APOBEC3A intratumoral DNA electroporation in mice.

Human APOBEC3A (A3A) cytidine deaminase shows pro-apoptotic properties resulting from hypermutation of genomic DNA, induction of double-stranded DNA breaks (DSBs) and G1 cell cycle arrest. Given this, we evaluated the antitumor efficacy of A3A by intratumoral electroporation of an A3A expression plasmid. DNA was repeatedly electroporated into B16OVA, B16Luc tumors of C57BL/6J mice as well as the aggressive fibrosarcoma Sarc2 tumor of HLA-A*0201/DRB1*0101 transgenic mice using noninvasive plate electrodes. Intratumoral electroporation of A3A plasmid DNA resulted in regression of ~50% of small B16OVA melanoma tumors that did not rebound in the following 2 months without treatment. Larger or more aggressive tumors escaped regression when so treated. As APOBEC3A was much less efficient in provoking hypermutation and DSBs in B16OVA cells compared with human or quail cells, it is likely that APOBEC3A would be more efficient in a human setting than in a mouse model.

Recurrent T cell receptor rearrangements in the cytotoxic T lymphocyte response in vivo against the p815 murine tumor.

P815 is a murine mastocytoma of DBA/2 origin which, although immunogenic, rapidly develops as a tumor in immunocompetent syngeneic hosts. In this report, we have studied, by a molecular approach, the in vivo alpha/beta T cell response to P815. Both situations of tumor growth after engraftment of naive animals or tumor rejection by preimmunized animals have been analyzed. The spectrum of T cell receptor beta chain rearrangements in the tumor-infiltrating lymphocytes was found to be highly variable among individual tumor-bearing mice. However, two rearrangements, one using V(beta)1 and J(beta)1.2 segments and one using the V(beta)1 and J(beta)2.5 segments, with conserved junctional regions, reproducibly emerge in most individuals. These two rearrangements thus correspond to "public" (recurrent) T cell clones, as opposed to "private" ones, which emerge in a seemingly stochastic fashion in immunized animals. Importantly, these public cells are observed in situations of either growth or rejection of the tumor. Quantification provides a clear increase in public T cells in secondary responses, but no obvious correlation provides between their level and primary tumor rejection. The V(beta)1- J(beta)1.2 rearrangement is borne by CTL directed against an antigen derived from P1A, a nonmutated mouse self protein which is expressed in P815 but not in normal mouse tissues except testis. A recurrent, public T cell response can thus be observed to an antigen derived from a self protein expressed by a tumor.

Fine mapping of epitopes by intradomain Kd/Dd recombinants.

11 intradomain recombinants between H-2Kd and H-2Dd were produced using an original technique based on in vivo recombination in Escherichia coli. After transfection into mouse L cells, all these recombinants were expressed at high levels on the cell surface. The specificities of 77 mAbs were examined on these cell lines. mAbs could be organized in 12 groups. In each group, a small number of amino acids participating in the recognized epitope(s) were identified. In a few instances, noncontinuous epitopes comprising amino acids belonging to different domains of the antigen were found. The data thus obtained are compatible with those produced in previous exon-shuffling experiments, but permit a much more precise definition of recognized epitope(s).

Cells expressing a major histocompatibility complex class I molecule with a single covalently bound peptide are highly immunogenic.

The major histocompatibility complex (MHC) class I molecules expressed at the cell surface are associated with a large number of different peptides so that the density of a given MHC-peptide complex is relatively low. Here we describe the properties of MHC class I molecules genetically attached to a single antigenic peptide. Cells expressing these fusion proteins are recognized by T cells specific for the particular MHC-peptide complex. Coculture of naive splenocytes with cells expressing these MHC-peptide fusion proteins and the B7.1 antigen allows the induction of primary cytotoxic T lymphocytes (CTL) in vitro. Injection of these cells into naive mice enhances the frequency of specific CTL precursors and protects against a subsequent challenge with a tumor or a virus bearing the antigenic peptide. Soluble MHC-peptide fusions were also produced in which all three components, that is, the heavy chain, beta 2-microglobulin and the peptide, have fused into a single-chain protein. The availability of MHC class I molecules bound to a single peptide provides valuable tools for the manipulation of CTL responses and the analysis of the selection processes in the thymus.

Human immunodeficiency virus (HIV) nef-specific cytotoxic T lymphocytes in noninfected heterosexual contact of HIV-infected patients.

We report on the detection of HIV-specific cytotoxic T lymphocytes (CTL) among 23 regular partners of HIV-infected individuals. 15 of the 46 individuals enrolled in the study were positive for HLA-A2.1 typing. Among the 23 contacts studied, 7 were seropositive and 16 were seronegative on repeated tests. None of the 16 seronegative contacts were positive for p24 antigenemia nor were they positive by the lymphocytes coculture assay, although, in two instances HIV-1 DNA could be detected by PCR (in one case using a gag SK 38/39 primer, and in the other using a primer for the pol P3/P4 primer). These two individuals remained seronegative for 18 and 36 mo, respectively. HIV-specific cytotoxicity was performed in the 15 HLA-A2.1 subjects (7 indexes, 2 seropositive contacts, and 6 seronegative contacts) and in 4 HLA-matched HIV negative donors. CTL specific for env, gag, or nef proteins could not be detected in unstimulated bulk cultures of peripheral blood lymphocytes in any of the six seronegative contacts. However, using a limiting dilution assay we found an usually high frequency of HIV nef-specific CTL precursors (CTLp) for HIV env and gag was very similar to that observed in seronegative HLA-matched healthy donors. Because no presence of HIV could be demonstrated in these individuals, these findings argue against the possibility of a silent HIV infection and suggest that a CTL response against nef may be involved in a rapid and effective clearance of the virus after sexual exposure.

Cytotoxic T cell response against the chimeric ETV6-AML1 protein in childhood acute lymphoblastic leukemia.

Cytotoxic T lymphocytes (CTL) are potent effector cells that could provide long term antitumor immunity if induced by appropriate vaccines. CTL recognize 8-14 amino acid-long peptides processed intracellularly and presented by MHC class I molecules. A well-characterized example of a potential tumor antigen in childhood pre-B Acute Lymphoblastic Leukemia (ALL) results from the chromosomal translocation 12;21 leading to the fusion of the ETV6 and AML1 genes. This translocation is observed in > 25% of ALL-patients. In this study, we have examined whether the chimeric ETV6-AML1 protein could serve as a tumor specific antigen for CTL in HLA-A2.1 individuals. We have identified a nonapeptide (RIAECILGM), encoded by the fusion region of the ETV6-AML1 protein, that binds to HLA-A2.1 molecules and induces specific primary CTL in peripheral blood lymphocytes from healthy donors. These CTL specifically lysed HLA-A2.1 tumor cells endogeneously expressing the ETV6-AML fusion protein. CTL with similar functional capacities were found with high frequencies and cloned from one patient's bone marrow indicating that ETV6-AML1-specific anti-ALL CTL are, at least in some patients, spontaneously stimulated and might participate to host antileukemia defense.

Cytotoxic T cell response against the chimeric p210 BCR-ABL protein in patients with chronic myelogenous leukemia.

Human chronic myelogenous leukemia (CML) is characterized by a translocation between chromosomes 9 and 22 that results in a BCR-ABL fusion gene coding for chimeric proteins. The junctional region of the BCR-ABLb3a2 molecule represents a potential leukemia-specific antigen which could be recognized by cytotoxic T lymphocytes (CTL). In fact, we identified a junctional nonapeptide (SSKALQRPV) which binds to HLA-A2.1 molecules. This peptide, as well as those binding to HLA-A3, -A11, and -B8 molecules (previously identified by others), elicits primary CTL responses in vitro from PBLs of both healthy donors and CML patients. Such CTL recognize HLA-matched, BCR-ABL-positive leukemic cells, implying efficient natural processing and presentation of these junctional peptides. Specific CTL were found at high frequency in 5 of 21 CML patients, suggesting that these epitopes are, to some extent, immunogenic in vivo during the course of the disease. These peptides could be useful for the development of specific immunotherapy in CML patients.

Analysis of T-cell defects in the specific immune response against acute lymphoblastic leukemia cells.

We previously showed that a specific antileukemia T-cytotoxic response is spontaneously elicited in acute lymphoblastic leukemia (ALL) patients and might contribute to host antileukemia defense, even though it is insufficient for tumor growth control. In this study, we report that multifactorial factors account for some of the acquired immune defects seen in ALL patients. In bone marrow of ALL patients, T cells do not express CD40L and CD25 markers, their apoptosis rate is increased, and a predominance of a CD4 cell subset expressing a Th2 phenotype is detected. A lack of expression of B7-1 molecules and other activation molecules is observed on all ALL blasts. These different parameters combined lead to in vivo dysfunction of T-cell proliferative and cytotoxic activity.

HIV-1 env, nef, and gag-specific T-cell immunity in mice: conserved epitopes in nef p27 and gag p25 proteins.

Cellular immunogenicity of env gp160, nef p27, and gag p55 proteins of human immunodeficiency virus type 1 (HIV-1) was studied in mice immunized with vaccinia virus recombinants. Proliferative responses of spleen cells were comparable against env gp160, nef p27, and gag p25 recombinant proteins. No specific activity was observed against gag p18 protein. Env, nef, and gag-specific T-cell lines were generated by repeated stimulation of immune spleen cells with recombinant HIV-1 proteins. They were CD4 positive, proliferative, and also cytotoxic against HIV-transfected target cells. Specificity of the T-cell response against nef and gag protein was analyzed with synthetic peptides. Peptides nef 15, nef 16, and gag AM-30 were, respectively, reactive in nef- and gag-specific proliferative and cytolytic assays. The three peptides described have a relatively conserved amino acid sequence among HIV isolates and appear broadly immunoreactive among species.

Alloantigen-specific regulation of cytotoxic T cell responses is mediated through the induction of clonal anergy of CD8+ T cells.

Priming mice with an alloantigen before immunization with this same alloantigen presented in association with a second one on an F1 stimulator cell inhibits the induction of cytotoxic response directed against the second alloantigen. This inhibition is associated with the induction of a strong cytotoxic T lymphocyte (CTL) response against the first priming alloantigen. For example, a specific suppression of anti-H-2b CTL responses could be induced in C3H/He mice (H-2k) by priming them with H-2d spleen cells before immunization with F1 (H-2dxb) spleen cells. In the present study, we have analyzed the mechanisms underlying this specific suppression of CTL responses. We have demonstrated that the reduction of H-2b-specific CTL responses is reflected by a decrease in the frequency of effector cells specific for H-2b antigen. However, there was no difference in the frequencies of precursor CTL in control and suppressed mice excluding clonal deletion as the mechanism maintaining low responsiveness. Co-culture experiments have shown that the suppression of anti-H-2b CTL responses was not due to suppressor cells but to the failure of CD8+ T cells of suppressed mice to collaborate with normal helper CD4+ T cells. The suppression was therefore ascribed to a functional impairment (clonal anergy) of the CD8+ T cell subset.

Primary cytotoxic T lymphocyte induction using peptide-stripped autologous cells.

MHC class I molecules bind short peptides derived from endogenously synthesized proteins. This binding occurs at neutral pH and MHC class I-peptide complexes dissociate at low or high pH. Here we show that MHC class I-peptide complexes expressed at the cell surface dissociate upon a brief and mild acid treatment without affecting cell viability or capacity of the peptide-stripped MHC molecules to re-bind exogenous peptides. Mouse or human blasts that have been peptide-stripped and reloaded with an exogenous peptide can induce in vitro peptide specific primary cytotoxic T lymphocytes (CTL) in mixed lymphocyte cultures. Mice immunized with syngeneic blasts that have been peptide-stripped and reloaded with a peptide derived from a tumor-associated antigen are protected against a subsequent challenge with a lethal dose of tumor cells. The importance of these findings for viral and tumor immunotherapy as well as for unravelling the mechanisms of induction of primary CTL responses are discussed.

Role for MHC class I molecules in selecting and protecting high affinity peptides in the presence of proteases.

Ag fragments derived from the cytosol are transported into the endoplasmic reticulum (ER) lumen, where they bind to nascent MHC class I molecules. However, it is not known whether only high affinity peptides enter the ER, or whether ER proteases must trim longer precursor peptides down to optimal size. To evaluate the feasibility of proteolytic fine trimming in vitro, soluble Kd and Kb were preincubated with peptides that bind to Kd or Kb and the mixture was exposed to three different proteases. Class I protected allele-specific peptides against proteolysis, whereas the other peptides were degraded to the amino acid level. When a Kd/S11E (SYIPSAEYILE) complex was immunoprecipitated after incubation with carboxypeptidase, both S11E and the optimal sized S9I (SYIPSAEYI) were found to be specifically bound to Kd. However, only S91 was recovered if S11E, Kd and carboxypeptidase were mixed at the same time, and there was no fine-trimming of bound S11E if high protease concentrations and short proteolysis times were used, which suggests that trimming occurs only in the unbound state. The amount of peptide that immunoprecipitated with Kd increased after carboxypeptidase treatment of Kd/S11E, implying that the peptide affinity had increased. Kd also protected S9I against proteolysis by a lysed microsome preparation, demonstrating that class I could also protect high affinity peptides in vivo. These results suggest that class I participates in the selection of high affinity peptides in the ER, by sampling transported unbound peptides are degraded by ER proteases or efflux back to the cytosol.

Retrovirus antigens recognized by cytolytic T lymphocytes activate tumor rejection in vivo.

We have initiated the molecular definition of the antigens recognized by Gross MuLV-specific cytolytic T lymphocytes on the surface of Gross MuLV-induced tumor cells. A panel of target cells was obtained by the double transfection and expression of a retrovirus gene and a foreign H-2 gene in recipient mouse fibroblasts. Our results show that class I H-2 transplantation antigens have a directive influence in determining the antigenicity of proteins encoded by the gag and env MuLV genes. Genes not linked to H-2 influence the intensity and the specificity of the cytolytic T lymphocyte response to Gross MuLV-induced tumors. Finally, MuLV-induced antigens expressed by transfected fibroblasts induce tumor immunity and lead to accelerated tumor rejection in vivo.

Molecular definition of retrovirus-induced antigens recognized by tumour-specific H-2-restricted cytolytic T lymphocytes.

The antigenic specificities of H-2-restricted, tumour-specific cytolytic T lymphocytes (CTL) were studied at the molecular level using CTL from BALB.B and C57BL/6 (H-2b) mice sensitized to an H-2b Gross murine leukaemia virus (MuLV)-induced tumour. Target cells were produced by the double transfection of mouse L cells (H-2k) with the cloned H-2Kb or H-2Db gene and retroviral DNA derived from a molecular clone of Akv MuLV (closely related to Gross MuLV). Doubly transfected L cells which express either H-2Kb or H-2Db antigen and retroviral antigens are lysed in a virus-specific manner by Gross MuLV-immune CTL. The existence of two independent Gross MuLV-immune CTL subpopulations, one restricted by H-2Kb and the other by H-2Db, is thus confirmed. Gross MuLV-immune CTL from both BALB.B and C57BL/6 mice killed L cells that express Akv MuLV gag gene products and H-2Kb or H-2Db antigen. In contrast, only CTL from C57BL/6 mice killed L cells that express Akv MuLV env gene products and H-2Kb or H-2Db. This indicates that specific recognition of MuLV-induced antigens by CTL can be selective and varies according to the origin of the CTL.

A novel approach to the induction of specific cytolytic T cells in vivo.

The induction of specific cytotoxic T lymphocytes (CTL) is one component in the immune response which can effectively protect the host against the progression of many viral infections. CTL are also known to play an important role in immune defence against tumour growth. CTL induction is dependent the presence of the specific antigen, appropriately presented, and interleukin-2 (IL2), provided by T helper lymphocytes. We studied the specific CTL response induced by tumour cells transfected with murine IL2. Our results show that tumour cells manipulated to secrete IL2 induce an improved specific anti-tumour response which results in tumour rejection in mice. To further investigate the effect of IL2 on the CTL response to different antigens, we introduced synthetic peptides into IL2-secreting tumour cells and determined the specific CTL induction in syngeneic mice immunized with these cells. We report here that such IL2-secreting cells can effectively prime peptide-specific CTL in vivo. Our data are relevant to immunotherapy and vaccine development and open up the possibility that autologous cells, manipulated to secrete IL2 and located with one or a cocktail of peptides, could be used to stimulate a specific CTL response.

Alloantigen pretreatment induces a down-regulation of the in vivo subsequent development of cytotoxic T lymphocytes directed against linked alloantigens.

In the present study, we describe a new regulatory system that influences the in vivo development of cytotoxic T lymphocytes (CTL) and that could be related to epitopic suppression. Epitopic suppression has been previously shown to occur when carrier-primed mice are subsequently immunized with a "new" epitope coupled to the priming carrier. The suppression specifically inhibited the antibody response to the "new" epitope without affecting the secondary antibody response to the carrier. In this report, using a carrier/hapten-carrier type of immunization protocol, we have demonstrated that a similar regulatory system could also affect the induction of CTL directed against allogeneic cells. Priming mice with an alloantigen 1 (carrier) inhibits the induction of alloantigen 2 (hapten)-specific cytotoxic responses when the alloantigen 2 is presented in association with the alloantigen 1 on an F1 stimulator cell (hapten-carrier conjugate). This has been demonstrated by the specific decrease of anti-H-2b or anti-H-2d CTL responses generated in C3H/He mice (H-2k) previously primed with, respectively, H-2d or H-2b spleen cells before immunization with F1 (H-2d x b) spleen cells. This suppression of the CTL responses against the second immunizing alloantigen is associated with a strong CTL response against the first priming alloantigen. The induction of the suppression is dependent on the dose of H-2d spleen cells administered before immunization with F1 spleen cells and is not related to antigen elimination since a strong suppression of the CTL response against H-2b antigens is shown following immunization with a mixture of F1 cells and H-2b-bearing cells of H-2d-primed animals.

Interleukin 2-dependent activation of tumor-specific cytotoxic T lymphocytes in vivo.

We have quantitatively studied the effect of interleukin (IL) 2 on the cytotoxic T lymphocyte (CTL) response to tumor cells in vivo. Mastocytoma P815 was transfected with murine IL 2 cDNA (P815-IL 2) and injected into syngeneic mice. The anti-tumor response was analyzed and compared with the response induced by the non-transfected cells. P815 parental cells are highly tumorigenic, causing death within 20-30 days. In contrast, IL 2-transfected cells were totally rejected. Co-injection of IL 2-secreting and parental cells resulted in the inhibition of growth of both type of tumors. In addition, the response induced by IL 2-secreting cells protected the mice against a subsequent challenge with P815. Long-term memory persisted in treated mice 3 months after tumor rejection. Frequencies of CTL precursors and CTL specific for P815 increased as a result of IL 2 secretion by the target cells. Estimates of CTL frequency at days 8 and 12 after injection were 2 to 3 times higher in mice inoculated with P815-IL 2 cells, and this correlated with tumor rejection.

Cytotoxic T cell immunity against telomerase reverse transcriptase in humans.

Telomerase is a ribonucleoprotein enzyme which has been linked to malignant transformation in human cells. Telomerase activity is increased in the vast majority of human tumors, making its gene product the first molecule common to all human tumors. The generation of endogenously processed telomerase peptides bound to Class I MHC molecules could therefore target cytotoxic T lymphocytes (CTL) to tumors of different origins. This could advance vaccine therapy against cancer provided that precursor CTL recognizing telomerase peptides in normal adults and cancer patients can be expanded through immunization. We demonstrate here that the majority of normal individuals and patients with prostate cancer immunized in vitro against two HLA-A2.1 restricted peptides from telomerase reverse transcriptase (hTRT) develop hTRT-specific CTL. This suggests the existence of precursor CTL for hTRT in the repertoire of normal individuals and in cancer patients. Most importantly, the CTL of cancer patients specifically lysed a variety of HLA-A2(+) cancer cell lines, demonstrating immunological recognition of endogenously processed hTRT peptides. Moreover, in vivo immunization of HLA-A2.1 transgenic mice generated a specific CTL response against both hTRT peptides. Based on the induction of CTL responses in vitro and in vivo, and the susceptibility to lysis of tumor cells of various origins by hTRT CTL, we suggest that hTRT could serve as a universal cancer vaccine for humans.

Epitope recognition of conserved HIV envelope sequences by human cytotoxic T lymphocytes.

CTL constitute an essential part of the immune response against the HIV. CTL recognize peptides derived from viral proteins together with the MHC class I molecules on the surface of infected cells. The CTL response could be important in prevention or control of infection with HIV by destroying virus-producing cells. In this study we have attempted to identify peptide epitopes recognized by HIV-specific CTL. Using synthetic peptides, we have identified six conserved peptidic epitopes on the gp120 envelope glycoprotein recognized by polyclonal human CTL in association with HLA-A2 class I transplantation Ag. These results were confirmed by two approaches: i) blocking of CTL activities with antibodies specific for three of these conserved peptides; and ii) construction of doubly transfected P815-A2 target cells, using deletions of the HIV env gene. Vaccination or immunotherapy in HLA-A2 individuals can thus be considered using highly conserved HIV env peptidic sequences.

Unusually high frequencies of HIV-specific cytotoxic T lymphocytes in humans.

CTL specific for the HIV belong to the CD8 subset of T lymphocytes, and their activity is restricted by class I HLA transplantation Ag. In this report, HIV-specific CTL and their precursor cells were quantified by limiting dilution analysis. CTL were recovered from the lungs, lymph nodes, and blood of asymptomatic seropositive carriers and of patients with AIDS. HIV was found to be very immunogenic. High frequencies of both HIV-specific CTL and CTL precursor cells were detected in infected individuals. These CTL killed autologous HIV-infected macrophages and T4 lymphoblasts. They also killed doubly transfected P815-A2-env-LAV mouse tumor cells, which express the human HLA-A2 gene and the HIV-1 env gene. In the longitudinal studies of two HIV-infected patients, CTL and CTL precursor cell frequencies decreased as the clinical and immunologic status of the patients deteriorated. Most surprisingly, PBL from seronegative donors also responded to HIV stimulation in vitro and generated large numbers of HLA-restricted, HIV-specific CTL.