Mechanism for down-regulation of CD28 by Nef.
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.
Swigut
T
T
Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.
Shohdy
N
N
Skowronski
J
J
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D018106:CD28 Antigens / Q000378:metabolism*; D015704:CD4 Antigens / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D015536:Down-Regulation; D004705:Endocytosis / Q000502:physiology; D011992:Endosomes / Q000378:metabolism; D015693:Gene Products, nef / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D019169:Jurkat Cells; D008565:Membrane Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D016296:Mutagenesis; D015302:Simian Immunodeficiency Virus
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D018106:CD28 Antigens; D015704:CD4 Antigens; D015693:Gene Products, nef; D008565:Membrane Proteins