The downregulation of CD4 and MHC‐I by primate lentiviruses: a paradigm for the modulation of cell surface receptors

Abstract

The human and simian immunodeficiency viruses (HIV and SIV) downregulate the cell surface expression of CD4, their primary receptor, and of class I histocompatibility complex (MHC-I), a critical mediator of immune recognition. While the first of these effects seems important to preserve viral infectivity, the second likely promotes immune evasion. Three HIV-1 proteins, Nef, Env and Vpu, contribute to downregulate CD4, Env forms a complex with CD4 in the endoplasmic reticulum, thereby retaining the receptor in this compartment. Nef and Vpu, on the other hand, act as connectors between CD4 and specific intracellular trafficking pathways, targeting the receptor for degradation in the lysosome and the proteasome, respectively. Some of the downstream partners of the viral proteins in these events have been identified, and include the adaptor complex of clathrin-coated pits, the beta subunit of COP-I coatomer, and the ubiquitin pathway-related h-beta TrCP protein. HIV-induced MHC-I downregulation, mostly the effect of Nef, also reflects a redistribution of this receptor, with its accumulation in the Golgi. The modalities of this process, however, are as yet imperfectly understood. New evidence indicates that the mechanisms employed by primate lentiviruses to downmodulate CD4 and MHC-I are also exploited by a number of cellular regulatory processes.

Direct Priming and Cross-Priming Contribute Differentially to the Induction of CD8+ CTL Following Exposure to Vaccinia Virus Via Different Routes

“…Pelleted splenocytes were resuspended in ACK lysis buffer (BioSource International, Camarillo, CA; 150 mM NH 4 Cl, 1 M KHCO 3 , and 10 mM EDTA, pH 7.2) and incubated at room temperature for 5 min to remove RBC. The splenocytes were washed three times and then resuspended in RPMI containing 10% FCS supplemented with antibiotics, counted, and used as effectors directly in a 51 Cr release assay or an ELISPOT assay.…”

“…Cells of the mastocytoma cell line P815 (H-2 d ) were infected with the control vaccinia vector vSC8 or with vUS11 at a multiplicity of infection of 10 for 6 h. Infected cells were labeled with 51 Cr for 2 h at 37°C, then washed three times to remove the excess 51 Cr. Splenocyte effectors from immunized mice were mixed with target cells at varying E:T cell ratios in V-bottom, 96-well plates.…”

“…For experiments with ␤-gal-specific CTL, P815 cells were infected with vaccinia virus that expressed (vSC8) or did not express ␤-gal (vvWR) at multiplicity of infection of 5 in a 100-l volume. After 2 h the target cells were resuspended in 2 ml RPMI 1640 medium supplemented with 10% FCS for a total of 18 h. The target cells were labeled with 51 Cr for 1.5 h, washed, counted, and resuspended at a concentration of 1.75 ϫ 10 4 cells/ml in RPMI 1640 medium supplemented with 10% FCS, penicillin, streptomycin, and glutamine. Live effector cells, as determined by trypan blue exclusion, were mixed with labeled target cells at E:T cell ratios of 100, 33, 11, and 3.7 in V-bottom, 96-well plates.…”

“…B10.A(5R) mice were used because the class I alleles expressed by this strain (H-2K b , H-2D d , H-2L d ) are all susceptible to US11-mediated rapid degradation (30). Seven days after infection, spleens were harvested from the immunized mice, and the splenocytes were used directly as effectors in a 51 Cr release assay with vSC8-or vUS11-infected P815 cells as targets (H-2D d -, H-2L drestricted lysis). By using relatively high E:T cell ratios and long (8-h) incubation periods, the induction of CTL can be directly measured without an in vitro stimulation (40).…”

RNA interference and HIV‐1 infection

“…In contrast, Vpu and Env interfere with the transportation of recently synthesised CD4 molecules to the cell surface by targeting them for proteasomic degradation [136,137] (Figure 1). In vitro and in vivo studies have highlighted the relevance of this phenomenon in virus pathogenesis and disease progression [138][139][140][141].…”