In vitro induction of primary, antigen-specific CTL from human peripheral blood mononuclear cells stimulated with synthetic peptides

A protocol for in vitro induction of primary, antigen-specific CTL from human peripheral blood mononuclear cells (PBMCs) was developed. Antigen presenting cells (APCs) consisted of Staphylococcus aureus Cowan-I (SAC-I) activated PBMCs treated with a citrate-phosphate buffer at pH 3 to release endogenous peptides bound to surface MHC. This treatment resulted in transient expression of empty class I molecules which could be subsequently stabilized with peptide and beta 2-microglobulin (beta 2m). SAC-I activated PBMCs from HLA-A2.1 normal donors loaded with HBV core 18-27 peptide following acid treatment were used to stimulate PBMCs depleted of CD4+ T cells, in the presence of recombinant interleukin-7 (rIL-7). After 12 days, cells were restimulated with autologous, peptide-pulsed, adherent cells and tested for CTL activity 7 days later. In 23 independent experiments from 13 different HLA-A2.1 donors, this protocol resulted in induction of primary CTL more than 90% of the time. As indicated by both the frequency and magnitude of the response against peptide-sensitized target cells, SAC-I activated PBMCs treated with acid were the most efficient stimulator APC. Thirteen per cent of the cultures generated were capable of lysing target cells transfected with the HBV core antigen and, in general, these CTL cultures exhibited high avidity for the HBV core peptide. This protocol is generally applicable to different antigens and class I alleles, and thus, may be utilized to screen large numbers of peptides to identify human CTL epitopes.     

The origin and erratic global spread of tuberculosis. How the past explains the present and is the key to the future

A variety of different methods for the in vitro restimulation of human cytotoxic T lymphocyte (CTL) precursors (CTLp) are in use. Our aim was to enhance the detection of circulating human CTLp in peripheral blood. We have developed a standardized and highly efficient method for restimulating CTLp. Synthetic peptides were used to restimulate cognate CTLp from peripheral blood mononuclear cells (PBMC), and effector CTL capable of lysing peptide-pulsed and virus infected targets were generated. The effects of several parameters on CTL specific for influenza A, EBV and HIV-1 were evaluated, and the optimum peptide concentration for CTL generation was established. Supplementation of initial cultures with IL-7 greatly enhanced peptide-specific lytic activity for all peptides tested and the dose-response relationship for IL-7 was delineated. A novel technique using peptide-MHC class I molecule tetramers to stain T cells bearing cognate T cell receptors permitted enumeration of antigen-specific CD8 + CTL during in vitro restimulation; IL-7 supplementation selectively expanded the population of peptide-specific CD8 + CTL. Importantly, this protocol, whilst enhancing the restimulation and lytic activity of secondary CTL, does not induce primary CTL in vitro. The improved efficiency with which CTL are generated in this system substantially enhances the sensitivity of CTL culture and the 51Cr release assay to detect low levels of CTL activity.     

Protective CD4+ and CD8+ T cells against influenza virus induced by vaccination with nucleoprotein DNA

DNA vaccination is an effective means of eliciting both humoral and cellular immunity, including cytotoxic T lymphocytes (CTL). Using an influenza virus model, we previously demonstrated that injection of DNA encoding influenza virus nucleoprotein (NP) induced major histocompatibility complex class I-restricted CTL and cross-strain protection from lethal virus challenge in mice (J. B. Ulmer et al., Science 259:1745-1749, 1993). In the present study, we have characterized in more detail the cellular immune responses induced by NP DNA, which included robust lymphoproliferation and Th1-type cytokine secretion (high levels of gamma interferon and interleukin-2 [IL-2], with little IL-4 or IL-10) in response to antigen-specific restimulation of splenocytes in vitro. These responses were mediated by CD4+ T cells, as shown by in vitro depletion of T-cell subsets. Taken together, these results indicate that immunization with NP DNA primes both cytolytic CD8+ T cells and cytokine-secreting CD4+ T cells. Further, we demonstrate by adoptive transfer and in vivo depletion of T-cell subsets that both of these types of T cells act as effectors in protective immunity against influenza virus challenge conferred by NP DNA.     

Generation of CD8+ and CD4+ T-cell response to dendritic cells genetically engineered to express the MART-1/Melan-A gene

Both CD8+ and CD4+ T cells have demonstrated roles in antitumor immune response in many animal tumor systems. In many human tumor systems, although abundant literature exists on the evidence of tumor antigen-specific CD8+ CTL response, only limited information is available on tumor antigen-specific CD4+ T-cell response. Using the MART-1/Melan-A (MART-1) antigen system as a prototype human tumor-associated antigen (TAA)- and dendritic cell (DC)-based MART-1 antigen presentation system (i.e., DCs transduced with an adenoviral vector-based construct carrying the MART-1 gene), we explored, in vitro, the feasibility of generating both CD8+ and CD4+ T-cell responses in the same individual. Here, we show that autologous DCs from both HLA-A2-positive melanoma patients and normal healthy individuals that are transduced with an adenoviral vector containing the MART-1 antigen are capable of inducing both MART-1-specific CD8+ and CD4+ T cells in in vitro coculture. After several rounds of stimulation, both the CD4+ and CD8+ T cells synthesized IFN-gamma when they were specifically stimulated. The CD8+ T cells generated in such cocultures also recognized the MART-1(27-35) peptide, AAGIGILTV, in 4-h cytotoxicity assays. These observations, therefore, suggest that Th1-type responses can be generated, in vitro, by stimulation with DCs that are genetically modified to express a TAA. Although the outcome of this type of genetically engineered DC-based stimulation may vary from system to system, this type of in vitro antigen presentation may be very useful in more comprehensive analyses of CD4+ T-cell response to defined TAAs, and such genetically engineered autologous DCs might be better candidates to serve as surrogate cancer vaccines.     

Ex vivo generation of human anti-pre-B leukemia-specific autologous cytolytic T cells

In contrast to other neoplasms, antigen-specific autologous cytolytic T cells have not been detected in patients with human pre-B-cell leukemias. The absence of efficient B7 family (B7-1/CD80; B7-2/CD86) -mediated costimulation has been shown to be a major defect in tumor cells' capacity to function as antigen-presenting cells. We show here the generation of autologous anti-pre-B-cell leukemia-specific cytolytic T-cell lines from the marrows of 10 of 15 patients with pre-B-cell malignancies. T-cell costimulation via CD28 is an absolute requirement for the generation of these autologous cytolytic T cells (CTL). Although costimulation could be delivered by either bystander B7 transfectants or professional antigen-presenting cells (indirect costimulation), optimal priming and CTL expansion required that the costimulatory signal was expressed by the tumor cell (direct costimulation). These anti-pre-B-cell leukemia-specific CTL lysed both unstimulated and CD40-stimulated tumor cells from each patient studied but did not lyse either K562 or CD40-stimulated allogeneic B cells. Cytolysis was mediated by the induction of tumor cell apoptosis by CD8+ T cells via the perforin-granzyme pathway. Although we were able to generate anti-leukemia-specific CTL from the bone marrow, we were unable to generate such CTL from the peripheral blood of these patients. These studies show that antigen-specific CTL can be generated from the bone marrow of patients with pre-B-cell leukemias and these findings should facilitate the design of adoptive T-cell-mediated immunotherapy trials for the treatment of patients with B-cell precursor malignancies.     

Generation and characterization of gp100 peptide-specific NK-T cell clones

MHC-restricted cytotoxic T lymphocytes (CTLs) specific for antigens expressed by malignant cells are important components of immune responses against human cancer. Peripheral blood monocytes of HLA-A2+ healthy donors were used to induce dendritic cells (DCs) by granulocyte-macrophage colony-stimulating factor and interleukin-4 and loaded with a gp100 peptide (YLEPGPVTA). By applying these peptide-loaded DCs, a CTL line that displayed high cytotoxic reactivity with peptide-loaded target cells was generated. A total of 11 gp100 peptide-specific CTL clones were generated from this cell line. Several of these CTL clones were studied in detail. Of particular interest was clone CTL-45, which, contrary to the parental cell line, displayed strong NK activity and, by flow-cytometric analysis, revealed a CD3+, TCR BV17, CD8+ and CD56+ phenotype. This clone was strictly peptide-specific and effectively killed a panel of melanoma cells expressing HLA-A2 and gp100. Tumor-specific T cells with this kind of dual function are potentially of great clinical importance as they have a backup mechanism that may go into action when tumor cells escape specific killing by losing their HLA-class I molecules.     

Bone marrow-generated dendritic cells pulsed with a class I-restricted peptide are potent inducers of cytotoxic T lymphocytes

It has previously been shown that bone marrow-generated dendritic cells (DC) are potent stimulators in allogeneic mixed leukocyte reactions and are capable of activating naive CD4+ T cells in situ in an antigen-specific manner. In this study we have investigated whether bone marrow-generated DC are capable of inducing antigen-specific CD8+ T cell responses in vivo. Initial attempts to induce specific cytotoxic T lymphocyte (CTL) responses in mice injected with bone marrow-generated DC pulsed with ovalbumin (OVA) peptide were frustrated by the presence of high levels of nonspecific lytic activity, which obscured, though not completely, the presence of Ag-specific CTL. Using conditions that effectively differentiate between antigen-specific and nonspecific lytic activity, we have shown that bone marrow-generated DC pulsed with OVA peptide are potent inducers of OVA-specific CTL responses in vivo, compared with splenocytes or RMA-S cells pulsed with OVA peptide, or compared with immunization with free OVA peptide mixed with adjuvant. Antibody-mediated depletion experiments have shown that the cytotoxic effector cells consist primarily of CD8+ cells, and that induction of CTL in vivo is dependent on CD4+ as well as on CD8+ T cells. These results provide the basis for exploring the role of bone marrow-generated DC in major histocompatibility class I-restricted immune responses, and they provide the rationale for using bone marrow-generated DC in CTL-mediated immunotherapy of cancer and infectious diseases.     

Simultaneous induction of multiple antigen-specific cytotoxic T lymphocytes in nonhuman primates by immunization with a mixture of four Plasmodium falciparum DNA plasmids

CD8(+) T cells have been implicated as critical effector cells in protective immunity against malaria parasites developing within hepatocytes. A vaccine that protects against malaria by inducing CD8(+) T cells will probably have to include multiple epitopes on the same protein or different proteins, because of parasite polymorphism and genetic restriction of T-cell responses. To determine if CD8(+) T-cell responses against multiple P. falciparum proteins can be induced in primates by immunization with plasmid DNA, rhesus monkeys were immunized intramuscularly with a mixture of DNA plasmids encoding four P. falciparum proteins or with individual plasmids. All six monkeys immunized with PfCSP DNA, seven of nine immunized with PfSSP2 DNA, and five of six immunized with PfExp-1 or PfLSA-1 DNA had detectable antigen-specific cytotoxic T lymphocytes (CTL) after in vitro restimulation of peripheral blood mononuclear cells. CTL activity was genetically restricted and dependent on CD8(+) T cells. By providing the first evidence for primates that immunization with a mixture of DNA plasmids induces CD8(+) T-cell responses against all the components of the mixture, these studies provide the foundation for multigene immunization of humans.     

Distinct T cell receptor signaling requirements for perforin- or FasL-mediated cytotoxicity

A diverse array of signals are generated in a cytotoxic T lymphocyte (CTL) after the T cell receptor (TCR) engages the class I major histocompatibility complex (MHC) peptide complex. These signals result in a multitude of CTL effector functions, including cellular cytotoxicity, cell surface receptor expression, and cytokine secretion. We have examined signaling through the TCR in a wild type CD8+, MHC-restricted, antigen-specific CTL clone, 14-7, and its interleukin 2-dependent variant clone 14-7FD. We report here that 14-7FD is unable to kill via the perforin mechanism of killing, yet is able to kill via the Fas ligand/Fas mechanism and secrete interferon-gamma in an antigen-specific manner. 14-7FD has cytolytic granules that contain perforin and serine esterases, which are secreted after phorbol ester and Ca2+ ionophore treatment. Lastly, to investigate which TCR signaling requirements were operational in 14-7FD, we examined TCR-triggered intracellular Ca2+ mobilization in the two clones. After TCR engagement, 14-7FD failed to mobilize intracellular Ca2+, which may be the cause for its inability to trigger the perforin/granule exocytosis mechanism of killing. These results indicate that the signal transduction events that trigger perforin killing and the signaling requirements to induce FasL expression are distinct. We hypothesize that these two distinct TCR signal transduction requirements allow for separate activation of these two mechanisms of killing relating to their role in eradication of infected cells or regulation of immune responses.     

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Crosslinking of CD28 receptors on resting T lymphocytes by B7 costimulatory molecules expressed by antigen-presenting cells (APCs) plays a critical role in T-cell activation. Human melanomas express major histocompatibility complex (MHC)-restricted tumor-associated antigens that can be recognized by cytotoxic T lymphocytes (CTL), yet they remain poorly immunogenic. One mechanism for the failure of T-cell response is the lack of expression of costimulatory molecules by human melanoma cells. We have transfected the B7-1 gene into three HLA-A2-expressing human melanoma cell lines, and studied their capacity to stimulate primary human T cells. B7-expressing melanoma cells were excellent inducers of T-cell proliferation, cytokine production, and cytolytic activity in allogeneic mixed lymphocyte cultures through a process dependent on the function of the T-cell receptor as well as interactions between B7:CD28, CD2:LFA-3, and LFA-1:ICAM-1. Subset analysis demonstrated that CD4+ T cells or addition of exogenous interleukin-2 was required for the induction of CD8+ CTL. Untransfected parental melanoma cells were inert as APCs in these cultures. Rotating stimulation of T cells with the three B7-expressing cell lines led to the generation of T-cell lines that were cytolytic for HLA-A2+ melanoma cells and other HLA-A2+ targets that were pulsed with HLA-A2-restricted MART-1 peptides. These data demonstrate that expression of B7-1 by human melanoma cells converts them into effective APCs for the in vitro induction of MHC-restricted, melanoma-specific CTL.     

Th1 cells induce and Th2 inhibit antigen-dependent IL-12 secretion by dendritic cells

Dendritic cells are the most relevant antigen-presenting cells (APC) for presentation of antigens administered in adjuvant to CD4+ T cells. Upon interaction with antigen-specific T cells, dendritic cells (DC) expressing appropriate peptide-MHC class II complexes secrete IL-12, a cytokine that drives Th1 cell development. To analyze the T cell-mediated regulation of IL-12 secretion by DC, we have examined their capacity to secrete IL-12 in response to stimulation by antigen-specific Th1 and Th2 DO11.10 TCR-transgenic cells. These cells do not differ either in TCR clonotype or CD40 ligand (CD40L) expression. Interaction with antigen-specific Th1, but not Th2 cells, induces IL-12 p40 and p75 secretion by DC. The induction of IL-12 production by Th1 cells does not depend on their IFN-gamma secretion, but requires direct cell-cell contact mediated by peptide/MHC class II-TCR and CD40-CD40L interactions. Th2 cells not only fail to induce IL-12 secretion, but they inhibit its induction by Th1 cells. Unlike stimulation by Th1, inhibition of IL-12 production by Th2 cells is mediated by soluble molecules, as demonstrated by transwell cultures. Among Th2-derived cytokines, IL-10, but not IL-4 inhibit Th1-driven IL-12 secretion. IL-10 produced by Th2 cells appears to be solely responsible for the inhibition of Th1 -induced IL-12 secretion, but it does not account for the failure of Th2 cells to induce IL-12 production by DC. Collectively, these results demonstrate that Th1 cells up-regulate IL-12 production by DC via IFN-gamma-independent cognate interaction, whereas this is inhibited by Th2-derived IL-10. The inhibition of Th1 -induced IL-12 production by Th2 cells with the same antigen specificity represents a novel mechanism driving the polarization of CD4+ T cell responses.     

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Induction and maintenance of peripheral tolerance are important mechanisms to maintain the balance of the immune system. In addition to the deletion of T cells and their failure to respond in certain circumstances, active suppression mediated by T cells or T-cell factors has been proposed as a mechanism for maintaining peripheral tolerance. However, the inability to isolate and clone regulatory T cells involved in antigen-specific inhibition of immune responses has made it difficult to understand the mechanisms underlying such suppression. Here, we show that chronic activation of both human and murine CD4+ T cells in the presence of interleukin (IL)-10 gives rise to CD4+ T-cell clones with low proliferative capacity, producing high levels of IL-10, low levels of IL-2 and no IL-4. These antigen-specific T-cell clones suppress the proliferation of CD4+ T cells in response to antigen, and prevent colitis induced in SCID mice by pathogenic CD4+CD45RBhigh splenic T cells. Thus IL-10 drives the generation of a CD4+ T-cell subset, designated T regulatory cells 1 (Tr1), which suppresses antigen-specific immune responses and actively downregulates a pathological immune response in vivo.     

In vitro induction of naive cytotoxic T lymphocytes with complexes of peptide and recombinant MHC class I molecules coated onto beads: role of TCR/ligand density

We previously reported that complexes of peptide with soluble single-chain recombinant MHC (SC-MHC) class I molecules are able to induce cytotoxic T lymphocytes (CTL) in vitro in a murine system with an efficiency comparable to that observed with peptide-pulsed dendritic cells as antigen-presenting cells. In this report, we have assessed the capacity of preformed peptide/SC-Kd complexes in monomeric or dimeric form as well as of peptide/SC-Kd-loaded beads to generate in vitro specific CTL responses from naive DBA/2 spleen cells. Peptide/SC-Kd-coated beads were consistently more efficient. We evaluated the role of costimulatory molecules, using monoclonal antibodies anti-CD80 or anti-CD86. In addition, the capacity of peptide/SC-Kd-coated beads to generate a CTL response from purified naive CD8+ T cells was ascertained. Taken together, the results indicate that, under our conditions, CTL priming does not require the participation of co-stimulatory molecules and is the consequence of a direct interaction between the cognate TCR on peptide-specific CTL precursors and the peptide/SC-Kd-loaded beads. Titration of the amount of preformed complexes of SC-Kd and peptide 170-179 of HLA-CW3 that need to be coated onto the beads to prime CTL precursors shows an activation threshold which can be calculated to be between 25000 and 50000 complexes. In effect, in cultures stimulated with specific peptide CW3/SC-Kd complexes representing less than 50% occupancy of the total (10(5)) complexes on the beads, no peptide-specific cytolytic activity was observed. These results suggest that the efficiency of the primary CTL induction depends on the density of specific peptide/SC-Kd complexes present on the beads.     

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This study addresses the nature of the pathogenic effector T cell in experimental autoimmune uveoretinitis and the effect of different cytokines on these cells in vitro. Lymph node cells of B10.RIII mice immunized with the uveitogenic peptide 161-180 of interphotoreceptor retinoid binding protein were cultured with the peptide with or without IL-12, IL-4, or anti-IL-4. An antigen-specific T cell line was subsequently derived from these cells. Primary cultures of immune lymph node cells stimulated with the peptide proliferated and produced IL-2 and some IL-4, but no IFN-gamma. The addition of recombinant IL-12 resulted in abundant production of IFN-gamma, which was blocked by the addition of IL-4 and was enhanced by anti-IL-4. Only those cultures that produced IFN-gamma in vitro were uveitogenic in vivo. A long-term uveitogenic T cell line, initially derived in the presence of IL-12, produced IFN-gamma and IL-2, but not IL-4, and was CD4+ (Th1-like). Antigen-specific proliferation and IFN-gamma production of the line were enhanced by exogenous IL-4, TGF-beta, IL-2, IL-6, IL-7, and IL-9 and were inhibited by IL-10 and TNF-alpha. Our results provide support for the hypothesis that the uveitogenic effector T cell has a Th1-like phenotype. Furthermore, the data suggest that the effects of the cytokine milieu on fully differentiated Th1 effectors may differ considerably from their effects on less mature stages of antigen-specific T cells.     

Expression of the costimulatory receptor CD30 is regulated by both CD28 and cytokines

Costimulation was originally defined and characterized during primary T cell activation. The signaling events that regulate subsequent antigen encounters by T cells are less well defined. In this study we examined the role of CD30 in T cell activation and defined factors that regulate expression of CD30 on T cells. We demonstrate that CD30 expression is restricted to activated T cells and regulated by CD28 signal transduction. In contrast to CD28-expressing TCR Tg cells, CD28-deficient TCR Tg cells did not express CD30 when cultured with peptide and APCs. However, rIL-4 reconstituted CD30 expression on CD28-deficient TCR Tg cells. Blockade of CD28 interactions or depletion of IL-4 inhibited the induction of CD30, suggesting that both CD28 and IL-4 play important roles in the induction of CD30 expression on wild-type cells. However, CD28 signaling did not up-regulate CD30 expression solely through its ability to augment IL-4 production because IL-4-deficient T cells stimulated with anti-CD3 and anti-CD28 expressed CD30. Induction of CD30 in the absence of IL-4 was not due to the IL-4-related cytokine IL-13. CD30, when expressed on an activated T cell, can act as a signal transducing receptor that enhances the proliferation of T cells responding to CD3 crosslinking. Collectively, the data suggest that T cell expression of CD30 is dependent on the presence of CD28 costimulatory signals or exogenous IL-4 during primary T cell activation. Once expressed on the cell surface, CD30 can serve as a positive regulator of mature T cell function.     

T-cell receptor peptides as immunotherapy for autoimmune disease

The observations in both mouse and rat models of experimental allergic encephalomyelitis (EAE) demonstrating restricted T-cell receptor (TCR) usage among pathogenic T cells has led to the generation of a new class of therapeutic vaccines composed of TCR V region peptides. Whether a similar approach will be of use in the treatment of human autoimmune disorders is still unclear. The experiments performed in our laboratory over the past several years have focused on two aspects of TCR peptide immunoregulation, namely, (1) how to identify the critical T-cell populations involved in the pathology of autoimmune disease, and (2) how to identify biologically relevant TCR peptides--those endogenous TCR peptides presented in association with MHC molecules on the surface of pathogenic T cells that are recognized by immunoregulatory T-cell populations. Results of our recently completed clinical studies regarding TCR V beta expression among CD4+ T cells in the cerebral spinal fluid (CSF) of patients with multiple sclerosis suggests that these cells may be an appropriate T-cell population to be targeted for TCR peptide therapy. In addition, our studies on the immune response to autologous, soluble TCR heterodimers may provide a strategy for the identification of new TCR peptide candidate vaccines.     

Direct activation of human CD8+ cytotoxic T lymphocytes by interleukin-18

Direct activation of human cytotoxic T lymphocytes (CTL) by interleukin (IL)-18 was observed in a system in which CTL effective against autologous tumor cells were generated. Peripheral blood mononuclear cells (PBMC) from tumor-bearing patients, after removal of natural killer (NK) cells, were cultured in a medium containing IL-1, -2, -4, and -6, with or without IL-18, and stimulated with autologous tumor cells. IL-18 increased the activity of the CTL and the proportion of autologous CD8+ T cells present after 28 days in the induction culture. When purified CD8+ T cells were cultured in the presence of IL-18 and IL-2 for 7 days, the CTL showed enhanced cytotoxic activity against autologous tumor cells. Moreover, a purified CD8+ T cell population, which did not exhibit any apparent cytotoxic activity against autologous tumor cells, displayed cytotoxic activity after 7-day incubation with IL-18. These results suggest that IL-18 may be useful to generate autologous CTL in humans and may thereby contribute to adoptive immunotherapy for tumors.     

Inhibition of human T-cell responses to house dust mite allergens by a T-cell receptor peptide

Recent analysis of the usage of T-cell receptor (TcR) beta chain variable region (V beta) gene elements by house dust mite (HDM)-reactive T cells from an atopic donor suggested that TcR-V beta 3 gene products may form a major component of the human T-cell repertoire reactive to this common allergen. In this study a peptide analog of the TcR-V beta 3 complementarity determining region 2 (CDR2) is shown to inhibit the polyclonal human T-cell response to HDM; this effect is specific because inhibition is dependent on the presence of V beta 3 + T cells. This experimental approach has been used to determine whether the pattern seen in T-cell clones derived from one atopic donor reflects TcR-V beta usage in the polyclonal response to allergen in the general population. Inhibition of more than 50% of the polyclonal response to allergen by V beta 3-CDR2 peptide was observed in 16 of 21 donors tested, suggesting that TcR-V beta 3 gene usage may form a major component of the human HDM repertoire and as such offer a suitable target for T cell-directed specific immunotherapy in HDM-allergic individuals. Depletion of CD8+ T cells abolishes peptide-mediated inhibition of CD4+ T-cell proliferation to HDM, suggesting that induction of a CD8+ regulatory T-cell subset by the CDR2 peptide may modulate HDM-specific allergic T-cell responses.