Generation of cytomegalovirus-specific human T-lymphocyte clones by using autologous B-lymphoblastoid cells with stable expression of pp65 or IE1 proteins: a tool to study the fine specificity of the antiviral response

Generation of good manufacturing practice grade virus-specific T cells for the management of post-transplant CMV infections

Adoptive transfer of antigen-specific T cells has recently emerged as a successful strategy to treat viral infections following hematopoietic cell transplantation (HCT). Ex-vivo expanded donor-derived virus-specific T cells (VSTs) can be safe and effective, devoid of all the drug-related adverse effects. The study aimed to manufacture cGMP grade VSTs from healthy donors, characterize the VST product and demonstrate its safety and efficacy. Peripheral blood mononuclear cells (PBMCs) collected from six healthy donors were stimulated with pepmix that mimics the pp65 antigenic epitope of CMV and cultured for 14 days in G-Rex culture tubes. Post pepmix exposure and expansion the median CD3% was 98.8% (range:95.5% to 99.9%) while the median CD4% and CD8% were 49.1% (range:21.3% to 86.6%) and 43.9% (range:12.7% to 75.5%) respectively. The percentage of IFNγ+ cells was much higher among the CD8+ T cells (median - 18.47%; range 6.50% - 45.82%) when compared to CD4+ T cells (median - 2.74%; range 0.47% - 18.58%) and there was a switch from the CD45RA+ naive phenotype to CD45RA- effector memory phenotype in the 4 samples that achieved a >5 fold expansion. The VSTs were cytotoxic to the pepmix pulsed lymphoblasts (efficacy) while they did not induce cytolysis in the lymphoblasts that were not exposed to the pepmix (safety). This feasibility exercise helped us optimize the starting cell dose for the culture and clinical grade culture strategies, subset characterization and cytotoxicity assays. The approach could be applied to the clinical practice where virus-specific T cell infusions could be given for post-transplant viral infections.

Multiplex Identification of Antigen-Specific T Cell Receptors Using a Combination of Immune Assays and Immune Receptor Sequencing

Monitoring antigen-specific T cells is critical for the study of immune responses and development of biomarkers and immunotherapeutics. We developed a novel multiplex assay that combines conventional immune monitoring techniques and immune receptor repertoire sequencing to enable identification of T cells specific to large numbers of antigens simultaneously. We multiplexed 30 different antigens and identified 427 antigen-specific clonotypes from 5 individuals with frequencies as low as 1 per million T cells. The clonotypes identified were validated several ways including repeatability, concordance with published clonotypes, and high correlation with ELISPOT. Applying this technology we have shown that the vast majority of shared antigen-specific clonotypes identified in different individuals display the same specificity. We also showed that shared antigen-specific clonotypes are simpler sequences and are present at higher frequencies compared to non-shared clonotypes specific to the same antigen. In conclusion this technology enables sensitive and quantitative monitoring of T cells specific for hundreds or thousands of antigens simultaneously allowing the study of T cell responses with an unprecedented resolution and scale.

Identification of transcription factor AML-1 binding site upstream of human cytomegalovirus UL111A gene

Human cytomegalovirus (HCMV) interleukin-10 (hcmvIL-10), encoded by HCMV UL111A gene, is a homolog of human IL-10. It exerts immunomodulatory effects that allow HCMV to evade host defense mechanisms. However, the exact mechanism underlying the regulation of hcmvIL-10 expression is not well understood. The transcription factor acute myeloid leukemia 1 (AML-1) plays an important role in the regulation of various genes involved in the differentiation of hematopoietic lineages. A putative AML-1 binding site is present within the upstream regulatory region (URR) of UL111A gene. To provide evidence that AML-1 is involved in regulating UL111A gene expression, we examined the interaction of AML-1 with the URR of UL111A in HCMV-infected human monocytic THP-1 cells using a chromatin immunoprecipitation assay. HcmvIL-10 transcription was detected in differentiated THP-1 cells, but not in undifferentiated ones. Furthermore, the URR of UL111A showed a higher intensity of AML-1 binding, a higher level of histone H3 acetyl-K9, but a lower level of histone H3 dimethyl-K9 in differentiated THP-1 cells than undifferentiated cells. Down-regulation of AML1 by RNA interference decreased the expression of the UL111A gene. Our results suggest that AML-1 may contribute to the epigenetic regulation of UL111A gene via histone modification in HCMV-infected differentiated THP-1 cells. This finding could be useful for the development of new anti-viral therapies.

Human natural killer cells promote cross-presentation of tumor cell-derived antigens by dendritic cells

Dendritic cells (DCs) cross-present antigen (Ag) to initiate T-cell immunity against most infections and tumors. Natural killer (NK) cells are innate cytolytic lymphocytes that have emerged as key modulators of multiple DC functions. Here, we show that human NK cells promote cross-presentation of tumor cell-derived Ag by DC leading to Ag-specific CD8(+) T-cell activation. Surprisingly, cytotoxic function of NK cells was not required. Instead, we highlight a critical and nonredundant role for IFN-γ and TNF-α production by NK cells to enhance cross-presentation by DC using two different Ag models. Importantly, we observed that NK cells promote cell-associated Ag cross-presentation selectively by monocytes-derived DC (Mo-DC) and CD34-derived CD11b(neg) CD141(high) DC subsets but not by myeloid CD11b(+) DC. Moreover, we demonstrate that triggering NK cell activation by monoclonal antibodies (mAbs)-coated tumor cells leads to efficient DC cross-presentation, supporting the concept that NK cells can contribute to therapeutic mAbs efficiency by inducing downstream adaptive immunity. Taken together, our findings point toward a novel role of human NK cells bridging innate and adaptive immunity through selective induction of cell-associated Ag cross-presentation by CD141(high) DC, a process that could be exploited to better harness Ag-specific cellular immunity in immunotherapy.

Human XCR1+ dendritic cells derived in vitro from CD34+ progenitors closely resemble blood dendritic cells, including their adjuvant responsiveness, contrary to monocyte-derived dendritic cells

Human monocyte-derived dendritic cell (MoDC) have been used in the clinic with moderately encouraging results. Mouse XCR1(+) DC excel at cross-presentation, can be targeted in vivo to induce protective immunity, and share characteristics with XCR1(+) human DC. Assessment of the immunoactivation potential of XCR1(+) human DC is hindered by their paucity in vivo and by their lack of a well-defined in vitro counterpart. We report in this study a protocol generating both XCR1(+) and XCR1(-) human DC in CD34(+) progenitor cultures (CD34-DC). Gene expression profiling, phenotypic characterization, and functional studies demonstrated that XCR1(-) CD34-DC are similar to canonical MoDC, whereas XCR1(+) CD34-DC resemble XCR1(+) blood DC (bDC). XCR1(+) DC were strongly activated by polyinosinic-polycytidylic acid but not LPS, and conversely for MoDC. XCR1(+) DC and MoDC expressed strikingly different patterns of molecules involved in inflammation and in cross-talk with NK or T cells. XCR1(+) CD34-DC but not MoDC efficiently cross-presented a cell-associated Ag upon stimulation by polyinosinic-polycytidylic acid or R848, likewise to what was reported for XCR1(+) bDC. Hence, it is feasible to generate high numbers of bona fide XCR1(+) human DC in vitro as a model to decipher the functions of XCR1(+) bDC and as a potential source of XCR1(+) DC for clinical use.

Combining next-generation sequencing and immune assays: a novel method for identification of antigen-specific T cells

In this study, we combined a novel sequencing method, which can identify individual clonotypes based on their unique T cell receptor (TCR) rearrangement, with existing immune assays to characterize antigen-specific T cell responses. We validated this approach using three types of assays routinely used to measure antigen-specific responses: pentamers which enable identification of T cells bearing specific TCRs, activation marker expression following antigen stimulation and antigen-induced proliferation to identify cytomegalovirus (CMV) specific clonotypes. In one individual, 8 clonotypes were identified using a pentamer reagent derived from the CMV pp65 protein. The same 8 clonotypes were also identified following sequencing of cells that upregulated an activation marker following incubation with an identical peptide derived from pp65. These 8 and an additional 8 clonotypes were identified using a more sensitive CFSE-based proliferation assay. We found clear sequence homology among some of the clonotypes identified, and the CDR3 region in one clonotype was identical to a previously published pp65-specific clonotype sequence. Many of these CMV-specific clonotypes were present at frequencies below 10(-5) which are undetectable using standard flow-cytometric methods. These studies suggest that an immune response is comprised of a diverse set of clones, many of which are present at very low frequencies. Thus, the combination of immune assays and sequencing depicts the richness and diversity of an immune response at a level that is not possible using standard immune assays alone. The methods articulated in this work provide an enhanced understanding of T cell-mediated immune responses at the clonal level.

Induction of antiviral cytotoxic T cells by plasmacytoid dendritic cells for adoptive immunotherapy of posttransplant diseases

Virus-associated hematologic malignancies (EBV lymphoproliferative disease) and opportunistic infections (CMV) represent a major cause of hematopoietic stem cell and solid organ transplantation failure. Adoptive transfer of antigen-specific T lymphocytes appears to be a major and successful immunotherapeutic strategy, but improvements are needed to reliably produce high numbers of virus-specific T cells with appropriate requirements for adoptive immunotherapy that would allow extensive clinical use. Since plasmacytoid dendritic cells (pDCs) are crucial in launching antiviral responses, we investigated their capacity to elicit functional antiviral T-cell responses for adoptive cellular immunotherapy using a unique pDC line and antigens derived from Influenza, CMV and EBV viruses. Stimulation of peripheral blood mononuclear cells from HLA-A*0201(+) donors by HLA-A0201 matched pDCs pulsed with viral-derived peptides triggered high levels of multi-specific and functional cytotoxic T-cell responses (up to 99% tetramer(+) CD8 T cells) in vitro. Furthermore, the central/effector memory cytotoxic T cells elicited by the pDCs strongly display antiviral activity upon adoptive transfer into a humanized mouse model that mimics a virus-induced malignancy. We provide a simple and potent method to generate virus-specific CTL with the required properties for adoptive cellular immunotherapy of post-transplant diseases.

Persistent viral infection in humans can drive high frequency low-affinity T-cell expansions

CD8 T cells that recognize cytomegalovirus (CMV) -encoded peptides can be readily detected by staining with human leucocyte antigen (HLA) -peptide tetramers. These cells are invariably highly differentiated effector memory cells with high avidity T-cell receptors (TCR). In this report we demonstrate an HLA-A*0201 restricted CMV-specific CD8 T-cell response (designated YVL) that represents several percent of the CD8 T-cell subset, yet fails to bind tetrameric major histocompatibility complex (MHC) ligands. However, these tetramer-negative cells are both phenotypically and functionally similar to other CMV-specific CD8 T cells. YVL peptide-specific CD8 T-cell clones were generated and found to be of high avidity in both cytotoxicity and interferon-γ (IFN-γ) assays, and comparable with other CMV peptide-specific CD8 T-cell clones. However, under conditions of CD8 blockade, the response was almost nullified even at very high ligand concentrations. This was also the case in IFN-γ experiments using peripheral blood mononuclear cells stimulated with peptide ex vivo. In contrast, all other CMV specificities (tetramer-positive) displayed minimal or only partial CD8 dependence. This suggests that YVL-specific responses depict a low-affinity TCR-MHC-peptide interaction, that is compensated by substantial CD8 involvement for functional purposes, yet cannot engage multivalent soluble ligands for ex vivo analysis. It is interesting that such a phenomenon is apparent in the face of a persistent virus infection such as CMV, where the responding cells represent an immunodominant response in that individual and may present a highly differentiated effector phenotype.

Cross-presentation of HCMV chimeric protein enables generation and measurement of polyclonal T cells

CD8(+) T cell immunity has a critical function in controlling human cytomegalovirus (HCMV) infection. In immunocompromized individuals, HCMV reactivation or disease can lead to increased morbidity and mortality, particularly in transplant recipients. In this setting, adoptive transfer of HCMV-specific CD8(+) T cells is a promising vaccine strategy to restore viral immunity, with most clinical approaches focussing on the use of peptides for the generation of single epitope-specific CD8(+) T cells. We show that using an IE1-pp65 chimeric protein as the antigen source promotes effective cross-presentation, by monocyte-derived dendritic cells (MoDCs), to generate polyclonal CD8(+) T cell epitopes. By exploring human leukocyte antigen (HLA)-restricted immunodominance hierarchies both within and across two immunodominant proteins, we show that HLA-B7 epitopes elicit higher CD8(+) T cell responses compared with HLA-A1, -A2 or -B8. This study provides important evidence highlighting both the efficacy of the IE1-pp65 chimeric protein and the importance of immunodominance in designing future therapeutic vaccines.

Cytomegalovirus-specific CD8(+) T cells targeting different HLA/peptide combinations correlate with protection but at different threshold frequencies

Cytomegalovirus (CMV) causes significant morbidity and mortality in patients after haematopoietic stem cell transplantation (HSCT). Due to limitations of current antiviral therapies, alternative approaches, involving transfer of donor-derived CMV-specific CD8(+) T cells, have been considered. Levels of such cells correlating with protection against CMV infection and disease have only been reported in patients expressing HLA-A*0201 and HLA-B*0702. This is despite an increasing number of reports describing cells targeting CMV peptides presented by other human leucocyte antigens (HLAs). Considering several frequent HLA alleles, our findings suggest that HLA-A*2402/pp65 (341-349)- and HLA-B*3501/pp65 (123-131)-specific CD8(+) T cells correlate with protection from CMV reactivation at significantly lower cell levels than HLA-A*0101/pp50 (245-253)- and HLA-A*0201/pp65 (495-503)-specific CD8(+) T cells, both in HSCT recipients post-transplant and in healthy CMV seropositive volunteers. This may result from a differing efficiency of the responses restricted by the two sets of HLA alleles. These findings add to the knowledge of immunodominance and differences in antigen processing that are coordinated in individuals with different HLA alleles and have direct implications for therapy and monitoring in patients.

Immune evasion proteins gpUS2 and gpUS11 of human cytomegalovirus incompletely protect infected cells from CD8 T cell recognition

Human cytomegalovirus (HCMV) encodes four glycoproteins, termed gpUS2, gpUS3, gpUS6 and gpUS11 that interfere with MHC class I biosynthesis and antigen presentation. Despite gpUS2-11 expression, however, HCMV infection is efficiently controlled by cytolytic CD8 T lymphocytes (CTL). To address the role of gpUS2 and gpUS11 in antigen presentation during viral infection, HCMV mutants were generated that expressed either gpUS2 or gpUS11 alone without coexpression of the three other proteins. Fibroblasts infected with these viruses showed reduced HLA-A2 and HLA-B7 surface expression. Surprisingly, however, CTL directed against the tegument protein pp65 and the regulatory IE1 protein still recognized and lysed mutant virus infected fibroblasts. Yet, suppression of IE1 derived peptide presentation by gpUS2 or gpUS11 was far more pronounced. The results show that gpUS2 and gpUS11 alone only incompletely protect HCMV infected fibroblasts from CTL recognition and underline the importance of studying infected cells to elucidate HCMV immune evasion.

Comparisons of CD8+ T cells specific for human immunodeficiency virus, hepatitis C virus, and cytomegalovirus reveal differences in frequency, immunodominance, phenotype, and interleukin-2 responsiveness

To better understand the components of an effective immune response to human immunodeficiency virus (HIV), the CD8(+) T-cell responses to HIV, hepatitis C virus (HCV), and cytomegalovirus (CMV) were compared with regard to frequency, immunodominance, phenotype, and interleukin-2 (IL-2) responsiveness. Responses were examined in rare patients exhibiting durable immune-mediated control over HIV, termed long-term nonprogressors (LTNP) or elite controllers, and patients with progressive HIV infection (progressors). The magnitude of the virus-specific CD8(+) T-cell response targeting HIV, CMV, and HCV was not significantly different between LTNP and progressors, even though their capacity to proliferate to HIV antigens was preserved only in LTNP. In contrast to HIV-specific CD8(+) T-cell responses of LTNP, HLA B5701-restricted responses within CMV pp65 were rare and did not dominate the total CMV-specific response. Virus-specific CD8(+) T cells were predominantly CD27(+)45RO(+) for HIV and CD27(-)45RA(+) for CMV; however, these phenotypes were highly variable and heavily influenced by the degree of viremia. Although IL-2 induced significant expansions of CMV-specific CD8(+) T cells in LTNP and progressors by increasing both the numbers of cells entering the proliferating pool and the number of divisions, the proliferative capacity of a significant proportion of HIV-specific CD8(+) T cells was not restored with exogenous IL-2. These results suggest that immunodominance by HLA B5701-restricted cells is specific to HIV infection in LTNP and is not a feature of responses to other chronic viral infections. They also suggest that poor responsiveness to IL-2 is a property of HIV-specific CD8(+) T cells of progressors that is not shared with responses to other viruses over which immunologic control is maintained.

Analysis of the human cytomegalovirus pp65-directed T-cell response in healthy HLA-A2-positive individuals

Human cytomegalovirus (HCMV) is contained by T-lymphocyte responses focused towards the major tegument protein pp65. To systematically identify T-cell epitopes, we applied the following strategy: 441 overlapping 15mer peptides spanning the entire HCMV pp65 antigen in 1-aa steps were screened in enzyme-linked immunospot (ELispot) assays for interferon gamma (IFN-gamma) secretion by peripheral blood mononuclear cells (PBMCs) from nine healthy HCMV-seropositive subjects expressing human leukocyte antigen (HLA)-A2. This analysis confirmed a number of previously known epitopes and revealed several new ones. A total of 26 epitopes were identified, including 14 HLA-A2, four HLA-B7, -B35, -812 and -B44 restricted class I epitopes, six class II epitopes, and two epitopes of unknown restriction. Three novel HLA-A2 epitopes were confirmed using T2-cells, and one peptide for which only binding data had been published so far was verified. Two novel class II epitopes were confirmed by intracellular cytokine staining. Responses were usually oligoclonal against up to seven HLA-A2 epitopes, albeit with a few dominating epitopes. Clusters of overlapping epitopes (hot-spots) were identified. These and the newly identified T-cell epitopes may be of great value for epitope-based immunotherapeutic approaches, including peptide vaccines.

Protective CD8+ T-cell responses to cytomegalovirus driven by rAAV/GFP/IE1 loading of dendritic cells

Background

Recent studies demonstrate that recombinant adeno-associated virus (rAAV)-based antigen loading of dendritic cells (DCs) generates in vitro, significant and rapid cytotoxic T-lymphocyte (CTL) responses against viral antigens.

Methods

We used the rAAV system to induce specific CTLs against CVM antigens for the development of cytomegalovirus HCMV) gene therapy. As an extension of the versatility of the rAAV system, we incorporated immediate-early 1 (IE1), expressed in HCMV. Our rAAV vector induced a strong stimulation of CTLs directed against the HCMV antigen IE1. We then investigated the efficiency of the CTLs in killing IE1 targeted cells.

Results

A significant MHC Class I-restricted, anti-IE1-specific CTL killing was demonstrated against IE1 positive peripheral blood mononuclear cells (PBMC) after one, in vitro, stimulation.

Conclusion

In summary, single PBMC stimulation with rAAV/IE1 pulsed DCs induces strong antigen specific-CTL generation. CTLs were capable to lyse low doses of peptides pulsed into target cells. These data suggest that AAV-based antigen loading of DCs is highly effective for generating human CTL responses against HCMV antigens.

Maintenance of large subpopulations of differentiated CD8 T-cells two years after cytomegalovirus infection in Gambian infants

Background

In a previously published study, we found that large differentiated subpopulations of CD8 T-cells emerged rapidly after CMV infection in young infants and persisted throughout the following year. Here we describe a follow-up study conducted on the same infants to establish whether the differentiated subpopulations continued through the second year post-infection.

Methodology / Principal Findings

CMV-specific cells identified using tetramers remained more activated and differentiated than the overall CD8 population. The large subpopulation of differentiated cytotoxic (CD28⁻CD62L⁻Bcl-2^lowCD95⁺perforin⁺) cells that emerged rapidly after infection remained stable after two years. No similar subpopulation was found in CMV-uninfected infants indicating that two years after infection, CMV remained a major factor in driving CD8 T-cell differentiation. Although markers of activation (CD45R0 and HLA-D) declined throughout the first year, HLA-D expression continued to decline during the second year and CD45R0 expression increased slightly. The age-related increase in IFNγ response observed during the first year continued but was non-significant during the second year, indicating that the rate of functional improvement had slowed substantially.

Conclusions / Significance

The large differentiated subpopulations of CD8 T-cells that had emerged immediately after CMV infection persisted through the second year post-infection, while levels of activation and functional capacity remained fairly constant.

Refinement of strategies for the development of a human cytomegalovirus dense body vaccine

Development of a vaccine against human cytomegalovirus (HCMV) infection has been identified as a high priority goal in biomedical research, yet no vaccine has been licensed until now. Recombinant subviral dense bodies (recDB) are a promising basis for the establishment of such a vaccine. In this article, strategies for the generation of recDB, based on recombination-mediated genetic engineering of the 230 kb HCMV DNA genome in E. coli are outlined. Analysis of viral mutants that were constructed in this process provided the proof-of-principle that heterologous antigens can be packaged into recDB and that these particles prime CD8 T cell responses against the recombinant antigen upon their application to HLA-A2 transgenic mice.

Refinement in the production and purification of recombinant HCMV IE1-pp65 protein for the generation of epitope-specific T cell immunity

Human cytomegalovirus (HCMV) remains one of the most common opportunistic infections causing disease following stem cell transplantation, despite the availability of anti-viral therapies. Adoptive immunotherapy has the potential to further aid in counteracting chronic viral reactivation and subsequent disease by restoring viral immunity through the transfer of virus-specific T cells from transplant donors to their recipients. Our study refines the production and purification of a recombinant HCMV protein containing two of the most immunodominant antigens (IE1 and pp65) for the generation of polyclonal HCMV-specific T cells. In doing so, a 6x His-tagged IE1-pp65 protein was generated using a serum-free baculovirus/insect cell expression system and soluble IE1-pp65 protein was subsequently purified using Ni-NTA affinity chromatography under stringent conditions to obtain a highly pure product. The ability of the recombinant IE1-pp65 protein to elicit a functional T cell mediated immune response was demonstrated by the vigorous reactivation and expansion of HLA-A2-restricted pp65(495-503)-specific CD8+ T cells. This recombinant IE1-pp65 protein can potentially generate a multitude of HLA-restricted HCMV-specific T cells, providing a better alternative to using costly overlapping peptides or HCMV lysates for expansion of T cells for use in adoptive immunotherapy strategies.

High throughput functional microdissection of pathogen-specific T-cell immunity using antigen and lymphocyte arrays

The analysis of the human T-cell response specific for relevant pathogens is useful for diagnostic purposes and for research. Several methods enumerate antigen specific T-cells and measure their functions. Since screening of numerous antigens from pathogens is often needed to evaluate immunocompetence, lymphocytes, labor and cost are limiting factors. To examine pathogen-specific T-cell immunity, we have miniaturized the analysis of T-cell responses using an array approach in 384- and 1536-well plates with as few as 10 x 10(3) PBMC per well instead of the 500 x 10(3) PBMC used for current assays. Secreted cytokines were detected in the same wells used for lymphocyte cultures. The method can detect about ten CMV specific T-cells diluted into 50 x 10(3) PBMC (0.02%), and can quantify secreted cytokines. The microarray approach allowed evaluation of T-cell immunity in children with a sensitivity higher than current methods. When applied to CMV epitope mapping, the data obtained with conventional methods were confirmed. The assay could be automated, allowing high throughput processing. The assay provides quantitative information on cytokines induced by antigen stimulation and can be applied in a simplified format as a field test to monitor T-cell immunity in vaccine trials or in veterinary medicine.

CMV pp65 and IE-1 T cell epitopes recognized by healthy subjects

Background

Adoptive immune and vaccine therapies have been used to prevent cytomegalovirus (CMV) disease in recipients of hematopoietic progenitor cell transplants, but the nature of T cell responses to CMV have not been completely characterized.

Methods

Peptide pools and individual peptides derived from the immune-dominant CMV proteins pp65 and IE-1 and antigen-specific, cytokine flow cytometry were used to characterize the prevalence and frequency of CD4+ and CD8+ memory T cells in 20 healthy CMV-seropositive subjects.

Results

CD8+ T cell responses to pp65 were detected in 35% of subjects and to IE-1 in 40% of subjects. CD4+ T cell responses to pp65 were detected in 50% of subjects, but none were detected to IE-1. Several new IE-1 HLA class I epitopes were identified, including 4 restricted to HLA-C antigens. One region of IE-1 spanning amino acids 300 to 327 was rich in class I epitopes. The HLA class I restrictions of IE-1 peptides were more promiscuous than those of pp65 peptides.

Conclusion

Since naturally occurring CD4+ and CD8+ T cell responses to pp65 were detectable in many subjects, but only CD8+ T cell responses to IE-1 were detected, pp65 may be better than IE-1 for use in vaccine and adoptive immune therapies.

Cytomegalovirus infection in Gambian infants leads to profound CD8 T-cell differentiation

Cytomegalovirus (CMV) infection is endemic in Gambian infants, with 62% infected by 3 months and 85% by 12 months of age. We studied the CD8 T-cell responses of infants to CMV following primary infection. CMV-specific CD8 T cells, identified with tetramers, showed a fully differentiated phenotype (CD28(-) CD62L(-) CD95(+) perforin(+) granzyme A(+) Bcl-2(low)). Strikingly, the overall CD8 T-cell population developed a similar phenotype following CMV infection, which persisted for at least 12 months. In contrast, primary infection was accompanied by up-regulation of markers of activation (CD45R0 and HLA-D) on both CMV-specific cells and the overall CD8 T-cell population and division (Ki-67) of specific cells, but neither pattern persisted. At 12 months of age, the CD8 T-cell population of CMV-infected infants was more differentiated than that of uninfected infants. Although the subpopulation of CMV-specific cells remained constant, the CMV peptide-specific gamma interferon response was lower in younger infants and increased with age. As the CD8 T-cell phenotype induced by CMV is indicative of immune dysfunction in the elderly, the existence of a similar phenotype in large numbers of Gambian infants raises the question of whether CMV induces a similarly deleterious effect.

Macrophage cultures are susceptible to lytic productive infection by endothelial-cell-propagated human cytomegalovirus strains and present viral IE1 protein to CD4+ T cells despite late downregulation of MHC class II molecules

The contribution of CD4(+) T cells to control of human cytomegalovirus (HCMV) has been shown and infected tissue macrophages might contribute to this response by antigen presentation. As shown previously, CD4(+) T cells recognize HCMV immediate-early antigen IE1 on glioblastoma cells manipulated to express MHC class II molecules. Here, the possible interference of virus-induced MHC class II downmodulation with the presentation of IE1 by natural target cells was analysed. The capacity of IE1-specific CD4(+) T-cell clones to recognize HCMV-infected monocyte-derived macrophages was tested. Various HCMV strains were used to achieve efficient infection of macrophages. Activation of CD4(+) T cells by infected macrophages was evaluated at different time points after infection. Endothelial-cell-adapted HCMV strains efficiently infected cultured human macrophages. However, the immediate-early and early phases of replication were prolonged. Infected cells entered the late replication phase only after 3 days of infection, which was associated with downmodulation of MHC class II molecules at the surface of infected cells. Strong stimulation of IE1-specific CD4(+) T cells resulted from endogenous de novo antigen production and presentation by infected macrophages during the first 3 days of virus replication, despite MHC class II downmodulation in the late replication phase. Therefore, infected macrophages are assumed to contribute to the antiviral immune response in infected organs.

Selection of Epstein-Barr virus specific cytotoxic T lymphocytes can be performed with B lymphoblastoid cell lines created in serum-free media

Epstein-Barr Virus (EBV)-transformed B lymphoblastoid cell lines (BLCL) are currently used for numerous applications in cellular immunology. Where protocols destined for clinical application are concerned, the final choice of assay is made according to a risk/benefit ratio analysis. In this balance the use of xenogenic or allogenic serum has always been a major concern, as it carries both an infectious and an immunological risk. So far, it is unknown whether serum can be omitted from the entire BLCL selection procedure. In addition, as BLCL have been described as heterogeneous, serum deprivation may affect their antigen-presenting capacity. In the present study, BLCL were generated in the absence or presence of fetal calf serum (referred to as BLCL0 or BLCL(FCS), respectively). Next, in order to assess the antigen-presenting capacity of these cells, we compared the ability of BLCL0 and BLCL(FCS) cells to stimulate the EBV-specific repertoire of the corresponding donor's peripheral blood mononuclear cells in vitro. Our results showed that addition of serum was not essential for BLCL infection and culture, and that as far as we could determine, BLCL0 cells were as effective as BLCL(FCS) in reactivating the EBV-specific T-cell repertoire in vitro. Notably, FCS-specific T-lymphocytes can be detected among the BLCL(FCS)-specific CD4+-CTL. Not only was this latter observation unexpected for an EBV-seropositive donor, but it implied that the BLCL had captured and processed the corresponding FCS-derived solubles antigens; taken together our results emphasized the interest of the possibility to generate BLCL0, both for research and for clinical applications.

Cytomegalovirus immune reconstitution occurs in recipients of allogeneic hematopoietic cell transplants irrespective of detectable cytomegalovirus infection

The question of when immune reconstitution of cytomegalovirus (CMV)-specific CD8 T cells occurs after hematopoietic cell transplantation and, more specifically, to which CMV targets this immunity is likely to be directed remains poorly understood. The dependence of immune reconstitution on CMV reactivation is even less clear. To better understand these events, 44 CMV-seropositive HLA-A*0201 subjects were followed up at approximately days 40, 90, 120, 150, 180, and 360 after hematopoietic cell transplantation for CMV immunity as measured by 2 types of assays: (1) an HLA-A*0201 tetramer-binding assay for both CMV pp65 (pp65) and immediate-early 1 (IE-1) or (2) intracellular cytokine interferon gamma responses induced by pp65 or IE-1-derived peptides. To verify the reliability of IE-1-specific assays relative to the pp65-based assays, a pilot study first compared the development of IE-1-specific immunity in a subgroup by using multiple HLA-A*0201-restricted peptides, and then these recipients were followed up for 1 year for immunologic function and for CMV infection. The IE-1-specific response occurred to each of the 3 HLA-A*0201-restricted peptides studied (IE-1-256, -297, and -316), and there was no predominant IE peptide response. However, the immunodominant HLA-A*0201-restricted pp65 peptide was recognized significantly more frequently than these IE-1 peptides. When this was compared with the occurrence of CMV infection, the overall immune reactivity, as measured by the mean or median number of CD8+ T cells reactive to either pp65 or IE-1 peptides by intracellular cytokine or tetramer binding assay, was not significantly different in those with and without CMV infection. For patients who demonstrated reconstituted immunity to CMV at 1 year, all were reconstituted by 6 months, and the timing of the first observed immune reactivity to either of the pp65 or the IE peptides was not different in those with and without detectable CMV infection.

Optimization of CD4+ T lymphocyte response to human cytomegalovirus nuclear IE1 protein through modifications of both size and cellular localization

We have previously reported that the CD4+ T lymphocyte response against nuclear human CMV IE1 protein depends in part on endogenous MHC class II presentation. To optimize presentation by HLA-DR of the nuclear IE1 protein and increase the response by CD4+ T cells, we have constructed two different adenovirus vectors containing mutant versions of IE1, containing a HLA-DR3 epitope, fused to GFP. The first construct consisted of a sequence of 46 aa encoded by exon 4, called GFP-IE1 (86-131). The second construct consisted of the whole IE1 mutated on exon 4 nuclear localization signals, identified in this study, and deleted of already known exon 2 nuclear localization signals (GFP-IE1M). Both of these IE1 vectors expressed proteins with cytoplasmic localization, as evidenced by GFP expression, as opposed to control GFP-IE1, which was nuclear. GFP-IE1 (86-131) induced IE1-specific CD4+ T cell clone response that was >30-fold more potent than that against GFP-IE1 and GFP-IE1M. The CD4+ T cell response was due to endogenous presentation followed by exogenous presentation at later time points. Presentation was dependent on both proteasome and acidic compartments. GFP-IE1 (86-131) was rapidly degraded by the APC, which may account for better presentation. Our data show potentiation of the CD4+ T cell response to a specific epitope through shortening and relocation of an otherwise nuclear protein and suggest applications in vaccination.

Transfection of dendritic cells with in vitro-transcribed CMV RNA induces polyclonal CD8+- and CD4+-mediated CMV-specific T cell responses

Transfection of dendritic cells (DCs) with RNA was shown to be effective in the generation of antigen-specific T cells, probably due to the induction of a polyclonal T cell response directed against multiple antigens. To verify this assumption we used DCs, generated from cytomegalovirus (CMV)-negative or -positive donors, that were electroporated with in vitro-transcribed RNA (in vitro transcript, IVT) coding for the CMV pp65 antigen. We found that transfection of DCs with pp65 IVT induces an expansion of polyclonal CD8(+) T lymphocytes that recognize peptide antigens presented on different HLA molecules. These T lymphocytes are able to lyse DCs pulsed with pp65-derived peptides or transfected with the cognate IVT. Furthermore, this approach allowed the identification of immunodominant epitopes presented upon IVT transfection. Interestingly, transfection of DCs with pp65 IVT resulted in the induction of CD4(+)-specific T cells. Cotransfection of DCs with IVTs coding for the CMV antigens pp65 and IE1 elicited polyclonal T lymphocytes specific for peptides derived from both antigens. More importantly, cytotoxic T cells could be generated in two of three CMV-negative donors. Finally, functional CMV-specific autologous cytotoxic T lymphocytes were successfully generated from immunosuppressed patients after allogeneic hematopoietic stem cell transplantation.

Stable human lymphoblastoid cell lines constitutively expressing hepatitis C virus proteins

The cellular immune response plays a central role in virus clearance and pathogenesis of liver disease in hepatitis C. The study of hepatitis C virus (HCV)-specific immune responses is limited by currently available cell-culture systems. Here, the establishment and characterization of stable human HLA-A2-positive B-lymphoblastoid×T hybrid cell lines constitutively expressing either the NS3–4A complex or the entire HCV polyprotein are reported. These cell lines, termed T1/NS3-4A and T1/HCVcon, respectively, were maintained in continuous culture for more than 1 year with stable characteristics. HCV structural and non-structural proteins were processed accurately, indicating that the cellular and viral proteolytic machineries are functional in these cell lines. Viral proteins were found in the cytoplasm in dot-like structures when expressed in the context of the HCV polyprotein or in a perinuclear fringe when the NS3–4A complex was expressed alone. T1/NS3-4A and T1/HCVcon cells were lysed efficiently by HCV-specific cytotoxic T lymphocytes from patients with hepatitis C and from human HLA-A2.1 transgenic mice immunized with a liposomal HCV vaccine, indicating that viral proteins are processed endogenously and presented efficiently via the major histocompatibility complex class I pathway. In conclusion, these cell lines represent a unique tool to study the cellular immune response, as well as to evaluate novel vaccine and immunotherapeutic strategies against HCV.

Simultaneous assessment of cytotoxic T lymphocyte responses against multiple viral infections by combined usage of optimal epitope matrices, anti- CD3 mAb T-cell expansion and "RecycleSpot"

The assessment of cellular anti-viral immunity is often hampered by the limited availability of adequate samples, especially when attempting simultaneous, high-resolution determination of T cell responses against multiple viral infections. Thus, the development of assay systems, which optimize cell usage, while still allowing for the detailed determination of breadth and magnitude of virus-specific cytotoxic T lymphocyte (CTL) responses, is urgently needed. This study provides an up-to-date listing of currently known, well-defined viral CTL epitopes for HIV, EBV, CMV, HCV and HBV and describes an approach that overcomes some of the above limitations through the use of peptide matrices of optimally defined viral CTL epitopes in combination with anti-CD3 in vitro T cell expansion and re-use of cells from negative ELISpot wells. The data show that, when compared to direct ex vivo cell preparations, antigen-unspecific in vitro T cell expansion maintains the breadth of detectable T cell responses and demonstrates that harvesting cells from negative ELISpot wells for re-use in subsequent ELISpot assays (RecycleSpot), further maximized the use of available cells. Furthermore when combining T cell expansion and RecycleSpot with the use of rationally designed peptide matrices, antiviral immunity against more than 400 different CTL epitopes from five different viruses can be reproducibly assessed from samples of less than 10 milliliters of blood without compromising information on the breadth and magnitude of these responses. Together, these data support an approach that facilitates the assessment of cellular immunity against multiple viral co-infections in settings where sample availability is severely limited.

Progress made towards the development of a CMV peptide vaccine

Cytomegalovirus disease still remains a major cause of morbidity and mortality in hematopoietic stem cell transplantation recipients. The cell-mediated immune response is essential in the maintenance of latency and the resolution of primary infections. The identification of immunodominant cytomegalovirus antigens has enabled researchers to determine the best candidate antigens to be included in a cytomegalovirus vaccine. Such a vaccine would have to stimulate both a cell-mediated and humoral immune response. Recent advances have enabled the rapid identification of minimal cytotoxic epitopes required to trigger such responses. Epitope mapping to date has mainly focused on the pp65 antigen but other antigens such as IE1 are starting to be mapped. A human leukocyte antigen allele hierarchy is starting to emerge that is dependent on the alleles present in an individual; this is relevant when considering what peptides should be included in a vaccine. This review looks at the current methods available for epitope mapping and the progress that has been made to date.

Human cytomegalovirus proteins pp65 and immediate early protein 1 are common targets for CD8+ T cell responses in children with congenital or postnatal human cytomegalovirus infection

Recombinant modified vaccinia Ankara- and peptide-based IFN-gamma ELISPOT assays were used to detect and measure human CMV (HCMV)-specific CD8(+) T cell responses to the pp65 (UL83) and immediate early protein 1 (IE1; UL123) gene products in 16 HCMV-infected infants and children. Age at study ranged from birth to 2 years. HCMV-specific CD8(+) T cells were detected in 14 (88%) of 16 children at frequencies ranging from 60 to >2000 spots/million PBMC. Responses were detected as early as 1 day of age in infants with documented congenital infection. Nine children responded to both pp65 and IE1, whereas responses to pp65 or IE1 alone were detected in three and two children, respectively. Regardless of the specificity of initial responses, IE1-specific responses predominated by 1 year of age. Changes in HCMV epitopes targeted by the CD8(+) T cell responses were observed over time; epitopes commonly recognized by HLA-A2(+) adults with latent HCMV infection did not fully account for responses detected in early childhood. Finally, the detection of HCMV-specific CD8(+) T cell responses was temporally associated with a decrease in peripheral blood HCMV load. Taken altogether, these data demonstrate that the fetus and young infant can generate virus-specific CD8(+) T cell responses. Changes observed in the protein and epitope-specificity of HCMV-specific CD8(+) T cells over time are consistent with those observed after other primary viral infections. The temporal association between the detection of HCMV-specific CD8(+) T cell responses and the reduction in blood HCMV load supports the importance of CD8(+) T cells in controlling primary HCMV viremia.

Recognition of cmv pp65 protein antigen by human cd4 t-cell lines induced with an immunodominant peptide pool

Cellular immunity against cytomegalovirus (CMV) is essential for recovery from infection and control of viral latency. In immunocompromised hosts, this balance between CMV and cellular immunity is lost. Accordingly, restoration of the CD8 compartment specific for CMV is beneficial for immunocompromised patients. It is clear that CMV-specific CD4 cells provide helper functions facilitating long-term persistence of CD8 cells. Considering the dearth of data on CMV-specific T-helper cells, we investigated the CD4 responses to the immunodominant protein pp65 to define antigenic peptides. Such peptides were pooled and used to generate long-term T-cell lines. The lines were responsive to CMV and pp65. T cells were selected with individual peptides to produce monospecific lines for accurate definition of fine epitope specificity and to confirm human leukocyte antigen HLA-DR restriction. Furthermore, these lines lost alloreactivity, suggesting that they can be generated from the allodonor for adoptive immunoreconstitution of stem cell graft recipients.

Attenuated poxviruses generate clinically relevant frequencies of CMV-specific T cells

Immunotherapeutic approaches to limit cytomegalovirus (CMV) morbidity and mortality after hematopoietic stem cell transplants (HSCTs) are currently under investigation as alternatives to antiviral drugs. In this context, we have inserted full-length and ubiquitin-modified CMV phosphoprotein 65 (pp65), phosphoprotein 150 (pp150), and immediate early protein 1 (IE1) immunodominant antigens into the virulent Western Reserve strain of vaccinia virus (VV) and the highly attenuated strain, modified vaccinia Ankara (MVA). Recombinant (r) VV or rMVA stimulated vigorous expansion of CMV-specific CD8+ T cells in CMV-positive donor peripheral blood mononuclear cells (PBMCs), which showed minimal alloreactivity and high levels of HLA tetramer binding, cytokine production, and cytotoxicity. Ubiquitinated antigens had a profound effect when expressed in VV. Single antigen rMVA expressing pp65 or IE1, either ubiquitin-modified or native, stimulated both cytotoxic T lymphocyte (CTL) populations to be expanded up to 500-fold in a 60-mL blood draw from the same donor. This result demonstrates the clinical feasibility of simultaneously amplifying multiple CMV-CTL populations. Transgenic HLA A2.1 (HHD II) mice, immunized with the same rMVA as used with human PBMCs, produced a robust cytotoxic response to both CMV pp65 and IE1. The specificity of the vigorous immunologic response to rMVA, both in vitro and in vivo, makes them candidates for clinical evaluation in the context of adoptive immunotherapy for hematopoietic stem cell transplant (HSCT) recipients or donor vaccination.

Identification of novel CTL epitopes of CMV-pp65 presented by a variety of HLA alleles

Cytomegalovirus (CMV)-specific T-cell immunity plays an important role in protection from CMV disease in immunocompromised patients. Identification of cytotoxic T-lymphocyte (CTL) epitopes is essential for monitoring T-cell immunity and also for immunotherapy. In this and previous studies, CMV-pp65-specific CTL lines were successfully generated from all of 11 CMV-seropositive healthy donors, using pp65-transduced CD40-activated B (CD40-B) cells as antigen-presenting cells. By use of enzyme-linked immunospot (ELISPOT) assays, individual CTL epitopes could be mapped with truncated forms of the pp65 gene. For human leukocyte antigen (HLA) alleles with a known binding motif, CTL epitopes within the defined regions were predicted by computer algorithm. For HLA alleles without a known binding motif (HLA-Cw*0801, -Cw*1202, and -Cw*1502), the epitopes were alternatively identified by step-by-step truncations of the pp65 gene. Through this study, a total of 14 novel CTL epitopes of CMV-pp65 were identified. Interestingly, 3 peptides were found to be presented by 2 different HLA class I alleles or subtypes. Moreover, use of CD40-B cells pulsed with a mixture of synthetic peptides led to generation of pp65-specific CTL lines from some of seronegative donors. The study thus demonstrated an efficient strategy for identifying CTL epitopes presented by a variety of HLA alleles.

A CTL epitope from human cytomegalovirus IE1 defined by combining prediction of HLA binding and proteasomal processing is the target of dominant immune responses in patients after allogeneic stem cell transplantation

OBJECTIVE AND METHODS

In an attempt to define HCMV IE1-derived, HLA-A(*)0201-restricted epitopes, an advanced computer-based epitope prediction combining HLA binding and proteasomal cleavages in silico was performed.

RESULTS

This prediction algorithm clearly confirmed VLEETSVML to be the most likely CTL epitope. By tetramer staining, HCMV pp65 NLVPMVATV-specific CD8(+) T cells were detectable in 18/24 HCMV seropositive HLA-A(*)0201-expressing individuals (median frequency 0.58%; range 0.1%-4.7%), and IE1 VLEETSVML-specific CD8(+) T cells in 5/24 (median frequency 2.1%; range 0.1%-4.3%), respectively (p<0.01). Also in recipients of an allogeneic SCT, VLEETSVML- and NLVPMVATV-specific CD8(+) T cells were detectable in comparable frequencies, but again the number of patients with detectable pp65-specific CD8(+) T cells was higher (p=0.014). In 4/15 individuals, all demonstrating IE1 VLEETSVML-specific CD8(+) T cells prior to peptide stimulation, VLEETSVML-specific T cell lines (purity of 42.6%-98.6% of all CD3(+)/CD8(+) T cells) were successfully generated after 2-4 weeks of culture using the IFN-gamma secretion assay.

CONCLUSION

In conclusion, this novel prediction strategy efficiently predicted an immunodominant viral T-cell epitope.

Mature CD8(+) T lymphocyte response to viral infection during fetal life

Immunization of newborns against viral infections may be hampered by ineffective CD8(+) T cell responses. To characterize the function of CD8(+) T lymphocytes in early life, we studied newborns with congenital human cytomegalovirus (HCMV) infection. We demonstrate that HCMV infection in utero leads to the expansion and the differentiation of mature HCMV-specific CD8(+) T cells, which have similar characteristics to those detected in adults. High frequencies of HCMV-specific CD8(+) T cells were detected by ex vivo tetramer staining as early as after 28 weeks of gestation. During the acute phase of infection, these cells had an early differentiation phenotype (CD28(-)CD27(+)CD45RO(+), perforin(low)), and they acquired a late differentiation phenotype (CD28(-)CD27(-)CD45RA(+), perforin(high)) during the course of the infection. The differentiated cells showed potent perforin-dependent cytolytic activity and produced antiviral cytokines. The finding of a mature and functional CD8(+) T cell response to HCMV suggests that the machinery required to prime such responses is in place during fetal life and could be used to immunize newborns against viral pathogens.

Mature CD8(+) T lymphocyte response to viral infection during fetal life

Immunization of newborns against viral infections may be hampered by ineffective CD8(+) T cell responses. To characterize the function of CD8(+) T lymphocytes in early life, we studied newborns with congenital human cytomegalovirus (HCMV) infection. We demonstrate that HCMV infection in utero leads to the expansion and the differentiation of mature HCMV-specific CD8(+) T cells, which have similar characteristics to those detected in adults. High frequencies of HCMV-specific CD8(+) T cells were detected by ex vivo tetramer staining as early as after 28 weeks of gestation. During the acute phase of infection, these cells had an early differentiation phenotype (CD28(-)CD27(+)CD45RO(+), perforin(low)), and they acquired a late differentiation phenotype (CD28(-)CD27(-)CD45RA(+), perforin(high)) during the course of the infection. The differentiated cells showed potent perforin-dependent cytolytic activity and produced antiviral cytokines. The finding of a mature and functional CD8(+) T cell response to HCMV suggests that the machinery required to prime such responses is in place during fetal life and could be used to immunize newborns against viral pathogens.

Cross-reactivity of HLA-B*1801-restricted T-lymphocyte clones with target cells expressing variants of the human cytomegalovirus 72kDa-IE1 protein

The impact of natural polymorphism in a cytomegalovirus-dominant HLA-B(*)1801-restricted epitope, IE1(199-206), on the specific responses of T-cell clones was assessed by measuring their cytolytic activity against target cells expressing mutated recombinant IE1 proteins. Our results suggest an in vivo selection of T lymphocytes that cross-react with multiple IE1 variants.

Ex vivo profiling of CD8+-T-cell responses to human cytomegalovirus reveals broad and multispecific reactivities in healthy virus carriers

Human cytomegalovirus (HCMV) can establish both nonproductive (latent) and productive (lytic) infections. Many of the proteins expressed during these phases of infection could be expected to be targets of the immune response; however, much of our understanding of the CD8(+)-T-cell response to HCMV is mainly based on the pp65 antigen. Very little is known about T-cell control over other antigens expressed during the different stages of virus infection; this imbalance in our understanding undermines the importance of these antigens in several aspects of HCMV disease pathogenesis. In the present study, an efficient and rapid strategy based on predictive bioinformatics and ex vivo functional T-cell assays was adopted to profile CD8(+)-T-cell responses to a large panel of HCMV antigens expressed during different phases of replication. These studies revealed that CD8(+)-T-cell responses to HCMV often contained multiple antigen-specific reactivities, which were not just constrained to the previously identified pp65 or IE-1 antigens. Unexpectedly, a number of viral proteins including structural, early/late antigens and HCMV-encoded immunomodulators (pp28, pp50, gH, gB, US2, US3, US6, and UL18) were also identified as potential targets for HCMV-specific CD8(+)-T-cell immunity. Based on this extensive analysis, numerous novel HCMV peptide epitopes and their HLA-restricting determinants recognized by these T cells have been defined. These observations contrast with previous findings that viral interference with the antigen-processing pathway during lytic infection would render immediate-early and early/late proteins less immunogenic. This work strongly suggests that successful HCMV-specific immune control in healthy virus carriers is dependent on a strong T-cell response towards a broad repertoire of antigens.

Use of transgenic HLA A*0201/Kb and HHD II mice to evaluate frequency of cytomegalovirus IE1-derived peptide usage in eliciting human CD8 cytokine response

Unlike the pp65 protein of human cytomegalovirus (CMV), which has an immunodominant peptide, pp65(495-503), recognized by human CD8(+) cells in the context of HLA A*0201, the fine peptide specificity for CMV IE1 has shown no such immunodominance. With the use of transgenic HLA A*0201/Kb and HHD II mice, a selected pool of IE1 peptides, including IE1(p256-264), IE1(p297-304), and IE1(p316-324), were shown to stimulate cytolytic T-lymphocyte lysis in the context of HLA A*0201. Based on an intracellular gamma interferon response, IE1(p297-304), a previously unrecognized CD8 epitope, triggered a prominent response to CMV IE1 in HLA A*0201 subjects.

Memory inflation: continuous accumulation of antiviral CD8+ T cells over time

CD8+ T lymphocytes play an important role in the control of intracellular pathogens during both acute and persistent infections. This is particularly true in the case of persistent herpesviruses such as human CMV, which are typified by large virus-specific CD8+ T cell populations during viral latency. To understand the origin of these populations and the factors shaping them over time, we investigated the CD8+ T cell response after murine CMV (MCMV) infection. The kinetics of the acute response were characterized by rapid expansion of activated T cells, followed by a contraction phase. Thereafter, we observed a striking pattern, where MCMV-specific memory CD8+ T cells steadily accumulated over time, with 20% of all CD8+ T cells at 1 year specific for one MCMV epitope. Accumulation of MCMV-specific CD8+ T lymphocytes was seen in all organs tested and was associated with continuous activation of specific CD8+ T lymphocytes, primarily within lymph nodes. The pattern of accumulation was observed in only two of five epitopes tested, and was accompanied by a gradual restriction in usage of the variable region of the TCR beta-chain over time. This novel pattern of a virus-specific CD8+ T cell response suggests that continuous or repetitive exposure to Ag can slowly mold memory T cell populations over time. This may be relevant for understanding the evolution of the large human CMV-specific CD8+ T cell populations seen in humans.

Modulation of HLA-A*0201-restricted T cell responses by natural polymorphism in the IE1(315-324) epitope of human cytomegalovirus

Cytotoxic T lymphocytes play a central role in the control of persistent human CMV (HCMV) infection and reactivation. In healthy virus carriers, the specific CD8(+) CTL response is almost entirely directed against the virion tegument protein pp65 and/or the 72-kDa major immediate early protein, IE1. Studies that included a large panel of HCMV(+) donors suggested that immunorelevance of pp65 and IE1 was directly related with individual HLA haplotype difference. Nevertheless, there are no data on the incidence of HCMV natural polymorphism on virus-specific CTL responses. To assess the impact of IE1 polymorphism on CTL response, we have sequenced in 103 clinical isolates the DNA region corresponding to IE1(315-324), an immunodominant epitope presented by HLA-A*0201 molecules. Seven peptidic variants were found with extensive difference in their frequencies. The response of four HLA-A*0201-restricted anti-IE1 T lymphocyte clones, which were previously generated from one donor against autologous B lymphoblastoid cells expressing a recombinant clinical variant of IE1, was then evaluated using target cells loaded with mutant synthetic peptides or expressing rIE1 variants. One of four clones, which have been sorted 19 times among 22 clones targeted against IE1(315-324), recognized six of the seven tested variant epitopes. All three other clones showed distinct reactivity patterns to target cells loaded with the different mutant peptides or expressing IE1 variants. Therefore, in the HLA-A2 context, clonal expansions of anti-IE1 memory CTLs may confer a protection against HCMV successive infections and reactivations by killing cells presenting most of the naturally occurring IE1(315-324) epitope variants.

Association of glycoprotein B and immediate early-1 genotypes with human leukocyte antigen alleles in renal transplant recipients with cytomegalovirus infection

BACKGROUND

Numerous risk factors for cytomegalovirus (CMV) infection or disease, or both, such as serostatus of donor and recipient, immunosuppressive regimen, or intensity of viral load, have been identified in renal transplant recipients. Additional parameters may be involved, notably, genetic variability of both host and virus, which could modulate the efficacy of the immune response.

METHODS

Active CMV infection was analyzed retrospectively in 634 renal transplant recipients, according to human leukocyte antigen (HLA)-A, HLA-B, and HLA-DR alleles; CMV serostatus; presentation of the disease; and variations in the coding sequences of glycoprotein (g) B and IE1 proteins.

RESULTS

Active infection occurred in 141 of 634 patients: seropositivity of the donor and the recipient were identified as risk factors. Patients carrying the HLA-A11, HLA-A32, or HLA-DR11 allele developed active infection more frequently, whereas none of the patients with the HLA-B16 or HLA-B55 allele was actively infected. Significant independent associations between some genotypes and particular HLA alleles were observed: gB1 was more frequent in the HLA-A24 or HLA-B7 context and underrepresented in patients with HLA-DR11; gB2 was more frequent in HLA-A32 or HLA-DR11 carriers; and an increased frequency of gB3 was observed in the HLA-A29 context. Considering the IE1-2 genotype, increased frequency was noted for HLA-A3 carriers, whereas this type was underrepresented for patients with the HLA-DR11 allele.

CONCLUSION

Data strongly suggest that differential presentation of polymorphic gB or IE peptides by HLA molecules or differential recognition by host CD8+ and CD4+ T lymphocytes, or both, should modulate immunologic response and then CMV pathogenesis in renal transplant patients.

Diagnostic potential of tetramer-based monitoring of cytomegalovirus-specific CD8+ T lymphocytes in allogeneic stem cell transplantation

Cytomegalovirus (CMV) infection remains a significant problem in allogeneic stem cell transplant (SCT) recipients despite the availability of effective antiviral drugs. This problem concerns patients which are unable to mount an effective T-lymphocyte response against CMV. Therefore, the development and use of tetramer technology to enumerate CMV-specific T cells will be valuable to identify these patients as early as possible. Here, we review clinical studies in which CMV-specific CD8(+) T cells have been monitored in allogeneic SCT recipients using tetramers in the context of similar studies that are based on functional assays of CMV-specific T cells. The results thus far warrant the further development of tetramer technology as a diagnostic tool to monitor CMV-specific T cells in SCT recipients and other groups of immunocompromised patients threatened by CMV.

Antigens and immunoevasins: opponents in cytomegalovirus immune surveillance

CD8+ T cells are the main effector cells for the immune control of cytomegaloviruses. To subvert this control, human and mouse cytomegaloviruses each encode a set of immune-evasion proteins, referred to here as immunoevasins, which interfere specifically with the MHC class I pathway of antigen processing and presentation. Although the concerted action of immunoevasins prevents the presentation of certain viral peptides, other viral peptides escape this blockade conditionally or constitutively and thereby provide the molecular basis of immune surveillance by CD8+ T cells. The definition of viral antigenic peptides that are presented despite the presence of immunoevasins adds a further dimension to the prediction of protective epitopes for use in vaccines.

Clinical trials with CMV-specific T cells

BACKGROUND

Reactivation of latent CMV following allogeneic hematopoietic stem cell transplantation remains a major cause of morbidity despite improvements in surveillance protocols and antiviral drug therapies. Selective restoration of anti-CMV cellular immunity is an attractive alternative approach if it can be achieved in a non-toxic manner that can be widely utilized. The application of CMV-specific adoptive cellular therapies following the initial proof of principle provided almost a decade ago has been limited be a number of factors including the practical difficulties of exporting technically demanding and labor-intensive methodology to smaller transplant centers.

METHODS

We review the lessons learnt from studies in the setting of EBV-associated post transplant lymphoproliferative disease and the advances in both understanding and technology that have allowed the development of a multitude of new approaches for the generation of CMV-specific T cells suitable for adoptive transfer. These include the use of monoclytes, dendritic cells and B-lymphoblastoid cell lines as the presenting cells of antigen delivered by pulsing with exogenous proteins or peptides, or of antigen processed endogenously following transduction with one of a variety of viral vectors. We also discuss some of the issues surrounding the planning and implementation of trial protocols incorporating these products, and the techniques necessary for monitoring the fate of the infused cells and their efficacy. Finally, we review the preliminary data that is available on the newer generation of clinical trials that are ongoing in this field.

DISCUSSION

Extensive characterization of many different systems used to culture CMV-specific cells has now been performed. These suggest that it is now possible to generate T cells with specificity for a number of different CMV-specific target antigens (most commonly the lower matrix protein pp65). Clinical trials using these products within the current regulatory guidelines ares till in their infancy. However, preliminary results are beginning to suggest that the newer generation of CMV-specific T cell products can be administered with a relatively low risk of graft-versus-host disease, and that antiviral activity can be demonstrated following infusion.

B cells immortalized by a mini–Epstein-Barr virus encoding a foreign antigen efficiently reactivate specific cytotoxic T cells

Lymphoblastoid cell lines (LCLs) are human B cells latently infected and immortalized by Epstein-Barr virus (EBV). Presenting viral antigens, they efficiently induce EBV-specific T-cell responses in vitro. Analogous ways to generate T-cell cultures specific for other antigens of interest are highly desirable. Previously, we constructed a mini-EBV plasmid that consists of less than half the EBV genome, is unable to cause virus production, but still immortalizes B cells in vitro. Mini-EBV–immortalized B-cell lines (mini-LCLs) are efficiently produced by infection of B cells with viruslike particles carrying only mini-EBV DNA. Mini-EBV plasmids can be engineered to express an additional gene in immortalized B cells. Here we present a mini-EBV coding for a potent CD8+ T-cell antigen, the matrix phosphoprotein pp65 of human cytomegalovirus (CMV). By means of this pp65 mini-EBV, pp65-expressing mini-LCLs could be readily established from healthy donors in a one-step procedure. We used these pp65 mini-LCLs to reactivate and expand effector T cells from autologous peripheral blood cells in vitro. When generated from cytomegalovirus (CMV)–seropositive donors, these effector T-cell cultures displayed strong pp65-specific HLA-restricted cytotoxicity. A large fraction of CD8+ T cells with pp65 epitope specificity was present in such cultures, as demonstrated by direct staining with HLA/peptide tetramers. We conclude that the pp65 mini-EBV is an attractive tool for CMV-specific adoptive immunotherapy. Mini-EBVs could also facilitate the generation of T cells specific for various other antigens of interest.

Identification of the HLA-A24 peptide epitope within cytomegalovirus protein pp65 recognized by CMV-specific cytotoxic T lymphocytes

Among cytomegalovirus (CMV) tegument proteins, phosphoprotein 65 (pp65) has been identified as the important target antigen of the cytotoxic T lymphocyte (CTL) response against the virus. We synthesized seven CMV-pp65-derived peptides carrying an HLA-A24-binding motif, and investigated the ability of these peptides to induce CMV-specific CTL. We identified one nonamer peptide (pp65113-121; VYALPLKML) able to bind HLA-A24 and induce CTL responses in vitro in peripheral blood mononuclear cells (PBMC) from CMV-seropositive individuals. The peptide-specific CTLs generated were capable of recognizing pp65 expressed on CMV-infected fibroblasts as well as pp65113-121 peptide bound to the surface of C1R-A*2402 cells in an HLA-A24-restricted manner. The pp65113-121 peptide thus might be considered a synthetic peptide vaccine in HLA-A24-positive individuals.