There are 3 classes of interferons: alpha, beta and gamma. Interferon-a exists as at least 15 subtypes, the genes for which shows 85% homology. IFN b1 shows 30% homology with IFNa. IFNb2 is now known as IL-6 and shows no homology with alpha or b1 types. IFN gamma is also a lymphokine and shows no homology with the other types. IFNs mediate their actions through specific receptors at hormone like concentrations. Interferon inducible response elements in the cellular genome are activated. There are 2 main types of IFN receptors, one for alpha and beta1 and the other for gamma.
IFNs are released form many cell types in response to virus infection, dsRNA, endotoxin, mitogenic and antigenic stimuli. DsRNA appears to be a particularly important inducer. Usually, good IFN inducers are viruses that multiply slowly and do not block the synthesis of host protein early or markedly damage the cells. IFN is usually assayed by determining its effect on the multiplication of a test virus, usually vesicular stomatitis virus, a rhabdovirus. Viral strains capable of high IFN production give rise to autointerference in endpoint assays. In general IFN gamma differs from the others in that it is released as a lymphokine from activated T-cells and occasionally from macrophages.
1. Mechanism of Action
The antiviral effects of IFNs are exerted through several pathways;-
(1) Increased expression of Class I and Class II MHC glycoproteins, thereby facilitating the recognition of viral antigens by the immune system.
(2) Immunoregulatory effects - activation of cells with the ability to destroy virus-infected targets; these include NK cells and macrophages. IFNs appear to drive a shift from humoral to cellular immunity.
(3) Direct inhibition of viral replication: several mechanisms contribute to the third pathway.
production of specific inhibitory proteins eg. the Mx protein which has specific anti-influenza action. It is likely that more specific inhibitory proteins will be identified.
inhibition of viral processes such as penetration, uncoating and budding from infected cells have been reported.
in vitro studies with extracts of IFN-treated cells show that the main target of IFN action is translation, which is blocked by 2 mechanisms, both requiring the presence of minute amounts of dsRNA;-
(i). activation of a dsRNA dependent protein kinase - this phosphorylates and inactivates the translation initiation factor eIF-2. The phosphorylation freezes the initiation complex formed by eIF-2, GTP and met-tRNAf with the small ribosomal subunit and mRNA. Because eIF-2 cannot be recycled, protein synthesis is inhibited or stopped.
(ii). activation of 2-5 oligo A synthetases ® synthesis of 2-5A ® activates endonuclease L (itself induced by IFN) ® degradation of mRNA ® inhibition of protein synthesis.
The combination of cell growth inhibition and enhancement of CMI accounts for the antitumour effect of IFN.
2. Protective role in virus infections
A protective role of IFN in animals is suggested by many observations;-
In mice recovering from influenza virus infection, the titre of IFN is maximal at the time when the virus titre begins to decrease and before a rise in Abs can be detected. At this stage, the IFN titre in the animal is sufficient to protect them against the lethal action of a togavirus.
Administration of a potent antiserum to IFN markedly increases the lethality of mouse hepatitis virus infection.
Suckling mice, which are susceptible to coxsackievirus B1, produces little IFN in response to this virus, whereas adult mice, which are resistant, produces large amounts.
These studies suggest that IFN has a major protective role in at least some viral infections. Much depends on the dynamics of the disease.
3. Possible therapeutic use
Clinically, effective prophylaxis was demonstrated against rhinovirus infection of human volunteers, with decreased incidence of infection and reduction of symptoms. Contacts of infected patients can be protected by intranasal spray of large doses of IFN. Also reduced is CMV reactivation in seropositive patients undergoing kidney transplants. IFNs could theoretically be ideal antiviral agents, since they act on many different viruses and have high activity. However, their therapeutic value is limited by various factors: IFNs are effective only during relatively short periods and have no effect on viral synthesis that is already initiated in a cell. Moreover, at high doses they have serious toxic effects on the host.
Attempts to use exogenous IFN for the treatment of human viral
diseases had been met with limited success. IFN-a had prophylactic effect against influenza
infection during epidemics, and local administration lessens the
severity of respiratory diseases, IFN had also reported to be
successful in treating genital warts and juvenile laryngeal
papillomatosis. More recently, synthetic alpha-interferon was
licensed for use in the treatment of hepatitis B carriers and it
is also being used for the treatment of hepatitis C carriers with
chronic active hepatitis.
4. Interferon Therapy for Chronic HBV Carriers
It has been postulated that chronic carriage of HBV is due mainly to inadequate production of interferon and the failure of the body to respond to interferon in the presence of acute HBV infection. Trials were conducted where a 16 week course of a -interferon is given to e +ve carriers. The results were very encouraging and a -interferon is now licensed for the treatment of chronic hepatitis B. Many patients lose their e Ag and go on to develop anti-e and eventually to lose their HBsAg and to develop anti-HBs. HBV viral DNA becomes undetectable. The reason why e +ve carriers are chosen is that interferon will not work once that the viral DNA is integrated. A cutoff period of 2 years after the initial infection by HBV is chosen for e +ve carriers. It is thought that after 2 years, HBV DNA is very likely to be integrated. The current criteria for treatment
The patient should be e +ve
The patient should have detectable HBV DNA
The patient should have chronic active hepatitis
The patient should not have been e +ve for more than 2 years, although children who have been e +ve for years should be considered for inclusion.
The 16 week course costs US$3000. Suitable subjects for receiving interferon therapy include congenitally infected babies and young children, women in child bearing ages and people with chronic active hepatitis. The response rate to IFN is higher in patients who have endogenous immune attack on infected liver cells. (manifested by elevation in SGPT level). Most oriental patients are infected perinatally and have some degree of immune tolerance to HBV and are unable to react against the virus. These patients, particularly children, have inactive liver disease, are free of symptoms, and have normal SGPT levels despite active viral replication. IFN has very little effect on these patients. The response rate for IFN therapy is as follows;-
1. Active viral replication
a. Immune reaction against the virus (elevated SGPT), response rate: 30 to 40%
b. No immune reaction (normal SGPT), response rate: 5 to 10%
2. Inactive viral replication: IFN therapy is not indicated
Therefore, in general IFN therapy should only be given to those patients with active viral replication (HBeAg+, HBV DNA+) and persistently elevated SGPT. It should not be given to patients with cirrhosis as the side effects of IFN may exacerbate the situation. A successful response is defined as (1). seroconversion from HBeAg+ to HBeAg-, (2). disappearance of HBV DNA from serum, and (3) normalisation of SGPT level. Most patients remain HBsAg+ and it is thought that responders are at lower risk of developing cirrhosis and HCC, although longer follow-up study is required.
The demonstration that IFN can eliminate active HBV replication raises the possibility that chronic HBV carriage may be secondary to a -interferon deficiency. a -interferon is rarely detected in the circulation of chronic hepatitis B. The terminal protein of the polymerase gene of HBV stopped cells from responding to interferon. The terminal protein appears to stop the E protein from activating the interferon cascade. Some non-responders to IFN have increased expression of TP, wheras responders have low expression levels of TP. The high incidence of chronic HBV carriage in babies born to HBeAg +ve mothers suggests that circulating e Ag in the mother induces immunotolerance in the baby.
Other antiviral agents are being investigated for the treatment of chronic hepatitis B infection. Those that had shown promise include lamivudine and famciclovir. Both can reduce serum HBV-DNA to levels below 1 pg/ml within weeks. These drugs have fewer side effects than interferon and thus may be particularly useful in severely ill patients such as bone marrow transplant recipients. Lamivudine has now been licensed for the treatment of chronic hepatitis B. Clinical trials with lamivudine as monotherapy have demonstrated that lamivudine causes a significant reduction in HBV DNA, enhances HbeAg seroconversion, and reduces progression of fibrosis. There are almost no side effects. However, YMDD drug resistant mutants begin to emerge after 36 weeks of therapy. So that at the end of the first year, 15% of strains are resistant, and at the end of the second year, 35% are resistant. However, it appears that the mutant virus is less replicative-competent. Another problem with lamivudine therapy is the tendency for patients to relapse after the discontinuation of treatment.
5. Interferon therapy of Hepatitis C Carriers
Early studies indicate that interferon and ribavirin are effective of cases of acute and chronic hepatitis C. A combination of interferon and ribavirin may be useful. There is more experience in the use of interferon for the treatment of hepatitis C. The current recommendation is that interferon treatment may be considered in those with chronic active hepatitis who are at risk of progression to cirrhosis and HCC. The recommended regimen is 3 MU tds sc or im for 6 months. The response rate is around 50%. However, approximately 50% of responders relapse upon cessation of treatment. At present, it is not clear what factors predict response to interferon therapy. There is some data to suggest that older patients and those with established cirrhosis respond less well. There is growing evidence that the genotype of the infecting HCV determines the response to IFN. Most responders will have significant reduction of SGPT level within 2 months of interferon therapy. One may try a higher dose such as 5 or 10 MU in non-responders although it is not certain whether the higher doses work. At present, it is not clear what factors predict relapse after treatment. For those who relapse after treatment, they may be offered a second course and then put on maintenance therapy for 6 to 12 months. There is data to suggest that combination therapy with interferon and ribavirin is more effective than interferon alone. In fact, a pharmaceutical preparation of both these agents together is available for this purpose.