Expression and function of HLA-A2.1 in transgenic mice

HLA-transgenic mouse models to study autoimmune central nervous system diseases

It is known that certain human leukocyte antigen (HLA) genes are associated with autoimmune central nervous system (CNS) diseases, such as multiple sclerosis (MS), but their exact role in disease susceptibility and etiopathogenesis remains unclear. The best studied HLA-associated autoimmune CNS disease is MS, and thus will be the primary focus of this review. Other HLA-associated autoimmune CNS diseases, such as autoimmune encephalitis and neuromyelitis optica will be discussed. The lack of animal models to accurately capture the complex human autoimmune response remains a major challenge. HLA transgenic (tg) mice provide researchers with powerful tools to investigate the underlying mechanisms promoting susceptibility and progression of HLA-associated autoimmune CNS diseases, as well as for elucidating the myelin epitopes potentially targeted by T cells in autoimmune disease patients. We will discuss the potential role(s) of autoimmune disease-associated HLA alleles in autoimmune CNS diseases and highlight information provided by studies using HLA tg mice to investigate the underlying pathological mechanisms and opportunities to use these models for development of novel therapies.

Development of a humanized HLA-A30 transgenic mouse model

Background

There are remarkable genetic differences between animal major histocompatibility complex (MHC) systems and the human leukocyte antigen (HLA) system. HLA transgenic humanized mouse model systems offer a much better method to study the HLA‐A‐related principal mechanisms for vaccine development and HLA‐A‐restricted responses against infection in human.

Methods

A recombinant gene encoding the chimeric HLA‐A30 monochain was constructed. This HHD molecule contains the following: α1‐α2 domains of HLA‐A30, α3 and cytoplasmic domains of H‐2D^b, linked at its N‐terminus to the C‐terminus of human β2m by a 15‐amino‐acid peptide linker. The recombinant gene encoding the chimeric HLA‐A30 monochain cassette was introduced into bacterial artificial chromosome (BAC) CH502‐67J3 containing the HLA‐A01 gene locus by Red‐mediated homologous recombination. Modified BAC CH502‐67J3 was microinjected into the pronuclei of wild‐type mouse oocytes. This humanized mouse model was further used to assess the immune responses against influenza A virus (H1N1) pdm09 clinically isolated from human patients. Immune cell population, cytokine production, and histopathology in the lung were analyzed.

Results

We describe a novel human β2m‐HLA‐A30 (α1α2)‐H‐2D^b (α3 transmembrane cytoplasmic) (HHD) monochain transgenic mouse strain, which contains the intact HLA‐A01 gene locus including 49 kb 5′‐UTR and 74 kb 3′‐UTR of HLA‐A01*01. Five transgenic lines integrated into the large genomic region of HLA‐A gene locus were obtained, and the robust expression of exogenous transgene was detected in various tissues from A30‐18# and A30‐19# lines encompassing the intact flanking sequences. Flow cytometry revealed that the introduction of a large genomic region in HLA‐A gene locus can influence the immune cell constitution in humanized mice. Pdm09 infection caused a similar immune response among HLA‐A30 Tg humanized mice and wild‐type mice, and induced the rapid increase of cytokines, including IFN‐γ, TNF‐α, and IL‐6, in both HLA‐A30 humanized Tg mice and wild‐type mice. The expression of HLA‐A30 transgene was dramatically promoted in tissues from A30‐9# line at 3 days post‐infection (dpi).

Conclusions

We established a promising preclinical research animal model of HLA‐A30 Tg humanized mouse, which could accelerate the identification of novel HLA‐A30‐restricted epitopes and vaccine development, and support the study of HLA‐A‐restricted responses against infection in humans.

Relationship between Poor Immunogenicity of HLA-A2-Restricted Peptide Epitopes and Paucity of Naïve CD8(+) T-Cell Precursors in HLA-A2-Transgenic Mice

We examined the immunogenicity of H-2 class I-restricted and HLA-A2-restricted epitopes through peptide immunization of HLA-A2-transgenic mice that also express mouse H-2 class I molecules. All four of the tested epitopes restricted by H-2 class I robustly elicited T-cell responses, but four of seven epitopes restricted by HLA-A2 did not induce T-cell responses, showing that HLA-A2-restricted peptide epitopes tend to be poorly immunogenic in HLA-A2-transgenic mice. This finding was confirmed in HLA-A2-transgenic mice infected with a recombinant vaccinia virus expressing hepatitis C virus proteins. We examined the precursor frequency of epitope-specific naïve CD8(+) T cells in HLA-A2-transgenic and conventional C57BL/6 mice and found that the poor immunogenicity of HLA-A2-restricted peptide epitopes is related to the paucity of naïve CD8(+) T-cell precursors in HLA-A2-transgenic mice. These results provide direction for the improvement of mouse models to study epitope repertoires and the immunodominance of human T-cell responses.

HLA-A*01:03, HLA-A*24:02, HLA-B*08:01, HLA-B*27:05, HLA-B*35:01, HLA-B*44:02, and HLA-C*07:01 monochain transgenic/H-2 class I null mice: novel versatile preclinical models of human T cell responses

We have generated a panel of transgenic mice expressing HLA-A*01:03, -A*24:02, -B*08:01, -B*27:05, -B*35:01, -B*44:02, or -C*07:01 as chimeric monochain molecules (i.e., appropriate HLA α1α2 H chain domains fused with a mouse α3 domain and covalently linked to human β2-microglobulin). Whereas surface expression of several transgenes was markedly reduced in recipient mice that coexpressed endogenous H-2 class I molecules, substantial surface expression of all human transgenes was observed in mice lacking H-2 class I molecules. In these HLA monochain transgenic/H-2 class I null mice, we observed a quantitative and qualitative restoration of the peripheral CD8(+) T cell repertoire, which exhibited a TCR diversity comparable with C57BL/6 WT mice. Potent epitope-specific, HLA-restricted, IFN-γ-producing CD8(+) T cell responses were generated against known reference T cell epitopes after either peptide or DNA immunization. HLA-wise, these new transgenic strains encompass a large proportion of individuals from all major human races and ethnicities. In combination with the previously created HLA-A*02:01 and -B*07:02 transgenic mice, the novel HLA transgenic mice described in this report should be a versatile preclinical animal model that will speed up the identification and optimization of HLA-restricted CD8(+) T cell epitopes of potential interest in various autoimmune human diseases and in preclinical evaluation of T cell-based vaccines.

Using HLA-A2.1 Transgenic Rabbit Model to Screen and Characterize New HLA-A2.1 Restricted Epitope DNA Vaccines

We have established an HLA-A2.1 transgenic rabbit /cottontail rabbit papillomavirus (CRPV) infection model. Using this novel transgenic animal model, we reported earlier that a multivalent epitope DNA vaccine (CRPVE1ep1-5) containing five HLA-A2.1 restricted epitopes from CRPVE1 (42-50, 149-157, 161-169, 245-253 and 303-311) was successful in providing strong and specific protective and therapeutic immunity. Among these five epitopes, two (161-169 and 303-311) have been proven to stimulate strong immunity in both HLA-A2.1 transgenic mouse and rabbit models. In the current study, we further identified the remaining three epitopes (CRPVE1/42-50,149-157, 245-253) in both animal models. CRPVE1/149-157 was able to induce specific CTL responses in HLA-A2.1 transgenic mice by DNA immunization but undetectable by peptide immunization. CRPVE1/42-50 and 245-253 failed to respond in HLA-A2.1 transgenic mice either by peptide or DNA immunization. All the three epitopes when administrated as DNA vaccines, however, were able to stimulate strong protective immunity in HLA-A2.1 transgenic rabbits in a dose dependent manner. Among the five epitopes, two (CRPVE1/ 303-311and CRPVE1/149-157) DNA vaccines also showed specific therapeutic effects in CRPV-infected HLA-A2.1 transgenic rabbits. Taken together, the HLA-A2.1 transgenic rabbit model recognized more epitopes than did the HLA-A2.1 transgenic mouse model. Our data demonstrate that the HLA-A2.1 transgenic rabbit model can complement the HLA-A2.1 transgenic mouse model for the development and testing of new HLA-A2.1 restricted prophylactic and therapeutic T cell based DNA vaccines.

HLA class I transgenic mice: development, utilisation and improvement

Classical major histocompatibility complex (MHC) class I antigens are trimeric molecules found on the surface of nucleated cells in all jawed vertebrates. MHC I are recognised by two families of receptors: clonotypic T cell receptors expressed on the surface of CD8+ cytotoxic T lymphocytes (CTLs), and monomorphic receptors expressed by both natural killer cells and CTLs. The production of MHC I molecules within the cells is a sequential process performed with the help of interacting proteins: proteases, chaperones, transporters and so on. Although largely homologous in their structure, organisation and function, the human and mouse MHC I antigen processing and presentation machineries show fine differences. Transgenesis and 'knockout' or 'knock-in' technologies permit the addition of relevant human genes or the replacement of mouse genes by their human orthologues in order to produce immunologically humanised mice. Such experimental animals are especially relevant for the comparative evaluation of immunotherapies and for the characterisation of MHC I peptide epitopes. This review presents the similarities and differences between mouse and human MHC I antigen processing machinery, and describes the development and utilisation of improving mouse models of human cytotoxic T cell immunity.

The use of reverse immunology to identify HLA-A2 binding epitopes in Tie-2

A potential target for a cancer vaccine would be receptors, such as Tie-2 which are over expressed on tumour endothelium. Using computer aided motif predictions for possible HLA class I epitopes, we have identified peptides from Tie-2 that should bind with a range of affinities to HLA-A*0201. No direct correlation between predicted values and actual binding affinities was observed. Although, the programs did produce a number of false positives, two epitopes were predicted that bound with relatively high affinity when compared with an influenza peptide. We have previously identified a Tie-2 epitope and shown that it was only immunogenic when we substituted preferred amino acids at key anchor residues to increase binding affinity. In this study we used a similar approach to generate modified epitopes. When HLA-A2 transgenic mice were immunised with peptides, CTL killing of the target cells was only achieved when the wild type epitope was presented at moderate levels. Moreover, the efficiency of immunisation was increased when we linked CD4 epitopes to CD8 epitopes. Caution should therefore be employed in the use of both reverse immunology and anchor modification of CTL epitopes in the identification of CTL epitopes for cancer vaccines.

Comparison of the immune response to a self antigen after DNA immunisation of HLA*A201/H-2Kb and HHD transgenic mice

HHD mice which express human HLA-A2 but no mouse class I, have been shown to have improved cytotoxic lymphocyte (CTL) responses to foreign antigens compared to HLA*A201/Kb transgenic mice. We have previously designed a DNA vaccine to a self antigen. Vaccination of A2.1/H-2Kb mice with this vaccine resulted in CTL in one third of the mice, but the frequency of antigen-specific cells was low. An increased frequency was observed in HHD mice, moreover CTL were obtained from all immunised mice. HHD mice are therefore useful for studying immunisation strategies for inducing and maintaining CD8 responses to over-expressed self antigens.

Identification of an HLA-A*0201 cytotoxic T lymphocyte epitope specific to the endothelial antigen Tie-2

Tie-2 stabilises pericyte-endothelial interactions during angiogenesis and is highly expressed on endothelium during several diseases, including arthritis, age-related macular degeneration and cancer. A vaccine that targets endothelium overexpressing Tie-2 may result in vessel damage and stimulate an inflammatory cascade resulting in disease regression. We have identified a region unique to Tie-2 (amino acids 1-196) that is homologous in humans and mice. Using computer algorithms, several HLA-A*0201 epitopes that are identical in mice and humans were predicted within this region; however, binding assays showed that the majority of these epitopes were of low affinity. Modification of the anchor residues of 4 epitopes enhanced HLA binding. These epitopes were incorporated by site-directed mutagenesis into a Tie-2 DNA construct. Immunisation of HLA*0201 transgenic mice with one of the modified Tie-2 constructs stimulated CTLs that recognised both wild-type and modified peptide-pulsed target cells. In contrast, no CTLs were generated in mice immunised with wild-type Tie-2 construct, demonstrating that the modified epitope was necessary in the generation of CTLs. Moreover, CTLs from mice immunised with the modified construct killed HLA-A*0201 endothelial cells overexpressing Tie-2. Our study demonstrates that it is possible to break tolerance to the endothelial antigen Tie-2, suggesting that it may be feasible to design a vaccine to activate CTLs to kill endothelial cells overexpressing Tie-2.

Optimization and immune recognition of multiple novel conserved HLA-A2, human immunodeficiency virus type 1-specific CTL epitopes

MHC-I-restricted cytotoxic responses are considered a critical component of protective immunity against viruses, including human immunodeficiency virus type 1 (HIV-1). CTLs directed against accessory and early regulatory HIV-1 proteins might be particularly effective; however, CTL epitopes in these proteins are rarely found. Novel artificial neural networks (ANNs) were used to quantitatively predict HLA-A2-binding CTL epitope peptides from publicly available full-length HIV-1 protein sequences. Epitopes were selected based on their novelty, predicted HLA-A2-binding affinity and conservation among HIV-1 strains. HLA-A2 binding was validated experimentally and binders were tested for their ability to induce CTL and IFN-gamma responses. About 69 % were immunogenic in HLA-A2 transgenic mice and 61 % were recognized by CD8(+) T-cells from 17 HLA-A2 HIV-1-positive patients. Thus, 31 novel conserved CTL epitopes were identified in eight HIV-1 proteins, including the first HLA-A2 minimal epitopes ever reported in the accessory and regulatory proteins Vif, Vpu and Rev. Interestingly, intermediate-binding peptides of low or no immunogenicity (i.e. subdominant epitopes) were found to be antigenic and more conserved. Such epitope peptides were anchor-optimized to improve immunogenicity and further increase the number of potential vaccine epitopes. About 67 % of anchor-optimized vaccine epitopes induced immune responses against the corresponding non-immunogenic naturally occurring epitopes. This study demonstrates the potency of ANNs for identifying putative virus CTL epitopes, and the new HIV-1 CTL epitopes identified should have significant implications for HIV-1 vaccine development. As a novel vaccine approach, it is proposed to increase the coverage of HIV variants by including multiple anchor-optimized variants of the more conserved subdominant epitopes.

HLA-A2 1 restricted peptides from the HBx antigen induce specific CTL responses in vitro and in vivo

The HBx-derived, HLA-A2.1 restricted peptides, XEP-3, XEP-4, and XEP-6, induced activation of specific CTLs from patients with HBV in vitro. XEP-6 peptide induced the strongest response among the three peptides in CTLs from the blood samples of patients that were HBsAg positive. It was not clear whether the stage of disease (chronic infection, cirrhosis or hepatoma) was related to the responsiveness of the CTLs to each peptide. We vaccinated HLA-A2/K(b) transgenic mice with these peptides encapsulated in pH-sensitive liposomes at various concentrations and tested their ability to protect against challenge with rVV-HBx. Mice immunized with encapsulated peptides were protected against viral challenge whereas those immunized with empty liposomes were not. In general, 5 micro g of each peptide per head inoculation was sufficient to give protection after 2 weeks. After 3 weeks, this protective effect was increased. This effect of time was more important on the level of protection than the initial dose of the peptide. To explain the protective effect, IFN-gamma secreting CD8(+) cells isolated from mice 3 weeks after immunization were analyzed ex vivo. There was little dose dependency of peptide on IFN-gamma secretion except for XEP-3. The variations in the results may reflect the chemical properties of the peptides, such as solubility and binding affinity. In conclusion, epitope peptides derived from HBx can induce specific CTL activation and lead to cellular immunity in vitro and in vivo by inducing the peptide-specific CD8(+) CTLs. Thus, pH-sensitive liposomes increase the immune response following immunization with a peptide vaccine. This could be used for the treatment of HBV-related disease.

Development of HLA-A2402/K(b) transgenic mice

HLA-transgenic mice have been developed to facilitate studies of HLA-restricted cytotoxic responses, e.g., for the identification of immunodominant HLA-restricted CTL epitopes and the optimization of peptide or DNA vaccine constructs for human use. We have developed HLA-A2402/K(b)-transgenic mice expressing chimeric human (alpha1 and alpha2 domains of HLA-A2402) and mouse (alpha3, transmembrane and cytoplasmic domains of H-2K(b)) class I molecules. Immunization of these HLA-A2402/K(b)-transgenic mice with various known HLA-A24-restricted immunodominant cancer CTL epitope peptides derived from gp100, MAGE-1, MAGE-3, Her2/neu, CEA and TERT induced HLA-A24-restricted, peptide-specific CTLs. Using these transgenic mice, we identified a novel HLA-A24-restricted CTL epitope, PSA(152-160), encoded by human prostate-specific antigen. Staining with HLA tetramers showed that the cytotoxic activity induced by immunizing with PSA(152-160) in HLA-A2402/K(b) transgenic mice was HLA-A2402-restricted and CD8-dependent. Therefore, PSA(152-160) might be a candidate peptide for vaccination of HLA-A24(+) patients with prostate cancer. Our results suggest that HLA-A2402/K(b) transgenic mice might be useful in the search for HLA-A24-restricted CTL epitopes functioning as human cancer antigens and for the development of peptide-based cancer immunotherapy.

Cancer immunotherapy: insights from transgenic animal models

A wide range of strategies in cancer immunotherapy has been developed in the last decade, some of which are currently being used in clinical settings. The development of these immunotherapeutical strategies has been facilitated by the generation of relevant transgenic animal models. Since the different strategies in experimental immunotherapy of cancer each aim to activate different immune system components, a variety of transgenic animals have been generated either expressing tumor associated, HLA, oncogenic or immune effector cell molecule proteins. This review aims to discuss the existing transgenic mouse models generated to study and develop cancer immunotherapy strategies and the variable results obtained. The potential of the various transgenic animal models regarding the development of anti-cancer immunotherapeutical strategies is evaluated.

Transgenic mice expressing human HLA and CD8 molecules generate HLA-restricted measles virus cytotoxic T lymphocytes of the same specificity as humans with natural measles virus infection

Control of primary measles virus (MV) infection in humans and continued maintenance of immune memory that protects against reinfection are mediated primarily through the anti-MV T cell response, as judged by observations of children with defects in antibody formation but competency in making T cells. Further, the failure of T cell responses in those infected with MV most often leads to overwhelming infection. To better define and manipulate the elements involved in human T cell responses to MV, we analyzed the generation of HLA-restricted cytotoxic T lymphocytes (CTL) in a small animal model. Transgenic mice expressing the human class I MHC antigen HLA-B27 in conjunction with human CD8 molecules produced vigorous HLA-restricted CTL responses to MV antigens, paralleling those in MV infection of humans. In addition, such humanized mice generated human CD8 coreceptor-dependent HLA-B27-restricted CTL with the same specificity for recognition of MV fusion (F) peptide RRYPDAVYL as reported for humans during natural MV infection. Neither murine beta(2)-microglobulin nor murine CD8 substituted adequately as coreceptors for the HLA-B27 heavy chain. By contrast, HLA-A2.1-restricted responses to measles could be generated in the absence of expression of human beta(2)-microglobulin or CD8(+) molecules in HLA-A2.1/K(b) transgenic mice. Thus a small animal model is now available for studying strategies for optimizing human CD8(+) T cell responses and for testing vaccines. This model offers the potential, when combined with the newly reported CD46 transgenic mouse model in which MV replicates in cells of the immune system, for uncoding the molecular mechanism of MV-induced immunosuppression.

Phenotypical and Functional Characterization of the CD8+ T Cell Repertoire of HLA-A2.1 Transgenic, H-2K b °D b ° Double Knockout Mice

Homozygous HLA-A2.1 transgenic H-2Kb°Db° double knockout (KO) mice were created. Their potential to develop HLA-A2.1-restricted cytolytic responses was compared with that of their classical transgenic counterparts, which still express H-2Kb, Db molecules. On cell surfaces, both strains express similar amounts of chimeric (α1α2 domains of human, α3 cytoplasmic domains of mouse) HLA-A2.1 molecules in noncovalent association with mouse β2-microglobulin. Compared with mice that are totally deprived of histocompatibility class Ia molecules (H-2Kb°Db° double KO), the expression of HLA-A2.1 in transgenic/double KO mice resulted in sizeable increase in the periphery of CD8+ T cells with a normally diversified TCR repertoire. A biased education in favor of HLA-A2.1, ascribable to the absence of H-2 class Ia molecules, was evidenced in these transgenic/double KO mice by their improved capacity to mount HLA-restricted cytolytic responses, regardless of whether they were virally infected or injected with synthetic epitopic peptide. HLA class I transgenic, H-2 class Ia KO mice should represent useful animal models for the preclinical evaluation of vaccine formulations aiming at the induction of HLA class I-restricted CTL responses.

Induction in transgenic mice of HLA-A2.1-restricted cytotoxic T cells specific for a peptide sequence from a mutated p21ras protein

Cytotoxic T cells (CTL) recognize short peptides that are derived from the proteolysis of endogenous cellular proteins and presented on the cell surface as a complex with MHC class I molecules. CTL can recognize single amino acid substitutions in proteins, including those involved in malignant transformation. The mutated sequence of an oncogene may be presented on the cell surface as a peptide, and thus represents a potential target antigen for tumour therapy. The p21ras gene is mutated in a wide variety of tumours and since the transforming mutations result in amino acid substitutions at positions 12, 13 and 61 of the protein, a limited number of ras peptides could potentially be used in the treatment of a wide variety of malignancies. A common substitution is Val for Gly at position 12 of p21ras. In this study, we show that the peptide sequence from position 5 to position 14 with Val at position 12-ras p5-14 (Val-12)-has a motif which allows it to bind to HLA-A2.1. HLA-A2.1-restricted ras p5-14 (Val-12)-specific CTL were induced in mice transgenic for both HLA-A2.1 and human beta2-microglobulin after in vivo priming with the peptide. The murine CTL could recognize the ras p5-14 (Val-12) peptide when they were presented on both murine and human target cells bearing HLA-A2.1. No cross-reactivity was observed with the native peptide ras p5-14 (Gly-12), and this peptide was not immunogenic in HLA-A2.1 transgenic mice. This represents an interesting model for the study of an HLA-restricted CD8 cytotoxic T cell response to a defined tumour antigen in vivo.

The use of human leucocyte antigen class I transgenic mice to investigate human immune function

HLA class I transgenic mice are a powerful research tool which have been used as models for human immune responses. This review describes the generation of the different HLA class I transgenic mice, the techniques used to improve expression of the transgene and use of the transgene product in immune responses. It also illustrates how HLA class I transgenic mice have provided insights into the nature of the allogeneic and xenogeneic response, the generation of CTL responses, the development of autoimmune diseases, and their use for the generation of anti-HLA class I antibodies. Despite these advances, the use of available HLA class I transgenic mice as models for human disease and immune responses has been limited. The development of new transgenic strains incorporating multiple human transgenes may allow the potential of HLA class I transgenic mice to be realised.

The effect of human beta2-microglobulin on major histocompatibility complex I peptide loading and the engineering of a high affinity variant. Implications for peptide-based vaccines

The ability to directly load cell surface major histocompatibility complex (MHC) class I molecules with peptides provides a potentially powerful approach toward the development of vaccines to generate cell-mediated immunity. We demonstrate that exogenous beta2-microglobulin (beta2m) stabilizes human cell surface MHC I molecules and facilitates their loading with exogenous peptides. Additionally, using three-dimensional crystal structures and known interaction sites between MHC I heavy chains and beta2m, we engineered variants of human beta2m (hbeta2m) with a single serine substitution at residue 55. This alteration was predicted to promote hydrophobic interactions at the MHC I heavy chain/beta2m interface and displace an ordered water molecule. Compared with hbeta2m, the serine to valine substitution at residue 55 had improved ability to bind to cell surface HLA-A1, HLA-A2, and HLA-A3 molecules, facilitate exogenous peptide loading, and promote recognition by peptide-specific T cells. The inclusion of hbeta2m or higher affinity variants when pulsing cells with MHC-restricted peptides increases the efficiency of peptide loading 50-80-fold. Therefore, the inclusion of hbeta2m in peptide-based vaccines may increase cell surface antigen densities above thresholds that allow recognition of peptide antigens by the immune system, particularly for cryptic, subdominant, or marginally antigenic peptides.

Cytotoxic T lymphocyte recognition of HLA-G in mice

Several features of HLA-G's sequence and expression pattern distinguish HLA-G from its classical counterparts. These features, including HLA-G's limited polymorphism and its expression at the maternal-fetal interface, have been used as a basis for suggesting a distinct functional role for this nonclassical class I HLA molecule. On the other hand, published data do demonstrate that HLA-G has much in common with its classical counterparts. It associates with beta 2-microglobulin and cytosolic peptides, it binds to CD8, and its presence can inhibit NK-cell-mediated lysis of HLA-G-bearing target cells. To develop a model in which HLA-G's function could be more thoroughly studied, we produced several HLA-G-expressing transgenic mouse strains. We report here the results of skin graft experiments which show that nontransgenic mice reject HLA-G-expressing transgenic murine skin as foreign and that this rejection is associated with the presence in the recipient of lymphocytes capable of specifically lysing HLA-G-expressing cells. In addition, experiments are described which demonstrate that HLA-G transgenic mice recognize HLA-G as a "self" molecule. Together the reported data demonstrate that HLA-G is capable of stimulating an HLA-G-restricted CTL response, that HLA-G molecules can serve as target molecules in lytic interactions with CTLs, and that HLA-G is involved in education of the lymphocytic repertoire of HLA-G transgenic mice.

HLA-A2.1-restricted education and cytolytic activity of CD8(+) T lymphocytes from beta2 microglobulin (beta2m) HLA-A2.1 monochain transgenic H-2Db beta2m double knockout mice

Three different HLA-A2.1 monochains were engineered in which either the human or mouse beta2-microglobulin (beta2m) is covalently linked to the NH2 terminus of the heavy chain by a 15- amino acid long peptide: HHH, entirely human, HHD, with the mouse H-2Db alpha3, transmembrane, and cytoplasmic domains, and MHD, homologous to HHD but linked to the mouse beta2mb. The cell surface expression and immunological capacities of the three monochains were compared with transfected cells, and the selected HHD construct was introduced by transgenesis in H-2Db-/- beta2m-/- double knockout mice. Expression of this monochain restores a sizable peripheral CD8(+) T cell repertoire essentially educated on the transgenic human molecule. Consequently, infected HHD, H-2Db-/- beta2m-/- mice generate only HLA-A2.1-restricted CD8(+) CTL responses against influenza A and vaccinia viruses. Interestingly, the CTL response to influenza A virus is mostly, if not exclusively, directed to the 58-66 matrix peptide which is the HLA-A2.1-restricted immunodominant epitope in humans. Such mice might constitute a versatile animal model for the study of HLA-A2.1-restricted CTL responses of vaccine interest.

HLA-G transgenic mice: a model for studying expression and function at the maternal/fetal interface

Experiments designed to identify all HLA class I genes led to the cloning of the HLA-G gene (Geraghty et al. 1987). Very low levels of HLA-G mRNA expression have been demonstrated in the eye, thymus, peripheral blood lymphocytes and in keratinocytes (Shukla et al. 1990, Ishitani & Geraghty 1992, Kirszenbaum et al. 1994, Ulbrecht et al. 1994). Higher levels of HLA-G expression were demonstrated in certain subpopulations of trophoblasts (Ellis et al. 1990, Kovats et al. 1990). Specifically, those subpopulations of trophoblasts in direct contact with decidua, i.e. the cytotrophoblasts of the cytotrophoblast shell and columns, invasive cytotrophoblasts, and cytotrophoblasts of the chorionic membrane, express HLA-G (Yelavarthi et al. 1991, Chumbley et al. 1993). HLA-G molecules on these cells are in a position to interact directly with the maternal immune system. HLA-G transgenic mice have been produced in an effort to produce a model in which antigen presentation by HLA-G can be studied. Studies utilizing these transgenic mice have led to the definition of a previously unknown regulatory region and have yielded some basic information about HLA-G's ability to function as a class I molecule (Schmidt et al., in press; manuscript in preparation). One of the HLA-G transgenic mouse lines produced, G.3.2, has been shown to have a cell-type specific extraembryonic HLA-G expression pattern paralleling that seen in human extraembryonic tissues (Schmidt et al, in press). In addition, the levels of HLA-G mRNA expression seen in the extraembryonic tissues from this transgenic mouse line are similar to the levels of HLA-G mRNA expression detectable in human extraembryonic tissues. As cells expressing HLA-G come in direct contact with decidual cells, these mice will serve as a model in which HLA-G's ability to present foreign antigens to the maternal immune system can be studied. The G.3.2 HLA-G transgenic mouse line also exhibits thymic HLA-G expression levels similar to those seen in human thymus (Schmidt et al., manuscript in preparation). HLA-G expression in the transgenic murine thymus is largely restricted to dendritic cells. It is possible that this thymic HLA-G mRNA expression is responsible for the tolerance to HLA-G which is seen in the HLA-G transgenic mice. Further studies will be necessary to determine whether the lymphocytic repertoire of the HLA-G transgenic mouse can recognize HLA-G molecules presenting foreign peptides.

The T cell response of HLA-DR transgenic mice to human myelin basic protein and other antigens in the presence and absence of human CD4

Analysis of HLA class II transgenic mice has progressed in recent years from analysis of single chain HLA class II transgenes with expression of mixed mouse/human heterodimers to double transgenic mice expressing normal human heterodimers. Previous studies have used either HLA transgenic mice in which there is a species-matched interaction with CD4 or mice which lack this interaction. Since both systems are reported to generate HLA-restricted responses, the matter of the requirement for species-matched CD4 remains unclear. We have generated triple transgenic mice expressing three human transgenes, DRA, DRB, and CD4, and compared HLA-restricted responses to peptide between human-CD4+ (Hu-CD4+) and Hu-CD4- littermates. We saw no difference between Hu-CD4+ and Hu-CD4- groups, supporting the notion that for some responses at least the requirement for species-matched CD4 may not be absolute. Evidence for positive selection of mouse T cell receptors in HLA-DR transgenic mice came both from the acquisition of new, HLA-restricted responses to various peptides and from an increased frequency of T cells using the TCR V beta 4 gene segment. An important goal with respect to the analysis of function in HLA transgenic mice is the clarification of mechanisms which underpin the recognition of self-antigens in human autoimmune disease. As a first step towards 'humanized' disease models in HLA transgenic mice, we analyzed the responses of HLA-DR transgenic mice to the human MPB 139-154 peptide which has been implicated as an epitope recognized by T cells of multiple sclerosis patients. We obtained T cell responses to this epitope in transgenic mice but not in nontransgenic controls. This study suggests that HLA transgenic mice will be valuable in the analysis of HLA-restricted T cell epitopes implicated in human disease and possibly in the design of new disease models.

Generation of an antibody to HLA-G in transgenic mice and demonstration of the tissue reactivity of this antibody

A method was devised to generate antibodies against the non-classical class I HLA-G antigen. This consisted of immunising HLA-A2/beta 2m double transgenic mice with HLA-G transfected into mouse Ltk- cells. A polyclonal antibody was obtained which appears to be specific for HLA-G. The staining pattern of this antibody was restricted solely to all populations of extravillous trophoblast. No fetal tissues reacted with this antibody, including those where HLA-G mRNA has been demonstrated, such as fetal eye, thymus and liver. This study confirms that HLA-G is a trophoblast-specific protein, although it remains a possibility that the technique of immunohistology is not sufficiently sensitive to detect low level HLA-G antigen expression in non-trophoblast tissues.

Human cytomegalovirus-infected cells have unstable assembly of major histocompatibility complex class I complexes and are resistant to lysis by cytotoxic T lymphocytes

Viruses which cause persistence in the naturally infected host are predicted to have evolved immune evasion mechanisms. Human cytomegalovirus (HCMV) causes significant morbidity and mortality in immunocompromised patients yet persists without clinical manifestations in seropositive individuals who have normal immune function. We report that HCMV infection in vitro impairs major histocompatibility complex class I (MHC-I) assembly accompanied by resistance to killing by cytotoxic CD8+ T lymphocytes. Pulse-chase metabolic labelling experiments show that MHC-I complexes continue to be assembled by both uninfected and HCMV-infected cells. However, MHC-I molecules are unstable in HCMV-infected cells and are rapidly broken down. Endoglycosidase H treatment of immunoprecipitates indicates that the breakdown of MHC-I complexes in HCMV-infected cells occurs primarily in a pre-Golgi compartment. Interference with normal MHC-I assembly and expression, if relevant in vivo, may have implications for the restriction of the diversity of the CD8+ cytotoxic T lymphocyte repertoire directed against HCMV antigens and may be an important mechanism of viral persistence.

Identification of the renal erythropoietin-producing cells using transgenic mice

Regulation of erythropoietin production by the kidneys is central to the control of erythropoiesis. Uncertainty about the identity of the renal cells involved has been a major obstacle to understanding this mechanism. We have used sequence from the mouse erythropoietin locus to direct expression of a marker gene, SV40 T antigen, to these cells in transgenic mice. The transgenic constructs contained an oligonucleotide marker (Epo-M) or SV40 sequence (Epo-TAg) in the 5' untranslated region of the mouse erythropoietin gene, flanked on each side by 9 and 7.5 kb of DNA from the mouse erythropoietin locus. Anemia-inducible expression of Epo-M and Epo-TAg was observed in the kidney. In one of thirteen lines, homologous integration of Epo-TAg into the mouse erythropoietin locus occurred. In transgenic mice bearing Epo-TAg at homologous and heterologous insertion sites, renal expression was restricted to a population of cells in the interstitium of the cortex and outer medulla. Immunohistochemical characterization by light and electron microscopy shows that these are the fibroblast-like type I interstitial cells.

Identification of the nonamer peptide from influenza A matrix protein and the role of pockets of HLA-A2 in its recognition by cytotoxic T lymphocytes

Influenza matrix peptide 58-66 is shown to be the optimal nonamer for binding to HLA-A2 and presentation to cytotoxic T lymphocytes (CTL). If titered out to 2 x 10(-10) - 4 x 10(-10) M in CTL-mediated lysis assays and to 3 x 10(-9) M in an HLA-A2 assembly-stabilization assay in cell lysates. The peptide was shown to make probable contacts with its carboxy terminus close to residue 116 in the floor of the cleft of HLA-A2, close to the F pocket. The side chain of the amino-terminal amino acid was unimportant, but its free amino and carbonyl groups in the A pocket appeared important in optimizing peptide presentation. The B pocket probably accommodates the side chain of residue 2 (isoleucine) and was shown to be critical in peptide presentation.

Analysis of the HLA-restricted influenza-specific cytotoxic T lymphocyte response in transgenic mice carrying a chimeric human-mouse class I major histocompatibility complex

Transgenic murine lines have been constructed that express a chimeric class I molecule composed of the alpha 1 and alpha 2 domains of HLA-A2.1 and the alpha 3, transmembrane, and cytoplasmic domains of H-2Kb. Upon immunization with influenza virus, transgenic mice developed a strong A2.1Kb-restricted cytotoxic T lymphocyte (CTL) response specific for the same matrix protein epitope that serves as the dominant A2.1-restricted determinant in the equivalent human response. Fine specificity analysis of CTL clones using truncated peptides revealed strong similarity between the response repertoire of transgenic mice and that previously reported using influenza-specific A2.1-restricted CTL clones from humans. This suggests that even when considering T cell responses by different species, the alpha 1 and alpha 2 domains of the restriction element play a dominant role in determining the CTL specific repertoire. Thus, substituting the alpha 3 domain of A2.1 with a murine counterpart has permitted development of a transgenic strain that should serve as an excellent model system in studies of HLA-restricted responses.