MM 526-541 and 145-152
1. To provide an awareness of the mode of action and potential clinical use of interferon.
2. To provide insight into the uses and misuses of antiviral vaccines.
3. To develop the concept of viral-mediated interference with viral replication.
4. To relate the lysogenic state to diseases thought to be caused by "dormant," "latent" or "slow" viruses.
5. To define the rational use of antiviral agents.
The physician's job is to interfere with viral replication in order to prevent or ameliorate the disease process. This can be done by manipulating the biological system of the patient or by utilizing antiviral antibiotics. In general, the more complex a system is (such as the viral replication cycle), the more easily that system is disrupted. However, in viral replication, the virus is utilizing mostly host cell protein-synthesizing systems. Disruption of the viral replication cycle can also affect cellular metabolism in not only the infected cell but also in the noninfected cell. However, there are ways to interfere with viral replication with minimal effect on the host cell.
Interferon is a class of glycoproteins which interferes with virus replication. There are numerous types of interferon and each is produced by an animal or by cultured animal cells. Interferon synthesis is induced by viruses or by certain biochemicals and can be of three types:
IFN-a =a -Interferon (20 subtypes) - from many different cell types
IFN -B =B - Interferon (2 subtypes) - from fibroblasts, macrophages
IFN -Y =Y - Interferon (3 subtypes) - from T-lymphocytes
a. Interferons are cell specific in both their production and their effects.
b. Interferons are virus non-specific.
c. Interferons are induced by viruses, chemicals, some species of bacteria and some extracts of fungi.
d. Although all animal cells appear able to produce one or more types of interferon, cells of the bone marrow, spleen, and macrophages, appear to play a special role, i.e., they produce a larger volume of interferon and a more potent interferon.
The nature of the stimulus to interferon production has been clarified by the discovery that double-stranded RNAs, such as reovirus RNA and certain synthetic polynucleotides, can induce a large production of interferon in many animals and in tissue cultures.
For most RNA viruses, double-stranded RNA segments produced during replication mediate the induction of interferon.
In virus-infected cells, the synthesis of interferon begins after viral maturation is initiated and then continues for many hours.
Interferons are synthesized on membrane-bound polysomes. As they are formed, they are segregated into vesicles and are glycosylated; from the vesicles they are excreted outside the cells. Therefore until they are excreted, interferons do not act on the cell that produces them.
Mechanism of interferon action
Suggested mechanisms for the antiviral action of interferon.
Interferons cause antiviral resistance not directly, but by activating cellular genes for antiviral proteins; they are ineffective in enucleated cytoplasts or in the presence of actinomycin D.
Interferons induce the cell response by interacting with the cell surface. At the cell surface, interferons bind to receptors containing gangliosides (glycosylated phospholipids); transformed cells, which are deficient in gangliosides, are less interferon-sensitive than normal cells. In human cells, genes specifying the receptors are present on chromatosome 21; 21-trisomic (Down's syndrome) cells are especially sensitive to interferon. The molecular mechanisms of interferon-induced anti-viral resistance are multiple, and probably differ in different cell-virus systems. However, in vitro studies with extracts of interferon-treated cells show that the main target of interferon action is translation, which is blocked by two mechanisms, involving a protein kinase and a nuclease. In both, the block requires the presence of minute amounts of double-stranded RNA (dsRNA), which seems to signal to the cells the presence of a viral infection. (As in the induction of interferon, the dsRNA may be that of a viral replicative intermediate, or it may result from symmetric transcription of the viral DNA). Both translation blocks are therefore specific for virus-infected cells (containing dsRNA), although they do not distinguish cellular and viral messengers within such cells.
Interferons exhibit a wide variety of cell regulatory activities. They can be considered a family of hormones involved in regulation of cell growth and differentiation. The cell regulatory activity of IFN - is much greater than that of IFN - a or IFN -B . Effects of interferon on the functions of the uninfected cells:
a. Inhibition of cell replication;
b. Inhibition of activation of spleen lymphocytes;
c. Inhibition of liver regeneration;
d. Inhibition of production of platelets and leukocytes;
e. Enhancement of the expression of histocompatibility antigens on the lymphocyte surface;
f. Induction of liver degeneration;
g. Reduction of antiviral antibody production by inhibition of B-cell activity;
h. Enhancement of T-cell activity; and
i. Increase in the cytotoxic activity of natural killer cells against virus-infected cells.
These effects denote a shift from humoral to cell-mediated immunity, which has a defensive role in many viral infections, but a pathogenic role in some. The combination of cell growth inhibition and enhancement of cell-mediated immunity accounts for the antitumor effect of interferon in experimental animals.
Viral vaccination refers to the administration of virus (inactivated or live), viral protein or antibody to a virus to a patient. Neither vaccination nor recovery from natural infection always results in total protection against a later infection with the same virus - REMEMBER THAT THE ANTIGENIC DETERMINANTS OF AN ENVELOPED VIRUS CAN VARY WITH THE TYPE OF CELL IN WHICH THE VIRUS GREW. Control is achieved by limiting the multiplication of virulent virus upon subsequent exposure and preventing its spread to target organs where the pathologic damage is done.
From Medical Microbiology, 19th Edition, Jawetz, Melnick and Adelberg's, pp. 404, Table 33-7. Reproduced with permission.
Inactivated virus vaccines - Inactivated vaccines generally stimulate the development of circulating antibody against the capsid or envelope proteins of the virus, conferring some degree of resistance. However, there are disadvantages to this type of vaccine:
a. Extreme care is required in their manufacture to make certain that no residual live virulent virus is present in the vaccine;
b. The immunity conferred is often brief and directed against only external antigens (capsid, envelope, spike); c. Parenteral administration of inactivated vaccine sometimes gives limited protection because local resistance is not induced adequately at the natural port of entry or primary site of multiplication of the wild virus infection; and
d. Some inactivated virus vaccines induce hypersensitivity to subsequent infection or booster shots.
Live attenuated virus vaccines. An attenuated virus is one which has lost its virulence or pathogenicity. This is usually accomplished by passing the virus through an animal host. A vaccine made with an attenuated virus has the advantage of acting like the natural infection with regard to its effect on immunity. The viruses multiply in the host and tend to stimulate longer-lasting antibody, antibody directed against all antigens of the virus (including internal antigens), and resistance at the portal of entry. The disadvantages of live attenuated vaccines include:
a. The risk of reversion to greater virulence during multiplication within the host;
b. Unrecognized adventitious agents latently infecting the culture substrate may enter the vaccine stocks; and
c. Limited shelf life.
Recombinant vaccines. These are live virus vaccines where the virus used has been altered via genetic engineering to produce an avirulent but immune organism. This is done in one of two ways:
a. The virus normally causing a disease is attenuated by deleting one or more of the genes required for virulence. This deletion may partially inactivate the virus but it is still able to reproduce; and
b. A virus that is not pathogenic, e.g., the vaccinia virus, is altered by the insertion of a gene from a pathogenic virus. The inserted gene codes for protein (e.g., the capsid protein) which elicits an immune reaction and protects against the pathogenic virus.
Purified protein vaccines. Viral genes are cloned into plasmids. That cloned DNA is then transformed into a bacterial cell where it can be expressed if appropriate genetic engineering techniques are used. As the bacterial cell grows and reproduces, it synthesizes large quantities of the cloned protein. The protein is purified and then used as a vaccine.
DNA vaccines (gene vaccines). Viral DNA is inserted into human cells where it codes for proteins that are expressed on the surface of the human cell, thus stimulating the immune system. C. Lysogenization, latency, dormancy. Normally when a virus enters a cell, it reproduces at a rapid rate and produces large numbers of progeny virus. In rare cases, the virus enters the cell and persists in the cell with little or no detectable effect on the host cell. This is gusually called a latent or dormant infection. These latent infections are sometimes activated by systemic shock. The basis for this latency has been attributed to:
1. Lysogenization;
2. Lack of pathogenicity of the virus; and
3. Suppression of reproduction by the immune system (humoral, cell mediated, interferon).
Antiviral agents. Because of the great variability among the reproductive cycles of viruses, it has not yet been possible to develop a broad spectrum anti-viral compound. However, there are a few narrow spectrum chemotherapeutic agents approved for the treatment of viral diseases.
1. Acycloguanosine (Acyclovir) (Zovirax)
This drug is used to treat herpesvirus skin infections. It inhibits viral DNA-dependent DNA polymerase activity.
2. Adenine arabinoside (Ara A) (Vidarabine)
This drug is used to treat herpesvirus eye infections, encephalitis, neonatal herpes, and herpes skin infections. It inhibits DNA polymerase.
3. Cytosine arabinoside (Ara C)
This drug is used to treat herpesvirus eye infections. It acts as an analog of deoxy-cytidine and thus inhibits DNA synthesis.
Licensed only as an anti-tumor drug.
4. Iododeoxyuridine (Idoxuridine)
This is an analog of thymidine and works by inhibiting DNA replication and transcription. It is used topically against herpesvirus keratitis.
5. Trifluorothymidine (Trifluridine)
This is a deoxyribonucleoside analog which inhibits viral DNA replication and transcription. It is used to treat herpes simplex keratitis.
6. Foscarnet Sodium (Phosphonoformic acid)
This drug is used to treat retinitis of cyto-megalovirus etiology.
7. Ganciclovir (Cytovene)
This drug is used to treat all types of cytomegalovirus infections in AIDS patients. Because of bone marrow toxicity, its use is limited to AIDS patients. It inhibits viral DNA polymerase.
8. Amantadine
This drug is used to treat influenza A. It interferes with penetration and/or uncoating of the virus.
9. Rimantadine
This is a structural analog of amantadine. It inhibits penetration, uncoating and release of the influenza A virus.
10. Isatin- -Thiosemicarbazone (Methisazone)
This drug is used to treat smallpox (a poxvirus disease) where it acts by inhibiting translation of late mRNA.
11. N-methyl-isatin- -Thiosemicarbazone (Marburan)
This is a methylated derivative of methisazone. It inhibits the translation of late messenger RNA of the smallpox virus.
12. Alpha interferon
This compound inhibits viral protein synthesis (translation) via phosphorylation of elongation factor 2 and/or induction of a ribonuclease which degrades mRNA. It is used to treat condylomata acuminata (genital warts).
13. Ribavirin (Virazole)
This drug is used to treat Lassa fever, respiratory syncytial disease and Hanta virus respiratory/renal disease (Muerto Canyon fever). It interferes with the synthesis of viral mRNA by inhibiting DNA-dependent RNA polymerase, inosine monophosphate dehydrogenase and various viral GTP-dependent enzymes. It has the broadest spectrum of activity of any of the anti-viral compounds.
14. Azidothymidine (AZT, Zidovudine, Retrovir)
This drug is used to treat AIDS. It is a thymidine analog. It interferes with viral RNA dependent DNA polymerase (reverse transcriptase).
A chain terminator activated via phosphorylation by thymidine kinase.
15. Dideoxyinosine (ddI) (Videx)
This drug is used to treat AIDS. It is a nucleoside analog that is converted to dideoxyadenosine triphosphate to inhibit the reverse transcriptase of the human immunodeficiency virus.
16. Dideoxycytidine (ddC)
This drug is used to treat AIDS. It is a nucleoside analog that is converted to dideoxycytidine triphosphate to inhibit the reverse transcriptase of the human immuno-deficiency virus.
17. Lamivudine - 3TC (Epivir)
This drug is used to treat AIDS, but it can only be used in combination with AZT and a protease inhibitor in patients who have never been treated previously with AZT. It is a nucleoside analog.
18. Saquinavir (Invirase) (Hoffman - LaRoche)
This drug is used to treat AIDS, but it can only be used in combination with AZT and Lamivudine - 3TC in patients who have never been treated previously with AZT. It is a protease inhibitor.
19. Ritonavir (Norvir (Abbott)
This drug is used to treat AIDS, but it can only be used in combination with AZT and Lamivudine - 3TC in patients who have never been treated previously with AZT. It is a protease inhibitor. 20. Indinavir (Crixivan) (Merck)
This drug is used to treat AIDS, but it can only be used in combination with AZT and Lamivudine - 3TC in patients who have never been treated previously with AZT. It is a protease inhibitor.
21. Viracept (Agouron Pharmaceuticals)
This drug is used to treat aids, but it can only be used on combination with AZT and Lamivudine - 3TC in patients who have never been previously treated with AZT. It is the only protease inhibitor approved for children as well as for adults.
1. When human cells become infected by a virus, they may produce interferon. Interferon is a class of glycoproteins which interfere with viral replication by inducing the synthesis of a translation inhibitory protein and a nuclease.
2. The induction of interferon is mediated by double-stranded RNA.
3. In uninfected cells interferons act like a family of hormones involved in regulation of cell growth and differentiation where they often effect a shift from humoral to cell-mediated immunity.
4. Effective vaccination for viral disease can be achieved using live viruses, inactivated viruses, viral antigens or antibody directed against viruses.
5. Pediatric patients are routinely immunized against the viral diseases of polio, measles (rubeola), mumps and rubella. Several other viral vaccines are available for special situations.
6. In general the live attenuated viral vaccines induce the broadest range of immunity (i.e., stimulate IgA, IgG and IgM production) and give the longest acting immunity.
7. Some viruses have the ability to be dormant or latent in a cell during which time they do not reproduce themselves. Most commonly they do this by integrating their DNA into the host cell DNA.
8. The antiviral agents currently available are narrow spectrum compounds, most of which are analogs of the purine or pyrimidine bases of DNA or RNA.
9. The antiviral drugs and the diseases they are effective against are:
A. Herpesvirus infections:
Acycloguanosine - herpesvirus skin infections
Adenine arabinoside - herpesvirus eye infections, skin infections, CNS infections, and neonatal herpes
Cytosine arabinoside - herpesvirus eye infections
Iododeoxyuridine - herpesvirus eye infections T
rifluorothymidine - herpesvirus keratitis
B. Cytomegalovirus infection
Foscarnet sodium - cytomegalovirus retinitis
Ganciclovir - cytomegalovirus retinitis
C. Influenza A virus infection
Amantadine
Rimantadine
D. Smallpox virus infection
Isatin-B -Thiosemicarbazone (Methisazone)
N-methyl-isatin- -Thiosemicarbazone (Marburan)
E. Papilloma virus infection (genital warts)
Alpha interferon
F. Lassa fever virus, respiratory syncytial disease virus, Muerto Canyon fever virus
Ribavirin
G. Human immunodeficiency virus (AIDS)
Azidothymidine (AZT)
Dideoxyinosine (ddI)
Dideoxycytosine (ddC)
Lamivudine - 3TC
Saquinavir
Ritonavir
Indinavir
Viracept
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