Gene transfer of two entry inhibitors protects CD4⁺ T cell from HIV-1 infection in humanized mice.

Targeting viral entry is the most likely gene therapy strategy to succeed in protecting the immune system from pathogenic HIV-1 infection. Here, we evaluated the efficacy of a gene transfer lentiviral vector expressing a combination of viral entry inhibitors, the C46 peptide (an inhibitor of viral fusion) and the P2-CCL5 intrakine (a modulator of CCR5 expression), to prevent CD4⁺ T-cell infection in vivo. For this, we used two different models of HIV-1-infected mice, one in which ex vivo genetically modified human T cells were grafted into immunodeficient NOD.SCID.γc⁻/⁻mice before infection and one in which genetically modified T cells were derived from CD34⁺ hematopoietic progenitors grafted few days after birth. Expression of the transgenes conferred a major selective advantage to genetically modified CD4⁺ T cells, the frequency of which could increase from 10 to 90% in the blood following HIV-1 infection. Moreover, these cells resisted HIV-1-induced depletion, contrary to non-modified cells that were depleted in the same mice. Finally, we report lower normalized viral loads in mice having received genetically modified progenitors. Altogether, our study documents that targeting viral entry in vivo is a promising avenue for the future of HIV-1 gene therapy in humans.

CD4 down modulation on T-cells: an 'immune' checkpoint for HIV.

Progressive decline of an efficient immune response during the asymptomatic phase of AIDS indicates a deletion of HIV-specific cells. The deletion of HIV-specific CD4(+) T cells may occur by direct or indirect cytopathic effects of productive infection. However, preceding cell death, several important cell surface molecules, such as MHC class I or CD4, are down modulated specifically in HIV-infected cells. Down modulation of the CD4 molecule on CD4(+) HIV-specific cells would abrogate efficient help to the cytotoxic arm of the immune response. Therefore, CD4 down modulation may represent an important checkpoint in HIV biology, conferring 'immune' protection to the pool of infected cells. Furthermore, down regulation of CD4 generates a population of infected T cells that should be considered as a possible reservoir for viral replication at all stages of the infection.

T cell-specific expression from Mo-MLV retroviral vectors containing a CD4 mini-promoter/enhancer.

BACKGROUND: Gene therapy of various immunological disorders will greatly benefit from improved retroviral vectors (RVs) with T cell specificity. Such vectors can be designed by placing a gene of therapeutic interest under the control of tissue-specific transcriptional elements. However, low titers and loss of specificity are frequently encountered with tissue-specific vectors. The aim of the present study was to develop a T cell-specific RV. METHODS: We constructed a series of Moloney murine leukemia virus (Mo-MLV)-based RVs expressing enhanced green fluorescent protein (EGFP) under the control of a mini-promoter/enhancer cassette derived from the CD4 gene (CD4pmE) and tested them in cell lines and peripheral blood lymphocytes. Expression of EGFP was monitored by fluorescence microscopy and analyzed by flow cytometry. RESULTS: The CD4pmE cassette was inserted between the viral long terminal repeats (LTRs) in self-inactivating vectors (SIN vectors) or was substituted to the 3' U3 viral promoter/enhancer (hybrid vectors). High vector titers but poor specific expression of EGFP were achieved when CD4pmE was inserted in sense orientation in SIN vectors. Low titers but high specificity were observed when the CD4pmE cassette was in anti-sense orientation. In contrast, high titers and good T cell specificity were obtained with hybrid vectors. CONCLUSION: An efficient T cell-specific retroviral vector was obtained.

Productive infection of double-negative T cells with HIV in vivo.

HIV induces CD4 down-regulation from the surface of infected cells by several independent mechanisms, suggesting an important biological role for this phenomenon. In vitro CD4 down-regulation generates T cells with a double-negative (DN) CD4(-)CD8(-) T cell receptor-alphabeta(+) phenotype. However, evidence that this down-regulation occurs in vivo in HIV-infected subjects is lacking, and viral load or viral production assays invariably focus on CD4(+) T cells. We show here that HIV infection can often be detected in sorted DN cells from peripheral blood and lymph nodes, even when plasma viral load is undetectable. DN T cells infected with HIV represented up to 20% of the cellular viral load in T cells, as determined by DNA PCR. In patients on successful highly active antiretroviral therapy, the viral load decreased in the plasma in CD4(+) and in DN T cells, suggesting that infected DN cells, like CD4(+) cells, contribute to viral production and are sensitive to highly active antiretroviral therapy. Indeed, HIV unspliced and multispliced RNAs were often detectable in DN T cells in spite of the small size of this subset. Infectious virus from DN T cells was transmitted efficiently in coculture experiments with uninfected T cell lymphoblasts, even when viral DNA in the DN cells was barely detectable. We conclude that a discrete population of infected DN T cells exists in HIV-positive subjects, even when the plasma viral load is undetectable. These cells may represent an important source of infectious virus.

Differentiation of human CD8 T cells: implications for in vivo persistence of CD8+ CD28- cytotoxic effector clones.

CD8 T cells contain a distinct subset of CD8+ CD28- cells. These cells are not present at birth and their frequency increases with age. They frequently contain expanded clones using various TCRalphabeta receptors and these clones can represent >50% of all CD8 cells, specially in old subjects or patients with chronic viral infections such as HIV-1. Herein, it is shown that a large fraction of CD8+ CD28- cells expresses intracellular perforin by three-color flow cytometry, in particular when this subset is expanded. Together with their known ability to exert potent re-directed cytotoxicity, this indicates that CD8+ CD28- T cells comprise cytotoxic effector cells. With BrdU labeling, we show that CD8+ CD28- cells derive from CD8+ CD28+ precursors in vitro. In addition, sorted CD8+ CD28+ cells gave rise to a population of CD8+ CD28- cells after allo-stimulation. Moreover, ex vivo CD8+ CD28+ cells contain the majority of CD8 blasts, supporting the notion that they contain the proliferative precursors of CD8+ CD28- cells. CD95 (Fas) expression was lower in CD8+ CD28- cells, and this subset was less prone to spontaneous apoptosis in ex vivo samples and more resistant to activation-induced cell death induced by a superantigen in vitro. Thus, the persistence of expanded clones in vivo in the CD8+ CD28- subset may be explained by antigen-driven differentiation from CD8+ CD28+ memory precursors, with relative resistance to apoptosis as the clones become perforin(+) effector cells.

Altered expression of CD4, CD54, CD62L, and CCR5 in primary lymphocytes productively infected with the human immunodeficiency virus.

Infection of T cells with HIV-1 induces loss of CD4 and HLA class I from the cell surface. In the present article we have investigated whether changes in expression of other cell surface molecules could be related to HIV infection. To detect HIV-infected cells at the single-cell level, peripheral blood lymphocytes were infected in vitro with HIV-HSA, a reporter virus encoding the murine heat-stable antigen. Expression of HSA on activated primary lymphocytes was an efficient indicator of productive infection. Expression of the majority of the cell surface proteins studied was unaffected by HIV infection (HLA class I, II, CD11a, CD18, CD25, CD27, CD28, CD29, CD30, CD31, CD38, CD44, CD45R0, CD49d, CD57, CD94, CD95, and CXCR4). However, phenotypic changes specific to the productively infected cells were detected. Expression of the CD4 molecule was progressively lost and this was closely associated with loss of CD62L expression, a molecule involved in T cell homing into the lymph nodes. By contrast, T cells productively infected with this T-tropic reporter virus were enriched for CD54, and for CCR5, the main coreceptor for M-tropic viruses. Given the roles of CD62L, CD54, and CCR5 in lymphocyte trafficking, these results suggest that cells productively infected with HIV might have altered homing patterns in vivo.

CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway.

Thymocytes with a low expression of CD4 and an intermediate density of the TCR (CD4low TCRint) were analyzed for phenotype, MHC dependence, production kinetics, and TCR repertoire to investigate their position in the intrathymic T cell maturation process. Comparison of normal and MHC-deficient mice showed that the CD4low TCRint cell subset was MHC class II dependent, as this subpopulation could not be defined in MHC class II- or double (class I and II)-deficient mice. These thymocytes were heat-stable Aghigh and CD69+, thus immature and recently engaged in a TCR interaction, probably with MHC class II molecules. Their generation kinetics were studied in two systems: development of exogenous bone marrow cells transferred into RAG-2-/- mice, and pulse labeling with bromodeoxyuridine. In both systems, CD4low TCRint cells were produced well before CD4low TCRhigh cells, the direct precursors of CD8 single-positive cells. Their production paralleled that of CD4high TCRint cells, but they were different than these thymocytes in their smaller cell size. Moreover, they had the same V beta 6 frequency in Mls-1a and Mls-1b mice, suggesting that these cells could be undergoing a negative selection process. The data here clearly demonstrate that CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway, and suggest that these cells could represent a MHC class II-restricted dead-end subset.

Persistence of V beta 6+ T cells in Mls-1a mice. A role for the third complementarity-determining region (CDR3) of the T cell receptor beta chain in superantigen recognition.

We have studied V alpha 2 and J beta usage by V beta 6+CD4+ peripheral T cells isolated from the congenic mice strains BALB/c (Mls-1b) and BALB.D2 (Mls-1a). We found that the TCR beta-chain of V beta 6+CD4+ T cells present in adult Mls-1a mice differed from those in Mls-1b mice; the fraction of V beta 6+CD4+T cells using the J beta 2.7 segment was reduced, while the number of V beta 6+CD4+ T cells using J beta 1.2 was augmented. These results indicate that the CDR3 region of the TCR beta-chain participates in recognition of the Mls superantigen. We also found that in Mls-1a mice an increased fraction of V beta 6+CD4+ T cells expressed the V alpha 2 chain. The study of J beta usage by V beta 6+CD4+V alpha 2+ and V beta 6+CD4+V alpha 2- T cells indicates that both J beta segment and TCR V alpha 2 chain expression confer complementary protection against deletion by Mls-1a superantigen. These results suggest a novel view of Mls-1a-driven selection, where the CDR3 region of the V beta chain modulates superantigen recognition, and the affinity/avidity of the TCR-MHC-superantigen complex determine the fate of the T cell.

Activation and 'deletion' of self-reactive mature and immature T cells during ontogeny of Mls-1a mice: implications for neonatal tolerance induction.

We assessed the kinetics of V beta 6+ T cell elimination in the lymph nodes and thymus during Mls-1a mouse ontogeny. Our results suggest that induction of tolerance to Mls-1a antigens involves mechanisms other than clonal deletion of immature T cells in the thymus. Mature CD4+CD8- (CD4SP) T cells were affected by Mls-1a antigens earlier than immature thymocyte populations. Up to 2 weeks after birth, reduced frequencies of V beta 6+ T cells were detected only in CD4SP cells from the thymus and lymph nodes, and generation of CD4SP cells in the thymus was blocked at least 1 week earlier than that of their CD4+CD8loTCRhi immature precursors. The number of V beta 6+CD4SP T cells increased during the first 2 weeks of life and remained constant thereafter. We thus found no evidence of deletion of mature V beta 6+CD4SP T cells, as the reduced frequencies in adult mice can be attributed to the dilution of previously generated cells in lymphoid organs of growing mice, which increase in cellularity after birth. V beta 6+CD4+ T cells were activated in vivo shortly after birth, as shown by a selective increase in IL-2 receptor alpha chain expression in the thymus and lymph nodes from day 0 to day 2 after birth. It is therefore likely that endogenous expression of Mls-1a antigen shortly after birth activates V beta 6+CD4SP T cells and renders them anergic. This process of tolerance induction may precede the clonal deletion of immature T cells in the thymus, described in the adult mouse.

Generation of mature T cell populations in the thymus: CD4 or CD8 down-regulation occurs at different stages of thymocyte differentiation.

We have studied the differentiation and repertoire selection during the maturation of CD4+CD8+ (DP) thymocytes into CD4+CD8- (CD4SP) and CD8+CD4- (CD8SP) T cells, in normal mice, mice transgenic for T cell receptor (TcR)-alpha beta restricted by either class I or class II major histocompatibility (MHC), and in mice deficient in class I or class II MHC expression. Our data suggest that mature CD4 and CD8 T cells derive from different pathways of T cell differentiation in the thymus. Thus, interaction of DP thymocytes with MHC class II leads to the immediate down-regulation of CD8, which occurs simultaneously with an increase in TcR expression; DPTcR(lo)HSA(hi) thymocytes mature into a CD4+CD8(lo) TcR(hi)HSA(hi) intermediate population. This cell population generates CD4SP thymocytes, the majority of which are still HSA(hi). In contrast, interaction with MHC class I induces the up-regulation of TcR, which precedes the down-regulation of CD4; DPTcR(lo) generate DPTcR(hi) thymocytes, the majority of which are the committed precursors of CD8SP cells. Further differentiation results in CD4 down-regulation and the transition from DPTcR(hi) into CD8+CD4(lo) TcR(hi)HSA(lo) and +D8SPTcR(hi)HSA- T cells. Since down-regulation of CD4 and CD8 occurs at different stages of thymocyte differentiation, our results do not support a stochastic/selective model of lineage commitment in the thymus.