Human CD19-specific switchable CAR T-cells are efficacious as constitutively active CAR T-cells but cause less morbidity in a mouse model of human CD19+ malignancy

Current Food and Drug Administration (FDA)-approved CD19-specific chimeric antigen receptor (CAR) T-cell therapies for B-cell malignancies are constitutively active and while efficacious, can cause morbidity and mortality. Their toxicities might be reduced if CAR T-cell activity was regulatable rather than constitutive. To test this, we compared the efficacies and morbidities of constitutively active (conventional) and regulatable (switchable) CAR (sCAR) T-cells specific for human CD19 (huCD19) in an immune-competent huCD19+ transgenic mouse model.Conventional CAR (CAR19) and sCAR T-cells were generated by retrovirally transducing C57BL/6 (B6) congenic T-cells with constructs encoding antibody-derived single chain Fv (sFv) fragments specific for huCD19 or a peptide neoepitope (PNE), respectively. Transduced T-cells were adoptively transferred into huCD19 transgenic hemizygous (huCD19Tg/0 ) B6 mice; healthy B-cells in these mice expressed huCD19Tg Prior to transfer, recipients were treated with a lymphodepleting dose of cyclophosphamide to enhance T-cell engraftment. In tumor therapy experiments, CAR19 or sCAR T-cells were adoptively transferred into huCD19Tg/0 mice bearing a syngeneic B-cell lymphoma engineered to express huCD19. To regulate sCAR T cell function, a switch protein was generated that contained the sCAR-specific PNE genetically fused to an anti-huCD19 Fab fragment. Recipients of sCAR T-cells were injected with the switch to link sCAR effector with huCD19+ target cells. Mice were monitored for survival, tumor burden (where appropriate), morbidity (as measured by weight loss and clinical scores), and peripheral blood lymphocyte frequency.CAR19 and sCAR T-cells functioned comparably regarding in vivo expansion and B-cell depletion. However, sCAR T-cells were better tolerated as evidenced by the recipients' enhanced survival, reduced weight loss, and improved clinical scores. Discontinuing switch administration allowed healthy B-cell frequencies to return to pretreatment levels.In our mouse model, sCAR T-cells killed huCD19+ healthy and malignant B-cells and were better tolerated than CAR19 cells. Our data suggest sCAR might be clinically superior to the current FDA-approved therapies for B-cell lymphomas due to the reduced acute and chronic morbidities and mortality, lower incidence and severity of side effects, and B-cell reconstitution on cessation of switch administration.

Donor and host B7-H4 expression negatively regulates acute graft-versus-host disease lethality

B7-H4 is a negative regulatory B7 family member. We investigated the role of host and donor B7-H4 in regulating acute graft-versus-host disease (GVHD). Allogeneic donor T cells infused into B7-H4-/- versus WT recipients markedly accelerated GVHD-induced lethality. Chimera studies pointed toward B7-H4 expression on host hematopoietic cells as more critical than parenchymal cells in controlling GVHD. Rapid mortality in B7-H4-/- recipients was associated with increased donor T cell expansion, gut T cell homing and loss of intestinal epithelial integrity, increased T effector function (proliferation, proinflammatory cytokines, cytolytic molecules), and reduced apoptosis. Higher metabolic demands of rapidly proliferating donor T cells in B7-H4-/- versus WT recipients required multiple metabolic pathways, increased extracellular acidification rates (ECARs) and oxygen consumption rates (OCRs), and increased expression of fuel substrate transporters. During GVHD, B7-H4 expression was upregulated on allogeneic WT donor T cells. B7-H4-/- donor T cells given to WT recipients increased GVHD mortality and had function and biological properties similar to WT T cells from allogeneic B7-H4-/- recipients. Graft-versus-leukemia responses were intact regardless as to whether B7-H4-/- mice were used as hosts or donors. Taken together, these data provide new insights into the negative regulatory processes that control GVHD and provide support for developing therapeutic strategies directed toward the B7-H4 pathway.

B7-H4 expression in donor T cells and host cells negatively regulates acute graftversus- host disease lethality

The B7 family members are critical in positive and negative regulation of immune responses by engaging various lymphocyte receptors. B7-H4 is a member of the B7 family that can negatively regulate T cell function. We investigated the role of host and donor B7-H4 in regulating acute graft-versus-host disease (GVHD). Allogeneic donor T cells infused into B7-H4−/− versus wild type (WT) recipients markedly accelerated GVHD-induced lethality in a C57BL/6 to BALB/c GVHD model. Chimera studies pointed toward B7-H4 expression on host hematopoietic cells as more critical than parenchymal cells in controlling GVHD. B7-H4−/− recipients had rapid mortality associated with increased donor T cell expansion, gut T cell homing and loss of intestinal epithelial integrity, increased Teffector function (proliferation, pro-inflammatory cytokines, cytolytic molecules) and reduced apoptosis. Higher metabolic demands of rapidly proliferating donor T cells in B7-H4−/− versus WT recipients required multiple metabolic pathways, increased extra-cellular acidification rates and oxygen consumption rates, and increased expression of fuel substrate transporters. Interestingly, during GVHD, B7-H4 expression was upregulated on allogeneic WT donor T cells. Consistent with these data, donor B7-H4−/− T cells given to WT recipients increased GVHD mortality and functioned similarly to WT T cells from allogeneic B7-H4−/− recipients. Graft-versus-leukemia responses were intact regardless as to whether B7-H4−/− mice were used as hosts or donors. Taken together, these data provide new insights into the negative regulatory processes that control GVHD and provide support for developing therapeutic strategies directed toward the B7-H4 pathway.

Human CD19-Targeted Mouse T Cells Induce B Cell Aplasia and Toxicity in Human CD19 Transgenic Mice

The clinical success of chimeric antigen receptor (CAR) T cell therapy for CD19+ B cell malignancies can be limited by acute toxicities and immunoglobulin replacement needs due to B cell aplasia from persistent CAR T cells. Life-threatening complications include cytokine release syndrome and neurologic adverse events, the exact etiologies of which are unclear. To elucidate the underlying toxicity mechanisms and test potentially safer CAR T cells, we developed a mouse model in which human CD19 (hCD19)-specific mouse CAR T cells were adoptively transferred into mice whose normal B cells express a hCD19 transgene at hemizygous levels. Compared to homozygous hCD19 transgenic mice that have ∼75% fewer circulating B cells, hemizygous mice had hCD19 frequencies and antigen density more closely simulating human B cells. Hemizygous mice given a lethal dose of hCD19 transgene-expressing lymphoma cells and treated with CAR T cells had undetectable tumor levels. Recipients experienced B cell aplasia and antigen- and dose-dependent acute toxicities mirroring patient complications. Interleukin-6 (IL-6), interferon γ (IFN-γ), and inflammatory pathway transcripts were enriched in affected tissues. As in patients, antibody-mediated neutralization of IL-6 (and IFN-γ) blunted toxicity. Apparent behavioral abnormalities associated with decreased microglial cells point to CAR-T-cell-induced neurotoxicity. This model will prove useful in testing strategies designed to improve hCD19-specific CAR T cell safety.

Humanized Mouse Xenograft Models: Narrowing the Tumor-Microenvironment Gap

Cancer research has long been hampered by the limitations of the current model systems. Both cultured cells and mouse xenografts grow in an environment highly dissimilar to that of their originating tumor, frequently resulting in promising treatments that are ultimately clinically ineffective. The development of highly immunodeficient mouse strains into which human immune systems can be engrafted can help bridge this gap. Humanized mice (HM) allow researchers to examine xenograft growth in the context of a human immune system and resultant tumor microenvironment, and recent studies have highlighted the increased similarities in attendant tumor structure, metastasis, and signaling to those features in cancer patients. This setting also facilitates the examination of investigational cancer therapies, including new immunotherapies. This review discusses recent advancements in the generation and application of HM models, their promise in cancer research, and their potential in generating clinically relevant treatments. This review also focuses on current efforts to improve HM models by engineering mouse strains expressing human cytokines or HLA proteins and implanting human bone, liver, and thymus tissue to facilitate immune cell maturation and trafficking. Finally, we discuss how these improvements may help direct future HM model cancer studies. Cancer Res; 76(21); 6153-8. ©2016 AACR.

XactMice: humanizing mouse bone marrow enables microenvironment reconstitution in a patient-derived xenograft model of head and neck cancer

The limitations of cancer cell lines have led to the development of direct patient derived xenograft (PDX) models. However, the interplay between the implanted human cancer cells and recruited mouse stromal and immune cells alters the tumor microenvironment and limits the value of these models. To overcome these constraints, we have developed a technique to expand human hematopoietic stem and progenitor cells (HSPCs) and use them to reconstitute the radiation-depleted bone marrow of a NOD/SCID/IL2rg^−/− (NSG) mouse on which a patient’s tumor is then transplanted (XactMice). The human HSPCs produce immune cells that home into the tumor and help replicate its natural microenvironment. Despite previous passage on nude mice, the expression of epithelial, stromal, and immune genes in XactMice tumors aligns more closely to that of the patient tumor than to those grown in non-humanized mice – an effect partially facilitated by human cytokines expressed by both the HSPC progeny and the tumor cells. The human immune and stromal cells produced in the XactMice can help recapitulate the microenvironment of an implanted xenograft, reverse the initial genetic drift seen after passage on non-humanized mice, and provide a more accurate tumor model to guide patient treatment.

Expansion of human and murine hematopoietic stem and progenitor cells ex vivo without genetic modification using MYC and Bcl-2 fusion proteins

The long-term repopulating hematopoietic stem cell (HSC) population can self-renew in vivo, support hematopoiesis for the lifetime of the individual, and is of critical importance in the context of bone marrow stem cell transplantation. The mechanisms that regulate the expansion of HSCs in vivo and in vitro remain unclear to date. Since the current set of surface markers only allow for the identification of a population of cells that is highly enriched for HSC activity, we will refer to the population of cells we expand as Hematopoietic Stem and Progenitor cells (HSPCs). We describe here a novel approach to expand a cytokine-dependent Hematopoietic Stem and Progenitor Cell (HSPC) population ex vivo by culturing primary adult human or murine HSPCs with fusion proteins including the protein transduction domain of the HIV-1 transactivation protein (Tat) and either MYC or Bcl-2. HSPCs obtained from either mouse bone marrow, human cord blood, human G-CSF mobilized peripheral blood, or human bone marrow were expanded an average of 87 fold, 16.6 fold, 13.6 fold, or 10 fold, respectively. The expanded cell populations were able to give rise to different types of colonies in methylcellulose assays in vitro, as well as mature hematopoietic populations in vivo upon transplantation into irradiated mice. Importantly, for both the human and murine case, the ex vivo expanded cells also gave rise to a self-renewing cell population in vivo, following initial transplantation, that was able to support hematopoiesis upon serial transplantation. Our results show that a self-renewing cell population, capable of reconstituting the hematopoietic compartment, expanded ex vivo in the presence of Tat-MYC and Tat-Bcl-2 suggesting that this may be an attractive approach to expand human HSPCs ex vivo for clinical use.

A new function for LAT and CD8 during CD8-mediated apoptosis that is independent of TCR signal transduction

The majority (>95%) of thymocytes undergo apoptosis during selection in the thymus. Several mechanisms have been proposed to explain how apoptosis of thymocytes that are not positively selected occurs; however, it is unknown whether thymocytes die purely by "neglect" or whether signaling through a cell-surface receptor initiates an apoptotic pathway. We have previously demonstrated that on double positive thymocytes the ligation of CD8 in the absence of TCR engagement results in apoptosis and have postulated this is a mechanism to remove thymocytes that have failed positive selection. On mature single positive T cells CD8 acts as a co-receptor to augment signaling through the TCR that is dependent on the phosphorylation of the adaptor protein, linker for activation of T cells (LAT). Here, we show that during CD8-mediated apoptosis of double positive thymocytes there is an increase in the association of CD8 with LAT and an increase in LAT tyrosine phosphorylation. Decreasing LAT expression and mutation of tyrosine residues of LAT reduced apoptosis upon crosslinking of CD8. Our results identify novel functions for both CD8 and LAT that are independent of TCR signal transduction and suggest a mechanism for signal transduction leading to apoptosis upon CD8 crosslinking.

A novel method for long term bone marrow culture and genetic modification of murine neutrophils via retroviral transduction

Neutrophils are a critical component of the innate immune response to invading microbial pathogens. However, an excessive and/or prolonged neutrophil response can result in tissue injury that is thought to underlie the pathogenesis of various inflammatory diseases. The development of novel therapeutic strategies for inflammatory diseases depends on an improved understanding of regulation of neutrophil function. However, investigations into neutrophil function have been constrained in part by the difficulty of genetically modifying neutrophils using current techniques. To overcome this, we have developed a novel method for the genetic modification of murine bone marrow derived progenitor cells using retroviral transduction followed by long term bone marrow culture to generate mature neutrophils. These neutrophils are functionally mature as determined by morphology, surface marker (Gr1, CD11b, CD62L and CXCR2) expression, and functional attributes including the ability to generate superoxide, exocytose granule contents, chemotax, and phagocytose and kill bacteria. Further, the in vitro matured neutrophils are capable of migrating to an inflammatory site in vivo. We utilized this system to express the Bcl-2 transgene in mature neutrophils using the retroviral vectors pMIG and pMIT. Bcl-2 overexpression conferred a substantial delay in spontaneous apoptosis of neutrophils as assessed by annexin V and 7-amino-actinomycin D (7AAD) staining. Moreover, Bcl-2 overexpression did not alter granulopoiesis, as assessed by morphology and surface marker expression. This system enables the genetic manipulation of progenitor cells that can be differentiated in vitro to mature neutrophils that are functional in vitro and in vivo.

Small-molecule inhibitors of Bcl-2 family proteins are able to induce tumor regression in a mouse model of pre-B-cell acute lymphocytic lymphoma

The overexpression of prosurvival members of the Bcl-2 family is commonly associated with the enhanced malignancy of hematological tumors. There has been great interest in a novel set of agents that are able to mimic the function of the BH3 domain by binding to the groove of Bcl-2-like proteins and initiating the cell death sequence. We sought to examine the efficacy of BH3 mimetics in a spontaneous mouse model of B-cell neoplasia. We evaluated the ability of the BH3 mimetics to preferentially target tumor cells while sparing normal cells. In addition, we examined the contributions of Bim and Puma to the sensitivity of tumor cells to the BH3 mimetics. We report here that two BH3 mimetics (HA-14-1 and BH3-I-2') were able to induce apoptosis of murine B-cell lymphoma cells in vitro and in vivo. Tumors that arose from transplantation of primary lymphoma cells regressed following 7 days of treatment with BH3-mimetic drugs. The long-term benefits of the transient treatment of tumor-bearing mice with the BH3 mimetics, however, could not be properly evaluated, due to the high levels of toxicity we observed in vivo with these drugs. Decreased expression of either Bim or Puma from B-cell tumor cells was able to protect these cells from the apoptosis induced by these BH3 mimetics, suggesting that they function through other means. We conclude that while the BH3-mimetic drugs are effective at inducing cell death of lymphoma cells in vitro and in vivo, their unclear molecular specificity and their ability to kill normal cells may limit their therapeutic uses in humans.

Farnesyl transferase inhibitors induce extended remissions in transgenic mice with mature B cell lymphomas

Background

We have used a mouse model based on overexpression of c-Myc in B cells genetically engineered to be self-reactive to test the hypothesis that farnesyl transferase inhibitors (FTIs) can effectively treat mature B cell lymphomas. FTIs are undergoing clinical trials to treat both lymphoid and non-lymphoid malignancies and we wished to obtain evidence to support the inclusion of B cell lymphomas in future trials.

Results

We report that two FTIs, L-744,832 and SCH66336, blocked the growth of mature B cell lymphoma cells in vitro and in vivo. The FTI treatment affected the proliferation and survival of the transformed B cells to a greater extent than naïve B cells stimulated with antigen. In syngeneic mice transplanted with the transgenic lymphoma cells, L-744,832 treatment prevented the growth of the tumor cells and the morbidity associated with the resulting lymphoma progression. Tumors that arose from transplantation of the lymphoma cells regressed with as little as three days of treatment with L-744,832 or SCH66336. Treatment of these established lymphomas with L-744,832 for seven days led to long-term remission of the disease in approximately 25% of animals.

Conclusion

FTI treatment can block the proliferation and survival of self-reactive transformed B cells that overexpress Myc. In mice transplanted with mature B cell lymphomas, we found that FTI treatment led to regression of disease. FTIs warrant further consideration as therapeutic agents for mature B cell lymphomas and other lymphoid tumors.

MHC class II structural requirements for the association with Igalpha/beta, and signaling of calcium mobilization and cell death

Emerging evidence indicates that in addition to their well-characterized role in antigen presentation, MHC II molecules transmit signals that induce death of APCs. Appropriately timed APC death is important for prevention of autoimmunity. Though the exact mechanism of MHC II-mediated cell death signaling is unknown, the response appears independent of caspase activation and does not involve Fas-FasL interaction. Here we investigated MHC II structural requirements for mediation of cell death signaling in a murine B cell lymphoma. We found that neither the transmembrane spanning regions nor the cytoplasmic tails of MHC II, which are required for MHC II-mediated cAMP production and PKC activation, are required for the death response. However, mutations in the connecting peptide region of MHC II alpha chain (alphaCP), but not the beta chain (betaCP), resulted in significant impairment of the death response. The alphaCP mutant was also unable to mediate calcium mobilization responses, and did not associate with Igalpha/beta. Knock-down of Igbeta by shRNA eliminated the MHC II-mediated calcium response but not cell death. We propose that MHC II mediates cell death signaling via association with an undefined cell surface protein(s), whose interaction is partially dependent on alphaCP region.

Function and regulation of SPLUNC1 protein in Mycoplasma infection and allergic inflammation

Respiratory infections, including Mycoplasma pneumoniae (Mp), contribute to asthma pathobiology. To date, the mechanisms underlying the increased susceptibility of asthmatics to airway Mp infection remain unclear. Short palate, lung, and nasal epithelium clone 1 (SPLUNC1) protein is a recently described large airway epithelial cell-derived molecule that was predicted to exert host defense activities. However, SPLUNC1 function and regulation in an infectious or allergic milieu are still unknown. We determined host defense and anti-inflammatory functions of SPLUNC1 protein in Mp infection and the regulation of SPLUNC1 by Mp and allergic inflammation (e.g., IL-13). SPLUNC1 function was examined in Mp or human airway epithelial cell cultures by using SPLUNC1 recombinant protein, overexpression and RNA interference. Human and mouse bronchial epithelial SPLUNC1 was examined using immunostaining, Western blotting, ELISA, laser capture microdissection, and real-time PCR. Mouse models of Mp infection and allergic inflammation and air-liquid interface cultures of normal human primary bronchial epithelial cells were used to study SPLUNC1 regulation by Mp and IL-13. We found that: 1) SPLUNC1 protein decreased Mp levels and inhibited epithelial IL-8 production induced by Mp-derived lipoproteins; 2) normal human and mouse large airway epithelial cells expressed high levels of SPLUNC1; and 3) although Mp infection increased SPLUNC1, IL-13 significantly decreased SPLUNC1 expression and Mp clearance. Our results suggest that SPLUNC1 serves as a novel host defense protein against Mp and that an allergic setting markedly reduces SPLUNC1 expression, which may in part contribute to the persistent nature of bacterial infections in allergic airways.

Bim and Bcl-2 mutually affect the expression of the other in T cells

The life and death of T cells is controlled to a large extent by the relative amounts of Bcl-2-related proteins they contain. The antiapoptotic protein Bcl-2 and the proapoptotic protein Bim are particularly important in this process with the amount of Bcl-2 per cell dropping by about one-half when T cells prepare to die. In this study we show that Bcl-2 and Bim each control the expression of the other. Absence of Bim leads to a drop in the amount of intracellular Bcl-2 protein, while having no effect on the amounts of mRNA for Bcl-2. Conversely, high amounts of Bcl-2 per cell allow high amounts of Bim, although in this case the effect involves increases in Bim mRNA. These mutual effects occur even if Bcl-2 is induced acutely. Thus these two proteins control the expression of the other, at either the protein or mRNA level.

The protooncogene MYC can break B cell tolerance

The protooncogene MYC has been implicated in both the proliferation and programmed cell death of lymphoid cells, and in the genesis of lymphoid tumors. Here, we report that overexpression of MYC, as found in many lymphomas, can break immune tolerance. Mice that would otherwise be tolerant to a transgenic autoantigen mounted an immune response to the antigen if MYC was vigorously expressed in the B cell lineage. The responsive B cells converted to an activated phenotype and produced copious amounts of autoantibody that engendered immune complex disease of the kidney. MYC was required to both establish and maintain the breach of tolerance. These effects may be due to the ability of MYC to serve as a surrogate for cytokines. We found that the gene could mimic the effects of cytokines on both B cell proliferation and survival and, indeed, was required for those effects. These findings demonstrate a critical role for MYC in the response of B cells to antigen and expand the potential contributions of MYC to the genesis of lymphomas.

Major Bronchial Trauma in the Pediatric Age Group

Tracheobronchial injuries are rare among all age groups and are extremely rare among the pediatric age group. Yet, the incidence has seemed to increase. Most of these patients die before reaching the hospital from severe associated injuries. Isolated bronchial injury is even more rare than tracheal injury, and it is the focus of the present study. A retrospective national survey was conducted among all tertiary referral and trauma centers in Israel regarding the period between the years 1983 and 1998. Only six cases (3 males and 3 females) of isolated bronchial rupture were found the throughout the country. Ages of the patients ranged from 2 to 14 years; all were involved in motor vehicle accidents, four of them as pedestrians. Ruptures occurred in the bronchus intermedicus (2 cases), left and right main bronchus (2 cases each). All but one patient went through primary repair. We give a full description of the procedure and discuss the literature regarding incidence, diagnosis, treatment, and outcome.

Interferon gamma is required for activation-induced death of T lymphocytes

The effector cytokine interferon gamma (IFN-gamma) may play a role in T cell homeostasis. We have examined the requirement for IFN-gamma in one mechanism that regulates T cell expansion and survival, activation-induced cell death (AICD). CD4(+) T cells lacking IFN-gamma or the Stat1 transcription factor are resistant to AICD. IFN-gamma is required for the production of caspases, and retrovirus-mediated expression of caspase-8 restores the sensitivity of Stat1-deficient T cells to AICD. In vitro, IFN-gamma limits the expansion of T cells that are stimulated through their antigen receptors. Thus, IFN-gamma may function to control the expansion and persistence of T cells by promoting caspase-8-dependent apoptosis.

IL-2 and related cytokines can promote T cell survival by activating AKT

The regulated elimination of T cells serves to maintain normal immune function and prevents autoimmune responses. IL-2 family cytokines play an important role in controlling the survival of immature and mature T cells. These molecules activate the protein kinase, AKT/PKB. AKT has been shown to transduce an antiapoptotic signal in numerous cell types. In this study, we show that an active form of AKT can protect T cells from apoptosis following growth factor withdrawal and that IL-2 family cytokines can promote T cell survival by activating this kinase. We also provide evidence that AKT does not block death receptor-mediated killing of lymphocytes. These data suggest that AKT may serve as a common signaling element by which members of the IL-2 family of cytokines promote T cell survival.

c-Myc regulates mammalian body size by controlling cell number but not cell size

Overexpression of the proto-oncogene c-myc has been implicated in the genesis of diverse human tumours. c-Myc seems to regulate diverse biological processes, but its role in tumorigenesis and normal physiology remains enigmatic. Here we report the generation of an allelic series of mice in which c-myc expression is incrementally reduced to zero. Fibroblasts from these mice show reduced proliferation and after complete loss of c-Myc function they exit the cell cycle. We show that Myc activity is not needed for cellular growth but does determine the percentage of activated T cells that re-enter the cell cycle. In vivo, reduction of c-Myc levels results in reduced body mass owing to multiorgan hypoplasia, in contrast to Drosophila c-myc mutants, which are smaller as a result of hypotrophy. We find that c-myc substitutes for c-myc in fibroblasts, indicating they have similar biological activities. This suggests there may be fundamental differences in the mechanisms by which mammals and insects control body size. We propose that in mammals c-Myc controls the decision to divide or not to divide and thereby functions as a crucial mediator of signals that determine organ and body size.

Retroviruses as tools to study the immune system

Retrovirus-based vectors provide an efficient means to introduce and express genes in cells of the immune system and have become a popular tool to study immune function. They are easy to manipulate and provide stable, long-term gene expression because they integrate into the genome. Current retroviral vectors do have limitations that affect their usefulness in certain applications. However, recent advances suggest a number of ways in which these vectors might be improved to extend their utility in immunological research.

Inducible chromosomal translocation of AML1 and ETO genes through Cre/loxP-mediated recombination in the mouse

Transgenic mice have been used to explore the role of chromosomal translocations in the genesis of tumors. But none of these efforts has actually involved induction of a translocation in vivo. Here we report the use of Cre recombinase to replicate in vivo the t(8;21) translocation found in human acute myeloid leukemia (AML). As in the human tumors, the murine translocation fuses the genes AML1 and ETO. We used homologous recombination to place loxP sites at loci that were syntenic with the break points for the human translocation. Cre activity was provided in mice by a transgene under the control of the Nestin promoter, or in cultured B cells by infecting with a retroviral vector encoding Cre. In both instances, Cre activity mediated interchromosomal translocations that fused the AML1 and ETO genes. Thus, reciprocal chromosomal translocations that closely resemble rearrangements found in human cancers can be achieved in mice.

Murine monoclonal antibodies biologically active against the amino region of HIV-1 gp120: isolation and characterization

The human immunodeficiency virus (HIV)-1 envelope glycoprotein is synthesized as a precursor (gp160) and subsequently cleaved to generate the external gp120 and transmembrane gp41 glycoproteins. Both gp120 and gp41 have been demonstrated to mediate critical functions of HIV, including viral attachment and fusion with the cell membrane. The antigenic variability of the HIV-1 envelope glycoprotein has presented a significant problem in the design of appropriate and successful vaccines and offers one explanation for the ability of HIV to evade immune surveillance. Therefore, the development and characterization of functional antibodies against conserved regions of the envelope glycoprotein is needed. Because of this need, we generated a panel of murine monoclonal antibodies (MuMabs) against the HIV-1 envelope glycoprotein. To accomplish this, we immunized Balb/C mice with a recombinant glycoprotein 160 (gp160) that was synthesized in a baculovirus expression system. From the growth-positive hybridomas, three MuMabs were generated that demonstrated significant reactivity with recombinant gp120 but failed to show reactivity against HIV-1 gp41, as determined by enzyme-linked immunosorbent assay (ELISA). Using vaccinia constructs that synthesize variant truncated subunits of gp160, we were able to map reactivity of all three of the Mabs (ID6, AC4, and AD3) to the first 204 residues of gp120 (i.e., the N terminus of gp120) via Western blot analysis. Elucidation of the epitopes for these Mabs may have important implications for inhibition of infection by HIV-1. Our initial attempts to map these Mabs with linear epitopes have not elucidated a specific antigenic determinant; however, several physical characteristics have been determined that suggest a continuous surface epitope. Although these antibodies failed to neutralize cell-free or cell-associated infection by HIV-1, they did mediate significant antibody-dependent cellular cytotoxicity (ADCC) activity, indicating potential therapeutic utility. In summary, these data suggest the identification of a potentially novel site in the first 200 aa of gp120 that mediates ADCC.

Autoimmunity as a consequence of retrovirus-mediated expression of C-FLIP in lymphocytes

The induction of apoptosis by death receptors serves to regulate immune responses by eliminating unwanted and harmful cells. Mature lymphocytes express FLICE inhibitory proteins (FLIPs) that block death receptor-induced cell death. Here, we show that both B and T cells downregulate c-FLIP upon activation in vitro. Retrovirus-mediated expression of c-FLIP blocks Fas-induced apoptosis of activated lymphocytes but does not affect cell death resulting from cytokine withdrawal. In vivo, c-FLIP expression results in defective superantigen-mediated elimination of T cells, the accumulation of activated B cells, the production of autoantibodies, and the development of autoimmune disease. No effect was seen on negative selection of thyomocytes. These results suggest that activation-dependent downregulation of c-FLIP renders mature lymphocytes sensitive to death receptor-mediated apoptosis and is required to maintain self-tolerance.

Uncoupling IL-2 signals that regulate T cell proliferation, survival, and Fas-mediated activation-induced cell death

IL-2 is an important growth and survival factor for T lymphocytes but also sensitizes these cells to Fas-mediated activation-induced cell death (AICD). The molecular basis of these different effects of IL-2 was studied by introducing wild-type and mutant forms of the IL-2 receptor beta (IL-2Rbeta) chain that lacked specific signaling capacities into receptor-deficient T cells by retroviral gene transfer. Activation of Stat5 by IL-2 was found to be involved in T cell proliferation and promoted Fas ligand (FasL) expression and AICD. T cell survival was dependent on a receptor region that activated Akt and the expression of Bcl-2. Thus, distinct IL-2Rbeta chain signaling modules regulate T cell fate by stimulating growth and survival or by promoting apoptosis.

Genetic models of abnormal apoptosis in lymphocytes

Apoptosis is a critical mechanism for regulating cell numbers during development, normal responses to hormones and other stimuli, and immune and inflammatory reactions. Recent advances in defining the biochemical mechanisms of cell death, and the development of animal models with isolated defects in cell death pathways, have led to an increasing appreciation of the pathophysiologic importance of lymphocyte apoptosis. In this article, we review our current understanding of the pathways and roles of apoptosis in lymphocytes, with an emphasis on transgenic and knockout models. We also summarize the relevance of these animal models to human diseases.

Role of cytokines in autoimmunity

Activation-induced apoptosis of T lymphocytes is an important mechanism for maintaining self-tolerance. The sensitivity of T cells to apoptosis by the Fas pathway is regulated by the exposure of these cells to different cytokines. IL-2 is a survival and growth factor for T cells, as well as a necessary potentiator of Fas-mediated cell death. The role of this cytokine in triggering death pathways is the likely explanation for the autoimmune disease that develops as a result of targeted disruption of the IL-2 or IL-2 receptor alpha or beta chain gene.

Biochemical mechanisms of IL-2-regulated Fas-mediated T cell apoptosis

Activation-induced cell death (AICD) of lymphocytes is an important mechanism of self-tolerance. In CD4+ T cells, AICD is mediated by the Fas pathway and is enhanced by IL-2. To define the mechanisms of this pro-apoptotic action of IL-2, we analyzed CD4+ T cells from wild-type and IL-2-/- mice expressing a transgenic T cell receptor. T cells become sensitive to AICD after activation by antigen and IL-2. IL-2 increases transcription and surface expression of Fas ligand (FasL) and suppresses transcription and expression of FLIP, the inhibitor of apoptosis. The ability of IL-2 to enhance expression of a pro-apoptotic molecule, FasL, and to suppress an inhibitor of Fas signaling, FLIP, likely accounts for the role of this cytokine in potentiating T cell apoptosis.

Protection of chimpanzees from high-dose heterologous HIV-1 challenge by DNA vaccination

Novel approaches for the generation of more effective vaccines for HIV-1 are of significant importance. In this report we analyze the immunogenicity and efficacy of an HIV-1 DNA vaccine encoding env, rev and gag/pol in a chimpanzee model system. The immunized animals developed specific cellular and humoral immune responses. Animals were challenged with a heterologous chimpanzee titered stock of HIV-1 SF2 virus and followed for 48 weeks after challenge. Polymerase chain reaction coupled with reverse transcription (RT-PCR) results indicated infection in the control animal, whereas those animals vaccinated with the DNA constructs were protected from the establishment of infection. These studies serve as an important benchmark for the use of DNA vaccine technology for the production of protective immune responses.

The glucocorticoid receptor type II complex is a target of the HIV-1 vpr gene product

The vpr gene of human immunodeficiency virus type 1 (HIV-1) encodes a 15-kDa virion-associated protein that functions as a regulator of cellular processes linked to the HIV life cycle. We report the interaction of a 41-kDa cytosolic viral protein R interacting protein 1 (Rip-1) with Vpr in vitro. Rip-1 displays a wide tissue distribution, including relevant targets of HIV infection. Vpr protein induced nuclear translocation of Rip-1, as did glucocorticoid receptor (GR)-II-stimulating steroids. Importantly, Vpr and Rip-1 coimmunoprecipitated with the human GR as part of an activated receptor complex. Vpr complementation of a vpr mutant virus was also mimicked by GR-II-stimulating steroids. Vpr and GR-II actions were inhibited by mifepristone, a GR-II pathway inhibitor. Together these data directly link the activity of the vpr gene product to the glucocorticoid steroid pathway and provide a biochemical mechanism for the cellular and viral activity of Vpr, as well as suggest that a unique class of antivirals, which includes mifepristone (RU486), may influence HIV-1 replication.

Extracellular Vpr protein increases cellular permissiveness to human immunodeficiency virus replication and reactivates virus from latency

The vpr gene product of human immunodeficiency virus (HIV) and simian immunodeficiency virus is a virion-associated regulatory protein that has been shown using vpr mutant viruses to increase virus replication, particularly in monocytes/macrophages. We have previously shown that vpr can directly inhibit cell proliferation and induce cell differentiation, events linked to the control of HIV replication, and also that the replication of a vpr mutant but not that of wild-type HIV type 1 (HIV-1) was compatible with cellular proliferation (D. N. Levy, L. S. Fernandes, W. V. Williams, and D. B. Weiner, Cell 72:541-550, 1993). Here we show that purified recombinant Vpr protein, in concentrations of < 100 pg/ml to 100 ng/ml, increases wild-type HIV-1 replication in newly infected transformed cell lines via a long-lasting increase in cellular permissiveness to HIV replication. The activity of extracellular Vpr protein could be completely inhibited by anti-Vpr antibodies. Extracellular Vpr also induced efficient HIV-1 replication in newly infected resting peripheral blood mononuclear cells. Extracellular Vpr transcomplemented a vpr mutant virus which was deficient in replication in promonocytic cells, restoring full replication competence. In addition, extracellular Vpr reactivated HIV-1 expression in five latently infected cell lines of T-cell, B-cell, and promonocytic origin which normally express very low levels of HIV RNA and protein, indicating an activation of translational or pretranslational events in the virus life cycle. Together, these results describe a novel pathway governing HIV replication and a potential target for the development of anti-HIV therapeutics.

Serum Vpr regulates productive infection and latency of human immunodeficiency virus type 1

In human immunodeficiency virus (HIV)-positive individuals, the vast majority of infected peripheral blood cells and lymph node cells may be latently or nonproductively infected. The vpr open reading frame of HIV-1 encodes a 15-kDa virion-associated protein, Vpr. The vpr gene has been shown to increase virus replication in T cells and monocyte/macrophages in vitro. We have previously reported that vpr expression in various tumor lines leads to growth inhibition and differentiation, indicating that Vpr may function as a regulator of cellular permissiveness to HIV replication. Here we show that Vpr protein is present in significant amounts in the serum of AIDS patients. Purified serum Vpr activated virus expression from five latently infected cell lines, U1, OM.10.1, ACH-2, J1.1, and LL58. Serum Vpr also activated virus expression from resting peripheral blood mononuclear cells of HIV-infected individuals. Together, these findings implicate serum Vpr in the activation of HIV replication in vivo and in the control of latency. Anti-Vpr antibodies inhibited Vpr activity, suggesting that humoral immunity modulates Vpr activity in vivo. These results have broad implications for the virus life cycle and for the prospective control of HIV replication and pathogenesis.

Gene inoculation generates immune responses against human immunodeficiency virus type 1

Recently, immunization techniques in which DNA constructs are introduced directly into mammalian tissue in vivo have been developed. In theory, gene inoculation should result in the production of antigenic proteins in a natural form in the immunized host. Here we present the use of such a technique for the inoculation of mice with a human immunodeficiency virus type 1 (HIV-1) envelope DNA construct (pM160). Mice were injected intramuscularly with pM160 and were subsequently analyzed for their anti-HIV envelope immune responses. Antisera collected from inoculated animals reacted with the recombinant HIV-1 envelope in ELISA and immunoprecipitation assays. The antisera also contained antibodies that were able to neutralize HIV-1 infection and inhibit HIV-1-mediated syncytium formation in vitro. Furthermore, splenic lymphocytes derived from pM160-inoculated animals demonstrated HIV-envelope-specific proliferative responses. The gene inoculation technique mimics features of vaccination with live attenuated viruses and, therefore, may ultimately prove useful in the rapid development of safe and efficacious vaccines as it provides for production of relevant antigen in vivo without the use of infectious agents.

Vertical transmission of human immunodeficiency virus (HIV) infection. Reactivity of maternal sera with glycoprotein 120 and 41 peptides from HIV type 1

The observation that approximately 70% of HIV-infected pregnant women do not transmit infection vertically suggests that antibody therapy may be effective in the prevention of transmission of HIV infection from mother to child. Currently, there is an incomplete understanding of the processes involved in vertical transmission of HIV infection. The elucidation of the serological basis of maternal immunity as it relates to protection from vertical transmission is the goal of this study. We have screened 20 maternal sera from HIV+ individuals of known vertical transmission status for reactivity with 31 peptides spanning the entire envelope glycoprotein of HIV-1. Of interest was reactivity to regions outside of the V3 loop of gp120. The findings have been examined in relationship to transmission status, as well as to in vitro anti-HIV-1 biological activity. Our results indicate that lack of vertical transmission is correlated with high viral neutralization activity, but not with antisyncytial activity nor with binding to the V3 peptides examined in this study. Also, the transmission group bound to fewer gp41 peptides when compared with the nontransmission group, suggesting that immune responses to gp41 may be important in preventing transmission. These findings may provide insights into the design of passive immunotherapies.