18.4: Inhibitors of Viral Protein Synthesis

Protein synthesis is indispensable for viral replication, as viruses lack the cellular machinery required for this process and must hijack the host's translational apparatus. In response, host cells deploy a critical innate immune defense involving interferons, specialized cytokines that play a central role in inhibiting viral propagation.

Upon viral detection, infected cells release interferons that bind to receptors on adjacent uninfected cells, activating the JAK-STAT signaling pathway and triggering the expression of interferon-stimulated genes (ISGs). These ISGs induce the expression of three key antiviral proteins: protein kinase R (PKR), 2',5'-oligoadenylate synthetase (OAS), and RNase L. Viral double-stranded RNA (dsRNA), a molecular pattern associated with infection, directly activates PKR and OAS.

Activated PKR phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α), halting the initiation of mRNA translation and suppressing both viral and host protein synthesis. Concurrently, OAS catalyzes the production of 2',5'-linked oligoadenylates, which activate RNase L. Active RNase L cleaves viral and cellular RNA, including ribosomal RNA, thereby exacerbating the shutdown of protein synthesis and amplifying the antiviral state within the cell.

Clinical Applications of Recombinant Interferons

Given their potent antiviral properties, recombinant interferons have been utilized in the clinical management of several viral diseases:

• Antiviral therapy: Interferon-alpha (IFN-α) has been a historical cornerstone in treating chronic hepatitis B and C infections, reducing viral load and delaying disease progression. In hepatitis C, pegylated interferon combined with ribavirin was standard therapy before the advent of direct-acting antivirals (DAAs), which now offer superior efficacy and tolerability. IFN-α is also used to treat human papillomavirus (HPV)-associated lesions and Kaposi's sarcoma linked to HIV.
• Oncology: IFN-α acts as an immunomodulatory agent in certain hematologic malignancies, such as hairy cell leukemia and chronic myeloid leukemia, malignant melanoma, and renal cell carcinoma.
• Immunology: Interferon-beta (IFN-β) is widely used to manage multiple sclerosis, reducing relapse frequency and delaying the progression of disability.

Despite their clinical benefits, interferon therapies are associated with side effects, including flu-like symptoms, fatigue, depression, and hematologic abnormalities, which may limit their use in some patients. Nonetheless, these diverse applications underscore the versatility and clinical significance of interferons across antiviral, oncologic, and immunologic domains.