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Journal of Virology, March 2008, p. 3139-3146, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.02102-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Selective Downregulation of Rhesus Macaque and Sooty Mangabey Major Histocompatibility Complex Class I Molecules by Nef Alleles of Simian Immunodeficiency Virus and Human Immunodeficiency Virus Type 2{triangledown}

M. Quinn DeGottardi,1 Anke Specht,2 Benjamin Metcalf,3 Amitinder Kaur,3 Frank Kirchhoff,2 and David T. Evans1*

Department of Microbiology and Molecular Genetics,1 Division of Immunology, New England Primate Research Center, Harvard Medical School, Southborough, Massachusetts 01772,3 Institute of Virology, Universitaetsklinikum, 89081 Ulm, Germany2

Received 21 September 2007/ Accepted 3 January 2008


   ABSTRACT
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 ABSTRACT
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Human immunodeficiency virus type 1 (HIV-1) Nef downregulates HLA-A and -B molecules, but not HLA-C or -E molecules, based on amino acid differences in their cytoplasmic domains to simultaneously evade cytotoxic T lymphocyte (CTL) and natural killer cell surveillance. Rhesus macaques and sooty mangabeys express orthologues of HLA-A, -B, and -E, but not HLA-C, and many of these molecules have unique amino acid differences in their cytoplasmic tails. We found that these differences also resulted in differential downregulation by primary simian immunodeficiency virus (SIV) SIVsmm/mac and HIV-2 Nef alleles. Thus, selective major histocompatibility complex class I downregulation is a conserved mechanism of immune evasion for pathogenic SIV infection of rhesus macaques and nonpathogenic SIV infection of sooty mangabeys.


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Downregulation of major histocompatibility complex (MHC) class I molecules by Nef is thought to be an important immune evasion mechanism of human immunodeficiency virus type 1 (HIV-1). By downregulating MHC class I molecules from the cell surface, Nef reduces the susceptibility of infected cells to recognition and destruction by virus-specific cytotoxic T lymphocytes (CTL) (13, 42, 51). However, removal of MHC class I molecules from the cell surface increases the potential susceptibility of infected cells to elimination by natural killer (NK) cells (23). HIV-1 appears to solve this dilemma by selectively downregulating those MHC class I molecules that are most important for CTL responses, while leaving others on the cell surface to prevent NK cell attack (12). Gene products of the highly polymorphic HLA-A and -B loci and to a lesser extent the HLA-C locus present viral peptides on the cell surface for recognition by CTL (37). HLA-C and -E molecules frequently serve as ligands to inhibit the activation of NK cells through interactions with molecules of the killer cell immunoglobulin-like receptor (KIR) and the CD94/NKG2 receptor families, respectively (23). HIV-1 Nef downregulates HLA-A and -B, but not HLA-C or -E, based on amino acid differences in the cytoplasmic domains of these molecules (12, 28, 50). By selectively downregulating HLA-A and -B, but not HLA-C or -E, HIV-1 reduces the susceptibility of infected cells to recognition by the majority of virus-specific CTL while simultaneously preventing their elimination by NK cells (12).

The simian immunodeficiency virus (SIV) Nef protein also downregulates MHC class I molecules from the cell surface (28, 29). However, the residues required for this activity differ from HIV-1 Nef (44). SIV Nef does not require the PxxP motif for MHC class I downregulation but instead uses unique amino acid sequences at the C-terminal end of the molecule that are not present in HIV-1 Nef (44). Following infection of rhesus macaques with SIV nef mutants that specifically disrupt these sequences, MHC class I downregulation was rapidly restored through nucleotide reversion or compensatory changes, indicating that there is strong selective pressure to retain this function of Nef in vivo (33, 43).

SIV infection of the rhesus macaque is an important animal model for lentiviral pathogenesis and AIDS vaccine development. However, the ability of the SIV Nef protein to differentially downregulate rhesus MHC class I molecules has not been addressed. Rhesus macaques express orthologues of the classical HLA-A and -B genes, but not orthologues of HLA-C (8). Orthologues of HLA-C have been identified in chimpanzees, gorillas, and orangutans (2, 24, 25), but not in Old World monkeys (1). Hence, the HLA-C locus appears to represent a recent duplication of the HLA-B locus that occurred after the divergence of apes from Old World monkeys (1, 8, 15). However, rhesus macaques have multiple duplications of the Mamu-A (Macaca mulatta [21]) and Mamu-B loci (8). There are at least four Mamu-A loci and an undefined and variable number of Mamu-B loci on any given rhesus MHC class I haplotype (36). Many molecules of the Mamu-B loci have unique amino acid differences in their cytoplasmic domains that are not found in humans (36). One of these duplicated Mamu-B loci, designated Mamu-I, has unusual features of both classical and nonclassical MHC class I loci (46). Alleles of this locus are present in all rhesus macaques and appear to be expressed in most tissues, but they exhibit limited sequence variation and show no evidence for diversifying selection in the peptide binding region of the molecule (46).

Rhesus macaques also have orthologues of the nonclassical HLA-E, -F, and -G genes. Gene products of the Mamu-E and -F loci are expressed in rhesus macaques and are well conserved relative to their human counterparts (7, 35). Mamu-G is a pseudogene in rhesus macaques whose function appears to have been assumed by Mamu-AG (4, 5), a product of recombination between ancestral Mamu-A and -B loci (6). Like HLA-G, Mamu-AG exhibits limited polymorphism, has a truncated cytoplasmic tail, and is expressed only in the placenta where it appears to play a role in maternal-fetal tolerance (4). Evidence of strong purifying selection on the peptide binding region of MHC Class I E molecules from different primate species, including macaques and humans (7, 22), suggests that Mamu-E and HLA-E serve the same function to present leader peptides derived from classical MHC class I molecules to NK cells bearing CD94/NKG2 receptors (9, 10, 26, 27).

Comparatively little is known about the MHC class I genes of the sooty mangabey. However, orthologues of HLA-A, -B, and -E (Cercocebus atys; Ceat-A, -B, and -E) have recently been identified (A. Kaur, unpublished data). Sooty mangabeys are natural hosts for SIVsmm infection (11), and despite high levels of persistent virus replication, are resistant to CD4+ lymphocyte depletion and progression to AIDS (39). Nevertheless, SIV-infected sooty mangabeys make virus-specific CD8+ T-cell responses that exert selective pressure on virus replication (19, 48). Phylogenetic, geographic, and epidemiological evidence strongly suggest that the pathogenic infections of humans with HIV-2 and macaques with SIVmac originated as a result of cross-species transmission of SIVsmm from sooty mangabeys (11, 17, 18, 31, 34). Thus, adaptations of SIVsmm Nef for immune evasion in the sooty mangabey are important to understanding the evolutionary origins of MHC class I downregulation for this group of primate lentiviruses.

Preliminary experiments demonstrated that the Nef protein of SIVmac239 was capable of differentially modulating the cell surface expression of full-length rhesus macaque MHC class I molecules based on amino acid differences in their cytoplasmic domains (data not shown). However, expression levels for the full-length MHC class I molecules were highly variable in the stable 721.221 cell lines used for these assays, making quantitative comparisons of Nef downregulation difficult. We therefore created a panel of stable Jurkat T-cell lines expressing the extracellular and transmembrane domains of the CD8{alpha} chain (residues 1 to 208) fused to cytoplasmic domain sequences of different rhesus macaque and sooty mangabey MHC class I molecules to investigate selective downregulation by Nef alleles of the SIVsmm/mac and HIV-2 lineages in these primate species. Cytoplasmic domain sequences that represented the gene products of multiple MHC class I alleles or known MHC class I loci were selected.

Amino acid sequence comparisons of rhesus macaque MHC class I molecules revealed very little variation within the cytoplasmic domains of Mamu-A molecules. The cytoplasmic domain of Mamu-A*01 was typical of the majority of Mamu-A molecules, and its sequence was identical to the HLA-A tail consensus sequence. In contrast, alignment of the predicted amino acid sequences for Mamu-B molecules revealed at least 20 different cytoplasmic tail variants (Fig. 1). While the most common sequence, represented by Mamu-B*01, was also identical to the HLA-B consensus sequence, several unique sequences were identified (Fig. 1). Many of these cytoplasmic domain sequences, including those for Mamu-B*03, -B*29, -B*19, and -I*01, were shared by the gene products of multiple alleles. Despite a high degree of sequence conservation for the extracellular domains of Mamu-E molecules, the cytoplasmic domain sequences for Mamu-E molecules were variable and differed from their human counterparts (Fig. 1).


Figure 1
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  		FIG. 1. Cytoplasmic domain sequences for rhesus macaque, sooty mangabey, and human MHC class I molecules. Amino acid sequences corresponding to the cytoplasmic tails of rhesus macaque (Macaca mulatta) (Mamu), sooty mangabey (Cercocebus atys) (Ceat), and human (HLA) MHC class I molecules were aligned to the cytoplasmic domain sequence of Mamu-A*01. MHC class I tail sequences indicated in bold type were selected for analysis of selective downregulation by Nef. Additional rhesus macaque and sooty mangabey MHC class I molecules with identical cytoplasmic tail sequences are listed to the right of each index sequence. The consensus cytoplasmic tail sequences for HLA-A, -B, -C, and -E are shown for comparison at the bottom of the figure. The positions of residues previously shown to contribute to selective HLA class I downregulation by HIV-1 Nef are shaded in gray (12). Amino acid identity is indicated with periods, a translational stop site is indicated with an asterisk, and residues are numbered according to the numbering system of Boyson et al. (8). The rhesus macaque MHC class I index sequences and GenBank accession numbers (shown in brackets) and references (shown in parentheses) are follows: Mamu-A*01 [AJ539307] (32), Mamu-B*01 [U42837] (52), Mamu-B*30 [AF157402] (45), Mamu-B*03 [U41825] (8), Mamu-B*44 [AJ556894] (36), Mamu-B*20 [AJ556878] (36), Mamu-B*29 (Mamu-B*29012) [AJ556884] (36), Mamu-B*47 [AJ556898] (36), Mamu-B*67 [AJ844598] (36), Mamu-B*46 [AJ556897] (36), Mamu-B*50 (Mamu-B*5002) [AJ620415] (36), Mamu-B*24 [AJ556881] (36), Mamu-B*26 [AJ844602] (36), Mamu-B*61 [AJ556906] (36), Mamu-B*05 [U41827] (8), Mamu-B*07 [U41829] (8), Mamu-B*27 [AJ556882] (36), Mamu-B*43 [AJ556893] (36), Mamu-B*64 [AJ556908] (36), Mamu-B*19 [AJ556877] (36), Mamu-I*01 (Mamu-I*01011) [AF161865] (46), Mamu-E*01 (Mamu-E*010102) [EF372279], Mamu-E*02 [EF546441], Mamu-E*0201 [EU109709], and Mamu-E*05 [U41837] (8).

 
Although the database for sooty mangabey MHC class I alleles is much more limited, a recent analysis of 17 MHC class I sequences from two different animals identified eight Ceat-A and -B alleles with predicted cytoplasmic domain sequences that were identical to the cytoplasmic domain sequences of rhesus macaque MHC class I molecules (Kaur, unpublished); two were identical to Mamu-A*01 (Ceat-A*02 and -A*04), two were identical to Mamu-B*01 (Ceat-B*02, and -B*07), three were identical to Mamu-B*03 (Ceat-B*03, -B*05, and -B*08), and one was identical to Mamu-B*29 (Ceat-B*12) (Fig. 1). Nine additional Ceat-A and -B sequences had unique cytoplasmic tail polymorphisms that were observed for only a single allele (data not shown). An allele of the Ceat-E locus, Ceat-E*01, was also identified; the predicted cytoplasmic tail sequence encoded by this locus was not found in either human or rhesus macaque MHC class I molecules (Fig. 1).

We selected the cytoplasmic tails of Mamu-A*01/Ceat-A*02, Mamu-B*01/Ceat-B*02, Mamu-B*03/Ceat-B*03, Mamu-B*29/Ceat-B*12, Mamu-B*07, Mamu-B*19, Mamu-I*01, Mamu-E*01, Mamu-E*05, and Ceat-E*01 as index sequences for the analysis of selective downregulation of rhesus macaque and sooty mangabey MHC class I molecules (Fig. 1). Stable Jurkat T-cell lines expressing CD8 fusions with each of these MHC class I tail sequences were infected with HIV-1 NL4-3-based vectors that express enhanced green fluorescent protein (eGFP) alone or together with the Nef proteins of different virus isolates. In each of these vectors, selected nef alleles were cloned into the nef locus of NL4-3 adjacent to a downstream internal ribosomal entry site, followed by the eGFP reporter gene (40). The HIV-1 long terminal repeat promoter was used to drive transcription of a spliced, bicistronic nef-eGFP mRNA that was the template for both Nef and eGFP translation. After 2 days, the cells were stained for CD8 surface expression, and the degree of downregulation was determined by dividing the mean fluorescence intensity (MFI) of CD8 staining on the eGFP-positive (eGFP+) cells infected with the nef-minus vector by the MFI of CD8 staining on the eGFP+ cells infected with vectors expressing each nef allele (40). The cells were also stained for endogenous HLA class I surface expression, and changes in the MFI of HLA class I staining were correlated with changes in CD8 staining. Ten different Nef alleles were tested to determine whether the specificity of MHC class I downregulation was conserved among virus isolates of the SIVsmm/mac and HIV-2 lineages. These included the Nef proteins expressed by SIVmac239, three different SIVsmm isolates obtained from naturally infected sooty mangabeys, and four independent isolates of HIV-2 (40). For controls, we also included the well-characterized Nef alleles of HIV-1 NL4-3 and NA7. An alignment of the amino acid sequences for each of these Nef proteins is shown in Fig. 2.


Figure 2
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  		FIG. 2. Amino acid sequence alignment of Nef proteins expressed by different isolates of SIV, HIV-2, and HIV-1. The amino acid sequence for SIVmac239 Nefopen was used as a reference (20, 38). Periods indicate amino acid identity, dashes indicate sequence gaps, and asterisks indicate translational stop sites. Residues known to play a role in MHC class I downregulation by the Nef proteins of HIV-1 and SIV are shaded gray (3, 16, 30, 43, 44). The Nef sequences and GenBank accession numbers (shown in brackets) are as follows: SIVmac239 [M33262], SIVsmmFYr1 [DQ092760], SIVsmmFWr1 [DQ092758], SIVsmmFFm1 [DQ092762], HIV-2 60415K [DQ092764], HIV-2 CBL-23 [DQ222472], HIV-2 BEN [M30502], HIV-2 310319 [DQ092766], HIV-1 NA7 [DQ242535] and HIV-1 NL4-3 [M19921] (40).

 
SIVmac239 Nef downregulated CD8 fusions with the Mamu-A*01, -B*01, and -B*03 tails, but not those with the Mamu-B*07, -B*19, -B*29, -I*01, -E*01, and -E*05 tails (Fig. 3). The cytoplasmic tails of Mamu-A*01 and -B*01 represent the majority of Mamu-A and -B molecules, and their sequences are identical to the consensus tail sequences for HLA-A and -B, while the cytoplasmic domain of Mamu-B*03 differs from Mamu-B*01 by a single amino acid (Fig. 1). Perhaps not coincidentally, all of the rhesus macaque MHC class I molecules known to bind CTL epitopes of SIV have cytoplasmic tails identical to one of these three molecules (14, 32, 47, 49). Little is known about the ligands for KIRs in rhesus macaques. However, the inability of SIVmac239 Nef to downregulate CD8 fusions with the cytoplasmic tails of Mamu-B*07, -B*19, -B*29, -I*01, -E*01, and -E*05 may reflect a role for these molecules in the inhibition of NK cell responses. Indeed, the cytoplasmic domains of Mamu-I*01 and -B*01 differ by only a single residue (D327N), and this same change was previously shown to be sufficient to abrogate HIV-1 Nef downregulation of HLA-C (12).


Figure 3
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  		FIG. 3. Downregulation of CD8 fusions with nine different cytoplasmic domain sequences common to rhesus macaque MHC class I molecules by Nef alleles of SIVmac, SIVsmm, HIV-2, and HIV-1. (A) The changes in cell surface expression for CD8 fusions with the cytoplasmic tails of Mamu-A*01, -B*01, -B*03, -B*07, -B*19, -B*29, -I*01, -E*01, and -E*05 are summarized and shown as a bar graph. Values reflect the means plus standard deviations (error bars) of downregulation (n-fold) for duplicate infections with each Nef-expressing vector relative to the Nef-minus control vector. The broken horizontal line indicates the position of no change (1-fold) in CD8 surface expression. (B) Representative flow cytometry data are shown for cell lines infected with vectors expressing selected Nef alleles and the Nef-minus control vector (Nef–). Values indicate the mean fluorescence intensity of HLA class I or CD8 staining on eGFP+ populations. (C) The downregulation of selected CD8 chimeras was compared to the downregulation of endogenous HLA class I molecules and to other CD8 chimeras by linear regression analysis. Comparisons of CD8 fusions with the cytoplasmic tails of Mamu-A*01, -B*01, -B*03, -I*01, and -E*05 versus HLA class I are shown in panels 1 to 5. Pairwise comparisons among the Mamu-A*01, -B*01, and -B*03 chimeras are shown in panels 6 to 8. Assays for CD8 and HLA class I downregulation were performed by infecting Jurkat cell lines with vesicular stomatitis virus G-pseudotyped, replication-competent HIV-1 NL4-3-based vectors that express Nef and eGFP as previously described (40, 41). Two days after infection, the cells were stained with phycoerythrin-conjugated antibodies to CD8 (BD Biosciences; clone RPA-T8) or HLA class I (Dako; clone W6/32), and the mean fluorescence intensity of CD8 and HLA class I surface expression on the eGFP+ cell population was determined by flow cytometry.

 
Two of the three SIVsmm Nef variants resulted in a pattern of downregulation similar to that of SIVmac239 Nef. The Nef proteins of SIVsmmFYr1 and SIVsmmFWr1 downregulated CD8 chimeras with the cytoplasmic tails of Mamu-A*01, -B*01, and -B*03, but not those with the cytoplasmic tails of Mamu-B*07, -B*19, -B*29, -I*01, and -E*01 (Fig. 3A). However, unlike SIVmac239 Nef, these SIVsmm Nef alleles also downregulated Mamu-E*05. Although Mamu-E*05 downregulation was only about twofold, this effect was reproducible and contrasts with the inability of these alleles to downregulate Mamu-E*01 (Fig. 3A). Thus, with the exception of SIVsmmFFm1 Nef, which did not significantly downregulate any of the rhesus MHC class I tail sequences, the pattern of downregulation was the same for two of the three SIVsmm Nef alleles and differed from SIVmac239 Nef only for Mamu-E*05.

The pattern and extent of downregulation were more complex for the four HIV-2 Nef variants tested. The magnitude of CD8 downmodulation varied from a marginal 1.2- to 2.5-fold by HIV-2 BEN Nef to an unusually high 5.6- to 12.4-fold by HIV-2 CBL-23 Nef for the Mamu-B*01 and -B*03 chimeras, respectively (Fig. 3). The breadth of downregulation also appeared to reflect their relative potency. HIV-2 CBL-23 Nef downregulated CD8 chimeras with each of the rhesus macaque MHC class I tail sequences except Mamu-I*01 and -E*01 (Fig. 3A). Likewise, HIV-2 60415K Nef, which was second only to HIV-2 CBL-23 Nef in CD8 and HLA class I downmodulation, downregulated all of the CD8 chimeras except Mamu-B*19, -I*01, and -E*01 (Fig. 3A). HIV-2 310319 Nef downregulated CD8 fusions with the cytoplasmic domains of Mamu-A*01, -B*01, -B*03, and -B*07, but not those with the cytoplasmic domains of Mamu-B*19, -B*29, -I*01, -E*01, and -E*05. Finally, the least potent allele, HIV-2 BEN Nef, downregulated fusions with the cytoplasmic tails of Mamu-A*01, -B*03, -B*07, and -E*05, but not those with the cytoplasmic tails of Mamu-B*01, -B*19, -B*29, -I*01, and -E*01 (Fig. 3A). Thus, the HIV-2 Nef variants were characterized by greater diversity in both the magnitude and breadth of downregulation. Since all of these HIV-2 Nef alleles were also previously shown to downregulate CD3, CD4, and CD28 (40) and the downregulation of these surface antigens did not correlate with the degree of HLA class I downregulation, differences in the potency of MHC class I downmodulation appear to reflect this specific function of Nef rather than general differences in Nef activity or protein expression levels.

As expected, both HIV-1 Nef variants downregulated CD8 fusions with the Mamu-A*01/HLA-A and Mamu-B*01/HLA-B cytoplasmic domains (Fig. 3A). In addition, these Nef variants also downregulated CD8 fusions with the cytoplasmic domains of Mamu-B*03 and -B*07 (Fig. 3A). Since the SIVsmm/mac Nef alleles did not downregulate the Mamu-B*07 chimera, Mamu-B*07 downregulation appears to be a distinctive property of HIV-1 Nef alleles. It is therefore tempting to speculate that this difference may reflect host-specific adaptation of HIV-1 and SIVsmm/mac Nef alleles for immune evasion in humans versus Old World monkeys, respectively.

CD8 downregulation correlated with endogenous HLA class I downregulation by linear regression analysis for the Mamu-A*01 (P = 0.0243), -B*01 (P = 0.0198), and -B*03 (P = 0.0401) chimeras, but not for the Mamu-I*01 (P = 0.1289) and -E*05 (P = 0.1870) chimeras (Fig. 3C). Additional correlations were observed for pairwise comparisons between the Mamu-A*01, -B*01, and -B*03 chimeras (Fig. 3C). These observations indicate that the downregulation of heterologous CD8/MHC class I fusion proteins accurately predict the downregulation of full-length MHC class I molecules and that the magnitude of downregulation is a function of the relative potency of each Nef variant.

Each of the Nef variants was also tested for downregulation of CD8 fusions with cytoplasmic domain sequences of sooty mangabey and human MHC class I molecules. With the exception of SIVsmmFFm1 Nef, which consistently failed to downregulate any of the CD8 chimeras, the Nef proteins expressed by SIVmac, SIVsmm, HIV-2, and HIV-1 all downregulated CD8 fusions with the cytoplasmic tails of Ceat-A*02, -B*02, and -B*03, but not those with the cytoplasmic tail of Ceat-E*01 (Fig. 4A). Likewise, each of the Nef variants, except SIVsmmFFm1 Nef, downregulated CD8 fusions with cytoplasmic tail sequences of HLA-A and -B molecules, but not those with cytoplasmic tail sequences of HLA-C and -E molecules (Fig. 4B). Thus, the pattern of selective downregulation for the known cytoplasmic tail variants of sooty mangabey MHC class I molecules and for cytoplasmic tail sequences typical of HLA class I molecules appears to be broadly conserved among the Nef proteins expressed by different primate lentiviruses.


Figure 4
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  		FIG. 4. Nef downregulation of CD8 fusions with the cytoplasmic tails of sooty mangabey and human MHC class I molecules. Bar graphs summarize the changes in cell surface expression for CD8 fusions with the cytoplasmic tails of sooty mangabey (A) and human (B) MHC class I molecules. Values reflect the means plus standard deviations (error bars) of the changes in cell surface expression (n-fold) for duplicate infections with each Nef-expressing vector relative to the Nef-minus control vector. The horizontal broken line indicates the position of no change (1-fold) in CD8 surface expression. (C) Representative flow cytometry data are shown for the downregulation of CD8 chimeras with the Ceat-B*03, Ceat-E*01, HLA-C, and HLA-E tails by selected Nef alleles relative to the Nef-minus control vector (Nef–).

 
These studies demonstrate that the SIVsmm/mac Nef proteins are capable of differentially modulating the cell surface expression of rhesus macaque and sooty mangabey MHC class I molecules. Three different SIVsmm/mac Nef variants downregulated CD8 chimeras with cytoplasmic tail sequences typical of Mamu-A and -B molecules known to bind SIV CTL epitopes in rhesus macaques but did not downregulate chimeras with the cytoplasmic tails of other Mamu-B or -E molecules. Since Old World monkeys do not have an MHC class I C locus (1, 8, 15), gene products of one or more of the duplicated B loci may serve as KIR ligands for the inhibition of NK cell responses. Furthermore, given the sequence conservation among MHC class I E alleles of different primate species (7, 22), it is likely that rhesus macaque and sooty mangabey MHC class I E molecules serve as ligands for CD94/NKG2 receptors on NK cells (9, 10, 26, 27). Thus, similar to HIV-1 infection of humans, SIVsmm/mac appears to selectively downregulate certain MHC class I molecules in infected rhesus macaques and sooty mangabeys to evade CTL recognition, while leaving others on the cell surface to prevent NK cell attack.

Similar patterns of downregulation were observed for Nef variants of SIVmac, SIVsmm, HIV-2, and HIV-1, suggesting that selective MHC class I downregulation is a broadly conserved functional activity of primate lentiviral Nef proteins. Indeed, no differences in the pattern of downregulation of sooty mangabey or human MHC class I tail sequences were observed for any of the Nef alleles tested. However, differences were observed in the ability to downregulate certain cytoplasmic tail sequences of rhesus macaque MHC class I molecules. The cytoplasmic tail of Mamu-B*07 was downregulated by Nef alleles of HIV-1, but not by SIVsmm/mac, and the Mamu-E*05 tail was downregulated by Nef alleles of SIVsmm, but not by SIVmac239 Nef. Thus, Nef may also have acquired host-specific adaptations for immune evasion in primate species that differ in the cytoplasmic domains of their MHC class I gene products.


   ACKNOWLEDGMENTS
 
We thank Michelle Connole, Daniela Krnavek, Kerstin Regensburger, and Martha Mayer for outstanding technical assistance.

This work was supported in part by Public Health Service grants AI52751, AI63993, AI67057, and AI49809 from the National Institutes of Health and by grants from the Wilhelm-Sander Foundation and the Deutsche Forchungsgemeinschaft. D.T. Evans is an Elizabeth Glaser Scientist supported by the Elizabeth Glaser Pediatric AIDS Foundation.


   FOOTNOTES
 
* Corresponding author. Mailing address: Department of Microbiology and Molecular Genetics, New England Primate Research Center, Harvard Medical School, One Pine Hill Drive, Southborough, MA 01772-9102. Phone: (508) 624-8025. Fax: (508) 786-3317. E-mail: devans{at}hms.harvard.edu Back

{triangledown} Published ahead of print on 16 January 2008. Back


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Journal of Virology, March 2008, p. 3139-3146, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.02102-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.




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