Deficient cellular immunity--finding and fixing the defects

CD8(-) T cell transfectants that express a high affinity T cell receptor exhibit enhanced peptide-dependent activation

T cells are activated by binding of the T cell receptor (TCR) to a peptide-major histocompatibility complex (MHC) complex (pMHC) expressed on the surface of antigen presenting cells. Various models have predicted that activation is limited to a narrow window of affinities (or dissociation rates) for the TCR-pMHC interaction and that above or below this window, T cells will fail to undergo activation. However, to date there have not been TCRs with sufficiently high affinities in order to test this hypothesis. In this report we examined the activity of a CD8-negative T cell line transfected with a high affinity mutant TCR (K(D) = 10 nM) derived from cytotoxic T lymphocyte clone 2C by in vitro engineering. The results show that despite a 300-fold higher affinity and a 45-fold longer off-rate compared with the wild-type TCR, T cells that expressed the mutant TCRs were activated by peptide. In fact, activation could be detected at significantly lower peptide concentrations than with T cells that expressed the wild-type TCR. Furthermore, binding and functional analyses of a panel of peptide variants suggested that pMHC stability could account for apparent discrepancies between TCR affinity and T cell activity observed in several prior studies.

Epstein-Barr virus-specific T cells for the management of Epstein-Barr virus lymphomas

Epstein-Barr virus (EBV), a human herpes virus, is associated with a variety of malignancies. In vitro, it is able to transform B cells, which will grow as lymphoblastoid cell lines in the absence of T cells. Patients with a variety of immunodeficiency diseases are subject to the development of B-cell lymphomas that express viral antigens on their cell surface. Development of EBV-associated B-cell lymphomas is seen in solid organ transplant and bone marrow transplant recipients receiving immunosuppressive therapy. Transfer of mature T cells from EBV-immune marrow donors has been demonstrated to be effective in controlling these EBV-associated B-cell tumors. Recently the demonstration that EBV transcripts are found in other lymphomas (including Hodgkin disease cells) has led to the suggestion that transfer of EBV-specific T cells may also be effective in managing these tumors. Current research involves optimizing methods to expand cells that recognize the EBV antigens expressed in the lymphoma cells.

Clonotypic structure of the human CD4+ memory T cell response to cytomegalovirus

High steady-state frequencies of CMV-specific CD4(+) memory T cells are maintained in CMV-exposed subjects, and these cells are thought to play a key role in the immunologic control of this permanent infection. However, the essential components of this response are poorly defined. Here, we report the use of a step-wise application of flow cytometric and molecular techniques to determine the number and size of the TCR Vbeta-defined clonotypes within freshly obtained CMV-specific CD4(+) memory T cell populations of four healthy, CMV-exposed human subjects. This analysis revealed a stable clonotypic hierarchy in which 1-3 dominant clonotypes are maintained in concert with more numerous subdominant and minor clonotypes. These dominant clonotypes accounted for 10-50% of the overall CMV response, and comprised from 0.3 to 4.0% of peripheral blood CD4(+) T cells. Two subjects displayed immunodominant responses to single epitopes within the CMV matrix phosphoprotein pp65; these single epitope responses were mediated by a single dominant clonotype in one subject, and by multiple subdominant and minor clonotypes in the other. Thus, the CMV-specific CD4(+) T cell memory repertoire in normal subjects is characterized by striking clonotypic dominance and the potential for epitope focusing, suggesting that primary responsibility for immunosurveillance against CMV reactivation rests with a handful of clones recognizing a limited array of CMV determinants. These data have important implications for the understanding of mechanisms by which a genetically stable chronic viral pathogen such as CMV is controlled, and offer possible insight into the failure of such control for a genetically flexible pathogen like HIV-1.

SV40-derived vectors provide effective transgene expression and inhibition of HIV-1 using constitutive, conditional,and pol III promoters

Vectors based on recombinant SV40 viruses (rSV40) are highly effective in delivering transgene expression driven by constitutive promoters. We tested here whether these vectors could be used with conditional promoters and promoters using RNA polymerase III transcription, with inhibition of HIV-1 by Tat activation response (TAR) decoys as a functional measure of effective transgene delivery and activity. TAR decoys inhibit HIV-1 Tat, a trans-activator of HIV-1 transcription. Tat acts early in the viral replicative cycle and is essential for efficient viral replication. We evaluated rSV40 gene delivery using two different inhibitors of Tat. One was a dual function polyTAR gene encoding 25 sequential TAR elements (TAR(25)), plus an antisense tat, driven either by HIV-1 long terminal repeat (HIV-LTR) as a conditional promoter, or by cytomegalovirus immediate-early promoter (CMV-IEP) as a constitutive promoter. The other inhibitor was a single TAR decoy, driven by the U6 small nuclear RNA promoter (U6-P). These decoys were delivered to unselected cells in two different human T lymphocyte lines and to unstimulated primary human peripheral blood mononuclear cells (pbmc). Gene delivery was confirmed by PCR, and expression by RT-PCR. By in situ hybridization analysis, >95% of cells were transduced. These transgene constructs protected all cell types tested from HIV-1, as measured by syncytia formation and p24 antigen release. Somewhat better inhibition of HIV-1 replication was achieved with HIV-1 long terminal repeat (HIV-1 LTR) as a conditional promoter than with the constitutive CMV-IEP. The U6-P was also very effective, driving a TAR(1) transcript. Cell viability was not detectably affected by TAR decoy expression. Thus, rSV40 vectors effectively deliver HIV-1-inhibitory RNAs using either constitutive or conditional pol II promoters, or using a pol III promoter. The versatility of this gene delivery system may prove to be useful in anti-HIV-1 therapeutics.

Haematopoietic cell transplantation as immunotherapy

The graft-versus-tumour effect seen after allogeneic (genetically different) haematopoietic cell transplantation for human malignancies represents the clearest example of the power of the human immune system to eradicate cancer. Recent advances in our understanding of the immunobiology of stem-cell engraftment, tolerance and tumour eradication are allowing clinicians to better harness this powerful effect.

Prospects for immunotherapy of acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is diagnosed in approximately 100000 people worldwide per year and 70% of the patients are children. Most children have a good prognosis, as almost 80% will be cured, however only 30% of adults are cured. Additionally, the current chemotherapies have long-lasting and severe side-effects. These findings indicate that the search for better and safer treatment modalities for ALL is still important. As leukemia directly affects the human immune cells, immunotherapeutic approaches have long been ignored as treatment options for this disease. However, increased knowledge of the immune system has opened new opportunities for immune modulation that could be of benefit to leukemia patients. Several recent advances towards immunotherapy of ALL will be discussed.

Immunopathogenesis of human immunodeficiency virus: implications for immune-based therapies

Human immunodeficiency virus (HIV) infection leads to a state of CD4 lymphopenia and generalized immune activation with subsequent development of opportunistic infections and neoplasms. The use of highly active antiretroviral treatment has dramatically improved the clinical outcome for HIV-infected patients, but the associated cost and toxicity and the eventual development of drug resistance have underscored the need for additional therapeutic strategies. Immune-based therapies, such as treatment with cytokines or immunosuppressants, adoptive immunotherapy, and therapeutic immunizations, are being intensely investigated as potential supplements to antiretroviral therapy. Although much data have been generated as a result of these efforts, to date there has been little evidence of the clinical efficacy of these strategies. Randomized clinical studies remain critical in evaluating the clinical significance and the role of immune-based therapies in the therapeutic armamentarium against HIV.

Stimulation via CD40 can substitute for CD4 T cell function in preventing reactivation of a latent herpesvirus

Reactivation of latent herpesviruses is a particular problem in immunocompromised individuals, such as AIDS patients, who lack effective CD4 T helper cell function. An important question is whether residual immune defenses can be mobilized to combat such opportunistic infections, in the absence of CD4 T cells. In the present study, we used a mouse model of opportunistic infection to determine whether stimulation via CD40 could substitute for CD4 T cell function in preventing reactivation of a latent herpesvirus. Treatment with an agonistic antibody to CD40 was highly effective in preventing reactivation of latent murine gammaherpesvirus (MHV-68) in the lungs of CD4 T cell-deficient mice. CD8(+) T cells were essential for this effect, whereas virus-specific serum antibody was undetectable and IFN-gamma production was unchanged. This demonstration that immunostimulation via CD40 can replace CD4 T cell help in controlling latent virus in vivo has potential implications for the development of novel therapeutic agents to prevent viral reactivation in immunocompromised patients.

Targeting antigen in mature dendritic cells for simultaneous stimulation of CD4+ and CD8+ T cells

Due to their potent immunostimulatory capacity, dendritic cells (DC) have become the centerpiece of many vaccine regimens. Immature DC (DCimm) capture, process, and present Ags to CD4(+) lymphocytes, which reciprocally activate DCimm through CD40, and the resulting mature DC (DCmat) loose phagocytic capacity, but acquire the ability to efficiently stimulate CD8(+) lymphocytes. Recombinant vaccinia viruses (rVV) provide a rapid, easy, and efficient method to introduce Ags into DC, but we observed that rVV infection of DCimm results in blockade of DC maturation in response to all activation signals, including CD40L, monocyte-conditioned medium, LPS, TNF-alpha, and poly(I:C), and failure to induce a CD8(+) response. By contrast, DCmat can be infected with rVV and induce a CD8(+) response, but, having lost phagocytic activity, fail to process the Ag via the exogenous class II pathway. To overcome these limitations, we used the CMV protein pp65 as a model Ag and designed a gene containing the lysosomal-associated membrane protein 1 targeting sequence (Sig-pp65-LAMP1) to target pp65 to the class II compartment. DCmat infected with rVV-Sig-pp65-LAMP1 induced proliferation of pp65-specific CD4(+) clones and efficiently induced a pp65-specific CD4(+) response, suggesting that after DC maturation the intracellular processing machinery for class II remains intact for at least 16 h. Moreover, infection of DCmat with rVV-Sig-pp65-LAMP1 resulted in at least equivalent presentation to CD8(+) cells as infection with rVV-pp65. These results demonstrate that despite rVV interference with DCimm maturation, a single targeting vector can deliver Ags to DCmat for the effective simultaneous stimulation of both CD4(+) and CD8(+) cells.

Functional CD4(+) and CD8(+) T-cell responses induced by autologous mitomycin C treated Epstein-Barr virus transformed lymphoblastoid cell lines

Epstein-Barr virus (EBV) gene expression in tumor cells of posttransplant lymphoproliferative disorder (PTLD) patients resembles that of EBV transformed B-cell lines (LCL). EBV-specific cytotoxic T-lymphocytes can be generated by stimulating peripheral blood lymphocytes with autologous LCL. We describe a standardized method for the growth inactivation and cryopreservation of LCL for optimal T-cell stimulation and analyzed the function and phenotype of responding T-cells. LCL growth was completely blocked by mitomycin C treatment (McLCL) and McLCL could be cryopreserved while retaining excellent APC function. McLCL stimulated both CD4(+) and CD8(+) T-cells as measured by HLA-DR and CD25 expression using FACS analysis. EBV-specific CTL activity and T-cell proliferation were induced and immunocytochemical staining showed CD4(+) and (granzyme B positive) CD8(+) T-cells rosetting with McLCL. Granzymes A and B, IFN-gamma, and IL-6 were detected at significant levels in the supernatant. Thus, ex vivo T-cell activation with cryopreserved McLCL results in activation of both CD4(+) and CD8(+) T-cells producing a Th1-like cytokine profile, making this a suitable protocol for adoptive therapy of PTLD.

Blood stem cell transplantation: factors influencing cellular immunological reconstitution

Recovery of immune function following stem cell transplantation is necessary for a good outcome. Immune recovery is facilitated by transplanting higher numbers of cells than neutrophil or platelet reconstitution requires. Estimates from studies in the allogeneic setting suggest the minimum stem cell dose to achieve optimal lymphocyte recovery is about 10(7) CD34+ cells/kg. Increasing the number of autologous stem cells infused potentially increases the risk of reinfusing tumor cells. Transplanted mature immune cells apparently have very limited early contribution to cellular immune recovery. Mobilizing cytokines permit collection of greater numbers of stem cells, but they also can polarize T cells with potentially significant consequences, for example, granulocyte colony-stimulating factor (G-CSF) decreases the antitumor cytotoxic effector functions of cells. Although this could be a disadvantage in the autologous setting, it might decrease graft versus host disease in the allogeneic setting. Thus, identification of cytokines, which alone or in combination provide the most potent mobilizing effect to permit the collection of the highest number of stem cells without inadvertent detrimental polarization of the responses of immune cells, and employment of cytokines posttransplantation, which direct differentiation of the stem cells along the most desirable pathways, for example, to generate antitumor immune responses, might improve immunological outcome. A future emphasis should be to better define the cytokines and target cell populations that provide optimal immune reconstitution rather than focusing solely on rapid hematological recovery. More complete immunological reconstitution in a greater proportion of patients should be accompanied by improvements in outcomes of blood stem cell transplantation.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Naive T cells transiently acquire a memory-like phenotype during homeostasis-driven proliferation

In a depleted lymphoid compartment, naive T cells begin a slow proliferation that is independent of cognate antigen yet requires recognition of major histocompatibility complex-bound self-peptides. We have followed the phenotypic and functional changes that occur when naive CD8(+) T cells undergo this type of expansion in a lymphopenic environment. Naive T cells undergoing homeostasis-driven proliferation convert to a phenotypic and functional state similar to that of memory T cells, yet distinct from antigen-activated effector T cells. Naive T cells dividing in a lymphopenic host upregulate CD44, CD122 (interleukin 2 receptor beta) and Ly6C expression, acquire the ability to rapidly secrete interferon gamma, and become cytotoxic effectors when stimulated with cognate antigen. The conversion of naive T cells to cells masquerading as memory cells in response to a homeostatic signal does not represent an irreversible differentiation. Once the cellularity of the lymphoid compartment is restored and the T cells cease their division, they regain the functional and phenotypic characteristics of naive T cells. Thus, homeostasis-driven proliferation provides a thymus-independent mechanism for restoration of the naive compartment after a loss of T cells.

Quantitative analysis of the antiviral activity of CD8(+) T cells from human immunodeficiency virus-positive asymptomatic patients with different rates of CD4(+) T-cell decrease

We have measured in 22 asymptomatic human immunodeficiency virus type 1-infected patients (10 rapid progressors and 12 slow progressors) the proviral load of CD4(+) T cells homogeneously superinfected by the same dose of a non-syncytium-inducing virus in the presence or in the absence of autologous CD8(+) T cells. We demonstrated that the antiviral activity of CD8(+) T cells was highly predictive of the rate of peripheral CD4(+) T-cell decline.

A murine leukemia virus (MuLV) long terminal repeat derived from rhesus macaques in the context of a lentivirus vector and MuLV gag sequence results in high-level gene expression in human T lymphocytes

We constructed human immunodeficiency virus type 1 (HIV-1) vectors that will allow higher levels of gene expression in T cells. Gene expression under the control of an internal cytomegalovirus (CMV) immediate-early promoter in a self-inactivating lentiviral vector (CSCG) is 4- to 15-fold lower in T-cell lines (SUPT1 and CEMX174) than in non-lymphoid-cell lines (HeLa and 293T). This is in contrast to a Moloney murine leukemia virus (MoMLV)-based retrovirus vector (SRalphaLEGFP). We therefore replaced the internal CMV promoter of CSCG with three different murine oncoretroviral long terminal repeat (LTR) promoters-murine sarcoma virus (MSV), MoMLV (MLV), and the LTR (termed Rh-MLV) that is derived from the ampho-mink cell focus-forming (AMP/MCF) retrovirus in the serum of one rhesus macaque monkey that developed T-cell lymphoma following autologous transplantation of enriched bone marrow stem cells transduced with a retrovirus vector preparation containing replication-competent viruses (E. F. Vanin, M. Kaloss, C. Broscius, and A. W. Nienhuis, J. Virol. 68:4241-4250, 1994). We found that the combination of Rh-MLV LTR and a partial gag sequence of MoMLV (Deltagag(871-1612)) in CS-Rh-MLV-E gave the highest level of enhanced green fluorescent protein (EGFP) gene expression compared with MLV, MSV LTR, phosphoglycerate kinase, and CMV promoters in T-cell lines, as well as activated primary T cells. Interestingly, there was a further two- to threefold increase in EGFP expression (thus, 10-fold-higher expression than with CMV) when the Rh-MLV promoter and Deltagag(871-1612) were used in a self-inactivating-vector setting that has a further deletion in the U3 region of the HIV-1 LTR. These hybrid vectors should prove useful in gene therapy applications for T cells.

Vaccination to treat persistent viral infection

Persistent infections caused by such agents as the human immunodeficiency virus, hepatitis B virus, Epstein-Barr virus, etc., present formidable medical problems. A defining characteristic of these infections is that anti-viral cytotoxic T lymphocytes (CTL) may be lost or, if present, fail to clear the infection. Here we report a vaccination strategy which was successful in generating lytic CTL in persistently infected mice. Vaccination with an immunodominant CTL epitope derived from the nucleoprotein of lymphocytic choriomeningitis virus (LCMV) delivered in the form of a lipopeptide incorporating a universal CD4 helper epitope successfully induced lytic MHC-restricted CTL in mice persistently infected with LCMV since birth. However, induction of such CTL did not eliminate the virus, most likely because the CTL were generated at low frequencies and had 2 to 3 logs lower affinity than CTL generated in uninfected mice inoculated with the vaccine. Both CTL populations from either uninfected or persistently infected mice produced significant and similar amounts of interferon-gamma and IL-6. Vaccine-induced low-affinity CTL were still inadequate at complete removal of the virus when combined with LCMV-specific CD4 helper T lymphocytes. Thus, our results establish that CTL can be generated in persistently infected mice and that a crucial factor for clearing viral infection is the affinity of the CTL.

Transfusion Medicine: New Clinical Applications of Cellular Immunotherapy

There is now clear clinical evidence that adoptive cellular immunotherapy can eradicate hematologic malignancy and cure otherwise lethal viral infections. With this knowledge comes the challenge of improving the effectiveness and safety of the approach and of simplifying the methodologies required whilst still meeting appropriate federal regulatory guidelines. This review provides an overview of the current status of cellular immunotherapies and addresses how they may be implemented and the future directions they are likely to take.

In Section I, Dr. Brenner with Drs. Rossig and Sili reviews the clinical experience to date with adoptive transfer of viral antigen-specific T cells for the successful treatment of Epstein-Barr virus-associated malignancies as well as viral infectious diseases. Genetic modification of the T cell receptor of the infused cells to potentiate such T cells as well as modifications to improve safety of the infusions are described.

In Section II, Dr. Young describes the hematopoietic lineages of human dendritic cells and some of their immunotherapeutic applications. The critical importance of dendritic cells to T cell immunity and the capacity to generate dendritic cells in large numbers has spawned enormous interest in the use of these specialized leukocytes to manipulate cellular immunity. Successful cytokine-driven differentiation of dendritic cells reveal two types, myeloid- and plasmacytoid or lymphoid-related dendritic cells. The effects of maturation on phenotype and function of the dendritic cells and their use as immune adjuvants in dendritic cell vaccines to elicit antitumor and antiviral immunity are reviewed.

In Section III, Professor Goulmy illustrates some current and future approaches towards tumor-specific cellular therapy of hematopoietic malignancy. Minor histocompatibility antigen (mHag) disparities between HLA-matched bone marrow donor and recipient can induce allo-responses that may participate in post bone marrow transplantation (BMT) graft-versus-leukemia (GVL) reactivities. A lack of such allo-reactivity may result in relapse of leukemia after BMT. In these patients, adoptive immunotherapy with cytotoxic T cells (CTLs) specific for hematopoietic system-restricted mHags may be used as an extension of current efforts using immunotherapy with donor lymphocyte infusions. Adoptive immunotherapy with CTLs specific for the hematopoietic system-restricted mHags, however, offers the prospect of greater and more predictable effectiveness in the absence of graft-versus-host disease.

Transfusion Medicine: New Clinical Applications of Cellular Immunotherapy

There is now clear clinical evidence that adoptive cellular immunotherapy can eradicate hematologic malignancy and cure otherwise lethal viral infections. With this knowledge comes the challenge of improving the effectiveness and safety of the approach and of simplifying the methodologies required whilst still meeting appropriate federal regulatory guidelines. This review provides an overview of the current status of cellular immunotherapies and addresses how they may be implemented and the future directions they are likely to take.

In Section I, Dr. Brenner with Drs. Rossig and Sili reviews the clinical experience to date with adoptive transfer of viral antigen-specific T cells for the successful treatment of Epstein-Barr virus-associated malignancies as well as viral infectious diseases. Genetic modification of the T cell receptor of the infused cells to potentiate such T cells as well as modifications to improve safety of the infusions are described.

In Section II, Dr. Young describes the hematopoietic lineages of human dendritic cells and some of their immunotherapeutic applications. The critical importance of dendritic cells to T cell immunity and the capacity to generate dendritic cells in large numbers has spawned enormous interest in the use of these specialized leukocytes to manipulate cellular immunity. Successful cytokine-driven differentiation of dendritic cells reveal two types, myeloid- and plasmacytoid or lymphoid-related dendritic cells. The effects of maturation on phenotype and function of the dendritic cells and their use as immune adjuvants in dendritic cell vaccines to elicit antitumor and antiviral immunity are reviewed.

In Section III, Professor Goulmy illustrates some current and future approaches towards tumor-specific cellular therapy of hematopoietic malignancy. Minor histocompatibility antigen (mHag) disparities between HLA-matched bone marrow donor and recipient can induce allo-responses that may participate in post bone marrow transplantation (BMT) graft-versus-leukemia (GVL) reactivities. A lack of such allo-reactivity may result in relapse of leukemia after BMT. In these patients, adoptive immunotherapy with cytotoxic T cells (CTLs) specific for hematopoietic system-restricted mHags may be used as an extension of current efforts using immunotherapy with donor lymphocyte infusions. Adoptive immunotherapy with CTLs specific for the hematopoietic system-restricted mHags, however, offers the prospect of greater and more predictable effectiveness in the absence of graft-versus-host disease.