Interferons and Viral Infections

Interferons

Interferons provide a frontline defense against viral infection.

"Interferons are cytokines that can induce an anti-viral state"

Infected cells produce interferons which are released and bind to receptors on uninfected cells. The binding of these receptors sends a signal to the uninfected cell’s nucleus to turn on some genes and turn other genes off, to put the cell in an anti-viral mode that will resist infection

a and b IFNs have three major functions: a) induce the resistance to viral replication by activating cellular genes that destroy mRNA and inhibit the translation of viral and host proteins, b) induce MHC 1 expression in uninfected cells thus Ý the resistance to NK cells and Ý the susceptibility of newly-infected cells to killing by CD-8 cytotoxic cells, c) activate NK cells, which selectively kill virus-infected cells

Classification of IFN’s:

Family	Chromosome	types	# of amino acids	% similarity
TYPE 1 a	9	>14	166	75-85%
b	9	1	166	30%
W	9	1	172	50-70%
TYPE 2 g	12		143	1%

IFN g is essentially unrelated to the other IFN’s

Type 1 and 2 IFN’s use different cell surface receptors but have partially overlapping signal transduction mechanisms.

Interferon Induction and Transduction

Inducers of Interferons

Double stranded RNA (dsRNA) will induce IFN a and b in virus infected cells.

Bacterial LPS will stimulate IFN a and b in monocytes and macrophages

IFN g synthesis is restricted to T-cells, large granular lymphocytes, and NK cells and is stimulated by mitogens and antigens.

How IFN’s relay the signal from outside to inside the cell

IFN receptors are made up of two polypeptide chains

IFN a /b receptor has IFNAR1 and IFNAR2

IFN g receptor has IFNGR1 and IFNGR2

Both types of receptors have JAKs (Janus kinases) associated with them.

a /b receptor (type 1)

Tyk2 (a JAK) on IFNAR1
JAK 1 on IFNAR2

g receptor (type 2)

JAK 1 on IFNGR1
JAK 2 on IFNGR2

When the receptors are bound they trans-phosphorylate and activate JAK kinases. The JAK kinases then go onto phosphorylate STAT’s (Signal Tranducers and Activators of Transcription). The STAT’s dimerize once phosphorylated, and are translocated to the nucleus where they bind upstream of IFN Stimulated Genes (ISG’s) and stimulate their transcription.

The g pathway: Type 2 receptors form a homo-dimer of STAT1.

STAT1 homo-dimers turn on GAS (gamma activated sequence) elements.

The a /b pathway: Type 1 receptors form STAT1 and STAT2 heterodimers.

Type 1 IFN receptors are pre-associated with JAK’s and STAT’s.

STAT1-STAT2 heterodimers can be associated with another transcription factor p48 and form an IFN-stimulated gene factor 3 (ISGF3) which then binds to IFN-stimulated regulatory elements (ISREs)

Antiviral Mechanisms of Interferon Action

PKR a dsRNA dependent serine/threonine protein kinase. Inhibits proteins synthesis so viruses cannot make their proteins. The activity of PKR is due to its autophosphorylation in response to viral dsRNA, followed by phosphorylation of the protein synthesis initiation factor eIF-2a , causing an inactive complex to form

Mx Protein Mx proteins are IFN-inducible GTPases in the dynamin superfamily.

Mx proteins impair the replication of influenza virus and some other negative strand RNA viruses at the level of viral transcription and at other steps.

Mx proteins are believed to interfere with the trafficking or activity of viral components during viral replication thereby interfering with viral replication.

2-5A System a multienzyme pathway in which IFN-inducible 2-5A synthetases are stimulated by dsRNA to produce a series of short 2’, 5’ oligoadenylates (2-5A) that activate RNase L. The active RNase L degrades viral and cellular RNA leading to suppression of viral replication.

Mechanisms by Which Viruses Inhibit Interferon Action

Different mechanisms are used: inhibitory viral RNA, inhibitory viral and cellular proteins, and proteolytic cleavage.

Examples:

HIV – tat protein directly inhibits PKR; tar RNA blocks activation of PKR
Polio – induces a protease which degrades PKR

Therapies using IFN’s

Malignancies – not a cure, used with anti-tumor drugs

Viral Diseases – to treat the Hepatitis diseases and papillomas

Immunomodulation- IFN b is used to treat MS, IFN g is used to treat Chronic Granulomatous Disease

Side Effects

similar to the flu (it is thought that maybe flu symptoms are IFN related)
fatigue, nausea/anorexia, Nervous system (depression, lethargy)

Improvements for therapies

increase signal transduction

combine with chemotherapy

dose intensification Þ PEG IFN used to reduce side effects

gene therapy: stimulated by IFN’s, used to make IFN’s

Studies Investigating IFN

Experiments with knockout mice:

With RNase L gene inactivated, infected mice died a few days earlier than wild type mice. If pre-treated with IFN-a , the knockout mice last longer, but not as long as controls.

2-5A system is also involved in signaling apoptosis in mice infected with a herpes virus.

Using Gene-chip technology a list of genes that are induced by IFN a , b , and g has been produced. These genes are targets for possible other antiviral pathways.