles. To validate this approach, mesenteric lymph node CD8+ αβ T cells were similarly analyzed. The transcription profiles show that, whereas lymph node CD8+ αβ T cells must be activated to become cytotoxic effectors, γδ IELs are constitutively activated and appear to use different signaling cascades. Our data suggest that γδ IELs may respond efficiently to a broad range of pathological situations irrespective of their diverse T cell antigen receptor repertoire. γδ IELs may modulate local immune responses and participate in intestinal lipid metabolism, cholesterol homeostasis, and physiology. This study provides a strong basis for further investigations of the roles of these cells as well as mucosal immune defense in general.

Antigen recognition by γδ T cells

Summary:  γδ T cells contribute to host immune competence uniquely. This is most likely because they have distinctive antigen‐recognition properties. While the basic organization of γδ T‐cell receptor (TCR) loci is similar to that of αβ TCR loci, there is a striking difference in how the diversity of γδ TCRs is generated. γδ and αβ T cells have different antigen‐recognition requirements and almost certainly recognize a different set of antigens. While it is unclear what most γδ T cells recognize, the non‐classical major histocompatibility complex class I molecules T10 and T22 were found to be the natural ligands for a sizable population (0.2–2%) of murine γδ T cells. The recognition of T10/T22 may be a way by which γδ T cells regulate cells of the immune system, and this system has been used to determine the antigen‐recognition determinants of γδ T cells. T10/T22‐specific γδ T cells have TCRs that are diverse in both V gene usage and CDR3 sequences. Their Vγ usage reflects their tissue origin, and their antigen specificity is conferred by a motif in the TCR δ chain that is encoded by V and D segments and by P‐nucleotide addition. Sequence variations around this motif modulate affinities between TCRs and T10/T22. That this CDR3 motif is important in antigen recognition is confirmed by the crystal structure of a γδ TCR bound to its ligand. The significance of these observations is discussed in the context of γδ T‐cell biology.

CD4 Augments the Response of a T Cell to Agonist but Not to Antagonist Ligands

The recognition of peptide variants by the T cell receptor (TCR) has revealed a wide range of possible responses. Here, using a series of CD4+ and CD4- variants of the same T cell hybridoma, we find that while the expression of CD4 converts weak agonists into full agonists, none of the antagonist peptides are efficiently recognized as agonists. Furthermore, in antagonist assays, little difference can be seen in the response of CD4+ and CD4- T cells. Together with previous work showing a marked difference in stability between TCR binding to agonist versus antagonist ligands, these data suggest that CD4 engagement occurs after a TCR-peptide/MHC complex has formed and that it requires a certain minimal half-life of the ternary complex to be fully engaged in signaling.