Expression of the HPV16E7 oncoprotein by thymic epithelium is accompanied by disrupted T cell maturation and a failure of the thymus to involute with age

Transgenic mice expressing the E7 protein of HPV16 from the keratin 14 promoter demonstrate increasing thymic hypertrophy with age. This hypertrophy is associated with increased absolute numbers of all thymocyte types, and with increased cortical and medullary cellularity. In the thymic medulla, increased compartmentalization of the major thymic stromal cell types and expansion of thymic epithelial cell population is observed. Neither an increased rate of immature thymocyte division nor a decreased rate of immature thymocyte death was able to account for the observed hypertrophy.

Thymocytes with reduced levels of expression of CD4 and/or CD8 were more abundant in transgenic (tg) mice and became increasingly more so with age. These thymic SP and DP populations with reduced levels of CD4 and/or CD8 markers had a lower rate of apoptosis in the tg than in the non-tg mice. The rate of export of mature thymocytes to peripheral lymphoid organs was less in tg animals relative to the pool of available mature cells, particularly for the increasingly abundant CD4^lo population. We therefore suggest that mature thymocytes that would normally die in the thymus gradually accumulated in E7 transgenic animals, perhaps as a consequence of exposure to a hypertrophied E7-expressing thymic epithelium or to factors secreted by this expanded thymic stromal cell population. The K14E7 transgenic mouse thus provides a unique model to study effects of the thymic epithelial cell compartment on thymus development and involution.

Polynucleotide viral vaccines: codon optimisation and ubiquitin conjugation enhances prophylactic and therapeutic efficacy

Papillomavirus infection is a major antecedent of anogenital malignancy. We have previously established that the L1 and L2 capsid genes of papillomavirus have suboptimal codon usage for expression in mammalian cells. We now show that the lack of immunogenicity of polynucleotide vaccines based on the L1 gene can be overcome with codon modified L1, which induces strong immune responses, including conformational virus neutralising antibody and delayed type hypersensitivity. Conjugation of a ubiquitin gene to a hybrid gene incorporating L1 and the E7 non-structural papillomavirus protein improved E7 specific CTL responses, and induced protection against an E7 expressing tumour, but induced little neutralising antibody. However, a mixture of ubiquitin conjugated and non-ubiquitin conjugated polynucleotides induced virus neutralising antibody and E7 specific CD8 T cells. An optimal combined prophylactic/therapeutic viral vaccine might therefore comprise ubiquitin conjugated and non-ubiquitinated genes, to induce prophylactic neutralising antibody and therapeutic cell mediated immune responses.

Tolerance or immunity to a tumor antigen expressed in somatic cells can be determined by systemic proinflammatory signals at the time of first antigen exposure

Mice transgenic for the E7 tumor Ag of human papillomavirus type 16, driven from a keratin 14 promoter, express E7 in keratinocytes but not dendritic cells. Grafted E7-transgenic skin is not rejected by E7-immunized mice that reject E7-transduced transplantable tumors. Rejection of recently transplanted E7-transgenic skin grafts, but not of control nontransgenic grafts or of established E7-transgenic grafts, is induced by systemic administration of live or killed Listeria monocytogenes or of endotoxin. Graft recipients that reject an E7 graft reject a subsequent E7 graft more rapidly and without further L. monocytogenes exposure, whereas recipients of an E7 graft given without L. monocytogenes do not reject a second graft, even if given with L. monocytogenes. Thus, cross-presentation of E7 from keratinocytes to the adaptive immune system occurs with or without a proinflammatory stimulus, but proinflammatory stimuli at the time of first cross-presentation of Ag can determine the nature of the immune response to the Ag. Furthermore, immune effector mechanisms responsible for rejection of epithelium expressing a tumor Ag in keratinocytes are different from those that reject an E7-expressing transplantable tumor. These observations have implications for immunotherapy for epithelial cancers.

Nonspecific down-regulation of CD8+ T-cell responses in mice expressing human papillomavirus type 16 E7 oncoprotein from the keratin-14 promoter

The E7 oncoprotein of human papillomavirus 16 (HPV16) transforms basal and suprabasal cervical epithelial cells and is a tumor-specific antigen in cervical carcinoma, to which immunotherapeutic strategies aimed at cytotoxic T-lymphocyte (CTL) induction are currently directed. By quantifying major histocompatibility complex class I tetramer-binding T cells and CTL in mice expressing an HPV16 E7 transgene from the keratin-14 (K14) promoter in basal and suprabasal keratinocytes and in thymic cortical epithelium, we show that antigen responsiveness of both E7- and non-E7-specific CD8+ cells is down-regulated compared to non-E7 transgenic control mice. We show that the effect is specific for E7, and not another transgene, expressed from the K14 promoter. Down-regulation did not involve deletion of CD8+ T cells of high affinity or high avidity, and T-cell receptor (TCR) Vbeta-chain usage and TCR receptor density were similar in antigen-responsive cells from E7 transgenic and non-E7 transgenic mice. These data indicate that E7 expressed chronically from the K14 promoter nonspecifically down-regulates CD8+ T-cell responses. The in vitro data correlated with the failure of immunized E7 transgenic mice to control the growth of an E7-expressing tumor challenge. We have previously shown that E7-directed CTL down-regulation correlates with E7 expression in peripheral but not thymic epithelium (T. Doan et al., J. Virol. 73:6166-6170, 1999). The findings have implications for the immunological consequences of E7-expressing tumor development and E7-directed immunization strategies. Generically, the findings illustrate a T-cell immunomodulatory function for a virally encoded human oncoprotein.

Papillomavirus virus-like particles for the delivery of multiple cytotoxic T cell epitopes

Chimeric papillomavirus (PV) virus-like particles (VLPs) based on the bovine papillomavirus type 1 (BPV-1) L1 protein were constructed by replacing the 23-carboxyl-terminal amino acids of the BPV1 major protein L1 with an artificial "polytope" minigene, containing known CTL epitopes of human PV16 E7 protein, HIV IIIB gp120 P18, Nef, and reverse transcriptase (RT) proteins, and an HPV16 E7 linear B epitope. The CTL epitopes were restricted by three different MHC class I alleles (H-2(b), H-2(d), HLA-A*0201). The chimeric L1 protein assembled into VLPs when expressed in SF-9 cells by recombinant baculovirus. After immunization of mice with polytope VLPs in the absence of adjuvant, serum antibodies were detected which reacted with both polytope VLPs and wild-type BPV1L1 VLPs, in addition to the HPV16E7 linear B cell epitope. CTL precursors specific for the HPV16 E7, HIV P18, and RT CTL epitopes were also detected in the spleen of immunized mice. Polytope VLPs can thus deliver multiple B and T epitopes as immunogens to the MHC class I and class II pathways, extending the utility of VLPs as self-adjuvanting immunogen delivery systems.

Potential strategies utilised by papillomavirus to evade host immunity

The co-evolution of papillomaviruses (PV) and their mammalian hosts has produced mechanisms by which PV might avoid specific and non-specific host immune responses. Low level expression of PV proteins in infected basal epithelial cells, together with an absence of inflammation and of virus-induced cell lysis, restricts the opportunity for effective PV protein presentation to immunocytes by dendritic cells. Additionally, PV early proteins, by a range of mechanisms, may restrict the efficacy of antigen presentation by these cells. Should an immune response be induced to PV antigens, resting keratinocytes (KC) appear resistant to interferon-gamma-enhanced mechanisms of cytotoxic T-lymphocyte (CTL)-mediated lysis, and expression of PV antigens by resting KC can tolerise PV-specific CTL. Thus, KC, in the absence of inflammation, may represent an immunologically privileged site for PV infection. Together, these mechanisms play a part in allowing persistence of PV-induced proliferative skin lesions for months to years, even in immunocompetent hosts.

The importance of pairwise interactions between peptide residues in the delineation of TCR specificity

A minimal, nonamer epitope (TEMEKEGKI) from the reverse transcriptase protein of HIV-1, restricted by H-2Kk, was identified and the function of individual residues determined. Besides classical anchor residues at positions 2 and 9, methionine at position 3 was identified as an important MHC anchor and improved binding of a different (malarial) nonamer epitope to H-2Kk, albeit while also abolishing CTL recognition. Lysine at position 5 was replaceable by alanine for CTL raised against wild-type peptide but abolished recognition for CTL raised against the variant 5ALA peptide, indicating a unidirectional cross-reactivity. Interestingly, one CTL line raised against the 5ALA substituted peptide was permissive for a double substitution at positions 5 and 6, in which lysine was permissive at position 5 only if the adjacent glutamic acid was replaced by alanine. Extensive analysis revealed three distinct patterns of responses with peptides doubly substituted in this region: recognition of both single substitutions but not the double substitution, recognition of only one single substitution but also the double substitution, or recognition of both single substitutions and the double substitution. A second complementary substitution can therefore restore function lost through a first substitution. Thus, no residue acts independently of its neighbors, and pairs of substitutions may give results not predictable from the effects of each taken singly. This finding may have bearing on viral infections (such as HIV), in which the accumulation of two mutations in the epitope may lead to the reengagement of memory CTL previously silenced by the initial mutation.

Split tolerance to a viral antigen expressed in thymic epithelium and keratinocytes

When expressed as a transgene from the keratin 14 (K14) promoter in an MHC class II-deficient mouse, I-Ab expressed in thymic cortical epithelium promotes positive but not negative selection of I-Ab-restricted CD4+ T cells (Laufer, T. M. et al., Nature 1996. 383:81-85). Transgenic mice expressing the E7 protein of human papilloma virus 16 from the K14 promoter were studied to determine the consequence of expression of a cytoplasmic/ nuclear protein from the K14 promoter. K14E7-transgenic mice express E7 in the thymus and skin without evidence for autoimmunity to E7. Repeated immunization of FVB(H-2q) or F1(C57BL/6JxFVB) mice with E7 elicited similar antibody responses to the defined B cell epitopes of E7 in K14E7-transgenic and non-transgenic animals. In contrast, for each genetic background, a single immunization with E7 elicited demonstrable T cell proliferative responses to the major promiscuous T helper epitope of E7 in the transgenic but not the non-transgenic animals. Further, E7-immunized non-transgenic F1 (FVBxC57BL/6J) animals developed strong E7-specific cytotoxic T lymphocyte (CTL) responses and were protected against challenge with E7+ tumors, whereas similarly immunized K14E7-transgenic animals had a markedly reduced CTL response to E7 and no E7-specific tumor protection was observed, although the antibody and CTL response to ovalbumin was normal. Expression of E7 protein as a transgene from the K14 promoter in the skin and thymus thus induces E7-specific tolerance in the cytotoxic T effector repertoire, together with expansion of the E7-specific T helper repertoire. These findings demonstrate that limited tissue distribution of an autoantigen may result in "split" tolerance to that autoantigen.

Cloning and expression of a cDNA encoding a nonintegrin laminin-binding protein from Echinococcus granulosus with localization of the laminin-binding domain

We have isolated a cDNA from the hydatid tapeworm, Echinococcus granulosus, encoding a protein that binds laminin. This is the first report of a helminth parasite laminin-binding protein and the first description of a cDNA encoding a laminin-binding protein from a parasite. The cDNA clone (egmo3) was isolated from an E. granulosus protoscolex cDNA expression library, and identified on the basis of sequence homology to the nonintegrin mammalian metastasis-associated 67-kDa laminin receptor (67-LR). The amino acid sequence predicted from the cDNA sequence is 268 residues long with a calculated molecular mass of 29.9 kDa. Southern blot analysis suggested that many copies of the gene may occur in the E. granulosus genome. A Northern blot revealed that the gene is expressed as a single transcript of approximately 1 kb consistent with the size of the cDNA insert. Antibodies raised to the purified protein interacted with a 30 kDa protein in whole E. granulosus protoscoleces. A Western blot of the purified and refolded recombinant protein specifically bound 125I-labelled laminin, as did a synthetic peptide derived from the inferred amino acid sequence of egmo3 which is similar in homology to peptide G, the active ligand-binding site of 67-LR. We also isolated the 3' end of the cDNA encoding the homologous protein from the closely related species, E. multilocularis. The polypeptide encoded by egmo3 also shares substantial identity with the acidic class of ribosomal proteins which are involved in protein synthesis. As such, the egmo3 protein may be multifunctional in E. granulosus, acting as a laminin-binding molecule but also playing a role in cell division and growth.

Cytotoxic T lymphocyte (CTL) adherence assay (CAA): a non-radioactive assay for murine CTL recognition of peptide-MHC class I complexes

Cytotoxic T lymphocytes (CTL) form an important immune surveillance system against intracellular pathogens. Here we describe a simple, visual assay for identifying peptides specifically recognized by CTL, based on the discovery that CTL develop increased adhesive properties upon TCR triggering. Several CTL lines were shown to pellet to the bottom of a round bottom 96-well plate in the absence of peptide. In contrast, these same CTL lines incubated with their cognate peptide, allowing them to present peptide to each other, adhered to the sides of the well and were readily distinguished by macroscopic visual examination of the plate after 4-5 h or overnight incubation. This CTL adherence assay (CAA) demonstrated peptide specificity and MHC restriction, and was titratable with peptide concentration. With this technique, a minimal-sized, malaria CTL epitope was correctly identified from a panel of overlapping nonamers, although the adherence pattern of two mono-substituted, variant peptides was less predictive of lytic activity. Also, substitutions in an HIV-1 envelope CTL epitope that reduced lytic activity were correctly predicted. Inhibitors of RNA and protein synthesis, upon preincubation, abrogated the adherence, indicating, at minimum, a need for live cells. Wortmannin, a PI-3 kinase inhibitor, inhibited the peptide specific adherence, consistent with a role for TCR or integrin signal transduction in CAA. Other cytoskeletal and metabolic inhibitors had no effect. Adherence of the T cells may involve low affinity, nonspecific interactions since wells coated with FCS, BSA or milk powder all produced an effective CAA in the presence of peptide under serum free conditions. Consequently, CAA may represent a rapid, simple method for screening large numbers of peptides to find cytolytic epitopes for a given CTL line and may identify additional epitopes causing T cell activation and adherence but not cytolytic activity.

The ultrastructural localization of Echinococcus granulosus antigen 5

Murine monoclonal and polyclonal antisera, raised against the 38 kDa subunit of Echinococcus granulosus antigen 5, were used to investigate the tissue distribution of the antigen in hydatid cysts. Immunoreactivity was visualized by indirect immunofluorescence on whole protoscoleces, and ultrastructural immunocytochemistry utilizing colloidal gold-based labelling procedures on unsectioned and cryosectioned brood capsules and protoscoleces. In protoscoleces, the 38 kDa subunit of antigen 5 was localized at the interface of parenchymal cells and associated extracellular matrices, as well as along the interface of the tegumentary syncytium in the somal region and its basal matrix. Cytoplasmic labelling of parenchymal cells was rare; when observed, it was associated with vesicles and membranes in cytoplasmic extensions of parenchymal cells. In brood capsules, the antigen was associated with the external face of the plasma of degenerating parenchymal cells. The 38 kDa subunit occurs along the extracellular face of cell membranes, suggesting that antigen 5 is either a component of the membranes or associated extracellular matrices.

Role of antigen, CD8, and cytotoxic T lymphocyte (CTL) avidity in high dose antigen induction of apoptosis of effector CTL

Experimental data suggest that negative selection of thymocytes can occur as a result of supraoptimal antigenic stimulation. It is unknown, however, whether such mechanisms are at work in mature CD8+ T lymphocytes. Here, we show that CD8+ effector cytotoxic T lymphocytes (CTL) are susceptible to proliferative inhibition by high dose peptide antigen, leading to apoptotic death mediated by TNF-alpha release. Such inhibition is not reflected in the cytolytic potential of the CTL, since concentrations of antigen that are inhibitory for proliferation promote efficient lysis of target cells. Thus, although CTL have committed to the apoptotic pathway, the kinetics of this process are such that CTL function can occur before death of the CTL. The concentration of antigen required for inhibition is a function of the CTL avidity, in that concentrations of antigen capable of completely inhibiting high avidity CTL maximally stimulate low avidity CTL. Importantly, the inhibition can be detected in both activated and resting CTL. Blocking studies demonstrate that the CD8 molecule contributes significantly to the inhibitory signal as the addition of anti-CD8 antibody restores the proliferative response. Thus, our data support the model that mature CD8+ CTL can accommodate an activation signal of restricted intensity, which, if surpassed, results in deletion of that cell.

Selective expansion of high- or low-avidity cytotoxic T lymphocytes and efficacy for adoptive immunotherapy

The conventional approach to cytotoxic T-lymphocyte (CTL) induction uses maximal antigen concentration with the intent of eliciting more CTL. However, the efficacy of this approach has not been systematically explored with regard to the quality of the CTLs elicited or their in vivo functionality. Here, we show that a diametrically opposite approach elicits CTLs that are much more effective at clearing virus. CTLs specific for a defined peptide epitope were selectively expanded with various concentrations of peptide antigen. CTLs generated with exceedingly low-dose peptide lysed targets sensitized with > 100-fold less peptide than CTLs generated with high-dose peptide. Differences in expression of T-cell antigen receptors or a number of other accessory molecules did not account for the functional differences. Further, high-avidity CTLs adoptively transferred into severe combined immunodeficient mice were 100- to 1000-fold more effective at viral clearance than the low-avidity CTLs, despite the fact that all CTL lines lysed virus-infected targets in vitro. Thus, the quality of CTLs is as important as the quantity of CTLs for adoptive immunotherapy, and the ability to kill virally infected targets in vitro is not predictive of in vivo efficacy, whereas the determinant density requirement described here is predictive. Application of these principles may be critical in developing effective adoptive cellular immunotherapy for viral infections and cancer.

Inactivation of human immunodeficiency virus (HIV)-1 envelope-specific CD8+ cytotoxic T lymphocytes by free antigenic peptide: a self-veto mechanism?

Free peptide has been found to inhibit cytotoxic T lymphocyte (CTL) activity, and veto cells bearing peptide-major histocompatibility complex (MHC) complexes have been found to inactivate CTL, but the two phenomena have not been connected. Here we show that a common mechanism may apply to both. CD8+ CTL lines or clones specific for a determinant of the human immunodeficiency virus (HIV) 1 IIIB envelope protein gp160, P18IIIB, are inhibited by as little as 10 min exposure to the minimal 10-mer peptide, I-10, within P18IIIB, free in solution, in contrast to peptide already bound to antigen-presenting cells (APC), which does not inhibit. Several lines of evidence suggest that the peptide must be processed and presented by H-2Dd on the CTL itself to the specific T cell receptor (TCR) to be inhibitory. The inhibition was not killing, in that CTL did not kill 51Cr-labeled sister CTL in the presence of free peptide, and in mixing experiments with CTL lines of different specificities restricted by the same MHC molecule, Dd, the presence of free peptide recognized by one CTL line did not inhibit the activity of the other CTL line that could present the peptide. Also, partial recovery of activity could be elicited by restimulation with cell-bound peptide, supporting the conclusion that neither fratricide nor suicide (apoptosis) was involved. The classic veto phenomenon was ruled out by failure of peptide-bearing CTL to inactivate others. Using pairs of CTL lines of differing specificity but similar MHC restriction, each pulsed with the peptide for which the other is specific, we showed that the minimal requirement is simultaneous engagement of the TCR and class I MHC molecules of the same cell. This could occur in single cells or pairs of cells presenting peptide to each other. Thus, mechanistically, the inhibition is analogous to veto, and might be called self-veto. As a clue to a possible mechanism, we found that free I-10 peptide induced apparent downregulation of expression of specific TCR as well as interleukin 2 receptor, CD69, lymphocyte function-associated antigen 1, and CD8. This self-veto effect also has implications for in vivo immunization and mechanisms of viral escape from CTL immunity.

Further characterization of the 38 kDa antigen from Echinococcus granulosus (hydatid disease) cyst fluid: evidence for antigenic heterogeneity and reactivity with anti-P1 antibodies

A panel of 5 IgM and 6 IgG1 monoclonal antibodies (MoAbs) was produced against a band, eluted from a reducing SDS-PAGE gel, containing the 38 kDa subunit of antigen 5 (Ag 5) from Echinococcus granulosus cyst fluid; seven of the MoAbs were shown subsequently to bind epitopes on Ag 5 but none recognized phosphorylcholine or periodate-sensitive carbohydrate epitopes. Differences in the fine specificity of the MoAbs were apparent and, upon reduction, heterogeneity in 38 kDa components from hydatid cyst fluids of different intermediate host origin was revealed by peptide fingerprinting and immunoblotting using the MoAbs. One of the IgG1 MoAbs (ED9) was able to distinguish a reduced 38 kDa molecule in cyst fluids from two distinct genotypes--the horse/dog and sheep/dog strains--of E. granulosus and this may have implications for hydatid serology, immunoepidemiology and strain typing. Furthermore, epitopes on this 38 kDa component or on a different molecule with the same or similar M(r) are reactive with anti-P1 blood group antigen antibodies and this could result in false-positive reactions where sera from P2 patients with suspected hydatid disease are tested by immunoblot or immunoprecipitation analysis.

Identification and diagnostic value of a major antibody epitope on the 12 kDa antigen from Echinococcus granulosus (hydatid disease) cyst fluid

An IgG1 monoclonal antibody (MoAb), designated C9E7H8, has been produced against an epitope on the 12 kDa antigen of Echinococcus granulosus cyst fluid, believed to represent the smallest subunit of antigen B. This MoAb, raised against purified 12 kDa antigen eluted from a reducing SDS-PAGE gel, demonstrated strong binding to native sheep cyst fluid in ELISA and recognition of all three subunits of antigen B (at 12, 16, 23 kDa) by immunoblot under both reducing and non-reducing conditions. Immunoblot analysis also indicated that the complementary epitope is conserved amongst cyst fluids from different intermediate hosts of E. granulosus, including fluids from cysts of two distinct strains, and is present in cyst fluid from E. multilocularis. The monoclonal displays binding to a cDNA clone, EgPS-3, which we have previously shown expresses part of the 12 kDa molecule. EgPS-3, expressed as a glutathione-S-transferase fusion protein, was successful in positive detection of 74% of cystic hydatid patients, although cross-reactions were observed with 25% of sera from alveolar hydatid and 22% of sera from schistosomiasis japonica patients. Three peptides, based on the predicted amino acid sequence of EgPS-3, showed increased specificity but slightly reduced sensitivity in the detection of antibody from E. granulosus patients. The predominant epitope recognized by human antibody occurs in the N-terminal 27 amino acids (peptide 65) of EgPS-3 which also correlates with the location of the monoclonal antibody epitope.

Sequence homology between two immunodiagnostic fusion proteins from Echinococcus multilocularis

Serological diagnosis of alveolar hydatid disease using crude parasite antigen is problematical with the result that several groups have addressed the question of specific serodiagnosis using defined native or recombinant antigens. Sequence data are available for two lambda gt 11 cDNA clones, designated EM4 and pEM10, which express E. multilocularis species-specific proteins in immunodiagnostic assays using human sera. Here, we have drawn attention to the extensive similarities between these antigens and have compared their characteristics since both may prove valuable in the future diagnosis of alveolar hydatidosis.

Serological evaluation of the 12 kDa subunit of antigen B in Echinococcus granulosus cyst fluid by immunoblot analysis

This study evaluated the 12 kDa (smallest) subunit of Echinococcus granulosus antigen B as a diagnostic molecule. Using immunoblotting, 90.9% of cystic hydatid patients, 40% of alveolar hydatid patients and 5.5% of cysticercosis patients showed sero-reactivity to this subunit. Human antibody response to the 12 kDa molecule appeared independent of factors such as parasite strain or host population responsiveness. The majority of infection sera, and some normal human controls, also recognized the 38 kDa subunit of antigen 5.