Human CD8 transgene regulation of HLA recognition by murine T cells

Peptide-Based Vaccines for Tuberculosis

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. As a result of the coronavirus disease 2019 (COVID-19) pandemic, the global TB mortality rate in 2020 is rising, making TB prevention and control more challenging. Vaccination has been considered the best approach to reduce the TB burden. Unfortunately, BCG, the only TB vaccine currently approved for use, offers some protection against childhood TB but is less effective in adults. Therefore, it is urgent to develop new TB vaccines that are more effective than BCG. Accumulating data indicated that peptides or epitopes play essential roles in bridging innate and adaptive immunity and triggering adaptive immunity. Furthermore, innovations in bioinformatics, immunoinformatics, synthetic technologies, new materials, and transgenic animal models have put wings on the research of peptide-based vaccines for TB. Hence, this review seeks to give an overview of current tools that can be used to design a peptide-based vaccine, the research status of peptide-based vaccines for TB, protein-based bacterial vaccine delivery systems, and animal models for the peptide-based vaccines. These explorations will provide approaches and strategies for developing safer and more effective peptide-based vaccines and contribute to achieving the WHO’s End TB Strategy.

Molecular mimicry: its evolution from concept to mechanism as a cause of autoimmune diseases

On a clonal level, certain antibodies and T cells can interact with dissimilar antigens found in microbes and in host cells. More than 5% of over 800 monoclonal antibodies derived from multiple RNA and DNA viruses, as well as from a large number of T cell clones, engage in such interactions. Several of these cross-reactions, which we termed molecular mimicry, are against unique host proteins involved in autoimmune responses and diseases. Thus, molecular mimicry initiated as a host response to a virus or a microbial infection, but alternatively cross-reacting with an appropriate host-antigen, can be a mechanism for instigating an autoimmune disease. Molecular mimicry provides an explanation for the genetic observation that identical twins rarely manifest the same autoimmune disease and the documented epidemiologic evidence that microbial and/or viral infections often precede autoimmune disorders.

Generation and utility of genetically humanized mouse models

Identifying in vivo models that are naturally predictive for particular areas of study in humans can be challenging due to the divergence that has occurred during speciation. One solution to this challenge that is gaining increasing traction is the use of genetic engineering to introduce human genes into mice to generate superior models for predicting human responses. This review describes the state-of-the-art for generating such models, provides an overview of the types of genetically humanized mouse models described to date and their applications in basic research, drug discovery and development and to understand clinical drug toxicity. We discuss limitations and explore promising future directions for the use of genetically humanized mice to further improve translational research.

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.

Opposing chemokine gradients control human thymocyte migration in situ

The ordered migration of thymocytes from the cortex to the medulla is critical for the appropriate selection of the mature T cell repertoire. Most studies of thymocyte migration rely on mouse models, but we know relatively little about how human thymocytes find their appropriate anatomical niches within the thymus. Moreover, the signals that retain CD4+CD8+ double-positive (DP) thymocytes in the cortex and prevent them from entering the medulla prior to positive selection have not been identified in mice or humans. Here, we examined the intrathymic migration of human thymocytes in both mouse and human thymic stroma and found that human thymocyte subsets localized appropriately to the cortex on mouse thymic stroma and that MHC-dependent interactions between human thymocytes and mouse stroma could maintain the activation and motility of DP cells. We also showed that CXCR4 was required to retain human DP thymocytes in the cortex, whereas CCR7 promoted migration of mature human thymocytes to the medulla. Thus, 2 opposing chemokine gradients control the migration of thymocytes from the cortex to the medulla. These findings point to significant interspecies conservation in thymocyte-stroma interactions and provide the first evidence that chemokines not only attract mature thymocytes to the medulla, but also play an active role in retaining DP thymocytes in the cortex prior to positive selection.

Effective elicitation of human effector CD8+ T Cells in HLA-B*51:01 transgenic humanized mice after infection with HIV-1

Humanized mice are expected to be useful as small animal models for in vivo studies on the pathogenesis of infectious diseases. However, it is well known that human CD8⁺ T cells cannot differentiate into effector cells in immunodeficient mice transplanted with only human CD34⁺ hematopoietic stem cells (HSCs), because human T cells are not educated by HLA in the mouse thymus. We here established HLA-B*51:01 transgenic humanized mice by transplanting human CD34⁺ HSCs into HLA-B*51:01 transgenic NOD/SCID/Jak3^−/− mice (hNOK/B51Tg mice) and investigated whether human effector CD8⁺ T cells would be elicited in the mice or in those infected with HIV-1 NL4-3. There were no differences in the frequency of late effector memory and effector subsets (CD27^lowCD28⁻CD45RA^+/−CCR7⁻ and CD27⁻CD28⁻CD45RA^+/−CCR7⁻, respectively) among human CD8⁺ T cells and in that of human CD8⁺ T cells expressing CX3CR1 and/or CXCR1 between hNOK/B51Tg and hNOK mice. In contrast, the frequency of late effector memory and effector CD8⁺ T cell subsets and of those expressing CX3CR1 and/or CXCR1 was significantly higher in HIV-1-infected hNOK/B51Tg mice than in uninfected ones, whereas there was no difference in that of these subsets between HIV-1-infected and uninfected hNOK mice. These results suggest that hNOK/B51Tg mice had CD8⁺ T cells that were capable of differentiating into effector T cells after viral antigen stimulation and had a greater ability to elicit effector CD8⁺ T cells than hNOK ones.

Beyond HLA-A*0201: new HLA-transgenic nonobese diabetic mouse models of type 1 diabetes identify the insulin C-peptide as a rich source of CD8+ T cell epitopes

Type 1 diabetes is an autoimmune disease characterized by T cell responses to β cell Ags, including insulin. Investigations employing the NOD mouse model of the disease have revealed an essential role for β cell-specific CD8(+) T cells in the pathogenic process. As CD8(+) T cells specific for β cell Ags are also present in patients, these reactivities have the potential to serve as therapeutic targets or markers for autoimmune activity. NOD mice transgenic for human class I MHC molecules have previously been employed to identify T cell epitopes having important relevance to the human disease. However, most studies have focused exclusively on HLA-A*0201. To broaden the reach of epitope-based monitoring and therapeutic strategies, we have looked beyond this allele and developed NOD mice expressing human β(2)-microglobulin and HLA-A*1101 or HLA-B*0702, which are representative members of the A3 and B7 HLA supertypes, respectively. We have used islet-infiltrating T cells spontaneously arising in these strains to identify β cell peptides recognized in the context of the transgenic HLA molecules. This work has identified the insulin C-peptide as an abundant source of CD8(+) T cell epitopes. Responses to these epitopes should be of considerable utility for immune monitoring, as they cannot reflect an immune reaction to exogenously administered insulin, which lacks the C-peptide. Because the peptides bound by one supertype member were found to bind certain other members also, the epitopes identified in this study have the potential to result in therapeutic and monitoring tools applicable to large numbers of patients and at-risk individuals.

Failure of effector function of human CD8+ T Cells in NOD/SCID/JAK3⁻/⁻ immunodeficient mice transplanted with human CD34+ hematopoietic stem cells

Humanized mice, which are generated by transplanting human CD34⁺ hematopoietic stem cells into immunodeficient mice, are expected to be useful for the research on human immune responses. It is reported that antigen-specific T cell responses occur in immunodeficient mice transplanted with both human fetal thymus/liver tissues and CD34⁺ fetal cells, but it remains unclear whether antigen-specific T cell responses occur in those transplanted with only human CD34⁺ hematopoietic stem cells (HSCs). Here we investigated the differentiation and function of human CD8⁺ T cells reconstituted in NOD/SCID/Jak3^−/− mice transplanted with human CD34⁺ HSCs (hNOK mice). Multicolor flow cytometric analysis demonstrated that human CD8⁺ T cells generated from the CD34⁺ HSCs comprised only 3 subtypes, i.e., CD27^highCD28⁺CD45RA⁺CCR7⁺, CD27⁺CD28⁺CD45RA⁻CCR7⁺, and CD27⁺CD28⁺CD45RA⁻CCR7⁻ and had 3 phenotypes for 3 lytic molecules, i.e., perforin(Per)⁻granzymeA(GraA)⁻granzymeB(GraB)⁻, Per⁻GraA⁺GraB⁻, and Per^lowGraA⁺GraB⁺. These CD8⁺ T cells failed to produce IFN-γ and to proliferate after stimulation with alloantigens. These results indicate that the antigen-specific T cell response cannot be elicited in mice transplanted with only human CD34⁺ HSCs, because the T cells fail to develop normally in such mice.

Immunological validation of the EpitOptimizer program for streamlined design of heteroclitic epitopes

One strategy to generate T-cell responses to tumors is to alter subdominant epitopes through substitution of amino acids that are optimal anchors for specific MHC molecules, termed heteroclitic epitopes. This approach is manually error-prone and time-consuming. In here, we describe a computer-based algorithm (EpitOptimizer) for the streamlined design of heteroclitic epitopes. Analysis of two cancer-related proteins showed that EpitOptimizer-generated peptides have enhanced MHC-I binding compared with their wild-type counterparts; and were able to induce stronger CD8+ T-cell responses against their native epitope. These data demonstrate that this approach can serve as the basis of epitope-engineering against cancer and intracellular pathogens.

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.

Optimization of a self antigen for presentation of multiple epitopes in cancer immunity

T cells recognizing self antigens expressed by cancer cells are prevalent in the immune repertoire. However, activation of these autoreactive T cells is limited by weak signals that are incapable of fully priming naive T cells, creating a state of tolerance or ignorance. Even if T cell activation occurs, immunity can be further restricted by a dominant response directed at only a single epitope. Enhanced antigen presentation of multiple epitopes was investigated as a strategy to overcome these barriers. Specific point mutations that create altered peptide ligands were introduced into the gene encoding a nonimmunogenic tissue self antigen expressed by melanoma, tyrosinase-related protein-1 (Tyrp1). Deficient asparagine-linked glycosylation, which was caused by additional mutations, produced altered protein trafficking and fate that increased antigen processing. Immunization of mice with mutated Tyrp1 DNA elicited cross-reactive CD8(+) T cell responses against multiple nonmutated epitopes of syngeneic Tyrp1 and against melanoma cells. These multi-specific anti-Tyrp1 CD8(+) T cell responses led to rejection of poorly immunogenic melanoma and prolonged survival when immunization was started after tumor challenge. These studies demonstrate how rationally designed DNA vaccines directed against self antigens for enhanced antigen processing and presentation reveal novel self epitopes and elicit multi-specific T cell responses to nonimmunogenic, nonmutated self antigens, enhancing immunity against cancer self antigens.

Highly conserved pattern of recognition of influenza A wild-type and variant CD8+ CTL epitopes in HLA-A2+ humans and transgenic HLA-A2+/H2 class I-deficient mice

As an in vivo model for studying human MHC (HLA) class I-restricted CTL responses to viral infection, we established a series of HLA Tg mice expressing HLA-A2, -B7 or -B27 human/mouse hybrid genes on a background deficient for H2 class I (Tg HLA(hyb)/H2 class I DKO). To determine whether CTL recognition of influenza A (flu) infection in Tg HLA-A2(hyb)/H2 DKO mice is similar to HLA-A2+ humans, we compared the HLA-A2-restricted Tg mouse and human CD8+ T-cell responses to an immunodominant flu epitope (wild-type [WT] M1 58-66), as well as a variant of this peptide (var. M1 58-66). Similar to HLA-A2+ humans, our results show WT M1 58-66 is likely the dominant CTL epitope recognized in infected Tg HLA-A2(hyb)/H2 DKO mice. Var. M1 58-66 was also recognized by WT peptide-reactive T cells from both HLA-A2+ humans and Tg mice, although slightly less efficiently than WT M1 58-66 in both cases. Reduced variant recognition was shown to be associated with reduced peptide/A2 binding, as well as a more limited repertoire of utilized TCR Vbeta chains. The similar pattern of recognition and cross reaction observed here for the WT and variant M1 58-66 epitopes with HLA-A2 by human and Tg HLA mouse CTLs indicates that A2-dependent events of Ag processing, presentation and recognition are well-conserved between species. These findings demonstrate that this Tg HLA-A2/H2 DKO model will aid identification and development of epitopes as vaccines for numerous viral and tumor antigens for the HLA-A2 supertype.

Molecular mimicry, microbial infection, and autoimmune disease: evolution of the concept

Molecular mimicry is defined as similar structures shared by molecules from dissimilar genes or by their protein products. Either several linear amino acids or their conformational fit may be shared, even though their origins are separate. Hence, during a viral or microbe infection, if that organism shares cross-reactive epitopes for B or T cells with the host, then the response to the infecting agent will also attack the host, causing autoimmune disease. A variation on this theme is when a second, third, or repeated infection(s) shares cross-reactive B or T cell epitopes with the first (initiating) virus but not necessarily the host. In this instance, the secondary infectious agents increase the number of antiviral/antihost effector antibodies or T cells that potentiate or precipitate the autoimmune assault. The formation of this concept initially via study of monoclonal antibody or clone T cell cross-recognition in vitro through its evolution to in vivo animal models and to selected human diseases is explored in this mini-review.

Poor immunogenicity of a self/tumor antigen derives from peptide-MHC-I instability and is independent of tolerance

Understanding the mechanisms underlying the poor immunogenicity of human self/tumor antigens is challenging because of experimental limitations in humans. Here, we developed a human-mouse chimeric model that allows us to investigate the roles of the frequency and self-reactivity of antigen-specific T cells in determination of the immunogenicity of an epitope (amino acids 209-217) derived from a human melanoma antigen, gp100. In these transgenic mice, CD8+ T cells express the variable regions of a human T cell receptor (hTCR) specific for an HLA-A*0201-restricted gp100(209-217). Immunization of hTCR-transgenic mice with gp100(209-217) peptide elicited minimal T cell responses, even in mice in which the epitope was knocked out. Conversely, a modified epitope, gp100(209-217(2M)), was significantly more immunogenic. Both biological and physical assays revealed a fast rate of dissociation of the native peptide from the HLA-A*0201 molecule and a considerably slower rate of dissociation of the modified peptide. In vivo, the time allowed for dissociation of peptide-MHC complexes on APCs prior to their exposure to T cells significantly affected the induction of immune responses. These findings indicate that the poor immunogenicity of some self/tumor antigens is due to the instability of the peptide-MHC complex rather than to the continual deletion or tolerization of self-reactive T cells.

I-TAC/CXCL11 is a natural antagonist for CCR5

The selective CXC chemokine receptor 3 (CXCR3) agonists, monokine induced by interferon-gamma (IFN- gamma)/CXC chemokine ligand 9 (CXCL9), IFN-inducible protein 10/CXCL10, and IFN-inducible T cell alpha chemoattractant (I-TAC)/CXCL11, attract CXCR3+ cells such as CD45RO+ T lymphocytes, B cells, and natural killer cells. Further, all three chemokines are potent, natural antagonists for chemokine receptor 3 (CCR3) and feature defensin-like, antimicrobial activities. In this study, we show that I-TAC, in addition to these effects, acts as an antagonist for CCR5. I-TAC inhibited the binding of macrophage-inflammatory protein-1alpha (MIP-1alpha)/CC chemokine ligand 3 (CCL3) to cells transfected with CCR5 and to monocytes. Furthermore, cell migration evoked by regulated on activation, normal T expressed and secreted (RANTES)/CCL5 and MIP-1beta/CCL4, the selective agonist of CCR5, was inhibited in transfected cells and monocytes, respectively. In two other functional assays, namely the release of free intracellular calcium and actin polymerization, I-TAC reduced CCR5 activities to minimal levels. Sequence and structure analyses indicate a potential role for K17, K49, and Q51 of I-TAC in CCR5 binding. Our results expand on the potential role of I-TAC as a negative modulator in leukocyte migration and activation, as I-TAC would specifically counteract the responses mediated by many "classical," inflammatory chemokines that act not only via CCR3 but via CCR5 as well.

In a Transgenic Model of Spontaneous Autoimmune Diabetes, Expression of a Protective Class II MHC Molecule Results in Thymic Deletion of Diabetogenic CD8+ T Cells

H-2(d) mice expressing both the influenza virus hemagglutinin (HA) as a transgene-encoded protein on pancreatic islet beta cells (InsHA), as well as the Clone 4 TCR specific for the dominant H-2K(d)-restricted HA epitope, can be protected from the development of spontaneous autoimmune diabetes by expression of the H-2(b) haplotype. Protection occurs due to the deletion of K(d)HA-specific CD8+ T cells. This was unexpected as neither the presence of the InsHA transgene nor H-2(b), individually, resulted in thymic deletion. Further analyses revealed that thymic deletion required both a hybrid MHC class II molecule, Ebeta(b) Ealpha(d), and the K(d) molecule presenting the HA epitope, which together synergize to effect deletion of CD4+CD8+ thymocytes. This surprising example of protection from autoimmunity that maps to a class II MHC molecule, yet effects an alteration in the CD8+ T cell repertoire, suggests that selective events in the thymus represent the integrated strength of signal delivered to each cell through recognition of a variety of different MHC-peptide ligands.

Fully Functional HLA B27-Restricted CD4+as well as CD8+T Cell Responses in TCR Transgenic Mice

The strong association of HLA B27 with spondyloarthropathies contrasts strikingly with most autoimmune diseases, which are HLA class II associated and thought to be mediated by CD4+ T lymphocytes. By introducing a human-derived HLA B27-restricted TCR into HLA B27 transgenic mice, we have obtained a functional TCR transgenic model, GRb, dependent on HLA B27 for response. Surprisingly, HLA B27 supported CD4+ as well as CD8+ T cell responses in vivo and in vitro. Further, HLA B27-restricted CD4+ T cells were capable of differentiation into a range of Th1 and Th2 T cell subsets with normal patterns of cytokine expression. The transgenic T cells were also able to enhance clearance of recombinant vaccinia virus containing influenza nucleoprotein in vivo. This is the first description of a human HLA class I-restricted TCR transgenic line. The existence of CD4+ MHC class I-restricted T cells has significant implications for immune regulation in autoimmunity and, in particular, in HLA B27-associated arthritis. We believe that this model provides a novel system for the study of unusual T cell behavior in vivo.

The interaction defect of accessory molecules is responsible for the poor ex vivo response to human antigens of mouse T helper cells

Mouse T cells fail to respond to xenogeneic pig and human antigens using the direct antigen-presenting pathway. The poor response by mouse CD8 cells is because of multiple defects in the molecular interactions between mouse CD8 cells and xenogeneic antigen-presenting cells (APCs). Using human CD4/DR3+, mouse CD4-/major histocompatibility complex (MHC) class II - mice, we investigated the defects in molecular interaction responsible for the poor response to xenogeneic antigens by naïve mouse CD4+ cells. Mouse CD4 cells failed to respond to human leucocyte antigen (HLA)-DR3 expressed on mouse APCs but developed a strong response to alloantigens, indicating a defect in the interaction between mouse CD4 and HLA-DR3 molecules. Human CD4+/mouse CD4- mouse T cells respond poorly to human and pig APCs but not allogeneic APCs, indicating that accessory molecular interactions are deficient across highly separated species. Adding mouse interleukin-2 (IL-2) to the mixed lymphocyte reaction system did not improve the poor response to human or pig antigens by mouse or human CD4+/mouse CD4- mouse T cells. Therefore, multiple molecular interactions deficient between mouse CD4 cells and human or pig APCs may lead to the poor response to xenogeneic antigens by naïve mouse CD4 cells.

Human MHC class I transgenic mice deficient for H2 class I expression facilitate identification and characterization of new HLA class I-restricted viral T cell epitopes

Although mice transgenic (Tg) for human MHC (HLA) class I alleles could provide an important model for characterizing HLA-restricted viral and tumor Ag CTL epitopes, the extent to which Tg mouse T cells become HLA restricted in the presence of endogenous H2 class I and recognize the same peptides as in HLA allele-matched humans is not clear. We previously described Tg mice carrying the HLA-B27, HLA-B7, or HLA-A2 alleles expressed as fully native (HLA(nat)) (with human beta(2)-microglobulin) and as hybrid human/mouse (HLA(hyb)) molecules on the H2(b) background. To eliminate the influence of H2(b) class I, each HLA Tg strain was bred with a H2-K(b)/H2-D(b)-double knockout (DKO) strain to generate mice in which the only classical class I expression was the human molecule. Expression of each HLA(hyb) molecule and HLA-B27(nat)/human beta(2)-microglobulin led to peripheral CD8(+) T cell levels comparable with that for mice expressing a single H2-K(b) or H2-D(b) gene. Influenza A infection of Tg HLA-B27(hyb)/DKO generated a strong CD8(+) T cell response directed at the same peptide (flu nucleoprotein NP383-391) recognized by CTLs from flu-infected B27(+) humans. As HLA-B7/flu epitopes were not known from human studies, we used flu-infected Tg HLA-B7(hyb)/DKO mice to examine the CTL response to candidate peptides identified based on the B7 binding motif. We have identified flu NP418-426 as a major HLA-B7-restricted flu CTL epitope. In summary, the HLA class I Tg/H2-K/H2-D DKO mouse model described in this study provides a sensitive and specific approach for identifying and characterizing HLA-restricted CTL epitopes for a variety of human disease-associated Ags.

Effect of the V3 loop deletion of envelope glycoprotein on cellular responses and protection against challenge with recombinant vaccinia virus expressing gp160 of primary human immunodeficiency virus type 1 isolates

The magnitude and breadth of cytotoxic-T-lymphocyte (CTL) responses induced by human immunodeficiency virus type 1 (HIV-1) envelope protein from which the hypervariable V3 loop had been deleted (DeltaV3) were evaluated in the HLA-A2/K(b) transgenic mice. It was demonstrated that vaccines expressing the DeltaV3 mutant of either HIV-1(IIIB) or HIV-1(89.6) envelope glycoprotein induced broader CD8(+) T-cell activities than those elicited by the wild-type (WT) counterparts. Specifically, the differences were associated with higher responses to conserved HLA-A2-restricted CTL epitopes of the envelope glycoprotein and could be correlated with an increased cell surface occupancy by the epitope-HLA-A2 complexes in target cells expressing the DeltaV3 mutant. Using recombinant vaccinia virus expressing heterologous gp160 of primary HIV-1 isolates in a murine challenge system, we observed that the extent of resistance to viral transmission was higher in animals immunized with the DeltaV3 than the WT envelope vaccine. The protection was linked to the presence of envelope-specific CD8(+) T cells, since depletion of these cells by anti-CD8 antibody treatment at the time of challenge abolished the vaccine-induced protection. The results from our studies provide insights into approaches for boosting the breadth of envelope-specific CTL responses.

Differential long-term and multilineage engraftment potential from subfractions of human CD34+ cord blood cells transplanted into NOD/SCID mice

Over the past decade xenotransplantation systems have been used with increasing success to gain a better understanding of human cells that are able to initiate and maintain the hematopoietic system in vivo. The nonobese diabetic/severe combined immunodeficiency (SCID) mouse has been a particularly useful model. Human cells capable of hematopoietic repopulation in this mouse, termed SCID-repopulating cells, have been assumed to represent the most primitive elements of the hematopoietic system, responsible for long-term maintenance of hematopoiesis. However, we demonstrate that SCID-repopulating cells present in the CD34(+) cell fraction of cord blood can be segregated into subpopulations with distinct repopulation characteristics. CD34(+)/CD38(+) progenitors can repopulate recipients rapidly, but can only maintain the graft for 12 weeks or less and have no secondary repopulation potential. Conversely, the more primitive CD34(+)/CD38(-) subpopulation repopulates recipients more gradually, can maintain the graft for at least 20 weeks, and contains cells with serial repopulation potential throughout the engraftment period. Additionally, a much higher frequency of T cell precursors are found among SCID-repopulating cells in the CD34(+)/CD38(-) subpopulation. These findings demonstrate that cells with variable repopulation potential comprise the human CD34(+) population and that short- and long-term potential of human precursors can be evaluated in the mouse model.

Rapid rejection of HLA-A2 transgenic skin graft due to indirect allorecognition

BACKGROUND

At present, it is not clear whether xenogeneic MHC molecules are recognized by T cells directly or indirectly through self-MHC-restricted presentation in a transplantation setting.

METHODS

We have transplanted skin from HLA-A2 transgenic (B6.A2) to nontransgenic C57BL/6 (B6) mice and investigated the subsequent mouse T-cell responses to HLA molecules, in vivo and in vitro.

RESULTS

Skin transplanted from transgenic B6.A2 to B6 mice was rejected rapidly, in 12-16 days. Although naive B6 mice did not respond to B6.A2 splenocytes in vitro, spleen cells from mice that underwent transplantation showed strong proliferative responses. An anti-B6.A2 T-cell line from mice that underwent transplantation made proliferative responses to B6.A2 splenocytes but did not recognize HLA-A2 on human cells or transfected allogeneic mouse cells. The indirect, self-H-2-restricted recognition of HLA-A2 implied by this was confirmed by the finding that lysates of HLA-A2-positive, but not HLA-A2-negative, human B cells were stimulatory when pulsed onto syngeneic antigen-presenting cells and by inhibition of anti-B6.A2 proliferation with both anti-mouse MHC class I and class II antibodies.

CONCLUSION

Our results suggest that indirect recognition of xenogeneic MHC antigen plays a predominant role in graft rejection.

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.

CD8+ T cells are necessary for recognition of allelic, but not locus-mismatched or xeno-, HLA class I transplantation antigens

Although HLA transgenic mice (HLA TgM) could provide a powerful approach to investigate human MHC-specific T cell responsiveness, the extent to which these molecules are recognized by the mouse immune system remains unclear. We established TgM expressing HLA class I alleles A2, B7, or B27 in their fully native form (HLAnat) or as hybrid molecules (HLAhyb) of the HLA alpha1/alpha2 domains linked to the H-2Kb alpha3, transmembrane, and cytoplasmic domains (i.e., to maintain possible species-specific interactions). Comparison of each as xeno- (i.e., by non-TgM) vs allo- (i.e., by TgM carrying an alternate HLA allele) transplantation Ags revealed the following: 1) Although HLAhyb molecules induced stronger xeno-CD8+ T cell responses in vitro, additional effector mechanisms must be active in vivo because HLAnat skin grafts were rejected faster by non-TgM; 2) gene knockout recipients showed that xenorejection of HLAnat and, unexpectedly, HLAhyb grafts doesn't depend on CD8+ or CD4+ T cells or B cells; 3) each HLAhyb strain developed tolerance to "self" but rejected allele- (-B27 vs -B7) and locus- (-B vs -A) mismatched grafts, the former requiring CD8+ T cells, the latter by CD8+ T cell-independent mechanisms. The finding that recognition of xeno-HLAhyb does not require CD8+ T cells while recognition of the identical molecule in a strictly allo context does, demonstrates an alpha1/alpha2 domain-dependent difference in effector mechanism(s). Furthermore, the CD8+ T cell-independence of locus-mismatched rejection suggests the degree of similarity between self and non-self alpha1/alpha2 determines the effector mechanism(s) activated. The HLA Tg model provides a unique approach to characterize these mechanisms and develop tolerance protocols in the context of human transplantation Ags.

Dense bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression

Infection of fibroblast cell cultures with human cytomegalovirus (HCMV) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB were in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutralizing antibodies detected 12 months following primary immunization. The HCMV-specific major histocompatibility complex class I-restricted cytolytic T-cell (CTL) response was assayed using mice transgenic for the human HLA-A2 molecule. Immunization with DB led to high levels of HCMV-specific CTL in the absence of de novo viral protein synthesis. Maximal total cytolytic activity in mice immunized with DB was nearly as efficient as the cytolytic activity induced by a standard immunization with murine cytomegalovirus. Furthermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Induction of humoral and cellular immune responses was achieved without the concomitant use of adjuvant. We thus propose that DB can serve as a basis for the future development of a recombinant nonreplicating vaccine against HCMV. Finally, such particles could be engineered for efficient delivery of antigens from other pathogens to the immune system.

Allele-specific and peptide-dependent recognition of swine leukocyte antigen class I by human cytotoxic T-cell clones

BACKGROUND

The T-cell mediated immune responses play a major role in xenograft rejection. However, the mechanisms behind human T-cell recognition of porcine xenoantigens remain to be elucidated.

METHODS

Human CD8+ T-cell lines were generated against porcine aortic endothelial cells (PAECs) from y/y and z/z haplotypes of Yucatan inbred swine. T-cell clones were obtained by limiting dilution. The human T-cell receptor (TCR)-swine leukocyte antigen (SLA) class I interaction was characterized.

RESULTS

The human CD8+ T-cell mediated direct recognition of PAECs was SLA haplotype-specific. The haplotype specificity was restricted by the SLA class I allelic polymorphism. To characterize the role of SLA-bound peptides in the human TCR-SLA class I interaction, we stripped peptides from SLA molecules by a brief acid treatment. Using z/z-specific CD8+ T cells as effectors, we demonstrated that the acid-treatment, which stripped SLA molecules of bound peptides, decreased the lysis of PAECs by 72%. Addition of peptides eluted from affinity purified z/z SLA class I molecules, but not from the irrelevant y/y SLA class I, restored the lysis of acid-treated z/z PAECs. In addition, the lysis of a human HLA class I negative cell line, 721.221, transfected with a relevant SLA class I allele derived from the z/z haplotype, was significantly increased with the addition of relevant z/z peptides. These experiments indicated that both SLA class I and bound peptides were required for recognition by human CD8+ T cells. Cloning studies identified two groups of xenoreactive T-cell clones. Group I clones recognized distinct porcine peptides in the context of SLA class I molecules, whereas group II clones recognized human endogenous cross-reactive peptides presented by SLA class I.

CONCLUSIONS

Our results demonstrated that, despite the differences in MHC molecules between species, human T-cell recognition of porcine MHC is similar to direct allo-recognition, that is, human TCR recognizes xenogeneic SLA-peptide complexes.

Immunodominance in major histocompatibility complex class I-restricted T lymphocyte responses

Of the many thousands of peptides encoded by a complex foreign antigen that can potentially be presented to CD8+ T cells (TCD8+), only a small fraction induce measurable responses in association with any given major histocompatibility complex class I allele. To design vaccines that elicit optimal TCD8+ responses, a thorough understanding of this phenomenon, known as immunodominance, is imperative. Here we review recent progress in unraveling the molecular and cellular basis for immunodominance. Of foremost importance is peptide binding to class I molecules; only approximately 1/200 of potential determinants bind at greater than the threshold affinity (Kd > 500 nM) associated with immunogenicity. Limitations in the TCD8+ repertoire render approximately half of these peptides nonimmunogenic, and inefficient antigen processing further thins the ranks by approximately four fifths. As a result, only approximately 1/2000 of the peptides in a foreign antigen expressed by an appropriate antigen presenting cell achieve immunodominant status with a given class I allele. A roughly equal fraction of peptides have subdominant status, i.e. they induce weak-to-nondetectable primary TCD8+ responses in the context of their natural antigen. Subdominant determinants may be expressed at or above levels of immunodominant determinants, at least on antigen presenting cells in vitro. The immunogenicity of subdominant determinants is often limited by immunodomination: suppression mediated by TCD8+ specific for immunodominant determinants. Immunodomination is a central feature of TCD8+ responses, as it even occurs among clones responding to the same immunodominant determinant. Little is known about how immunodominant and subdominant determinants are distinguished by the TCD8+ repertoire, or how (and why) immunodomination occurs, but new tools are available to address these questions.

Orientation of the Ig Domains of CD8αβ Relative to MHC Class I

The cell surface glycoprotein CD8 functions as a coreceptor with the TCR for interaction with MHC class I. The cocrystal structure of the CD8αα-MHC complex showed that one CD8 Ig domain provided the majority of the contact with MHC class I and that residue R4 of that domain contacted the α2 domain of MHC class I. We previously showed by mutational analysis that this residue was critical for binding to MHC class I. To determine which of the Ig domains for the CD8αβ heterodimer would make the most contact with class I MHC, we expressed single-chain or dimeric forms of CD8 on COS-7 cells and measured the adhesion of MHC class I positive cells. We found that when one of the R4 residues was mutated in a CD8αα homodimer binding comparable to that of wild type was observed, whereas a double R4 mutant severely impaired binding. However, when mutant CD8α (R4K) was coexpressed with wild-type CD8β, binding was not observed. These results support the model in which it is CD8α, not CD8β, that is making the most of the contact with MHC class I, including the α2 domain. In addition, they demonstrate that a single-chain form of CD8αα can bind to MHC class I.

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.

Alloreactive T Cells That Do Not Require TCR and CD8 Coengagement Are Present in Naive Mice and Contribute to Graft Rejection

Class I alloreactive CTL populations have been defined as either CD8 dependent or CD8 independent, based upon their ability to kill target cells in the presence of Ab to CD8. The CD8-dependent population uses CD8 in a coreceptor role with the TCR, and mutations in the class I molecule that destroy the CD8 binding site abrogate CTL killing, even if the target cell expresses other allelic forms of class I molecules with an intact binding site for CD8. The CD8-independent population apparently does not require CD8, as Ab to CD8 has no effect on the ability of these cells to kill appropriate target cells. We have isolated a third population of CTL that is inhibited by the addition of CD8 Ab yet can kill target cells that express the alloantigenic molecule incapable of binding CD8, provided that the target cells also express non antigenic class I molecules that contain an intact binding site for CD8. We refer to these cells as CD8 bystander-dependent CTL. Many (10 of 12) of these CTL were able to kill H-2Kb-expressing transfectants of T2 cells, consistent with the idea that they recognize a peptide-independent determinant that may be expressed at a high density on the cell surface. These CD8 bystander-dependent CTL are only readily detectable in vitro when spleen cells from mice primed in vivo with a skin graft are used.

Tolerance to p53 by A2.1-restricted cytotoxic T lymphocytes

Elevated levels of the p53 protein occur in approximately 50% of human malignancies, which makes it an excellent target for a broad-spectrum T cell immunotherapy of cancer. A major barrier to the design of p53-specific immunotherapeutics and vaccines, however, is the possibility that T cells may be tolerant of antigens derived from wild-type p53 due to its low level of expression in normal thymus and lymphohemopoetic cells. The combination of p53 deficient (p53-/-) and p53+/+ HLA-A2.1/Kb transgenic mice was used as a model to explore the possibility that A2.1-restricted cytotoxic T lymphocytes (CTL) are functionally tolerant of self peptides derived from the wild-type p53 tumor suppressor protein. A2.1-restricted CTL specific for a naturally processed p53 self-epitope spanning residues 187-197 were completely aborted in p53+/+ as opposed to p53-/- transgenic mice. In contrast, CTL specific for a second self-epitope spanning residues 261-269 of the murine p53 sequence were detected in both p53-/- and p53+/+ A2.1/Kb transgenic mice. However, the avidity of the CTL effectors obtained from p53+/+ mice was 10-fold lower than that obtained from p53-/- mice, again suggesting elimination of CTL with high avidity for the A2.1-peptide complex. The circumvention of functional tolerance of high avidity CTL may therefore be a necessary prerequisite for optimizing immunotherapy against A2.1-restricted wild-type p53 epitopes in humans.

Identification of Her-2/Neu CTL epitopes using double transgenic mice expressing HLA-A2.1 and human CD.8

The Her-2/neu protooncogene is associated with malignant transformation and aggressive disease. Because of its overexpression in tumor cells and because it has been shown to be immunogenic, this protein represents an excellent target for T-cell immunotherapy. By identifying potential HLA-A2.1-binding peptides from the Her-2/neu sequence, peptides were selected as candidate T-cell epitopes. The immunogenicity of each peptide was evaluated by priming double transgenic mice expressing both the human (hu) CD8 and HLA-A2.1 molecules with synthetic peptides corresponding to these sequences. Because of the lack of interaction between murine CD8 and HLA-A2.1, expression of huCD8 on murine cells facilitates recognition of HLA molecules on human tumor cell lines. This led to the identification of two peptides that elicit an A2-restricted CTL response, one of which has not been previously identified. Both peptide-specific CTL populations were able to specifically lyse A2.1 and Her-2/neu expressing human tumor cells originating from a variety of tissues, demonstrating the utility of this murine model in identifying peptides presented by human cells. However, several Her-2/neu peptides previously reported to be immunogenic for human CTL were found not to be immunogenic in transgenic mice. The basis for these discrepancies is discussed.

Glu227-->Lys substitution in the acidic loop of major histocompatibility complex class I alpha 3 domain distinguishes low avidity CD8 coreceptor and avidity-enhanced CD8 accessory functions

Cytotoxic T lymphocyte (CTL) activation requires specific T cell receptor (TCR)-class I major histocompatibility complex (MHC) antigen complex interactions as well as the participation of coreceptor or accessory molecules on the surface of CTL. CD8 can serve as a coreceptor in that it binds to the same MHC class I molecules as the TCR to facilitate efficient TCR signaling. In addition, CD8 can be "activated" by TCR stimulation to bind to class I molecules with high avidity, including class I not recognized by the TCR as antigenic complexes (non-antigen [Ag] class I), to augment CTL responses and thus serve an accessory molecule function. A Glu/Asp227-->Lys substitution in the class I alpha 3 domain acidic loop abrogates lysis of target cells expressing these mutant molecules by alloreactive CD8-dependent CTL. Lack of response is attributed to the destruction of the CD8 binding site in the alpha 3 domain which is likely to disrupt CD8 coreceptor function. The relative importance of the class I alpha 3 domain acidic loop Glu227 in coreceptor as opposed to accessory functions of CD8 is unclear. To address this issue, we examined CTL adhesion and degranulation in response to immobilized class I-peptide complexes formed in vitro from antigenic peptides and purified class I molecules containing wild-type or Glu227-->Lys substituted alpha 3 domains. The alpha 3 domain mutant class I-peptide complexes were bound by CTL and triggered degranulation, however to much lower levels than wild-type class I-peptide complexes. In further experiments, it is directly demonstrated that the alpha 3 domain mutant class I molecules, which lack the Glu227 CD8 binding site, still serve as TCR-activated, avidity-enhanced CD8 accessory ligands. However, mutant class I peptide Ag complexes failed to effectively serve as CD8 coreceptor ligands to initiate TCR-dependent signals required to induce avidity-enhanced CD8 binding to coimmobilized non-Ag class I molecules. Thus the Glu227-->Lys mutation effectively distinguishes CD8 coreceptor and avidity-enhanced CD8 accessory functions.