Self-nonself discrimination by T cells

The alpha beta T cell receptor (TCR) recognizes antigens that are presented by major histocompatibility complex (MHC)-encoded cell surface molecules by binding to both the antigen and the MHC molecules. Discrimination of self from nonself antigens and MHC molecules is achieved by negative and positive selection of T cells in the thymus: potentially harmful T cells with receptors that bind to self antigens plus self MHC molecules are deleted before they can mount immune responses. In contrast, the maturation of useful T cells with receptors that bind foreign antigens plus self MHC molecules requires the binding of their receptor to MHC molecules on thymic epithelium in the absence of foreign antigen. The binding of the TCR to either class I or class II MHC molecules directs differentiation of the selected cells into either CD4-8+ (killer) or CD4+8- (helper) T cells, respectively.     

Peripheral selection of the T cell repertoire

T lymphocytes undergo selection events not only in the thymus, but also after they leave the thymus and reside in the periphery. Peripheral selection was found to be dependent on T cell receptor (TCR)-ligand interactions but to differ from thymic selection with regard to specificity and mechanism. Unlike thymic selection, peripheral selection required binding of antigen to the TCR, and it induced expansion of T cell clones. Tolerance to self antigens that are restricted to the periphery occurred through the elimination of self-reactive T cells and by the clonal anergy, which was associated with down-regulation of the alpha beta TCR and CD8.     

Anti-idiotypic network induced by T cell vaccination against experimental autoimmune encephalomyelitis

In a study of the mechanism of resistance to autoimmune disease induced by T cell vaccination, rats were vaccinated against experimental autoimmune encephalomyelitis (EAE) by injecting them once in the hind footpads with a subencephalitogenic dose (10(4)) of a clone of T lymphocytes specific for myelin basic protein (BP). The response to vaccination was assayed by challenging the rats with an encephalitogenic dose (3 X 10(6)) of T lymphocytes of this BP-specific clone. Five to six days after vaccination, the cells responsible for mediating resistance to adoptively transferred EAE were concentrated in the popliteal lymph nodes draining the vaccination site. Transfer of the draining lymph node cells to unvaccinated rats led to loss of resistance in the donor rats and acquisition of resistance by the recipient rats. Limiting-dilution cultures of the draining lymph node cells were established with irradiated cells of the BP-specific clone as stimulators. Two sets of T lymphocytes specifically responsive to the BP-specific T cells from the clone were isolated: CD4+CD8- helper and CD4-CD8+ suppressor cells. The helper T cells, like the BP antigen, specifically stimulated the BP-specific vaccinating clone. In contrast, the suppressor T cells specifically suppressed the response of the BP-specific vaccinating clone to its BP antigen. These results suggest that T cell vaccination induces resistance to autoimmune disease by activating an antiidiotypic network.     

Defective CD8 T cell memory following acute infection without CD4 T cell help

The CD8+ cytotoxic T cell response to pathogens is thought to be CD4+ helper T cell independent because infectious agents provide their own inflammatory signals. Mice that lack CD4+ T cells mount a primary CD8 response to Listeria monocytogenes equal to that of wild-type mice and rapidly clear the infection. However, protective memory to a challenge is gradually lost in the former animals. Memory CD8+ T cells from normal mice can respond rapidly, but memory CD8+ T cells that are generated without CD4 help are defective in their ability to respond to secondary encounters with antigen. The results highlight a previously undescribed role for CD4 help in promoting protective CD8 memory development.     

Antigen bias in T cell cross-priming

Activated CD8+ T cells detect virally infected cells and tumor cells by recognition of major histocompatibility complex class I-bound peptides derived from degraded, endogenously produced proteins. In contrast, CD8+ T cell activation often occurs through interaction with specialized antigen-presenting cells displaying peptides acquired from an exogenous cellular source, a process termed cross-priming. Here, we observed a marked inefficiency in exogenous presentation of epitopes derived from signal sequences in mouse models. These data indicate that certain virus- and tumor-associated antigens may not be detected by CD8+ T cells because of impaired cross-priming. Such differences in the ability to cross-present antigens should form important considerations in vaccine design.     

CD8alphaalpha-mediated survival and differentiation of CD8 memory T cell precursors

Memory T cells are long-lived antigen-experienced T cells that are generally accepted to be direct descendants of proliferating primary effector cells. However, the factors that permit selective survival of these T cells are not well established. We show that homodimeric alpha chains of the CD8 molecule (CD8alphaalpha) are transiently induced on a selected subset of CD8alphabeta+ T cells upon antigenic stimulation. These CD8alphaalpha molecules promote the survival and differentiation of activated lymphocytes into memory CD8 T cells. Thus, memory precursors can be identified among primary effector cells and are selected for survival and differentiation by CD8alphaalpha.     

Alloantigen recognition is preceded by nonspecific adhesion of cytotoxic T cells and target cells

T-cell receptors bind antigens only when the antigens are exposed on the cell surface. This can be studied best in the interaction of cytolytic T lymphocytes (CTL) with target cells because the recognition and binding event can be separated from the lytic phase. Studies with CTL clones specific for HLA-A2 and HLA-B7 demonstrated that conjugates of CTL's and target cells can be formed in the absence of specific antigen recognition. Furthermore, T-cell receptor and target antigen cannot interact unless there is conjugate formation. This indicates that nonspecific conjugate formation between CTL's and target cells precedes the recognition of specific antigen by the T-cell receptor.     

Selective stimulation of T cell subsets with antibody-cytokine immune complexes

Interleukin-2 (IL-2), which is a growth factor for T lymphocytes, can also sometimes be inhibitory. Thus, the proliferation of CD8+ T cells in vivo is increased after the injection of a monoclonal antibody that is specific for IL-2 (IL-2 mAb), perhaps reflecting the removal of IL-2-dependent CD4+ T regulatory cells (T regs). Instead, we show here that IL-2 mAb augments the proliferation of CD8+ cells in mice simply by increasing the biological activity of preexisting IL-2 through the formation of immune complexes. When coupled with recombinant IL-2, some IL-2/IL-2 mAb complexes cause massive (>100-fold) expansion of CD8+ cells in vivo, whereas others selectively stimulate CD4+ T regs. Thus, different cytokine-antibody complexes can be used to selectively boost or inhibit the immune response.     

Structure and specificity of a class II MHC alloreactive gamma delta T cell receptor heterodimer

Two distinct CD3-associated T cell receptors (TCR alpha beta and TCR gamma delta) are expressed in a mutually exclusive fashion on separate subsets of T lymphocytes. While the specificity of the TCR alpha beta repertoire for major histocompatibility complex (MHC) antigens is well established, the diversity of expressed gamma delta receptors and the ligands they recognize are less well understood. An alloreactive CD3+CD4-CD8- T cell line specific for murine class II MHC (Ia) antigens encoded in the I-E subregion of the H-2 gene complex was identified, and the primary structure of its gamma delta receptor heterodimer was characterized. In contrast to a TCR alpha beta-expressing alloreactive T cell line selected for similar specificity, the TCR gamma delta line displayed broad cross-reactivity for multiple distinct I-E-encoded allogeneic Ia molecules.     

The human immunodeficiency virus: infectivity and mechanisms of pathogenesis

Infection with the human immunodeficiency virus (HIV) results in a profound immunosuppression due predominantly to a selective depletion of helper/inducer T lymphocytes that express the receptor for the virus (the CD4 molecule). HIV also has tropism for the brain leading to neuropsychiatric abnormalities. Besides inducing cell death, HIV can interfere with T4 cell function by various mechanisms. The monocyte serves as a reservoir for HIV and is relatively refractory to its cytopathic effects. HIV can exist in a latent or chronic form which can be converted to a productive infection by a variety of inductive signals.     

Antigen-specific helper function of cell-free T cell products bearing TCR V beta 8 determinants

Although the T cell receptor (TCR) alpha beta heterodimer and its encoding genes have been characterized, a cell-free form of this receptor, which is needed for the study of functional or ligand-binding properties of the receptor, has not previously been isolated. When the cell-free supernatant products of activated cloned T helper (TH) cells were found to mediate helper activity with antigen specificity identical to that of intact T cells, experiments were carried out to determine whether this functional activity was mediated by a cell-free form of TCR-related material. A disulfide-linked dimer indistinguishable from the T cell surface alpha beta heterodimer was precipitated from cell-free supernatants of cloned TH cells with F23.1, a monoclonal antibody specific for a TCR V beta 8 determinant. Moreover, when cell-free TH products were bound to and eluted from immobilized F23.1, these affinity-purified materials had antigen-specific and major histocompatibility complex-restricted helper activity that synergized with recombinant lymphokines in the generation of B cell antibody responses. These findings suggest that the factor isolated from T cell supernatants is a cell-free form of the TCR alpha beta dimer.     

The role of B cells for in vivo T cell responses to a Friend virus-induced leukemia

B cells can function as antigen-presenting cells and accessory cells for T cell responses. This study evaluated the role of B cells in the induction of protective T cell immunity to a Friend murine leukemia virus (F-MuLV)-induced leukemia (FBL). B cell-deficient mice exhibited significantly reduced tumor-specific CD4+ helper and CD8+ cytotoxic T cell responses after priming with FBL or a recombinant vaccinia virus containing F-MuLV antigens. Moreover, these mice had diminished T cell responses to the vaccinia viral antigens. Tumor-primed T cells transferred into B cell-deficient mice effectively eradicated disseminated FBL. Thus, B cells appear necessary for efficient priming but not expression of tumor and viral T cell immunity.     

Synergism between HIV gp120 and gp120-specific antibody in blocking human T cell activation

The human immunodeficiency virus (HIV) binds to CD4-positive cells through interaction of its envelope glycoprotein (gp120) with the CD4 molecule. CD4 is a prominent immunoregulatory molecule, and chronic exposure to antibody against CD4 (anti-CD4) has been shown to cause immunodeficiency in mice. T cell-dependent in vitro immune responses can also be inhibited by anti-CD4. Experimental findings reported here indicate that CD4-bound gp120 attracts gp120-specific antibodies derived from the blood of HIV-seropositive individuals to form a trimolecular complex with itself and CD4. Thus targeted to CD4, the gp120-specific antibody functions as an antibody to CD4; it cross-links and modulates the CD4 molecules and suppresses the activation of T cells as measured by mobilization of intracellular calcium (Ca2i+). The synergism between gp120 and anti-gp120 in blocking T cell activation occurs at low concentrations of both components. Neither gp120 nor anti-gp120 inhibits T cell activation by itself in the concentrations tested.     

Differential clustering of CD4 and CD3zeta during T cell recognition

Whereas T helper cells recognize peptide-major histocompatibility complex (MHC) class II complexes through their T cell receptors (TCRs), CD4 binds to an antigen-independent region of the MHC. Using green fluorescent protein-tagged chimeras and three-dimensional video microscopy, we show that CD4 and TCR-associated CD3zeta cluster in the interface coincident with increases in intracellular calcium. Signaling-, costimulation-, and cytoskeleton-dependent processes then stabilize CD3zeta in a single cluster at the center of the interface, while CD4 moves to the periphery. Thus, the CD4 coreceptor may serve primarily to "boost" recognition of ligand by the TCR and may not be required once activation has been initiated.     

Reciprocal control of T helper cell and dendritic cell differentiation

It is not known whether subsets of dendritic cells provide different cytokine microenvironments that determine the differentiation of either type-1 T helper (TH1) or TH2 cells. Human monocyte (pDC1)-derived dendritic cells (DC1) were found to induce TH1 differentiation, whereas dendritic cells (DC2) derived from CD4+CD3-CD11c- plasmacytoid cells (pDC2) induced TH2 differentiation by use of a mechanism unaffected by interleukin-4 (IL-4) or IL-12. The TH2 cytokine IL-4 enhanced DC1 maturation and killed pDC2, an effect potentiated by IL-10 but blocked by CD40 ligand and interferon-gamma. Thus, a negative feedback loop from the mature T helper cells may selectively inhibit prolonged TH1 or TH2 responses by regulating survival of the appropriate dendritic cell subset.     

A role for CD40 expression on CD8+ T cells in the generation of CD8+ T cell memory

The delivery of CD4 help to CD8+ T cell responses requires interactions between CD40 and CD40 ligand and is thought to occur through antigen-presenting cell (APC) activation. Here we show that generation of memory CD8+ T cells displaying an enhanced capacity for cell division and cytokine secretion required CD4 help but not CD40 expression by the APCs. Activated CD4+ and CD8+ T cells expressed CD40; and in the absence of this protein, CD8+ T cells were unable to differentiate into memory cells or receive CD4 help. These results suggest that, like B cells, CD8+ T cells receive CD4 help directly through CD40 and that this interaction is fundamental for CD8+ T cell memory generation.     

Regulation of antigen-specific CD8+ T cell homeostasis by perforin and interferon-gamma

T cell memory depends on factors that regulate expansion and death of these cells after antigenic stimulation. Mice deficient in perforin and interferon-gamma (IFN-gamma) exhibited increased expansion, altered immunodominance, and decreased death of antigen-specific CD8+ T cells after infection with an attenuated strain of Listeria monocytogenes, which was cleared from these mice. Expansion of CD8+ T cells was controlled by perforin, whereas IFN-gamma regulated immunodominance and the death phase. Thus, perforin and IFN-gamma regulate distinct elements of CD8+ T cell homeostasis independently of their role as antimicrobial effector molecules.