Priming of CTL responses by DNA vaccines: direct transfection of antigen presenting cells versus cross-priming

DNA vaccines can induce cytotoxic T lymphocyte (CTL) responses in various species including mice, non-human primates and humans. It is now well established that antigen presenting cells (APCs) are required for induction of these responses. However, it is not yet known whether this is a function of antigen expression within or acquisition of antigen by these cells, or a combination of both. Cross-priming has been demonstrated to occur from cells (including muscle cells) to APCs in vivo. In addition, there is evidence that APCs can be transfected after DNA vaccination. Hence, efforts to facilitate cross-priming and to increase transfection of APCs will be important for increasing the potency of DNA vaccines.

Induction of potent immune responses by cationic microparticles with adsorbed human immunodeficiency virus DNA vaccines

The effectiveness of cationic microparticles with adsorbed DNA at inducing immune responses was investigated in mice, guinea pigs, and rhesus macaques. Plasmid DNA vaccines encoding human immunodeficiency virus (HIV) Gag and Env adsorbed onto the surface of cationic poly(lactide-coglycolide) (PLG) microparticles were shown to be substantially more potent than corresponding naked DNA vaccines. In mice immunized with HIV gag DNA, adsorption onto PLG increased CD8(+) T-cell and antibody responses by approximately 100- and approximately 1,000-fold, respectively. In guinea pigs immunized with HIV env DNA adsorbed onto PLG, antibody responses showed a more rapid onset and achieved markedly higher enzyme-linked immunosorbent assay and neutralizing titers than in animals immunized with naked DNA. Further enhancement of antibody responses was observed in animals vaccinated with PLG/DNA microparticles formulated with aluminum phosphate. The magnitude of anti-Env antibody responses induced by PLG/DNA particles was equivalent to that induced by recombinant gp120 protein formulated with a strong adjuvant, MF-59. In guinea pigs immunized with a combination vaccine containing HIV env and HIV gag DNA plasmids on PLG microparticles, substantially superior antibody responses were induced against both components, as measured by onset, duration, and titer. Furthermore, PLG formulation overcame an apparent hyporesponsiveness of the env DNA component in the combination vaccine. Finally, preliminary data in rhesus macaques demonstrated a substantial enhancement of immune responses afforded by PLG/DNA. Therefore, formulation of DNA vaccines by adsorption onto PLG microparticles is a powerful means of increasing vaccine potency.

Relative potency of cellular and humoral immune responses induced by DNA vaccination

DNA vaccines can prime broad-based immune responses in small animal models. In the present study, we sought to evaluate the relative ability of DNA vaccines to induce humoral and cellular immune responses. Using a DNA vaccine encoding HIV gag in mice, we observed that CD8+ T cell responses were primed more readily than were antibody responses, particularly at low doses of DNA. These CD8+ T cell responses were detected in spleen cells, as well as at local sites such as the lung and draining lymph nodes. The potency of the HIV gag DNA vaccine used was sufficient to prime strong CTL responses in macaques, but only low to undetectable antibody responses. Therefore, DNA vaccines appear able to prime strong, broad CTL but only modest antibody responses. These results may have implications on the development of vaccines against infectious diseases where both CTL and antibody responses are desired, such as HIV.

Infection of human dendritic cells by a sindbis virus replicon vector is determined by a single amino acid substitution in the E2 glycoprotein

The ability to target antigen-presenting cells with vectors encoding desired antigens holds the promise of potent prophylactic and therapeutic vaccines for infectious diseases and cancer. Toward this goal, we derived variants of the prototype alphavirus, Sindbis virus (SIN), with differential abilities to infect human dendritic cells. Cloning and sequencing of the SIN variant genomes revealed that the genetic determinant for human dendritic cell (DC) tropism mapped to a single amino acid substitution at residue 160 of the envelope glycoprotein E2. Packaging of SIN replicon vectors with the E2 glycoprotein from a DC-tropic variant conferred a similar ability to efficiently infect immature human DC, whereupon those DC were observed to undergo rapid activation and maturation. The SIN replicon particles infected skin-resident mouse DC in vivo, which subsequently migrated to the draining lymph nodes and upregulated cell surface expression of major histocompatibility complex and costimulatory molecules. Furthermore, SIN replicon particles encoding human immunodeficiency virus type 1 p55(Gag) elicited robust Gag-specific T-cell responses in vitro and in vivo, demonstrating that infected DC maintained their ability to process and present replicon-encoded antigen. Interestingly, human and mouse DC were differentially infected by selected SIN variants, suggesting differences in receptor expression between human and murine DC. Taken together, these data illustrate the tremendous potential of using a directed approach in generating alphavirus vaccine vectors that target and activate antigen-presenting cells, resulting in robust antigen-specific immune responses.

Distribution of DNA vaccines determines their immunogenicity after intramuscular injection in mice

Intramuscular injection of DNA vaccines elicits potent humoral and cellular immune responses in mice. However, DNA vaccines are less efficient in larger animal models and humans. To gain a better understanding of the factors limiting the efficacy of DNA vaccines, we used fluorescence-labeled plasmid DNA in mice to 1) define the macroscopic and microscopic distribution of DNA after injection into the tibialis anterior muscle, 2) characterize cellular uptake and expression of DNA in muscle and draining lymph nodes, and 3) determine the effect of modifying DNA distribution and cellular uptake by volume changes or electroporation on the magnitude of the immune response. Injection of a standard 50-microl dose resulted in the rapid dispersion of labeled DNA throughout the muscle. DNA was internalized within 5 min by muscle cells near the injection site and over several hours by cells that were located along muscle fibers and in the draining lymph nodes. Histochemical staining and analysis of mRNA expression in isolated cells by RT-PCR showed that the transgene was detectably expressed only by muscle cells, despite substantial DNA uptake by non-muscle cells. Reduction of the injection volume to 5 microl resulted in substantially less uptake and expression of DNA by muscle cells, and correspondingly lower immune responses against the transgene product. However, expression and immunogenicity were restored when the 5-microl injection was followed by electroporation in vivo. These findings indicate that distribution and cellular uptake significantly affect the immunogenicity of DNA vaccines.

Increased DNA vaccine delivery and immunogenicity by electroporation in vivo

DNA vaccines have been demonstrated to be potent in small animals but are less effective in primates. One limiting factor may be inefficient uptake of DNA by cells in situ. In this study, we evaluated whether cellular uptake of DNA was a significant barrier to efficient transfection in vivo and subsequent induction of immune responses. For this purpose, we used the technique of electroporation to facilitate DNA delivery in vivo. This technology was shown to substantially increase delivery of DNA to cells, resulting in increased expression and elevated immune responses. The potency of a weakly immunogenic hepatitis B surface Ag DNA vaccine was increased in mice, as seen by a more rapid onset and higher magnitude of anti-hepatitis B Abs. In addition, the immunogenicity of a potent HIV gag DNA vaccine was increased in mice, as seen by higher Ab titers, a substantial reduction in the dose of DNA required to induce an Ab response, and an increase in CD8+ T cell responses. Finally, Ab responses were enhanced by electroporation against both components of a combination HIV gag and env DNA vaccine in guinea pigs and rabbits. Therefore, cellular uptake of DNA is a significant barrier to transfection in vivo, and electroporation appears able to overcome this barrier.

Quantification of the number of cytotoxic T cells specific for an immunodominant HCV-specific CTL epitope primed by DNA immunization

Priming of strong cellular immune responses to hepatitis C (HCV) is thought to be important for eradication of infection. Although productive infection of HCV occurs only reproducibly in humans and chimpanzees, definition of HCV-specific T cell epitopes in mice is necessary to screen efficiently HCV vaccine strategies for their ability to prime cellular immune responses. Out of seven strains of mice screened for immunodominant CTL epitopes against HCV-1a Core, E2, NS5a and NS5b, only one epitope (p214K9) in only one mouse strain was identified. Enumeration of p214K9-specific CD8+ cells by flow cytometry revealed that the number of epitope specific CTL primed by 'naked' DNA immunization was lower than that reported during viral infection. The p214K9 epitope described here, combined with analysis of CTL responses by flow cytometry, should be instrumental in ranking various HCV vaccine strategies for their ability to prime CTL responses.

Increased expression and immunogenicity of sequence-modified human immunodeficiency virus type 1 gag gene

A major challenge for the next generation of human immunodeficiency virus (HIV) vaccines is the induction of potent, broad, and durable cellular immune responses. The structural protein Gag is highly conserved among the HIV type 1 (HIV-1) gene products and is believed to be an important target for the host cell-mediated immune control of the virus during natural infection. Expression of Gag proteins for vaccines has been hampered by the fact that its expression is dependent on the HIV Rev protein and the Rev-responsive element, the latter located on the env transcript. Moreover, the HIV genome employs suboptimal codon usage, which further contributes to the low expression efficiency of viral proteins. In order to achieve high-level Rev-independent expression of the Gag protein, the sequences encoding HIV-1(SF2) p55(Gag) were modified extensively. First, the viral codons were changed to conform to the codon usage of highly expressed human genes, and second, the residual inhibitory sequences were removed. The resulting modified gag gene showed increases in p55(Gag) protein expression to levels that ranged from 322- to 966-fold greater than that for the native gene after transient expression of 293 cells. Additional constructs that contained the modified gag in combination with modified protease coding sequences were made, and these showed high-level Rev-independent expression of p55(Gag) and its cleavage products. Density gradient analysis and electron microscopy further demonstrated that the modified gag and gag protease genes efficiently expressed particles with the density and morphology expected for HIV virus-like particles. Mice immunized with DNA plasmids containing the modified gag showed Gag-specific antibody and CD8(+) cytotoxic T-lymphocyte (CTL) responses that were inducible at doses of input DNA 100-fold lower than those associated with plasmids containing the native gag gene. Most importantly, four of four rhesus monkeys that received two or three immunizations with modified gag plasmid DNA demonstrated substantial Gag-specific CTL responses. These results highlight the useful application of modified gag expression cassettes for increasing the potency of DNA and other gene delivery vaccine approaches against HIV.

Regulation of gene expression in vivo following transduction by two separate rAAV vectors

Control of gene expression is important to gene therapy for purposes of both dosing and safety. In vivo regulation of gene expression was demonstrated following co-injection of two separate recombinant adeno-associated virus vectors, one encoding an inducible murine erythropoietin transgene and the other a transcriptional activator, directly into the skeletal muscle of adult immunocompetent mice. Transcription was controlled by systemic administration or withdrawal of tetracycline over an 18 week period, demonstrating that the two vectors were capable of transducing the same cell. Cellular or humoral immune responses against the transactivator protein were not detected.

Analysis of homozygous mutant chimeric mice: deletion of the immunoglobulin heavy-chain joining region blocks B-cell development and antibody production

Using a recently described method for efficiently deriving homozygous targeted alleles in embryonic stem cells, we produced chimeric mice whose tissues were derived partially from embryonic stem cells bearing homozygous deletion of the mouse immunoglobulin heavy-chain joining (JH) region. Characterization of these chimeric mice indicated that homozygous JH deletion leads to arrest of B-cell development at an early stage, resulting in a total lack of peripheral B cells and serum IgM. These results were confirmed in mice containing the homozygous JH deletion in their germ line. This novel B-cell-deficient mouse strain provides a tool for studying the recombination and expression of exogenous immunoglobulin genes introduced into the mouse germ line.

Peptide and beta 2-microglobulin regulation of cell surface MHC class I conformation and expression

We have examined the roles of peptide and beta 2-microglobulin (beta 2m) in regulating the conformation and expression level of class I molecules on the cell surface. Using a cell line synthesizing H-2Dd H chain and mouse beta 2m but defective in endogenous peptide loading, we demonstrate the ability of either exogenous peptide or beta 2m alone to increase surface H-2Dd expression at both 25 degrees C and 37 degrees C. Peptide and beta 2m show marked synergy in their abilities to increase surface class I expression, with minimal increases promoted by peptide in the absence of free beta 2m. Low temperature-induced molecules have indistinguishable rates of loss of beta 2m and alpha 1/alpha 2 domain conformational epitopes during culture at 37 degrees C. However, the rate of alpha 3 epitope loss is much slower, indicating a minimum of two steps in class I loss from the cell surface: 1) loss of beta 2m binding to H chain and unfolding of the alpha 1/alpha 2 region; then 2) denaturation, degradation, or internalization of the free H chains possessing alpha 3 epitopes. These data show for the first time that free H chains survive for a finite time on the membrane in a form capable of refolding into alpha 1/alpha 2 epitope positive molecules upon addition of beta 2m and peptide. This refolding in the presence of beta 2m and peptide can explain the reported requirement for both components in sensitizing cells for class I-dependent CTL lysis. It also indicates that such conformational changes in class I molecules are not strictly dependent on either newly synthesized H chains or on intracellular chaperons. The study of H chain-peptide-beta 2m interaction on the cell surface may be relevant to understanding intracellular peptide loading events.

Peptide and beta 2-microglobulin regulation of cell surface MHC class I conformation and expression

We have examined the roles of peptide and beta 2-microglobulin (beta 2m) in regulating the conformation and expression level of class I molecules on the cell surface. Using a cell line synthesizing H-2Dd H chain and mouse beta 2m but defective in endogenous peptide loading, we demonstrate the ability of either exogenous peptide or beta 2m alone to increase surface H-2Dd expression at both 25 degrees C and 37 degrees C. Peptide and beta 2m show marked synergy in their abilities to increase surface class I expression, with minimal increases promoted by peptide in the absence of free beta 2m. Low temperature-induced molecules have indistinguishable rates of loss of beta 2m and alpha 1/alpha 2 domain conformational epitopes during culture at 37 degrees C. However, the rate of alpha 3 epitope loss is much slower, indicating a minimum of two steps in class I loss from the cell surface: 1) loss of beta 2m binding to H chain and unfolding of the alpha 1/alpha 2 region; then 2) denaturation, degradation, or internalization of the free H chains possessing alpha 3 epitopes. These data show for the first time that free H chains survive for a finite time on the membrane in a form capable of refolding into alpha 1/alpha 2 epitope positive molecules upon addition of beta 2m and peptide. This refolding in the presence of beta 2m and peptide can explain the reported requirement for both components in sensitizing cells for class I-dependent CTL lysis. It also indicates that such conformational changes in class I molecules are not strictly dependent on either newly synthesized H chains or on intracellular chaperons. The study of H chain-peptide-beta 2m interaction on the cell surface may be relevant to understanding intracellular peptide loading events.

MHC class I surface expression in embryo-derived cell lines inducible with peptide or interferon

It has long been recognized that the absence of expression of products of the major histocompatibility complex (MHC) during early development might allow the fetus to escape recognition by maternal lymphocytes. In addition to the MHC class I heavy chain and beta 2-microglobulin, antigenic peptide is an essential structural component of the class I molecule. Indeed, there is evidence that MHC-linked genes encoding peptide transporter molecules and possibly components of a proteolytic complex are necessary for MHC class I assembly and stability at the cell surface. Here we demonstrate that embryonic cells in general show a defect in MHC class I assembly. Surface expression was rescued in the presence of an appropriate antigenic peptide, or by treatment with interferon. Consistent with this, HAM1 messenger RNA was not constitutively expressed, but was inducible by interferon, and during differentiation in vitro. Thus, tolerance of the fetal allograft may in part be controlled at the level of peptide-dependent MHC class I assembly.

Defective assembly of class I major histocompatibility complex molecules in an embryonic cell line

Developmentally regulated expression of the products of the major histocompatibility complex (MHC) is thought to play a key role in maternal tolerance of the fetal allograft. Here we analyze a cell line (EE2H3), derived from early post-implantation-stage mouse embryos, that is defective for MHC class I assembly. To follow expression of a single well-defined class I product, we introduced the H-2Dd gene under control of the human beta-actin promoter. We found that the transfected EE2H3 cells expressed abundant levels of H-2Dd heavy chains and beta 2-microglobulin protein, but only small amounts of H-2Dd surface protein. Surface expression was rescued by the addition of an appropriate antigenic peptide, or by culturing the cells at low temperature. The phenotype exhibited by EE2H3 is thus remarkably similar to that described for class I-negative somatic cell variants selected using antibodies and complement. However, a striking difference was that surface expression in H-2Dd-transfected EE2H3 cells was markedly enhanced in response to treatment with interferon. Thus, we have identified a novel class I assembly-defective cell line. Considering that EE2H3 was established from primary cultures of mouse embryo cells without immunoselection, and is therefore likely to represent a cell population normally present in post-implantation-stage embryos, these findings raise the possibility that expression of class I surface antigens during early development may in part be controlled post-translationally at the level of MHC class I assembly.

Split anergy in a CD8+ T cell: receptor-dependent cytolysis in the absence of interleukin-2 production

Engagement of the antigen-specific receptor (TCR) of CD4+ T lymphocytes without a second (costimulatory) signal prevents the subsequent production of interleukin-2 (IL-2) by these cells. Because IL-2 is a key immunoregulatory lymphokine and is also produced by a subset of CD8+ T cells that are able to kill target cells, the effect of engaging the TCR of one such clone in the absence of costimulatory signals was examined. The capacity for TCR-dependent IL-2 production was lost, indicating comparable costimulator-dependent signaling requirements for IL-2 production in CD4+ and CD8+ T cells. However, TCR-mediated cytotoxicity was not impaired, implying that costimulation is required for only certain TCR-dependent effector functions.

[Break-even points in the costs of care to the elderly]

Following a preliminary study in 1987 for the Central Committee of Residential Homes on the extent to which the elderly make use of home and health care institutions and at what costs, a study was conducted on the relation between the costs of home and health care for the elderly and the level of dependency taking into account the housing situation. In addition attention has been given to the question whether residential care at a given level of dependency incurs less costs than home care: the so-called break-even points. It appeared that in the case of physical disability, break-even points clearly exist when all costs are considered, but differ from those, if any, calculated on the basis of costs with which the elderly themselves are faced. It appears that old people belonging to the lower income groups will be inclined or rather forced to give up their independent life. It was calculated that about 40% of the elderly with physical handicaps are financially better off if they apply for residential care. On the other hand the higher income group--the remaining 60%--is financially better off if they remain in their own homes. It is recommended to search for a solution to these problems, so that old people with a low income may enjoy their independence to a larger extent while avoiding a social loss of resources as a result of too early admission to an institution.

Interleukin 2 and concanavalin A stimulate interferon-gamma production in a murine cytolytic T cell clone by different pathways

We identified a variant murine cytolytic T lymphocyte (CTL) clone which, in contrast to the parent clone and all other murine T cell populations tested, was found to have acquired spontaneously the ability to produce interferon-gamma (IFN-gamma) in response to recombinant interleukin 2 (rIL-2). IFN-gamma production in response to concanavalin A (Con A), which was characteristic of all T cell populations tested, was preserved in this variant. The IFN produced by the variant in response to either stimulus was active in both a macrophage-activating factor assay and an anti-viral assay. Both activities induced by either stimulus could be blocked by monoclonal anti-IFN-gamma antibodies. Upon Northern blot analysis using an IFN-gamma-specific cDNA probe, the IFN-gamma RNA isolated from variant cells stimulated with Con A or IL-2 were found to migrate equivalently. The unusual pattern of responsiveness in this variant CTL was exploited to compare the mechanisms involved in induction of IFN-gamma production by Con A or IL-2. Striking differences were observed. Unlike IFN-gamma production induced by Con A, IFN-gamma production induced by IL-2 was not accompanied by an elevation of intracellular Ca2+ levels, did not require physiologic extracellular Ca2+ levels, and was not inhibited by the immunosuppressive agent cyclosporin A. Thus, in this variant CTL clone, conditions that have ordinarily been associated in an obligate manner with lymphokine gene expression were found instead to be related to the specific mode of stimulation.

Interleukin 2 and concanavalin A stimulate interferon-gamma production in a murine cytolytic T cell clone by different pathways

We identified a variant murine cytolytic T lymphocyte (CTL) clone which, in contrast to the parent clone and all other murine T cell populations tested, was found to have acquired spontaneously the ability to produce interferon-gamma (IFN-gamma) in response to recombinant interleukin 2 (rIL-2). IFN-gamma production in response to concanavalin A (Con A), which was characteristic of all T cell populations tested, was preserved in this variant. The IFN produced by the variant in response to either stimulus was active in both a macrophage-activating factor assay and an anti-viral assay. Both activities induced by either stimulus could be blocked by monoclonal anti-IFN-gamma antibodies. Upon Northern blot analysis using an IFN-gamma-specific cDNA probe, the IFN-gamma RNA isolated from variant cells stimulated with Con A or IL-2 were found to migrate equivalently. The unusual pattern of responsiveness in this variant CTL was exploited to compare the mechanisms involved in induction of IFN-gamma production by Con A or IL-2. Striking differences were observed. Unlike IFN-gamma production induced by Con A, IFN-gamma production induced by IL-2 was not accompanied by an elevation of intracellular Ca2+ levels, did not require physiologic extracellular Ca2+ levels, and was not inhibited by the immunosuppressive agent cyclosporin A. Thus, in this variant CTL clone, conditions that have ordinarily been associated in an obligate manner with lymphokine gene expression were found instead to be related to the specific mode of stimulation.

Toxicity of methyldopa (Aldomet) to mouse neuroblastoma cells in vivo

The adrenergic blocking agent methyldopa (Aldomet) is toxic to C-1300 neuroblastoma cells in vivo. Four injections of Aldomet at a dose of 7.5 mg/injection were given over a period of 24 hr to C-1300 neuroblastoma-bearing mice. This treatment killed a significant proportion of the C-1300 neuroblastoma cells. Flow cytometric data suggest that sensitivity of tumor cells to Aldomet is not related to the cell cycle.

Two color light scattering identifies physical differences between lymphocyte subpopulations

Forward angle light scattering of two different wavelengths by cells in a flow cytometer was used to investigate physical differences between lymphocytes of different lineage, functional subclass and developmental stage. Correlation of the ultraviolet (UV: 351 nm and 364 nm) and 488 nm light scattering signals produced by lymphoid cells demonstrated that the two signals were not equivalent and that they placed different emphasis on the physical parameters characterizing lymphocytes. Both small T and B lymphocytes from peripheral lymphoid tissues and mitogenically activated large T and B lymphocyte blasts were discriminated by both wavelengths. Differences between the Lyt-2 negative and Lyt-2 positive T lymphocyte subsets were also apparent. Two color light scattering could also discriminate between immature thymocytes and mature peripheral T cells and between small bone marrow cells and mature peripheral B cells. In bone marrow an increase in UV light scattering coincided with the appearance of cell surface immunoglobulin on small cells. These data establish that two color light scattering is a sensitive probe for distinguishing cells of apparently similar morphology and that it can be used to study the physical changes that occur during lymphoid cell differentiation.