Janice M. Riberdy

Measuring the diaspora for virus-specific CD8+ T cells.

The CD8+ T cell diaspora has been analyzed after secondary challenge with an influenza A virus that replicates only in the respiratory tract. Numbers of DbNP366- and DbPA224-specific CD8+ T cells were measured by tetramer staining at the end of the recall response, then followed sequentially in the lung, lymph nodes, spleen, blood, and other organs. The extent of clonal expansion did not reflect the sizes of the preexisting memory T cell pools. Although the high-frequency CD8+ tetramer+ populations in the pneumonic lung and mediastinal lymph nodes fell rapidly from peak values, the “whole mouse” virus-specific CD8+ T cell counts decreased only 2-fold over the 4 weeks after infection, then subsided at a fairly steady rate to reach a plateau at about 2 months. The largest numbers were found throughout in the spleen, then the bone marrow. The CD8+DbNP366+ and CD8+DbPA224+ sets remained significantly enlarged for at least 4 months, declining at equivalent rates while retaining the nucleoprotein > acid polymerase immunodominance hierarchy characteristic of the earlier antigen-driven phase. Lowest levels of the CD69 “activation marker” were detected consistently on virus-specific CD8+ T cells in the blood, then the spleen. Those in the bone marrow and liver were intermediate, and CD69hi T cells were very prominent in the regional lymph nodes and the nasal-associated lymphoid tissue. Any population of “resting” CD8+ memory T cells is thus phenotypically heterogeneous, widely dispersed, and subject to broad homeostatic and local environmental effects irrespective of epitope specificity or magnitude.

In vivo selection of retrovirally transduced hematopoietic stem cells

One of the main impediments to effective gene therapy of blood disorders is the resistance of human hematopoietic stem cells to stable genetic modification. We show here that a small minority of retrovirally transduced stem cells can be selectively enriched in vivo, which might be a way to circumvent this obstacle. We constructed two retroviral vectors containing an antifolate-resistant dihydrofolate reductase cDNA transcriptionally linked to a reporter gene. Mice were transplanted with transduced bone marrow cells and then treated with an antifolate-based regimen that kills unmodified stem cells. Drug treatment significantly increased the percentage of vector-expressing peripheral blood erythrocytes, platelets, granulocytes, and T and B lymphocytes. Secondary transplant experiments demonstrated that selection occurred at the level of hematopoietic stem cells. This system for in vivo stem-cell selection provides a means to increase the number of genetically modified cells after transplant, and may circumvent an substantial obstacle to successful gene therapy for human blood diseases.

Diminished Primary and Secondary Influenza Virus-Specific CD8+ T-Cell Responses in CD4-Depleted Ig−/− Mice

ABSTRACT Optimal expansion of influenza virus nucleoprotein (DbNP366)-specific CD8+ T cells following respiratory challenge of naive Ig−/− μMT mice was found to require CD4+ T-cell help, and this effect was also observed in primed animals. Absence of the CD4+population was consistently correlated with diminished recruitment of virus-specific CD8+ T cells to the infected lung, delayed virus clearance, and increased morbidity. The splenic CD8+set generated during the recall response in Ig−/− mice primed at least 6 months previously showed a normal profile of gamma interferon production subsequent to short-term, in vitro stimulation with viral peptide, irrespective of a concurrent CD4+T-cell response. Both the magnitude and the localization profiles of virus-specific CD8+ T cells, though perhaps not their functional characteristics, are thus modified in mice lacking CD4+ T cells.

Profound Protection against Respiratory Challenge with a Lethal H7N7 Influenza A Virus by Increasing the Magnitude of CD8+ T-Cell Memory

ABSTRACT The recall of CD8+ T-cell memory established by infecting H-2b mice with an H1N1 influenza A virus provided a measure of protection against an extremely virulent H7N7 virus. The numbers of CD8+ effector and memory T cells specific for the shared, immunodominant DbNP366epitope were greatly increased subsequent to the H7N7 challenge, and though lung titers remained as high as those in naive controls for 5 days or more, the virus was cleared more rapidly. Expanding the CD8+ memory T-cell pool (<0.5 to >10%) by sequential priming with two different influenza A viruses (H3N2→H1N1) gave much better protection. Though the H7N7 virus initially grew to equivalent titers in the lungs of naive and double-primed mice, the replicative phase was substantially controlled within 3 days. This tertiary H7N7 challenge caused little increase in the magnitude of the CD8+ DbNP366 + T-cell pool, and only a portion of the memory population in the lymphoid tissue could be shown to proliferate. The great majority of the CD8+ DbNP366 + set that localized to the infected respiratory tract had, however, cycled at least once, though recent cell division was shown not to be a prerequisite for T-cell extravasation. The selective induction of CD8+ T-cell memory can thus greatly limit the damage caused by a virulent influenza A virus, with the extent of protection being directly related to the number of available responders. Furthermore, a large pool of CD8+ memory T cells may be only partially utilized to deal with a potentially lethal influenza infection.

USE OF THE GREEN FLUORESCENT PROTEIN AS A MARKER TO IDENTIFY AND TRACK GENETICALLY MODIFIED HEMATOPOIETIC CELLS

Use of the green fluorescent protein as a marker to identify and track genetically modified hematopoietic cells

An Early CD4+ T Cell–dependent Immunoglobulin A Response to Influenza Infection in the Absence of Key Cognate T–B Interactions

Contact-mediated interactions between CD4+ T cells and B cells are considered crucial for T cell–dependent B cell responses. To investigate the ability of activated CD4+ T cells to drive in vivo B cell responses in the absence of key cognate T–B interactions, we constructed radiation bone marrow chimeras in which CD4+ T cells would be activated by wild-type (WT) dendritic cells, but would interact with B cells that lacked expression of either major histocompatibility complex class II (MHC II) or CD40. B cell responses were assessed after influenza virus infection of the respiratory tract, which elicits a vigorous, CD4+ T cell–dependent antibody response in WT mice. The influenza-specific antibody response was strongly reduced in MHC II knockout and CD40 knockout mice. MHC II–deficient and CD40-deficient B cells in the chimera environment also produced little virus-specific immunoglobulin (Ig)M and IgG, but generated a strong virus-specific IgA response with virus-neutralizing activity. The IgA response was entirely influenza specific, in contrast to the IgG2a response, which had a substantial nonvirus-specific component. Our study demonstrates a CD4+ T cell–dependent, antiviral IgA response that is generated in the absence of B cell signaling via MHC II or CD40, and is restricted exclusively to virus-specific B cells.

Human CD4+CD25+ Regulatory T Cells Share Equally Complex and Comparable Repertoires with CD4+CD25− Counterparts

CD4+CD25+ T cells are critical mediators of peripheral immune tolerance. However, many developmental and functional characteristics of these cells are unknown, and knowledge of human regulatory T cells is particularly limited. To better understand how human CD4+CD25+ T cells develop and function, we examined the diversity of CD4+CD25+ and CD4+CD25− T cell repertoires in both thymus and peripheral blood. Levels of T receptor excision circles (TREC) were comparable in purified CD4+CD25+ and CD4+CD25− thymic populations, but were significantly higher than those in samples derived from peripheral blood, consistent with murine studies demonstrating thymic development of CD4+CD25+ regulatory T cells. Surprisingly, CD4+CD25− T cells isolated from peripheral blood had greater TREC quantities than their CD4+CD25+ counterparts, supporting the possibility of extrathymic expansion as well. CD4+CD25+ and CD4+CD25− T cells from a given individual showed overlapping profiles with respect to diversity by Vβ staining and spectratyping. Interestingly, CD4+CD25+ T cells have lower quantities of CD3 than CD4+CD25− T cells. Collectively, these data suggest that human CD4+CD25+ T cells recognize a similar array of Ags as CD4+CD25− T cells. However, reduced levels of TCR on regulatory T cells suggest different requirements for activation and may contribute to how the immune system regulates whether a particular response is suppressed or augmented.

Localization of CD4+ T cell epitope hotspots to exposed strands of HIV envelope glycoprotein suggests structural influences on antigen processing

The spectrum of immunogenic epitopes presented by the H2-IAb MHC class II molecule to CD4+ T cells has been defined for two different (clade B and clade D) HIV envelope (gp140) glycoproteins. Hybridoma T cell lines were generated from mice immunized by a sequential prime and boost regime with DNA, recombinant vaccinia viruses, and protein. The epitopes recognized by reactive T cell hybridomas then were characterized with overlapping peptides synthesized to span the entire gp140 sequence. Evidence of clonality also was assessed with antibodies to T cell receptor Vα and Vβ chains. A total of 80 unique clonotypes were characterized from six individual mice. Immunogenic peptides were identified within only four regions of the HIV envelope. These epitope hotspots comprised relatively short sequences (≈20–80 aa in length) that were generally bordered by regions of heavy glycosylation. Analysis in the context of the gp120 crystal structure showed a pattern of uniform distribution to exposed, nonhelical strands of the protein. A likely explanation is that the physical location of the peptide within the native protein leads to differential antigen processing and consequent epitope selection.

Therapeutic levels of fetal hemoglobin in erythroid progeny of β-thalassemic CD34+ cells after lentiviral vector-mediated gene transfer.

β-Thalassemia major results from severely reduced or absent expression of the β-chain of adult hemoglobin (α₂β₂;HbA). Increased levels of fetal hemoglobin (α₂γ₂;HbF), such as occurs with hereditary persistence of HbF, ameliorate the severity of β-thalassemia, raising the potential for genetic therapy directed at enhancing HbF. We used an in vitro model of human erythropoiesis to assay for enhanced production of HbF after gene delivery into CD34(+) cells obtained from mobilized peripheral blood of normal adults or steady-state bone marrow from patients with β-thalassemia major. Lentiviral vectors encoding (1) a human γ-globin gene with or without an insulator, (2) a synthetic zinc-finger transcription factor designed to interact with the γ-globin gene promoters, or (3) a short-hairpin RNA targeting the γ-globin gene repressor, BCL11A, were tested. Erythroid progeny of normal CD34(+) cells demonstrated levels of HbF up to 21% per vector copy. For β-thalassemic CD34(+) cells, similar gene transfer efficiencies achieved HbF production ranging from 45% to 60%, resulting in up to a 3-fold increase in the total cellular Hb content. These observations suggest that both lentiviral-mediated γ-globin gene addition and genetic reactivation of endogenous γ-globin genes have potential to provide therapeutic HbF levels to patients with β-globin deficiency.

Sustained high-level polyclonal hematopoietic marking and transgene expression 4 years after autologous transplantation of rhesus macaques with SIV lentiviral vector-transduced CD34+ cells.

We previously reported that lentiviral vectors derived from the simian immunodeficiency virus (SIV) were efficient at transducing rhesus hematopoietic repopulating cells. To evaluate the persistence of vector-containing and -expressing cells long term, and the safety implications of SIV lentiviral vector-mediated gene transfer, we followed 3 rhesus macaques for more than 4 years after transplantation with transduced CD34+ cells. All 3 animals demonstrated significant vector marking and expression of the GFP transgene in T cells, B cells, and granulocytes, with mean GFP+ levels of 6.7% (range, 3.3%-13.0%), 7.4% (4.2%-13.4%), and 5.6% (3.1%-10.5%), respectively. There was no vector silencing in hematopoietic cells over time. Vector insertion site analysis of granulocytes demonstrated sustained highly polyclonal reconstitution, with no evidence for progression to oligoclonality. A significant number of clones were found to contribute at both 1-year and 3- or 4-year time points. No vector integrations were detected in the MDS1/EVI1 region, in contrast to our previous findings with a gamma-retroviral vector. These data show that lentiviral vectors can mediate stable and efficient long-term expression in the progeny of transduced hematopoietic stem cells, with an integration profile that may be safer than that of standard Moloney murine leukemia virus (MLV)-derived retroviral vectors.