What are DNA Vaccines

What are DNA Vaccines?

DNA Vaccines

Genetic/ DNA immunization is a novel technique used to efficiently stimulate humoral and cellular immune responses to protein antigens. The direct injection of genetic material into a living host causes a small amount of its cells to produce the introduced gene products. This inappropriate gene expression within the host has important immunological consequences, resulting in the specific immune activation of the host against the gene delivered antigen (Koprowski et al, 1998 ).

Traditional Vaccines: The development of vaccination against harmful pathogenic microorganisms represents an important advancement in the history of modern medicine. In the past, traditional vaccination has relied on two specific types of microbiological preparations to produce material for immunization and generation of a protective immune response. These two categories involve either living infectious material that has been manufactured in a weaker state and therefore inhibits the vaccine from causing disease, or inert, inactivated, or subunit preparations.                               Immunization

Live attenuated vaccines stimulate protective immune responses when they replicate in the host. The viral proteins produced within the host are released into the extracellular space surrounding the infected cells and are then acquired, internalized and digested by scavenger cells that circulate the body. These cells are called antigen

presenting cells (APCs) and include macrophages, dendritic cells, and B cells, which work together to expand immune response. The APCs recirculate fragments of the digested the antigen to their surface, attached to MHC class II antigens. This complex of foreign peptide antigen plus host MHC class II antigens forms part of the specific signal with which APCs along with the MHC peptide complex, trigger the action of of immune cells, the T helper lymphocytes. The second part of the activation signal comes from the APCs themselves, which display on their cell surface constimulatory molecules along with MHC-antigen complexes. Both drive T call expansion and activation through interaction with their respective ligands, the T cell receptor complex (TCR) and the constimulatory receptors CD28/CTLA4, present on the the T cell surface. Activated T cells secrete molecules that act as powerful activates of immune cells. Also as viral proteins are produced within the host cells, small parts of these proteins surface, chaperoned by host cell MHC class I antigens. These complexes together are recognized by a second class of T cells, killer or

cytotoxic cells. This recognition, along with other stimulation by APCs and production of cytokine by stimulated T cells, is responsible for the developments of mature cytotoxic T cells (CTL) capable of destroying infected cells. In most instances live infection induces life long immunity. Although live attenuated preparations are the vaccines of choice they do pose the risk of reversion to their pathogenic form, causing infection.                                                                                                                                                                                                                                                                         Immune Response

In contrast, when inoculated nonlive vaccines composed of whole or even partial viruses are not produced within the host cells, they mainly end up in the extracellular space.  They provide protection by directly generating T helper and humoral immune responses against the pathogenic immunogen. In the absence of the cellular production of the foreign antigen, these vaccines are usually devoid of the ability to induce significant T cytotoxic responses. In addition, these vaccines are not actually produced in the host, and therefore, they are not customized by the host. The immunity induced by their vaccines frequently decreases during the life of the host and may require additional boosters to achieve lifelong immunity. However, nonlive vaccines offer some important advantages over live vaccines: they are produced earlier, and they can be designed to contain only the specific antigenic target of the pathogen that is involved in the development of protective immunity and exclude all other viral components.

Genetic Immunization: Since its early applications in the 1950's, DNA-based immunization has become a novel approach to vaccine development. Direct injection of naked plasmid DNA induces strong immune responses to the antigen encoded by the gene vaccine. Once the plasmid DNA construct is injected the host cells take up the foreign DNA, expressing the viral gene and producng the corresponding viral protein inside the cell. This form of antigen presentation and processing induced both MHC and class I and class II restricted cellular and humoral immune responses (Encke , J. et al , 1999).

History: The use of genetic material to deliver genes for therapeutic purposes has been practiced for many years. Experiments outlining the transfer of DNA into cells of living animals were reported as early as 1950. Later experiments using purified genetic material only further confirmed that the direct DNA gene injection in the absence of viral vectors results in the expression of the inoculated genes in the host. There have been additional experiments that extend these findings to recombinant DNA molecules, further illustrating the idea that purified nucleic acids could be directly delivered into a host and proteins would be produced. In 1992, scientists Tang and Johnson reported that the delivery of human growth hormone in a expression cassette in vivo resulted in production of detectable levels of the growth hormone in host mice. They also found that these inoculated mice developed antibodies against the human growth hormone; they termed this immunization procedure "genetic immunization", which describes the ability of inoculated genes to be individual immunogens (Koprowski et al, 1998 ).

                                                                                                                                                                                                                                                                   DNA Vaccines

Construction:  DNA vaccines are composed of a bacterial plasmids. Expression plasmids used in DNA-based vaccination normally contain two unites: the antigen expression unit composed of promoter/enhancer sequences, followed by antigen-encoding and polyadenylation sequences and the production unit composed of of bacterial sequences necessary for plamid amplification and selection (Schirmbeck, R., 2001). The construction of bacterial plasmids with vaccine inserts is accomplished using recombinant DNA technology.  Once constructed, the vaccine plasmid is transformed into bacteria, where bacterial growth produces multiple plasmid copies. The plasmid DNA is then purified from the bacteria, by separating the circular plasmid from the much larger bacterial DNA and other bacterial impurities. This purifies DNA acts as the vaccine (AAM , 1996 ).

DNA vaccine plasmid

Administration- Over the past decade of clinical research and trials, several possible routs of plasmid delivery have been found. Successful immunization has been demonstrated after delivery of plasmids through intramuscular, intradermal and intravenous injection. The skin and mucous membranes being considered the best site for immunization due to the high concentrations of dendritic cells (DC), macrophages and lymphocytes (Raz ,E., 1998 ). Intradermal injection of DNA-coated gold particles with a gene gun have been used. The plasmid DNA can be diluted in distilled water, saline or sucrose. There has also been positive demonstration of proinjection or codelivery with various drugs (Encke et al, 1999 ).

Mechanisms: A plasmid vector that expresses the protein of interest (e.g. viral protein) under the control of an appropriate promoter is injected into the skin or muscle of the the host. After uptake of the plasmid, the protein is produced endogenously and intracellularly processed into small antigenic peptides by the host proteases. The peptides then enter the lumen of the endoplasmic reticulum (E.R.) by membrane-associated transporters. In the E.R., peptides bind to MHC class I molecules.  These peptides are presented on the cell surface in the

context of the MHC class I. Subsequent CD8+ cytotoxic T cells (CTL) are stimulated

and they evoke cell-mediated immunity. CTLs inhibit viruses through both cytolysis of infected cells and noncytolysis mechanisms such as cytokine production (Encke et al, 1999).

   The foreign protein can also be presented by the MHC class II pathway by APCs which elicit helper T cells (CD4+) responses. These CD4+ cells are able to recognize the peptides formed from exogenous proteins that were endocytosed or phagocytosed by APC, then degraded to peptide fragments and loaded onto MHC class II molecules. Depending on the the type of CD4+ cell that binds to the complex, B cells are stimulated and antibody production is stimulated. This is the same manner in which traditional vaccines work (Schirmbeck et al ., 2001 ).

                                                              DNA Vaccine Mechanism  

Advantages: DNA immunization offers many advantages over the traditional forms of vaccination. It is able to induce the expression of antigens that resemble native viral epitopes more closely than standard vaccines do since live attenuated and killed vaccines are often altered in their protein structure and antigenicity. Plasmid vectors can be constructed and produced quickly  and the coding sequence can be manipulated in many ways. DNA vaccines encoding several antigens or proteins can be delivered to the host in a single dose, only requiring a microgram of plasmids to induce immune responses. Rapid and large-scale production are available at costs considerably lower than traditional vaccines, and they are also very temperature stable making storage and transport much easier. Another important advantage of genetic vaccines is their therapeutic potential for ongoing chronic viral infections.  DNA vaccination may provide an important tool for stimulating an immune response in HBV, HCV and HIV patients. The continuos expression of the viral antigen caused by gene vaccination in an environment containing many APCs may promote successful therapeutic immune response which cannot be obtained by other traditional vaccines (Encke et al , 1999 ). This is a subject that has generated a lot of interest in the last five years.

Limitations: Although DNA can be used to raise immune responses against pathogenic proteins, certain microbes have outer capsids that are made up of polysaccharides.  This limits the extent of the usage of DNA vaccines because they cannot substitute for polysaccharide-based subunit vaccines (AMM, 1996).

Future- It has recently been discovered that the transfection of myocytes can be amplified by pretreatment with local anesthetics or with cardiotoxin, which induce local tissue damage and initiate myoblast regeneration. Gaining a full understanding of this mechanism of DNA uptake could prove helpful in improving applications for gene therapy and gene vaccination. Both improved expression and better engineering of the DNA plasmid may enhance antibody response to the gene products and expand the applications of the gene vaccines (Raz , E., 1998 ).

DNA Vaccines for Prevention of HBV

Immunogenicity of HBV: Even though both humoral and cell-mediated immunity (CMI) result from natural HBV infection, the presence of antibodies alone is enough to provide protection from infection or reinfection. The exact role of CTLs in protection or clearance of the HBV infection is not yet known. After natural infection, antibodies (anti-HBs) are detected against the HBsAg and viral core antigen (HBcAg, anti-HBc). There is a very clear role for anti-HBs in providing immunity and all vaccines used in humans have been designed to evoke this. In humans a level of 10 mIU/ml (milli-international units/ml) is considered sufficient to confer protection. The role for anti-HBc in providing immunity is less clear.  It is known that HBc is highly immunogenic but the protective effect of the anti-HBc antibodies is still in question since chronic carriers generally have high titers of anti-HBc, but not anti-HBs (Davis et al ., 1998 ).

Problems with current HBV vaccines: The vaccines presently used to immunize patients against HBV are subunit vaccines consisting of empty subviral particles composed solely of HBV envelope protein. Initially these were obtained from the plasma of chronic carriers of the virus, but their use was restricted by a limited supply of chronically infected plasma and by concerns about their safety. Currently, the majority of HBV vaccines contain recombinant HBsAg protein produced in stable transfected yeast or hamster ovary cells (Jilg , 1998 ). Although these recombinant protein vaccines are highly effective, they are expensive to produce, which inhibits

there use in the third world areas where they are needed most. These vaccines also have disadvantages; multiple doses are required  that each dose must be kept cold,

both of which further increase their cost in areas where refrigeration is not common. Other shortcomings of the subunit vaccines include non-response which occurs when there is no detectable level of anti-HBs in the patient after vaccination, or hypo-response where there

is less than the sufficent level of anti-HBs required for protection despite repeat injections. Reseachers have been working dilligently for the past few years to find an effective yet inexpensive vaccine. It is not possible to use a whole killed or live attenutaed vaccines due to the difficulty of growing HBV in culture and also due to the undesirable potential for improper reactions in immunocompromised patients. There is also a great need around the world for a therapeutic vaccine which could be used instead of the current HBV treatments using interferons, which are only at best partially effective, to treat chronically infected individuals (Daivs , 1998) .

                                           Recombinant HBV Vaccine

HBV DNA vaccine model: H.L. Davis et al. (1998) presented a model for a HBV DNA vaccine that used four vectors that expressed one or more of the HBV envelope proteins that are encoded by a single gene within the HBV genome. This gene is divided into S, pre-S2 and pre-S1 regions by three ATG start codons. Three of these four vectors used for the vaccine utilized the immediate early promoter of cytomegolovirus (CMV) to drive the expression of the S (pCMV-S) pre-S2 + S (pCMV-S2.S) or pre-S1 + pre-S2 + S (pCMV-S1.S2.S) envelope gene. The forth vector uses the endogenous HBV promoter within the pre-S1 domain to drive expression (pHBV-S2.S) (Davis et al. , 1998 ). This model used again by H.L Davis, and then by M-L Michel et al. in 2001. In 2001, R. Schirmbeck et al. (2001) developed a modified version of this model, using the commercially available pCI expression vector instead of the pCMV as a promoter to drive the expression of soley the S vector (pCl-S). A similar model using pCI was also used by A. Thermet et al. in 2003.   All of these five studies used intramuscular injection of the HBsAg-expression plasmids in murines used to model human response. R. Schirmbeck et al. also opted to use subcutaneous injections and DNA coated gold particles administered with a gene gun (Shirmbeck et al ., 2001 ) .

                                           HBsAg (Davis, 1998)                                                                                     pCMV-S (Davis, H.L., et al ., 1998)

In vivo synthesis of HBsAg and anti-HBs: H.L. Davis et al. (1998) found that HBs-Ag expression muscle fibers retained a normal histological appearance 5 days after DNA injection, but were seen to breakdown at about 10 days and completely disappear after 30 days. They determined that the disappearance of these  muscle fibers was probably due to attack by HBsAg specific CTL. Low levels of circulating antigen were detected early on in the study and undetectable amounts of HBsAg were found in the serum later, which reflects the neutralization of antigen by antibody as well as a decrease in the production of HBsAg with the destruction of the fibers (Davis et al. , 1998 ). Similar findings were described again by Davis in her study of DNA vaccine immunization against HBV in animal models. R. Schirmbeck et al. found similar serum antibody levels, noting that the highest antibody production was found when just the HBsAg antigen were injected instead of the vectors encoding for HBsAg (Schirmbeck et al ., 2001 ). A. Thermet et al.  (2003) also recorded similar findings, describing a significant increase in anti HBsAg antibodies after the initial injection of the protein encoding vectors (Thermet et al. , 2003 ).

                                        (Schirmbeck, S. et al.)                                                                                                        (Davis, H.L., et al. )

(Davis, H.L.)

Humoral response: Immunization of murines by interamuscular injection of plasmid DNA that express HBV envelope proteins and HBsAg stimulate fast, strong and long lasting humoral immune responses. Antibodies, which are initially of the IgM then IgG isotype, are produced; they are able to recognize several of the B-cell epitopes on the S, preS2 or preS1 domains of the envelope. (Michel, et al., 2001). With S-encoding DNA, there is a predominance of group-specific antibodies, which are capable of providing protection against certain strains of HBV. After injection, the pre-S2 domain predominates, especially in the early period. Pre-S2 antibodies are believed to to be beneficial for prophylactic vaccination due to the fact that they are known to produce protective immunity in chimpanzees and other animal models (Davis et al ., 1998 ). High titers of anti-HBs were found by M.L. Michel et al. (2001)

by 4-8 weeks and persisted for at least 17 months after a single DNA injection. Antibody concentration was boosted ten-fold after the second injection of DNA, and somewhat less by injection of a recombinant HBsAg protein (Michel et al ., 2001 ).

Cell mediated responses: DNA-based immunization using HBsAg plasmids result in strong CTL response. Murines injected with vectors expressing any of the three forms of the HBsAg protein produced high levels of CTL and CTL precursors. After prolonged stimulation with HbsAg-expression cells, spleen cells from DNA-immunized murines were capable of specific lysis of the present antigen, indicating high levels of CTL precursors. CTL activity developed between 3 and 6 days after DNA injection and reached maximal levels by day 12, expressing peak activity for months (Michel et al ., 2001 ).

Potential for HBV DNA vaccines in humans: Results following injection of HBsAg-encoding plasmid DNA in murine models indicate that the DNA vaccine should be feasible for vaccination in humans against HBV. A DNA vaccine would be superior to recombinant HBsAg for prophylactic vaccination; this is especially true in cases of non- or hypo- responsiveness, or for the induction of a rapid humoral response in infants born to chronic carriers (Davis et al ., 1998 ). There have been a few promising clinical trials which reported that DNA vaccines appear to be well tolerated in human hosts. Despite the induction of significant CTL responses, vaccination via needle or gene gun administration of DNA failed to induce antigen-specific antibodies. Increased efficiency could come from improvements in the tranfection or in the induction of better immune responses through the coexpression of cytokines or combinations with other non-DNA vaccines. However, due to the induction of strong CTL, DNA vaccines may be effective for the treatment of chronic HBV patients (Michel et al ., 2001 ).

Therapeutic Applications of a DNA-based HBV vaccine

    Due to the induction of strong CTL, DNA vaccines may be effective for the treatment of chronic carriers of HBV.  J. Encke et al. (1999) found a very strong therapeutic potential of genetic immunizations using a transgenic mouse model with

chronic HBV infection expressing HBV envelope proteins in the liver. After a singe injection of plasmids encoding HBsAg, anti-HBs antibodies were produced that completely cleared circulating HBsAg.  This total disappearance of antigens lasted for 20 weeks and was due to neutralizing antibodies and the complete loss of HBV mRNA in the liver.  This loss of HBV mRNA in the liver was likely caused by HBSAg-specific T cells mediated through cytokine-mediated mechanisms. Immune responses were also observed in Peeking duck and chimpanzee models (Encke, J. et al., 1999).

    M-L Michel et al. found that the endogenous synthesis of antigen, process into relevant epitopes and the subsequent induction of CD8+ CTLs caused by DNA vaccines may make them useful for the treatment of chronic HBV patients.  This team also used transgenic mice that continually expressed the HBsAg antigen in the liver as a model to study the possibility of inducing immune response in chronically infected individuals. By using plasmid DNA that encoded both small and middle HBV envelope proteins, anti-HBs antibodies where produced which were responsible for the clearance of circulating HBsAg. Identical to the findings of J. Encke et al. (1999), M-L Michel et al. also found that the elimination of HBsAg was also due to the disappearance of HBV mRNA from the liver.  Also confirming that it was CD4+ and CD8+ T cells  that were controlling the transgene expression, even in the absence of antibody production. It was also found that the regulation of the HBV envelope mRNA was mediated by type 1 cytokines produced by three activated T lymphocytes. In addition no liver damage was found after induction or transfer of HBsAg-specific CTL into the mice which suggests the the T lymphocytes induced in vivi in the mice after the DNA based immunization were able to cure haptocytes of HBV without killing them.  Their results were further confirmed using a model for duck HBV (DHBV). This points to the possibility of designing a more effective way to treat HBV using DNA based vaccination (Michel et al. , 2001 ).

A. Thermet et al. evaluated the long-term therapeutic efficiency of DNA vaccine in a group of six chronic DHBV carrying ducks.   They were immunized four times with plasmid encoding the large envelope protein

instead of the small and medium proteins targeted in the previous two experiments. Twelve ducks were also used as controls. The results showed the DNA immunization against the large envelope protein was able to significantly decrease and even completely eliminate viral replication in chronically DHBV-infected ducks. It was also noted that in two animals which had shown particularly low pre-treatment viremia levels the virus was completely eliminated; this suggests the potential positive effects of combination antiviral drugs with DNA immunization to for chronic hepatitis B therapy. To further investigate the possibility of such combination therapy, chronically infected ducks were injected with DHBV envelope protein encoding plasmids with lamivudine, an known effective HBV revers transcriptase inhibitor. The a viremia analysis showed a marketed drop in DHBV concentrations in the lamivudine- treated ducks compared with the untreated group.  The benefit of combination therapy was more pronounced in the group that received DNA immunization to envelope protein since 38% cleared intrahaptic DNA. This shows a interesting and novel approach for immunotherapy of chronic hepatitis B infection (Thermet et al., 2003).

Image Gallery

HCC

Gene Gun

HBV DNA-Base Vaccine

HBV with Anitbodies

Hepatitis B Particle Types

HBV Vaccine cycle  

Heptocytes infected with HBV

Fulminant fatal acute viral hepaptitis  

References

American Academy of Microbiology. The Scientific Future of DNA for Immunization. 1996. [ONLINE] http://www.asmusa.org/acasrc/Colloquia/dnareprt.pdf [4/27/03 last day accessed].

The Big Book of Viruses. [ONLINE] http://www.virology.net/Big_Virology/BVHomePage.html [04/13/03, last day accessed.]

Center for Disease Control. [ONLINE] http://www.cdc.gov/ncidod/diseases/hepatitis/b/index.htm [4/24/03, last day accessed]

Davis, Heather L., and Cynthia L. Brazolot Millan. 1998. DNA-based immunization against hepatitis B virus. In Gene Vaccination: Theory and Practice, ed. Raz, E.. Springer-Verlag, Heidelburg, pp93-108.

Davis, Heather L. 1998. DNA-based Immunization Against Hepatitis B: Experience with Animal Models. In DNA Vaccination/ Genetic Vaccination, ed. Koprowski, H, and D.B. Weiner.Spriner-Verlag, Heidelberg, pp57-66.

DNAvaccine.com [ONLINE] http://dnavaccine.com/ [4/28/03, last day accessed]

Encke, J., Jasper zu Putlitz, and Jack R. Wands. 1999. DNA Vaccines. Intervirology 1999;42:117-124.

Flint, S.J. et al. 2000. Principles of Virology: Molecular Biology, Pathogenesis, and Control. ASM Press, Washington, D.C., 804p.

Hepatitis B. [ONLINE] http://cpmcnet.columbia.edu/dept/gi/hepB.html [4/13/03, last day accessed.]

Garces, Robert. The Hepatitis B Virus Page. [ONLINE] http://www.globalserve.net/~harlequin/HBV/ [4/14/03, last day accessed.]

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Jilg, Wolfgang. 1998. Novel hepatitis B vaccines. Vaccine, 16: S65-S68.

Koprowski, H, and D.B. Weiner. 1998. DNA Vaccination/ Genetic Vaccination. Spriner-Verlag, Heidelberg, 198p.

Lu, Mengji and M. Roggendorf. 2001. Evaluation of New Approaches to Prophylactic and Therapeutic Vaccinations against Hepatitis B Viruses in the Woodchuck model. Intervirology, 44:124-131.

Michel, Marie-Louise and Delphine Loirat. 2001. DNA Vaccines for Prophylactic or Therapeutic Immunization against Hepatitis B. Intervirology, 44:78-87.

Poynard, Thierry. 2002. Hepatitis B and C: Management and Treatment. Martin Dunitz Ltd, London,148p.

Raz, Eyal. 1999. Gene Vaccination: THEORY AND PRACTICE. Springer-Verlag, Heidelburg, 169p.

Schirmbeck, R. and Jorg Reimann. April 2001. Revealing the Potential of DNA-based Vaccination: Lessons Learned from the Hepatitis B Virus Surface Antigen. Biol. Chem., 382:543-552.

Stares, James H. and Ellen G. Stares. 2002. Viruses and Human Disease. Academic Press, San Diego, California, 383p.

Thermet, Alexandre, et al. 2003. Progress in DNA vaccine for prophylaxis and therapy of hepatitis B. Vaccine, 21:659-662.

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Glossaryantigen-a usually protein or carbohydrate substance (as a toxin or enzyme) capable of stimulating an immune response

antigen presenting cells- a heterogeneous group of immunocompetent cells that mediate cellular immune response by processing and presenting antigens to the T-cell receptor. Traditional antigen-presenting cells include macrophages, dendritic cells, langerhans cells, and B- lymphocytes.

CCC DNA- covalently closed circular (CCC) duplex DNA located in the nucleus of an infected host that is stably complexed with a variety of host proteins and forms a viral chromatin species

constimulatory molecules- molecules such as B7-1, B7-2, and 4-1BB ligand to augment the generation of  anti-tumour and other more general  immunity

cytokines- a unique family of growth factors. They are secreted primarily from leukocytes and stimulate both the humoral and cellular immune responses, as well as the activation of phagocytic cells. Cytokines that are secreted from lymphocytes are termed lymphokines. A large family of cytokines are produced by various cells of the body

cytotoxic T cells- a subset of T lymphocytes that can kill body cells infected by viruses or transformed by cancer.

epitope- a molecular region on the surface of an antigen capable of eliciting an immune response and of combining with the specific antibody produced by such a response

hepatocelluar carcinoma (HCC)- cancer that arises from hepatocytes, the major cell type of the liver. It is especially prevalent in parts of Asia and Africa. About 80% of people with hepatocellular carcinomas have cirrhosis. Chronic infection with the hepatitis B virus and hepatitis C virus also increases the risk of developing hepatocellular carcinoma.

humoral immune response- the part of immunity or the immune response that involves antibodies secreted by B cells and circulating in bodily fluids

immunocompromised- a condition in which the immune system is not functioning normally because it was previously damaged by disease or medications

interferon- a protein produced by animal cells when they are invaded by viruses that is released into the bloodstream or intercellular fluid to induce healthy cells to manufacture an enzyme that counters the infection.

major histocompatibility complex (MHC) class I- molecules that bind peptide fragments derived from proteolytically degraded proteins endogenously synthesized by a cell. Small peptides are transported into the endoplasmic reticulum where they associate with nascent MHC class I molecules before being routed through the Golgi apparatus and displayed on the surface for recognition by cytotoxic TC lymphocytes.MHC class I expression is widespread on virtually every cell of the body. This is consistent with the protective function of cytotoxic TC lymphocytes which continuously survey cell surfaces and kill cells harboring metabolically active microorganisms.

major histocompatibility complex (MHC) class II-  molecules that bind peptide fragments derived from proteolytically degraded proteins exogenously internalized by "antigen presenting cells," including macrophages, dendritic cells, and B cells. The

resulting peptide fragments are compartmentalized in the endosome where they associate with MHC class II molecules before being routed to the cell surface for recognition by helper TH lymphocytes. MHC class II expression is restricted to "antigen presenting cells." This is consistent with the functions of helper TH lymphocytes which are locally activated wherever these cells encounter macrophages, dendritic cells, or B cells that have internalized and processed antigens produced by pathogenic organisms.

nucleocapsid-a unit of viral structure, consisting of a capsid (protein coat) with the enclosed nucleic acid; some simple viruses are naked nucleocapsids, while in others the nucleocapsids form part of a more complex structure.

oligonucleotide-a polymer made up of a few (2-20) nucleotides. In molecular genetics, a short sequence synthesized to match a region where a mutation is known to occur, and then used as a probe (oligonucleotide probe).

polyadenylation-the addition of several hundred A nucleotides to the 3' ends of mRNAs

prophylactic vaccination- a vaccination which provides protection from disease cauing agents

pregenome- generally refered to as 3.5-kb RNA and has a length exceeding that of the genome due to a short terminal redundancy caused by transcriptional readthrough of a circular genome.

T cell receptor (TCR)-  a complex of integral membrane proteins that participates in the activation of T cells in response to the presentation of antigen.

viremia- the presence of virus in the blood of a host

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Researchers use gene therapy to treat spinal muscular atrophy

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Hologic Human Papillomavirus (HPV) High-Risk Test Approved for Use in The Netherlands Population Cervical Screening Programme

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Aldevron receives research grant for hantavirus vaccine

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Gene Therapy treatment extends lives of mice with fatal disease

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