Co-stimulation in T cell responses

Src family tyrosine kinases associate with and phosphorylate CTLA-4 (CD152)

CTLA-4 (CD152) transduces inhibitory signals for T cell activation. Phosphorylation and dephosphorylation of tyrosine residue (Y)-165 in the cytoplasmic region of CTLA-4 play an important role in the signal transduction and in the cell surface. While signaling molecules such as SHP-2 and the p85 subunit of PI3 kinase associate with this tyrosine residue through SH2 domains upon phosphorylation, the adapter complex AP-2 interacts with the same tyrosine when dephosphorylated, leading to clathrin-mediated endocytosis of CTLA-4. We searched for the tyrosine kinase responsible for the phosphorylation of CTLA-4. Src family tyrosine kinases Fyn, Lyn, and Lck associate with CTLA-4 and phosphorylate both Y-165 and Y-182 that are mainly responsible for interaction with Fyn through its SH2 domain. SHP-2 associates with CTLA-4, in a Fyn-dependent manner. Our observations show that src family tyrosine kinases associate with and phosphorylate CTLA-4 and thereby have an important role in the signal transduction and the endocytosis of CTLA-4.

DNA immunization targeting the skin: molecular control of adaptive immunity

DNA-based immunization represents a novel approach for vaccine development. Recombinant DNA techniques are used to clone DNA sequences encoding antigens of choice into eukaryotic expression plasmids, which are readily and economically amplified in bacteria and recovered with a high degree of purity. For immunization, plasmid DNA is either coated onto microscopic gold particles and bombarded into skin using a gene gun or injected into skin or muscle. Expression of administered genes results in the induction of humoral and cellular immune responses against the encoded antigen. DNA immunization is capable of inducing protective immunity in a number of animal models of infectious disease and cancer. Recent studies suggest that antigen-specific cytotoxic T lymphocyte induction occurs through the presentation of appropriate peptides in the context of major histocompatibility complex molecules on bone marrow-derived professional antigen presenting cells. Following DNA inoculation into the skin, Langerhans cells and/or dermal dendritic cells are believed to acquire the newly synthesized antigen, either through direct transfection or via antigen uptake from transfected keratinocytes, and migrate to regional lymph nodes where they stimulate primary T cell responses. The nature of the immune response depends on the route, method, and timing of DNA delivery and can also be influenced by co-delivery of plasmids encoding immunomodulating cytokines like IFN-alpha, IL-2, or IL-12 and costimulatory molecules like B7-1. While many aspects of the biology of cutaneous DNA immunization remain unknown, the skin appears to offer unique potential as a target for DNA-based immunization.

Direct exposure to gallium arsenide upregulates costimulatory activity of murine macrophages

Gallium arsenide (GaAs) is an intermetallic semiconductor compound used in the electronics industry. Acute exposure of animals to GaAs systemically suppresses several immune functions while paradoxically causing inflammation at the exposure site. We investigated the effect of GaAs on costimulatory activity of murine peritoneal macrophages, 5 days after ip exposure. Costimulation by macrophages was determined by activation of CD4(+) helper T cell hybridomas to secrete interleukin-2 in the presence of immobilized monoclonal anti-CD3 antibody. Both peritoneal exudate cells (PEC) and resident peritoneal cells exposed to GaAs provided greater costimulation to the T cells than vehicle control cells. Resident peritoneal cells exposed to GaAs were also more efficient than latex bead-exposed cells, indicating that phagocytosis alone did not cause the GaAs effect. Double immunofluorescence staining and flow cytometric analysis revealed that GaAs-exposed PEC had increased cell surface expression of costimulatory B7-1 and B7-2 molecules and intracellular adhesion molecule-1 (ICAM-1) compared to controls. In addition to these molecules, resident peritoneal macrophages exposed to GaAs also expressed significantly higher levels of heat-stable antigen (HSA). Monoclonal antibodies specific for these costimulatory molecules significantly inhibited T cell activation, demonstrating that the molecules on GaAs-exposed cells were functional. In contrast, GaAs did not upregulate costimulatory molecules on splenic macrophages. These findings suggest that direct GaAs exposure improves macrophage costimulatory activity, possibly by activating the cells, which may contribute to respiratory inflammation caused by inhalation of GaAs particles.

Epitope spreading: the role of self peptides and autoantigen processing by B lymphocytes

A complex interplay of cells, soluble macromolecules, and antigen lead to a productive immune response that evolved for the survival of species. While the immune system is intended to protect from foreign agents, such as bacterial and viral infection, the presence of autoimmune diseases indicates that the system is not perfect in differentiating antigen that may cause harm from benign self constituents. The concept of epitope spreading, where many determinants on an offending antigen are the focus of immune attack, is an efficient means of clearing an infectious agent. However, the same mechanisms that lead to a diverse immune response may be harmful when the targets of attack are self tissues or self macromolecules. This review will examine the forms of self antigens that may initiate autoimmunity and the potential role of B lymphocytes, as autoantigen-presenting cells, as one mechanism by which diversification of autoimmunity may occur.

Cutting Edge: Induction of Antigen-Specific Hyporesponsiveness by Transplantation of Hemopoietic Cells Containing an MHC Class I Transgene Regulated by a Lymphocyte-Specific Promoter

We explored a novel approach to tolerance induction by the transplantation of bone marrow (BM) cells (BMCs) that themselves do not express a foreign histocompatibility Ag, but which give rise to mature lymphocytes that do so. Lines of transgenic (FVB) mice were generated that contained an MHC class I Dd cDNA regulated by a CD2 promoter. Because the CD2 promoter is lymphocyte-specific and activated relatively late in lymphocyte ontogeny, Dd is expressed on most mature lymphocytes in the periphery but only on developing B cells in the BM of transgenic mice. Transgenic BMCs are tolerogenic and reproducibly engraft in nontransgenic mice using a conditioning regimen that is nonpermissive for the engraftment of conventional (MHC promoter) Dd-transgenic BMCs. Engrafted BMCs generate transgene-expressing lymphocytes and confer a state of Ag-specific hyporesponsiveness on the host that is primarily attributable to a peripheral mechanism. The strategies by which tolerance can be optimized in this system are discussed.

CD28 affects the earliest signaling events generated by TCR engagement

The efficiency and magnitude of T cell responses are influenced by ligation of the co-stimulatory receptor CD28 by B7 molecules expressed on antigen-presenting cells (APC). In contrast to most previous studies in which agonistic anti-TCR/CD3 and anti-CD28 antibodies were employed, here we have investigated the contribution of CD28 to T cell activation under physiological conditions of antigen presentation. Jurkat T cells and primary T cells from TCR-transgenic mice stimulated with superantigen and antigen, respectively, presented by B7-expressing APC were utilized. In both systems we show that inhibiting CD28/B7 interaction resulted in impaired TCR-induced tyrosine phosphorylation of the signal-transducing zeta chain and ZAP-70. Consistent with a blockade of TCR-proximal signaling events, Jurkat cells stimulated in the absence of CD28 ligation were found to have strongly diminished tyrosine phosphorylation of cellular substrates and downstream signaling pathways such as Ca2+/calcineurin, ERK/MAPK and JNK. Our results provide evidence for a role of CD28 in enhancing TCR signaling capacity during the earliest stages of T cell:APC interaction.

Requirements for B7-CD28 Costimulation in Mucosal IgA Responses: Paradoxes Observed in CTLA4-Hγ1 Transgenic Mice

The block in the CD80/CD86-CD28/CTLA-4 pathway in CTLA4-Hγ1 transgenic (Tg) mice results in strongly impaired systemic IgG immunity and failure to develop germinal center reactions. By contrast, here we report that mucosal immunity and IgA B cell differentiation are not affected by this block. We found abundant germinal centers and evidence of IgA switch differentiation in Peyer’s patches, normal total IgA levels, and normal numbers of IgA-labeling cells in the gut mucosa. The distribution of B-1 and B-2 cells and the relative contribution of B-1 cells to the total IgA B cells were similar in Tg and wild-type mice. Despite this, oral immunizations with keyhole limpet hemocyanin plus cholera toxin adjuvant failed to stimulate Ag-specific mucosal IgA responses in CTLA4-Hγ1 Tg mice. This was not due to a lack of adjuvant activity of cholera toxin in Tg mice, nor was this secondary to an inability to take up Ag from the gut lumen. Rather, CD4+ T cells stimulated by oral immunization in Tg mice appeared to be inappropriately primed, as evidenced by a significantly reduced level of CD40 ligand and CD44 expression and an increased expression of CD95 compared to those in wild-type mice. This study reveals a paradox in the regulation of mucosal IgA responses.

Toward a predictive understanding of molecular recognition

T cells circulate in blood and the lymphatic system, continually engaging cells through transient non-specific adhesion. In a normally functioning immune system, these interactions permit sufficient time for T-cell receptors (TCRs) to sample major histocompatibility complex (MHC)-peptide complexes for the presence of foreign antigen, with detection of the latter to some extent being triggered by a longer dwell time of the receptor on the complex. Precisely how this incremental stability, which may be relatively small, leads to activation is unclear, but it appears to be related to diffusion-mediated formation of ternary complex dimers. The formation of stable dimers can explain the high sensitivity of the response, but leaves a number of questions unaddressed, including the following: i) How can high sensitivity be reconciled with high specificity, and how can a short TCR dwell time be reconciled with a comparably short time for ternary complex pair formation? ii) What is the nature of the early signals on the plasma membrane that determine alternative responses e.g. proliferation at one extreme and apoptosis at the other? iii) What are the cell-surface correlates of biphasic dose response functions i.e. of responses that peak as a function of dose and then descend? This paper has two loosely coupled goals. One is to review and assess the mathematical and computational methods available for analyzing reactions with and between mobile membrane-bound receptors. These methods range from phenomenological to mechanistic, the latter being based on the details of atomic structure. The other is to apply these methods to address biological questions, such as those raised above, part of whose answer may lie in the kinetic competition between alternative reaction paths.

Negative regulation of T cell activation

T cell activation is negatively regulated by cytotoxic T lymphocyte antigen 4 (CTLA-4) and the killer cell inhibitory receptors. Endocytosis and signaling of CTLA-4 are regulated by tyrosine phosphorylation. While T cell activation is mediated by phosphorylation of immunoreceptor tyrosine-based activation motifs and tyrosine kinases, inhibitory signals are delivered by tyrosine phosphatases. Unresponsiveness is also induced by modulation of signaling components of the T cell receptor complex.

Non-glycosylated human B7-1(CD80) retains the capacity to bind its counter-receptors

Though the cell surface-associated costimulator B7-1(CD80) is known to be highly N-glycosylated, the functional significance of this N-glycosylation has not been evaluated. Two experimental approaches were taken to assess the influence of N-glycosylation on human B7-1 function. First, stable K562 transfectants expressing human B7-1 were treated with the N-glycosylation inhibitor tunicamycin. This treatment reduced the levels of B7-1 at the cell surface as judged by both indirect immunofluorescence/flow cytometry and immunoprecipitation analyses. Significantly, the non-glycosylated cell surface-associated B7-1 on tunicamycin-treated cells retained the capacity to bind CTLA-4 x Ig, a soluble derivative of the CTLA-4(CD152) counter-receptor. Second, experiments were performed with bacterially-produced non-glycosylated derivatives of human B7-1, comprising either the complete B7-1 extracellular domain (hB7-1 x ed) or the membrane-proximal IgC-homologue domain of B7-1 in isolation (hB7-1 x IgC). While the hB7-1 x IgC derivative failed to bind to CTLA-4, the larger hB7-1 x ed derivative associated with CTLA-4 x Ig in cell-free binding assays. Futhermore, recombinant hB7-1 x ed effectively blocked B7-1-mediated costimulation in an in vitro T cell proliferation assay, suggesting that this soluble non-glycosylated B7-1 derivative is capable of engaging CD28, the B7 counter-receptor implicated in T cell activation. Taken together, these data indicate that the N-glycosylation of B7-1 is not required for its association with counter-receptors. Moreover, the findings pave the way for the therapeutic use of recombinant bacterial B7-1 derivatives as competitive inhibitors of B7-mediated signals.

Direct Evidence That Functionally Impaired CD4+ T Cells Persist In Vivo Following Induction of Peripheral Tolerance

A small population of CD4+ OVA-specific TCR transgenic T cells was tracked following the induction of peripheral tolerance by soluble Ag to address whether functionally unresponsive, or anergic T cells, persist in vivo for extended periods of time. Although injection of OVA peptide in the absence of adjuvant caused a transient expansion and deletion of the Ag-specific T cells, a population that showed signs of prior activation persisted in the lymphoid tissues for several months. These surviving OVA-specific T cells had long-lasting, but reversible defects in their ability to proliferate in lymph nodes and secrete IL-2 and TNF-α in vivo following an antigenic challenge. These defects were not associated with the production of Th2-type cytokines or the capacity to suppress the clonal expansion of a bystander population of T cells present in the same lymph nodes. Therefore, our results provide direct evidence that a long-lived population of functionally impaired Ag-specific CD4+ T cells is generated in vivo after exposure to soluble Ag.

T-cell activation in autoimmune and inflammatory diseases

Recent progress in experimental models and human genetic linkage studies have provided new insight into the pathogenesis of autoimmunity. Both antigen-specific and antigen-nonspecific signals are crucial in the development of autoimmune disease. Interestingly, several of the single gene loci that have been identified as potential causes of autoimmune disease encode molecules that regulate antigen-nonspecific modulation of immunity. The focus of this review is the role of the opposing signals transduced by the CD28 and cytotoxic T-lymphocyte antigen-4 receptors that bind the B7 costimulatory ligands. Recent studies suggest that CD28 signals activate T cells, whereas cytotoxic T-lymphocyte antigen-4 signals deactivate T cells. importantly, both signals contribute to the induction of autoimmunity and offer novel targets for future therapeutic strategies to treat autoimmune disease.

Costimulatory CD80 (B7-1) and CD86 (B7-2) on cerebrospinal fluid cells in multiple sclerosis

The costimulatory CD80 and CD86 molecules were measured by flow cytometry on cerebrospinal fluid (CSF) and blood lymphocytes from patients with possible first attacks of multiple sclerosis (MS, n = 25), clinically definite MS (n = 16), and noninflammatory neurological disease control subjects (n = 30). In patients with demyelinating diseases more CSF B cells expressed CD80 than in control subjects whereas the expression of CD86 by T cells in CSF was low in patients with demyelinating disease and highly variable in the control subjects. In patients with possible first attacks of MS the expression pattern of CD80 and CD86 differed significantly between patients with or without intrathecal synthesis of IgG. Increased expression of the CD80 molecule on CSF B cells may be of importance in the pathogenesis of MS. In contrast, CSF T cell expression of CD86 may be associated with protection from MS.

Progression to the G1b phase of the cell cycle is required for completion of human immunodeficiency virus type 1 reverse transcription in T cells

Successful infection by human immunodeficiency virus type 1 (HIV-1) requires the activation of target cells. Infection of quiescent peripheral CD4 lymphocytes by HIV-1 results in incomplete, labile, reverse transcripts. In the present study, we isolated highly purified quiescent T cells and utilized the CD3/CD28 activation pathways as well as cell cycle inhibitors to further define the role of costimulation and cell cycle progression in HIV-1 reverse transcription. Activation with alphaCD3 alone resulted in cell cycle progression into only G1a and incomplete HIV-1 reverse transcription. Costimulation through the CD28 receptor and transition into G1b was required to efficiently complete the reverse transcription process. These findings have relevance to immune activation in vivo, since lymphocytes rendered anergic by a single activation signal would be nonpermissive for productive infection with HIV-1. Importantly, these data also suggest that HIV vector-based genetic transduction strategies might be successful only in target cells that transition into the G1b phase of the cell cycle.

Interleukin 2 restores CD3-zeta chain expression but fails to generate tumour-specific lytic activity in tumour-infiltrating lymphocytes derived from human colorectal hepatic metastases

Metastatic colorectal cancer is usually progressive despite infiltration of the tumours by T lymphocytes, suggesting that these tumour-infiltrating lymphocytes (TILs) are functionally deficient. Recently, TILs from other tumours have been shown to express reduced levels of the T-cell receptor signal-transducing CD3-zeta chain. We were interested to determine whether a similar abnormality existed in TILs from human colorectal hepatic metastasis (CHM) and, if so, whether correcting the abnormality in vitro would restore anti-tumour activity and provide support for the development of immunotherapy for colorectal hepatic metastases. Twelve of 19 TILs from colorectal hepatic metastases were successfully expanded in vitro in high-dose recombinant interleukin 2 (rlL-2) and their specific anti-tumour cytolytic activity was determined. CD3-positive (CD3+) TILs were HLA-Drhigh and CD69high, suggesting that they had been activated by exposure to antigen but expressed low levels of CD25, CD71 and the nuclear proliferation antigen Ki-67. Furthermore, they showed reduced expression of CD3-zeta compared with autologous peripheral blood T cells (PBTs) and failed to proliferate in the absence of high-dose rIL-2. Expansion of TILs in rIL-2 resulted in restoration of CD3-zeta expression and the ability to lyse K562 and Daudi cells but not autologous tumour cells. The absence of autologous tumour-specific cytolytic T-cell (CTL) activity may be due to the poor immunogenicity of colorectal tumour cells, which we found expressed only low levels of MHC I antigens and CD54 and failed to express MHC II antigens or the co-stimulatory molecules CD80, CD86 or CD106. The inability of rIL-2 to generate tumour-specific CTLs despite restoration of CD3-zeta expression and the presence of an intact lytic mechanism suggests that successful immunotherapy may require the development of strategies to increase the immunogenicity of this tumour.

Non-responsiveness of antigen-experienced CD4 T cells reflects more stringent co-stimulatory requirements

We recently reported that previously activated T cells, irrespective of the nature of the first stimulus they encountered, are unable to respond to Staphylococcal enterotoxin B (SEB), nor to soluble anti-CD3 monoclonal antibody (mAb) presented by splenic antigen-presenting cells (APC). Such previously activated T cells are, however, fully capable of responding to plate-bound anti-CD3 plus splenic APC. These data suggest differential integration of the T-cell receptor (TCR) and co-stimulatory signalling pathways in naive versus antigen-experienced T cells. Consistent with this hypothesis, anti-CD28 mAb restores the proliferative capacity of resting ex vivo CD45RBlo CD4+ T cells (representing previously activated T cells) to both soluble anti-CD3 mAb and SEB. Interestingly, mAb-mediated engagement of cytotoxic T-lymphocyte antigen-4 (CTLA-4) completely negates the rescue effects mediated by anti-CD28 mAb in CD45RBlo cells. Nevertheless, the non-responsiveness of CD45RBlo CD4+ T cells cannot be reversed by anti-CTLA-4 Fab fragments, indicating that it is not related to negative regulatory effects of CTLA-4 engagement itself. Interestingly, the addition of interleukin-2 (IL-2) restores the proliferative capacity of CD45RBlo CD4+ T cells to SEB and soluble anti-CD3 mAb. Moreover, when rescued by IL-2, the cells are less susceptible to the negative regulatory effects of CTLA-4 engagement. Together, these findings suggest that the non-responsiveness of CD45RBlo CD4+ T cells to certain stimuli may be related to inadequate TCR signalling, primarily affecting IL-2 production.

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) regulates the unfolding of autoimmune diabetes

Evidence has been accumulating that shows that insulin-dependent diabetes is subject to immunoregulation. To determine whether cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is involved, we injected anti-CTLA-4 mAb into a TCR transgenic model of diabetes at different stages of disease. When injected into young mice, months before they would normally become diabetic, anti-CTLA-4 induced diabetes rapidly and essentially universally; this was not the result of a global activation of T lymphocytes, but did reflect a much more aggressive T cell infiltrate in the pancreatic islets. These effects were only observed if anti-CTLA-4 was injected during a narrow time window, before the initiation of insulitis. Thus, engagement of CTLA-4 at the time when potentially diabetogenic T cells are first activated is a pivotal event; if engagement is permitted, invasion of the islets occurs, but remains quite innocuous for months, if not, insulitis is much more aggressive, and diabetes quickly ensues.

Cutting Edge: CTLA-4 Ligation Delivers a Unique Signal to Resting Human CD4 T Cells That Inhibits Interleukin-2 Secretion but Allows Bcl-XL Induction

We have assessed the functional effects of a panel of CTLA-4 mAbs on resting human CD4+ T cells. Our results demonstrate that some CTLA-4 mAbs can inhibit proliferative responses of resting CD4+ cells and cell cycle transition from G0 to G1. The inhibitory effects of CTLA-4 were evident within 4 h, at a time when cell surface CTLA-4 expression remained undetectable. Other CTLA-4 mAbs had no detectable inhibitory effects, indicating that binding of Ab to CTLA-4 alone is not sufficient to mediate down-regulation of T cell responses. Interestingly, while IL-2 production was shut off, inhibitory anti-CTLA-4 mAbs permitted induction and expression of the cell survival gene bcl-XL. Consistent with this observation, cells remained viable and apoptosis was not detected after CTLA-4 ligation.

T-cell regulation: Thy-1 - hiding in full view

Thy-1 is a highly abundant glycoprotein on the surface of thymocytes and neurons. A null mutation in the gene encoding Thy-1 deregulates T-cell receptor signaling and causes abnormal thymocyte development; does this mean that Thy-1 has a signaling function?

Lymphoproliferation in CTLA-4-deficient mice is mediated by costimulation-dependent activation of CD4+ T cells

CTLA-4-deficient animals develop a fatal lymphoproliferative disorder. The cellular mechanism(s) responsible for this phenotype have not been determined. Here, we show that there is a preferential expansion of CD4+ T cells in CTLA-4(-/-) mice, which results in a skewing of the CD4/CD8 T cell ratio. In vivo antibody depletion of CD8+ T cells from birth does not alter the onset or the severity of the CD28-dependent lymphoproliferative disorder. In contrast, CD4+ T cell depletion completely prevents all features characteristic of the lymphoproliferation observed in CTLA-4-deficient mice. These results demonstrate that CD4+ T cells initiate the phenotype in the CTLA-4(-/-) mice. Further, these results suggest that the role of CTLA-4 in peripheral CD4+ versus CD8+ T cell homeostasis is distinct.