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Mechanism for down-regulation of CD28 by Nef
T Swigut1, N Shohdy, J Skowronski
1Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.
Abstract:
SIV and HIV Nef proteins disrupt T-cell receptor machinery by down-modulating cell surface expression of CD4 and expression or signaling of CD3-TCR. Nef also down-modulates class I major histocompatibility complex (MHC) surface expression. We show that SIV and HIV-1 Nefs down-modulate CD28, a major co-stimulatory receptor that mediates effective T-cell activation, by accelerating CD28 endocytosis. The effects of Nef on CD28, CD4, CD3 and class I MHC expression are all genetically separable, indicating that all are selected independently. In cells expressing a Nef-green fluorescent protein (GFP) fusion, CD28 co-localizes with the AP-2 clathrin adaptor and Nef-GFP. Mutations that disrupt Nef interaction with AP-2 disrupt CD28 down-regulation. Furthermore, HIV and SIV Nefs use overlapping but distinct target sites in the membrane-proximal region of the CD28 cytoplasmic domain. Thus, Nef probably induces CD28 endocytosis via the AP-2 pathway, and this involves a ternary complex containing Nef, AP-2 and CD28. The likely consequence of the concerted down-regulation of CD28, CD4 and/or CD3 by Nef is disruption of antigen-specific signaling machineries in infected T cells following a productive antigen recognition event.
Insights
Simian and human immunodeficiency virus Nef proteins accelerate the endocytosis of CD28, a key T-cell activation receptor. This disruption of T-cell signaling pathways is crucial for viral pathogenesis.
Area of Science:
- * Virology
- * Immunology
- * Molecular Biology
Background:
- * Simian and Human Immunodeficiency Viruses (SIV and HIV) utilize Nef proteins to subvert host immune responses.
- * Nef proteins are known to down-modulate cell surface expression of CD4, CD3-TCR, and MHC class I molecules, disrupting T-cell receptor (TCR) machinery.
- * CD28 is a critical co-stimulatory receptor essential for effective T-cell activation.
Purpose of the Study:
- * To investigate the mechanism by which SIV and HIV Nef proteins down-regulate CD28 surface expression.
- * To determine if Nef-mediated down-regulation of CD28, CD4, CD3, and MHC class I are independent processes.
- * To elucidate the molecular interactions involved in Nef-induced CD28 endocytosis.
Main Methods:
- * Analysis of CD28 surface expression in cells expressing SIV and HIV Nef proteins.
- * Co-localization studies of Nef-green fluorescent protein (GFP) fusion proteins with CD28, AP-2 clathrin adaptor, and other cellular components.
- * Mutational analysis of Nef proteins to identify critical interaction sites with CD28 and AP-2.
Main Results:
- * SIV and HIV Nef proteins accelerate CD28 endocytosis, leading to reduced surface expression.
- * The down-regulation of CD28, CD4, CD3, and MHC class I by Nef are genetically separable, indicating independent selection.
- * Nef-GFP co-localizes with CD28 and the AP-2 clathrin adaptor, and mutations disrupting Nef-AP-2 interaction impair CD28 down-regulation.
- * Overlapping yet distinct target sites in the CD28 cytoplasmic domain are utilized by SIV and HIV Nefs.
Conclusions:
- * Nef proteins likely induce CD28 endocytosis through the AP-2 pathway, involving a ternary complex of Nef, AP-2, and CD28.
- * The concerted down-regulation of CD28, CD4, and CD3 by Nef disrupts T-cell signaling, impairing immune responses in infected cells.
- * Understanding these mechanisms provides insights into viral pathogenesis and potential therapeutic targets.