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by �rjan
Stranneg�rd, Department of Clinical Virology, G�teborg University,
Sweden
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Interferon (IFN) was originally described more than 40 years ago
by Isaacs and Lindeman as a substance that is produced upon
stimulation of cells by viruses, and that has the ability to
protect cells from infection with viruses of any kind, that is to
interfere with viral infections. The antiviral activity of IFN was
thus found to be non-specific, a fact that led to the idea that IFN
might be used therapeutically against all kinds of viral
infections. The revolution that antibiotics had meant for bacterial
infections, it was reasoned, might be paralleled by therapeutic use
of IFN in viral infections
The optimism regarding the potential of IFN as an antiviral
therapeutic agent has not been fulfilled and for various reasons it
is not until during the last decade that IFN has been established
as a potent antiviral agent in chronic viral infections. In
parallel with being the object of antiviral research, however, IFN
has also been studied with regard to its anti-tumour properties,
and it is today becoming a standard treatment in certain malignant
diseases.
Different types of
interferons
Although IFN was initially thought to be a single entity, later
research has shown that there are multiple molecular species of
IFN. Thus, there are three main classes of human IFN:s called
alpha, beta and gamma interferons (IFN-alpha, IFN-beta and
IFNgamma) and a minor class called omega-IFN (IFN-omega). There are
13 genes, two of which are identical, for IFN-alpha, of which there
are thus 12 subtypes, but only one gene, and no subtypes for each
of IFN-beta and IFNgamma. IFN-alpha subtypes consist of 165 or 166,
IFN-beta of 165 and IFN-gamma of 142 amino acid residues. IFN-alpha
and IFN-beta were formerly called type I interferons and IFN-gamma
type II or immune IFN.
The reason why there are so many subtypes of IFN-alpha remains
enigmatic. However, the various subtypes of IFN-alpha vary markedly
regarding their biological activities. Thus, for instance, the most
pronounced antiviral activity on a molar basis is found in
IFN-alpha8, and IFNalpha1 has certain immunological activities that
are absent among other subtypes. It therefore seems plausible that
the different IFN-alpha subtypes are indeed separate cytokines
which share some activities, notably the antiviral capacity, but
otherwise have different functional profiles.
Cellular origin and
production of interferons
Although most cells in the body are capable of producing IFN:s,
the different classes of IFN:s are preferentially produced by
certain cell types. Thus, IFN-alpha subtypes are preferentially
produced by monocyte/macrophages and special ?natural
interferon-producing? cells that have characteristics in common
with natural killer (NK) cells. IFN-beta is preferentially produced
by fibroblasts and IFN-gamma by T cells and NK cells.
IFN:s are part of the innate immunological defence and are
produced after introduction in to the body of foreign substances.
In particular, viruses have the ability to evoke production of
IFN:s but also bacteria, fungi and other ?non-self? agents may
induce IFN production. IFN-alpha and IFN-beta are very rapidly
produced upon stimulation whereas IFN-gamma, which is a major
constituent of the antigen-specific T cell response, may be
produced at a somewhat later stage of immune responses. However,
since NK cells, which are not induced in an antigen-specific
manner, also produce IFN-gamma, the formation of this cytokine may
also be an early event following stimulation with infectious
agents.
Mechanisms of
action
After induction of IFN:s they react with cells that possess
specific receptors for the various IFN:s. IFN-alpha and IFN-beta
react with the same receptor, which, however, is completely
different from the IFN-gamma receptor. Following this interaction a
complex series of signal transduction events takes place,
resulting, in the end, in the production of a multitude of proteins
with different actions. Some of these have the ability to induce
antiviral states, others have antiproliferative effects and still
others have a variety of immunological effects. Together, all these
proteins help to defend the host against various intruders and,
also, to suppress the growth of cells with an exaggerated growth
potential, such as cancer cells, that arise in the body.
The immunological effects of IFN:s are particularly pronounced
regarding IFN-gamma. This cytokine is an important product of the
so called T helper type 1 (Th1) cells which are the major effector
cells of the cell-mediated immune system. IFN-gamma, that is
produced by Th1 cells or NK cells, can induce the expression of
both class I and class II histocompatibility (HLA) antigens on
various cells and thereby stimulate immune responses. IFN-alpha and
-beta can induce class I but not class II antigens. All interferons
have the ability to stimulate cellular cytotoxity, evoked by
cytotoxic T cells, monocytes or NK cells. Together, these actions
make interferons powerful immunological modulators. The capacity to
stimulate immune responses may explain the fact that autoimmune
disorders may be worsened and in certain cases possibly induced
during IFN therapy.
Association of
interferons with diseases
In acute viral infections there is usually a rapid IFN response
with appearance of sometimes large amounts of IFN-alpha in the
blood. This response, however, is short-lived and the IFN
disappears from the circulation within a few hours or days. In
chronic viral infections there may be a persistent IFN response.
Thus, in e.g. HIV infection, IFN-alpha is readily demonstrable in
serum in late stages of the disease at levels that increase with
progressing disease.
Autoimmune diseases, like systemic lupus erythematosus, are
frequently associated with a persistent occurrence of IFN-alpha in
the circulation. Since IFN-alpha has immunostimulatory properties
and autoantibodies, and sometimes even autoimmune disease, may
appear during IFN therapy, it seems possible that IFN-alpha could
have a causative role in autoimmune diseases. However, the
cause-and-effect relationship is in this case difficult to assess
and the pathogenetic mechanisms in autoimmune disease are varying
and not fully clarified.
Large scale
production of interferon
In order to use IFN:s for therapeutic purposes, methods to
produce large amounts of the substances are required. Early trials
with IFN utilised semi-purified preparations of IFN-alpha obtained
from human cultures of leukocytes that had been stimulated with a
paramyxovirus (Sendai virus). After the cloning of IFN genes it
became possible to use DNA recombinant techniques to produce large
amounts of purified IFN:s in an inexpensive way. Using another line
of investigation it was demonstrated that certain lines of
lymphoblastoid cells constitutively produced large amounts of
IFN-alpha, and these could thus be used for the production of large
amounts of IFN.
Presently, there are the following three main types of IFN-alpha
preparations that are commercially available.
- Recombinant IFN-alpha2 preparations, which
dominate the market are available in different forms of which
IFN-alpha2b (Intron A, Schering-Plough) and IFN-alpha2a (Roferon,
Roche) have undergone the most extensive clinical trials and are
the most widely used.
- Lymphoblastoid IFN-alpha (e.g. Wellferon,
Glaxo-Wellcome) contains a variety of IFN-alpha subtypes and is
produced by lymphoblastoid cells that are grown in large
tanks.
- Leukocyte or natural IFN-alpha is produced
by buffy coat cells derived from blood donors and stimulated by
Sendai virus. Although highly purified natural IFN-alpha
preparations are available (e.g. IFN-alphaN3, Interferon Sciences,
USA, or Interferon Alfanative, Bionative, Sweden) these
preparations are generally less well studied and the full potential
of their therapeutic use has not been established as yet.
IFN-beta has been available in a form produced by cultural human
fibroblasts but today mostly recombinant forms, with slightly
varying amino acid sequences are used for therapeutic purposes.
IFN-gamma is available only as a recombinant substance.
It is sometimes claimed that natural or lymphoblastoid IFN-alpha
preparations should be expected to be therapeutically superior to
recombinant proportions for two reasons; firstly because of their
contents of multiple subtypes of IFN-alpha and secondly because of
the fact that recombinant IFN:s produced by bacteria are not
properly glycosophated in comparison with IFN:s produced by human
cells. Since only very few comparative studies have been performed
regarding recombinant vs. lymphoblastoid IFN-alpha or recombinant
vs. natural IFN-alpha, it is not possible, at present to draw any
firm conclusions regarding this issue. However, drug resistance may
sometimes develop following therapy with recombinant IFN-alpha, due
to the formation of antibodies to the particular subtype contained
in the preparation. In these cases lymphoblastoid or, in
particular, natural IFN-alpha may offer a therapeutic
advantage.
Therapeutic use of
interferons
Side effects
Interferons are substances that are produced in the body and
have potent biological actions. Many of the symptoms associated
with acute viral infections are apparently caused by interferons
that are produced in large quantities during the infection, since
the symptoms can be reproduced after parenteral administration of
exogenous interferon. The common side effects of IFN treatment i.e.
the ?influenza-like? symptoms fever, chills, nausea fatigue,
myalgia and loss of appetite are thus expected events that occur in
most subjects, with a severity that depend on the dosage used.
These types of side effects usually show a tendency to be less
severe with time and are usually tolerable. Other side effects
include mental depression which will prompt discontinuation of
treatment, alopecia and weight loss. These side effects are,
however uncommon, as is also appearance of thyroid dysfunction. The
latter, that is associated with appearance of thyroid
autoantibodies, usually disappears after cessation of IFN-alpha
therapy. The risk for autoimmunity appears to be grater following
treatment with IFN-gamma than with IFN-alpha, however, and
IFN-gamma treatment has even been described to be associated with
development of systemic herpes erythematodes.
Use of IFN-alpha in
viral diseases
Acute viral infections
Several trials have been performed in acute viral infections
most notably in upper respiratory infections. Since the peak of
viral replication, and also the peak of the body?s own interferon
response usually occurs simultaneously with or before the
appearance of symptoms, therapy with exogenous interferon at this
stage is not likely to markedly change the course of the infection.
In accordance with this notion topical application of IFN-alpha in
e.g. viral upper respiratory infections has not been considered to
be worthwhile. In very severe acute infections, such as Lassa
fever, IFN-alpha therapy has been advocated and in some cases
claimed to be successful.
Although IFN is usually relatively ineffective in acute viral
infections it has been successfully used for the prevention of
respiratory infections in exposed individuals. The side effects
following intranasal application of IFN-alpha, such as nose
bleeding, however, have precluded its general use as a preventive
measure, and likewise, the side effects appearing after parenteral
administration of IFN-alpha are usually considered to outweigh the
potential benefits of preventive use of IFN:s.
Persistent viral
infection
In contrast to acute infections, chronic viral infections are
often amenable to IFN therapy. Among these hepatitis C virus (HCV)
and hepatitis B virus (HBV) infections are the most important and
IFN-alpha is currently a standard treatment in HCV and HBV
infections.
Large clinical trials have been performed using various types
and dosages of IFN-alpha in HCV infections. The trials have been
monitored using two types of markers, i.e. serum transaminases and
HCV-RNA, the latter being a measure of the virus itself.
Quantitative measurements have shown that IFN-alpha therapy results
in a rapid initial decline of HCV serum levels followed by a phase
of more slowly decreasing levels, that eventually leads to total
disappearance of HCV-RNA after several weeks or months of continued
treatment in responding individuals. Even after complete
virological (disappearance of HCV-RNA) and biochemical
(normalisation of liver transaminases) responses, however, there is
frequently a relapse of the infection following cessation of
treatment.
Using the initially tried dose of 3 million units of IFN-alpha
three times a week for a period of 6 months, there was usually a
response rate of 40-50% at end of treatment but at follow-up 6
months later about half of the initially responding patients had
relapsed. Later studies have shown that it is possible to increase
the rate of sustained response significantly by increasing the time
of treatment to 12 months and/or by using an initial, so called
induction, treatment with daily administration of higher doses of
IFN-alpha.
Recently the standard treatment for HCV infection has been
changed to include Ribavirin as an addition to IFN-alpha. As
compared to 6 month monotherapy with IFN-alpha the sustained
response rates using the combination therapy have been found to
generally be at least twice as high i.e. 40-50%. Combination
therapy is recommended in particular for patients with a high viral
load (i.e. more than 2-3 million HCV-RNA copies per ml) and for
those who have relapsed following IFN-alpha monotherapy. It is also
claimed that patients harbouring certain genotypes of HCV that are
considered difficult to treat (notably genotype 1b) should undergo
primary treatment with the combination of IFN-alpha +
Ribavirin.
Although the presently used therapeutic regimen�s in HCV
infection frequently fail to eradicate the virus, they may still be
worthwhile. Several studies, most notably one from Japan, have now
suggested that the risk of developing hepatocellular cancer may be
considerably reduced by IFN treatment, even in cases of persistent
infection.
HBV infections may be treated with IFN-alpha. A multitude of
studies have showed that treatment with IFN-alpha results in an
average response of about 40% measured as a change from e-antigen
positivity to anti-e positivity (seroconversion) or disappearance
of detectable HBV-DNA from the blood. The patients that respond
best to treatment are those that have an ongoing immunological
response manifested as raised aminotransferase levels and moderate
amounts of HBV-DNA in their blood. Generally, it is considered that
patients in other stages of the disease, such as in the
immunotolerant phase or in late cirrhotic stage respond less well
or are resistant to IFN-alpha treatment.
The doses used in HBV infection are usually higher than those
generally used in HCV infection and the duration of treatment is
usually shorter. Similar response rates, i.e. 40% or higher, may be
obtained in children.
During recent years new drugs, in particular nucleoside
analogues like Lamivudine or acyclovir-like drugs such as
Famciclovir have been found to have profound effects on the
replication of HBV. However, the effects of such drugs are usually
not long-lasting. Thus, although HBV-DNA has been found to rapidly
decrease following institution of antiviral therapy it has promptly
reappeared in the circulation following cessation of treatment. The
difference between IFN-alpha and the antiviral drugs in this
respect has been interpreted to indicate that the effect of
IFN-alpha in HBV-infection is primarily immunomodulatory rather
than antiviral. In order to obtain a persistent effect on the
hepatitis an immunological action, resulting in a seroconversion
(from e- to an-e- positivity) would be necessary. Therefore, trials
have been made with a combination of IFN-alpha and antiviral drugs.
Future therapy in hepatitis B may possibly, similarly as in
hepatitis C, be based on combination therapy.
Hepatitis B and C are the only viral infections where,
presently, IFN-alpha is used as a standard treatment. Clear effects
on the replication in vivo of HIV-1 have been obtained but since
the effect of IFN-alpha in HIV infection generally are inferior to
those of currently used nucleoside analogues and protease
inhibitors, IFN:s have not become a part of the standard treatment
in this disease. In HIV-infection complicated by Kaposi�s sarcoma,
however, IFN-alpha may be used therapeutically. Advantage is in
this case taken of both the antitumoral and antiviral and perhaps
also of the immunological effects of IFN-alpha.
For similar reasons as in Kaposi�s sarcoma IFN-alpha can be used
for treatment of papilloma virus infections, both in cases of
larynx papilloma and in cases of skin or genital warts. In larynx
papilloma, however, a permanent cure is difficult to achieve with
this treatment and intralesional injection of IFN-alpha into warts,
which is the drug administration of choice is cumbersome and
frequently only partially effective.
Interferon-beta in
multiple sclerosis
Although multiple sclerosis (MS) is considered to be an
immunological disease, it has several traits that suggest that the
causal agent may be a virus. Early trials with interferons revealed
that IFN-gamma may aggravate, and IFN-beta ameliorate disease
symptoms. This was taken to indicate that the beneficial effect of
IFN-beta are due to the immunomodulatory effects of this substance.
It is certainly possible, however, that the antiviral property of
IFN-beta may contribute to the therapeutic effect if viruses are
indeed involve in the pathogenesis of MS.
Recent large clinical studies have convincingly demonstrated the
beneficial effect of two types of recombinant IFN-beta, the
glycosylated IFN-beta1a and the non-glycosylated IFN-beta1b
carrying one amino acid deletion. Both of these have been found to
decrease relapse rates and cause reduction of lesions, detectable
by magnetic resonance. Also the progression of neurological
disability has been shown in some studies.
The obvious beneficial effects of IFN-beta in MS might be
paralleled by similar effects of IFN-alpha. This, however, has not
been sufficiently well explored and presently, IFN-alpha is not
recommended for use in MS.
Therapeutic use of
IFN-gamma
IFN-gamma is a potent stimulator of cell-mediated (Th1)
immunity. As a consequence of this it may have detrimental effects
on Th1-cell mediated autoimmune diseases, such as MS or type 1
diabetes mellitus. On the other hand, any disease that is
associated with defective cell-mediated immunity may be a potential
target for IFN-gamma therapy. The most prominent indication for
IFN-gamma, and the one that has been considered to justify approval
by medical products agencies in many countries, is chronic
granulomatous disease that is associated with a severe lack of IFN
production. Good results with IFN-gamma therapy have been obtained
e.g. in severe mycobacterial infections and mycoses. IFN-gamma has
also been tried in atopic dermatitis and in rheumatoid arthritis,
but the results have not been uniformly convincing
Interferon therapy in
malignant diseases
Studies of IFN therapy in tumors have in the majority of cases
been conducted using various preparations of IFN-alpha. Early
studies in osteosarcoma gave promising results and showed that
rather crude preparations of leukocyte (natural) IFN-alpha at doses
of 3 million units 3 times a week could be given for months-years
without causing intolerable side effects. Similarly, the early
trials showed that larynx papilloma could be successfully treated
with these preparations although relapses after cessation of
treatment were common.
With the advent of recombinant DNA technology it became possible
to produce large quantities of IFN and trials in various malignant
diseases were greatly extended. Excellent results using IFN-alpha
therapy have then been obtained particularly in hairy cell leukemia
and in chronic myelogeneous leukemia. To the list of diseases that
constitute indications for IFN-alpha treatment have in many
countries been added multiple myeloma, carcinoid tumors, follicular
lymphoma, polycythemia vera and malignant melanoma. There are other
drugs for treatment of most of these diseases and combinations of
IFN-alpha with such drugs have in many cases proved
advantageous.
Generally the doses of IFN-alpha given in malignant diseases are
much higher than used in viral diseases, but the duration of
treatment is shorter. High doses are chosen because it is believed
that the action of IFN-alpha is that of an antiproliferative,
cytostatic, drug. Although this may be true in most tumors it is
also possible that immunologic effects, such as stimulation of
cytotoxic cells, may plan an important role in certain tumors, and
lower doses of IFN-alpha may then possibly be required to obtain
good effects.
An example of the use of lower doses of IFN-alpha is the
currently performed clinical studies of triple therapy with
low-dose IFN-alpha, interleukin-2 and histamine in malignant
melanoma. This disease is poorly sensitive to treatment with
IFN-alpha alone but using the mentioned combination of drugs, which
is designed to evoke a maximal stimulation of natural killer (NK)
cells, a very significant effect on the survival can be
achieved.
IFN-alpha has been used as adjuvant therapy, i.e. after surgical
removal of primary tumors to prevent development of metastases.
Significant effects on the time to relapse and on overall survival
of recombinant IFN-alpha have been achieved in malignant melanoma.
Generally, however, the side effects associated with the high dose
regimen are severe. Several studies are presently performed in
order to explore the possibilities of using lower doses of
recombinant or natural IFN-alpha for adjuvant therapy in malignant
melanoma.
Achievements made during recent years indicate that IFN-alpha
should be regarded as an important part of the therapeutic arsenal
in malignant diseases. It seems likely that during the coming years
the indications for therapy with IFN-alpha, alone and in
combination with a variety of other drugs will be extended to
comprise a large variety of malignant diseases.
References
- Kirkwood JM. Systemic adjuvant treatment of high-risk melanoma;
the role of interferon alfa-2b and other immunotherapies. Eur J
Cancer 1998; 34 suppl 3: 12-17
- De Mayer E and De Mayer-Guignard J. Interferons; in The
cytokine handbook, ed. A Thomson Academic Press, San Diego 1998, pp
491-516
- Murray JA. Interferon therapy for hepatitis B and C. Postgrad
Med 1998; 104: 25-28
- Okanne T, Itok Y, Minami M et al. Interferon therapy lowers the
rate of progression to hepatocellular carcinoma in chronic
hepatitis C but not significantly in advanced stage: a
retrospective study in 1148 patients. Viral Hepatitis Study Group.
J Hepatol 1999; 30: 653-659
- Vilcek J, Sen GC. Interferons and other cytokines. Fields
Virology, 3rd edition ed. B.N. Fields et al, Lippincott-Roven
Publishers, Philadelphia 1996, pp 375-399
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