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| BACTERIOLOGY | IMMUNOLOGY | MYCOLOGY | PARASITOLOGY | VIROLOGY | |
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Interferon response to an acute virus infection |
Interferons play an important role in the first line of defense against viral infections. They are part of the non-specific immune system and are induced at an early stage in viral infection – before the specific immune system has had time to respond. Interferons are made by cells in
response to an appropriate stimulus, and are released into the surrounding
medium; they then bind to receptors on target cells and induce transcription of
approximately 20-30 genes in the target cells, and this results in an
anti-viral state in the target cells. TYPES OF
INTERFERON
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Effects of interferon |
INTERFERON-alpha
AND INTERFERON-beta (TYPE I INTERFERONS) These interferons induce about
20-30 proteins, and the function of many of these is not fully understood.
However, three of the proteins that appear to play an important role in the
induction of the anti-viral state have been intensively studied. Expression of
one of these proteins (2’5’ oligo A synthase) results in activation of the
second of these proteins (a ribonuclease) which can break down mRNA, and
expression of the third protein (a protein kinase) results in inhibition of the
initiation step of protein synthesis. These activities target viral protein
synthesis, but also result in inhibition of host protein synthesis. Thus
it is important that these proteins are only made and activated when needed. Interferon treatment induces the synthesis of the inactive form of these proteins in the target cell. Double-stranded RNA is needed for activation of these proteins. It directly activates 2’5’ oligo A synthase and protein kinase R, and indirectly activates ribonuclease L (since this needs 2'5'oligo A, the product of 2’5’ oligo A synthase, for activation). Thus, these potentially toxic pathways are only activated in the interferon-treated cell if double-stranded RNA is made, this will usually only happen if virus infection actually occurs. The activation of these proteins may sometimes result in the death of the cell, but at least the progress of the infection is prevented.
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OTHER EFFECTS OF INTERFERONS The pathway described above is by no means the only way
that interferons protect cells against viruses and other pathogens. All three interferons increase
expression of class I MHC molecules and thus promote recognition by cytotoxic T
cells. All three interferons can activate NK cells which can then kill
virus-infected cells. Interferon-gamma
increases expression of class II MHC molecules on antigen-presenting cells and
thus promotes presentation of antigens to helper T cells. Interferon-gamma
can also activate the ability of macrophages to resist viral infection
(intrinsic antiviral activity) and to kill other cells if they are infected
(extrinsic antiviral activity). Interferons have many other
effects on gene expression, not all of which are understood.
THERAPEUTIC USES OF INTERFERONS Interferons-alpha and -beta have been used to treat various viral infections. One currently approved use for various types of interferon-a is in the treatment of certain cases of acute and chronic hepatitis C and chronic hepatitis B. Interferon-gamma has been used to treat a variety of disease in which macrophage activation might play an important role in recovery, eg. lepromatous leprosy, leishmaniasis, toxoplasmosis. Since interferons have anti-proliferative effects, they have also been used to treat certain tumors such as melanoma and Kaposi’s sarcoma. |
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SIDE EFFECTS OF INTERFERONS Common side effects of interferons
High levels of interferons can cause kidney, liver, bone marrow and heart toxicity. VIRAL DEFENSES AGAINST THE NON-SPECIFIC AND SPECIFIC IMMUNE SYSTEMS Not surprisingly, some viruses have developed defenses against the interferon-induced antiviral response and other aspects of the immune defense system. For example, viruses may code for proteins which block interferon binding to cells, inhibit the action of the interferon-induced protein kinase, inhibit NK function, interfere with cell surface expression of MHC, block complement activation, prevent the host cell committing apoptosis, etc.
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