Which herpesvirus encodes a viral homologue of IL-10 that acts as an immunosuppressant and is considered a key immune evasion strategy?

Explanation

## Viral Immune Evasion Mechanisms in Herpesviruses **Key Point:** Epstein-Barr virus (EBV) encodes **BCRF1**, a viral homologue of interleukin-10 (vIL-10), which suppresses T-cell–mediated immunity and promotes viral persistence by downregulating pro-inflammatory cytokines. This is the canonical, textbook-cited example of a herpesvirus vIL-10 homologue. ### EBV's vIL-10 (BCRF1) — The Prototypic Viral IL-10 Homologue EBV's BCRF1 gene product shares ~70% amino acid identity with human IL-10 and exerts identical immunosuppressive functions: 1. **Inhibits Th1 differentiation** — suppresses IFN-γ and IL-2 production by T cells 2. **Blocks NK cell activation** — reduces cytotoxic killing of EBV-infected B cells 3. **Promotes B-cell survival** — facilitates latent infection and B-cell transformation (EBV-associated lymphomas) 4. **Downregulates MHC class II** — impairs antigen presentation to CD4+ T cells ### Why Not CMV? CMV does encode a protein with partial IL-10–like activity (UL111A / cmvIL-10), but **EBV's BCRF1 is the prototypic and most extensively characterised viral IL-10 homologue** in standard microbiology and immunology textbooks (Fields Virology, Murray's Medical Microbiology). The question asks which herpesvirus "encodes a viral homologue of IL-10" as its **key** immune evasion strategy — this description maps unambiguously to EBV/BCRF1 in the primary literature and exam syllabi. ### Comparison of Immune Evasion Across Herpesviruses | Virus | Key Immune Evasion Mechanism | Mechanism Type | |-------|------------------------------|----------------| | **EBV** | **BCRF1 (vIL-10)**, LMP-1 (NF-κB activation) | Cytokine mimicry, B-cell transformation | | **CMV** | UL111A (cmvIL-10, partial), MHC-I downregulation (US2/US11) | Antigen presentation block, partial cytokine mimicry | | **HSV-1/2** | ICP47 (MHC-I inhibitor), LAT (apoptosis block) | Antigen presentation, apoptosis inhibition | | **VZV** | ORF4 (IL-18 binding), glycoprotein E (complement inhibition) | Cytokine sequestration, complement evasion | **High-Yield:** EBV's BCRF1/vIL-10 is the classic example tested in NEET PG / INI-CET. It was the **first** viral IL-10 homologue discovered and remains the paradigm for cytokine mimicry as an immune evasion strategy (Fields Virology, 6th ed.; Murray's Medical Microbiology, 9th ed.). **Clinical Pearl:** EBV-driven vIL-10 production contributes to the immunosuppressive tumour microenvironment in EBV-associated malignancies (Burkitt lymphoma, Hodgkin lymphoma, nasopharyngeal carcinoma), explaining in part why these tumours evade immune surveillance. **Warning:** Do not confuse EBV's BCRF1 (the prototypic vIL-10) with CMV's UL111A — while CMV does encode a partial IL-10 homologue, EBV's BCRF1 is the textbook answer for "viral IL-10 homologue as key immune evasion strategy."