Engineering Cellular Resistance to HIV-1 Infection In Vivo Using a Dual Therapeutic Lentiviral Vector.

We described earlier a dual-combination anti-HIV type 1 (HIV-1) lentiviral vector (LVsh5/C46) that downregulates CCR5 expression of transduced cells via RNAi and inhibits HIV-1 fusion via cell surface expression of cell membrane-anchored C46 antiviral peptide. This combinatorial approach has two points of inhibition for R5-tropic HIV-1 and is also active against X4-tropic HIV-1. Here, we utilize the humanized bone marrow, liver, thymus (BLT) mouse model to characterize the in vivo efficacy of LVsh5/C46 (Cal-1) vector to engineer cellular resistance to HIV-1 pathogenesis. Human CD34+ hematopoietic stem/progenitor cells (HSPC) either nonmodified or transduced with LVsh5/C46 vector were transplanted to generate control and treatment groups, respectively. Control and experimental groups displayed similar engraftment and multilineage hematopoietic differentiation that included robust CD4+ T-cell development. Splenocytes isolated from the treatment group were resistant to both R5- and X4-tropic HIV-1 during ex vivo challenge experiments. Treatment group animals challenged with R5-tropic HIV-1 displayed significant protection of CD4+ T-cells and reduced viral load within peripheral blood and lymphoid tissues up to 14 weeks postinfection. Gene-marking and transgene expression were confirmed stable at 26 weeks post-transplantation. These data strongly support the use of LVsh5/C46 lentiviral vector in gene and cell therapeutic applications for inhibition of HIV-1 infection.

Introduction of exogenous T-cell receptors into human hematopoietic progenitors results in exclusion of endogenous T-cell receptor expression.

Current tumor immunotherapy approaches include the genetic modification of peripheral T cells to express tumor antigen-specific T-cell receptors (TCRs). The approach, tested in melanoma, has led to some limited success of tumor regression in patients. Yet, the introduction of exogenous TCRs into mature T cells entails an underlying risk; the generation of autoreactive clones due to potential TCR mispairing, and the lack of effective negative selection, as these peripheral cells do not undergo thymic selection following introduction of the exogenous TCR. We have successfully generated MART-1-specific CD8 T cells from genetically modified human hematopoietic stem cells (hHSC) in a humanized mouse model. The advantages of this approach include a long-term source of antigen specific T cells and proper T-cell selection due to thymopoiesis following expression of the TCR. In this report, we examine the molecular processes occurring on endogenous TCR expression and demonstrate that this approach results in exclusive cell surface expression of the newly introduced TCR, and the exclusion of endogenous TCR cell surface expression. This suggests that this stem cell based approach can provide a potentially safer approach for anticancer immunotherapy due to the involvement of thymic selection.

Using the BLT humanized mouse as a stem cell based gene therapy tumor model.

Small animal models such as mice have been extensively used to study human disease and to develop new therapeutic interventions. Despite the wealth of information gained from these studies, the unique characteristics of mouse immunity as well as the species specificity of viral diseases such as human immunodeficiency virus (HIV) infection led to the development of humanized mouse models. The earlier models involved the use of C. B 17 scid/scid mice and the transplantation of human fetal thymus and fetal liver termed thy/liv (SCID-hu) (1, 2) or the adoptive transfer of human peripheral blood leukocytes (SCID-huPBL) (3). Both models were mainly utilized for the study of HIV infection. One of the main limitations of both of these models was the lack of stable reconstitution of human immune cells in the periphery to make them a more physiologically relevant model to study HIV disease. To this end, the BLT humanized mouse model was developed. BLT stands for bone marrow/liver/thymus. In this model, 6 to 8 week old NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) immunocompromised mice receive the thy/liv implant as in the SCID-hu mouse model only to be followed by a second human hematopoietic stem cell transplant (4). The advantage of this system is the full reconstitution of the human immune system in the periphery. This model has been used to study HIV infection and latency (5-8). We have generated a modified version of this model in which we use genetically modified human hematopoietic stem cells (hHSC) to construct the thy/liv implant followed by injection of transduced autologous hHSC (7, 9). This approach results in the generation of genetically modified lineages. More importantly, we adapted this system to examine the potential of generating functional cytotoxic T cells (CTL) expressing a melanoma specific T cell receptor. Using this model we were able to assess the functionality of our transgenic CTL utilizing live positron emission tomography (PET) imaging to determine tumor regression (9). The goal of this protocol is to describe the process of generating these transgenic mice and assessing in vivo efficacy using live PET imaging. As a note, since we use human tissues and lentiviral vectors, our facilities conform to CDC NIH guidelines for Biosafety Level 2 (BSL2) with special precautions (BSL2+). In addition, the NSG mice are severely immunocompromised thus, their housing and maintenance must conform to the highest health standards (http://jaxmice.jax.org/research/immunology/005557-housing.html).

In vivo suppression of HIV by antigen specific T cells derived from engineered hematopoietic stem cells.

The HIV-specific cytotoxic T lymphocyte (CTL) response is a critical component in controlling viral replication in vivo, but ultimately fails in its ability to eradicate the virus. Our intent in these studies is to develop ways to enhance and restore the HIV-specific CTL response to allow long-term viral suppression or viral clearance. In our approach, we sought to genetically manipulate human hematopoietic stem cells (HSCs) such that they differentiate into mature CTL that will kill HIV infected cells. To perform this, we molecularly cloned an HIV-specific T cell receptor (TCR) from CD8+ T cells that specifically targets an epitope of the HIV-1 Gag protein. This TCR was then used to genetically transduce HSCs. These HSCs were then introduced into a humanized mouse containing human fetal liver, fetal thymus, and hematopoietic progenitor cells, and were allowed to differentiate into mature human CD8+ CTL. We found human, HIV-specific CTL in multiple tissues in the mouse. Thus, genetic modification of human HSCs with a cloned TCR allows proper differentiation of the cells to occur in vivo, and these cells migrate to multiple anatomic sites, mimicking what is seen in humans. To determine if the presence of the transgenic, HIV-specific TCR has an effect on suppressing HIV replication, we infected with HIV-1 mice expressing the transgenic HIV-specific TCR and, separately, mice expressing a non-specific control TCR. We observed significant suppression of HIV replication in multiple organs in the mice expressing the HIV-specific TCR as compared to control, indicating that the presence of genetically modified HIV-specific CTL can form a functional antiviral response in vivo. These results strongly suggest that stem cell based gene therapy may be a feasible approach in the treatment of chronic viral infections and provide a foundation towards the development of this type of strategy.

HIV latency in the humanized BLT mouse.

Even after extended treatment with powerful antiretroviral drugs, HIV is not completely eliminated from infected individuals. Latently infected CD4(+) T cells constitute one reservoir of replication-competent HIV that needs to be eliminated to completely purge virus from antiretroviral drug-treated patients. However, a major limitation in the development of therapies to eliminate this latent reservoir is the lack of relevant in vivo models that can be used to test purging strategies. Here, we show that the humanized BLT (bone marrow-liver-thymus) mouse can be used as both an abundant source of primary latently infected cells for ex vivo latency analysis and also as an in vivo system for the study of latency. We demonstrate that over 2% of human cells recovered from the spleens of HIV-infected BLT mice can be latently infected and that this virus is integrated, activation inducible, and replication competent. The non-tumor-inducing phorbol esters prostratin and 12-deoxyphorbol-13-phenylacetate can each induce HIV ex vivo from these latently infected cells, indicating that this model can be used as a source of primary cells for testing latency activators. Finally, we show activation-inducible virus is still present following suppression of plasma viral loads to undetectable levels by using the antiretroviral drugs zidovudine, indinavir sulfate, and didanosine, demonstrating that this model can also be used to assess the in vivo efficacy of latency-purging strategies. Therefore, the HIV-infected BLT mouse should provide a useful model for assessment of HIV latency activators and approaches to eliminate persistent in vivo HIV reservoirs.

Antitumor activity from antigen-specific CD8 T cells generated in vivo from genetically engineered human hematopoietic stem cells.

The goal of cancer immunotherapy is the generation of an effective, stable, and self-renewing antitumor T-cell population. One such approach involves the use of high-affinity cancer-specific T-cell receptors in gene-therapy protocols. Here, we present the generation of functional tumor-specific human T cells in vivo from genetically modified human hematopoietic stem cells (hHSC) using a human/mouse chimera model. Transduced hHSC expressing an HLA-A*0201-restricted melanoma-specific T-cell receptor were introduced into humanized mice, resulting in the generation of a sizeable melanoma-specific naïve CD8(+) T-cell population. Following tumor challenge, these transgenic CD8(+) T cells, in the absence of additional manipulation, limited and cleared human melanoma tumors in vivo. Furthermore, the genetically enhanced T cells underwent proper thymic selection, because we did not observe any responses against non-HLA-matched tumors, and no killing of any kind occurred in the absence of a human thymus. Finally, the transduced hHSC established long-term bone marrow engraftment. These studies present a potential therapeutic approach and an important tool to understand better and to optimize the human immune response to melanoma and, potentially, to other types of cancer.

Differentially stimulated CD4+ T cells display altered human immunodeficiency virus infection kinetics: implications for the efficacy of antiviral agents.

The activation state of CD4(+) T cells plays a crucial role in the establishment of a productive human immunodeficiency virus infection. Here, we show that T cells stimulated for 1 day demonstrated delayed kinetics of viral reverse transcription and integration compared to cells stimulated for 2 days prior to infection. As a result, the efficiency of reverse transcription and integration inhibitors differs in these differentially stimulated cells. These studies increase our understanding of how T cells support viral replication and provide insight regarding the efficiency of antiretroviral therapy in lymphoid compartments.

Primary cell model for activation-inducible human immunodeficiency virus.

Quiescent T lymphocytes containing latent human immunodeficiency virus (HIV) provide a long-lived viral reservoir. This reservoir may be the source of active infection that is reinitiated following the cessation of antiretroviral therapy. Therefore, it is important to understand the mechanisms involved in latent infection to develop new strategies to eliminate the latent HIV reservoir. We have previously demonstrated that latently infected quiescent lymphocytes can be generated during thymopoiesis in vivo in the SCID-hu mouse system. However, there is still a pressing need for an in vitro model of HIV latency in primary human cells. Here, we present a novel in vitro model that recapitulates key aspects of dormant HIV infection. Using an enhanced green fluorescent protein-luciferase fusion protein-containing reporter virus, we have generated a stable infection in primary human CD4(+) CD8(+) thymocytes in the absence of viral gene expression. T-cell activation induces a >200-fold induction of reporter activity. The induced reporter activity originates from a fully reverse-transcribed and integrated genome. We further demonstrate that this model can be useful to study long terminal repeat regulation, as previously characterized NF-kappaB response element mutations decrease the activation of viral gene expression. This model can therefore be used to study intricate molecular aspects of activation-inducible HIV infection in primary cells.

Immediate activation fails to rescue efficient human immunodeficiency virus replication in quiescent CD4+ T cells.

Unlike activated T cells, quiescent CD4+ T cells have shown resistance to human immunodeficiency virus (HIV) infection due to a block in the early events of the viral life cycle. To further investigate the nature of this block, we infected quiescent CD4+ T cells with HIV-1(NL4-3) and immediately stimulated them. Compared to activated (prestimulated) cells, these poststimulated cells showed slightly decreased viral entry and delays in the completion of reverse transcription. However, the relative efficiency of integration was similar to that of prestimulated cells. Together, this resulted in decreased expression of tat/rev mRNA and synthesis of viral protein. Furthermore, based on cell cycle staining and BrdU incorporation, poststimulated cells expressing viral protein failed to initiate a second round of their cell cycle, independently of Vpr-mediated arrest. Together, these data demonstrate that the early stages of the HIV life cycle are inefficient in these poststimulated cells and that efficient replication cannot be induced by subsequent activation.

Anti-human immunodeficiency virus hematopoietic progenitor cell-delivered ribozyme in a phase I study: myeloid and lymphoid reconstitution in human immunodeficiency virus type-1-infected patients.

A phase I gene transfer clinical study was undertaken to examine the ability to introduce a potential anti-human immunodeficiency virus (HIV) gene therapeutic into hematopoietic progenitor cells (HPC), thereby contributing to multilineage engraftment. The potential therapeutic effect of genetically modifying HPC with protective genes in HIV-infected adults depends in part on the presence of adult thymic activity and myeloid capacity in the setting of HIV replication. Herein we report the presence and expression of a retroviral vector encoding an anti-HIV-1 ribozyme in mature hematopoietic cells of different lineages, and de novo T-lymphocyte development ensuing from genetically engineered CD34(+) HPC. Sustained output of vector-containing mature myeloid and T-lymphoid cells was detected even in patients with multidrug-resistant infection. In addition, the study showed that the degree of persistence of gene-containing cells was dependent on transduced HPC dose. These novel findings support the concept of gene therapy as a modality to effect immune reconstitution with cells engineered to inhibit HIV replication and this report represents the first demonstration of long-term maintenance of a potential therapeutic transgene in HIV disease.