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Journal of Virology, January 2007, p. 349-361, Vol. 81, No. 1
0022-538X/07/$08.00+0     doi:10.1128/JVI.01841-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Simian Immunodeficiency Virus SIVmac239 Infection of Major Histocompatibility Complex-Identical Cynomolgus Macaques from Mauritius

Roger W. Wiseman,^1, Jason A. Wojcechowskyj,^1, Justin M. Greene,¹ Alex J. Blasky,¹ Tobias Gopon,¹ Taeko Soma,¹ Thomas C. Friedrich,¹ Shelby L. O'Connor,² and David H. O'Connor^1,2*

Wisconsin National Primate Research Center, University of Wisconsin—Madison, Madison, Wisconsin 53706,¹ Department of Pathology and Laboratory Medicine, University of Wisconsin—Madison, Madison, Wisconsin 53706²

Received 23 August 2006/ Accepted 2 October 2006

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Nonhuman primates are widely used to study correlates of protective immunity in AIDS research. Successful cellular immune responses have been difficult to identify because heterogeneity within macaque major histocompatibility complex (MHC) genes results in quantitative and qualitative differences in immune responses. Here we use microsatellite analysis to show that simian immunodeficiency virus (SIV)-susceptible cynomolgus macaques (Macaca fascicularis) from the Indian Ocean island of Mauritius have extremely simple MHC genetics, with six common haplotypes accounting for two-thirds of the MHC haplotypes in feral animals. Remarkably, 39% of Mauritian cynomolgus macaques carry at least one complete copy of the most frequent MHC haplotype, and 8% of these animals are homozygous. In stark contrast, entire MHC haplotypes are rarely conserved in unrelated Indian rhesus macaques. After intrarectal infection with highly pathogenic SIVmac239 virus, a pair of MHC-identical Mauritian cynomolgus macaques mounted concordant cellular immune responses comparable to those previously reported for a pair of monozygotic twins infected with the same strain of human immunodeficiency virus. Our identification of relatively abundant SIV-susceptible, MHC-identical macaques will facilitate research into protective cellular immunity.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Nonhuman primates are important models for major human infectious diseases, including AIDS (6). As vaccine candidates increasingly focus on eliciting cell-mediated immunity against human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), there is intense interest in the genes of the major histocompatibility complex (MHC) that define the specificity of the cellular immune response. In humans, the genetics of the MHC are well defined, with only a single polymorphic HLA-A, HLA-B, and HLA-C locus per chromosome. In contrast, MHC haplotypes in macaques contain a variable number of expressed polymorphic class I (7, 9, 21, 29, 39, 40) and class II (10, 12) loci.

AIDS research has motivated study of MHC genetics in nonhuman primates, most notably in rhesus macaques of Indian origin. More than 130 MHC class I alleles and 160 MHC class II alleles as well as two genomic sequences of the MHC region from rhesus macaques are currently in GenBank. The large number of defined MHC alleles highlights the heterogeneity of these animals. Unfortunately for SIV research, this diversity generally limits investigators to MHC matching animals for single class I alleles, such as Mamu-A*01, rather than entire MHC haplotypes (6, 14). Shared MHC haplotypes, comprising MHC class IA and IB genes, MHC class II genes, and tightly linked genes involved in antigen processing and inflammation (16), have been identified only in rhesus macaques related by descent (31, 42). Therefore, it is exceedingly difficult to study the influence of the entire gene-dense MHC region on SIV pathogenesis in unrelated rhesus macaques.

We became interested in Mauritian cynomolgus macaques (MCM) as a model of SIV pathogenesis because of their unique natural history. Historical records suggest that European seafarers introduced cynomolgus macaques to the small Indian Ocean island of Mauritius within the last 500 years (36). Mitochondrial and Y chromosome DNA analyses indicate that the current MCM population of between 25,000 and 35,000 monkeys descended from a very small founder population that is most likely to have originated from Sumatra and has remained isolated for approximately 80 to 100 generations (22; A. J. Tosi and C. S. Coke, submitted for publication). In the contemporary Finnish human population, which descends from a limited number of ancestors within approximately the same number of generations as MCM, entire shared MHC haplotypes are common (17). Thus, we hypothesized that the unusual natural history of MCM might portend the presence of high-frequency MHC haplotypes (20). We discovered that six high-frequency haplotypes encompassing both the MHC class I and class II loci account for almost all MHC diversity in MCM. We also demonstrated broadly similar cellular immune responses in MHC-identical MCM infected with SIVmac239.

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Animals and SIVmac239 challenge. Blood samples from feral MCM were purchased directly for genetic analyses (Charles River BRF, Houston, TX). Our initial microsatellite analysis focused on five MCM that were examined previously in an MHC class I allele discovery study (20). Subsequently, blood from another 112 feral MCM was obtained in two independent shipments from Charles River BRF for MHC class I genotyping.

A pair of male MHC-identical MCM (CY0111 and CY0113) was selected based on microsatellite and reference strand conformation analysis. Both animals were challenged intrarectally with a single dose of 5 x 10⁴ TCID₅₀ (tissue culture dose sufficient to infect 50% of cells) of molecularly cloned SIVmac239 Nef open virus (19). SIV-infected animals were cared for according to the regulations and guidelines of the University of Wisconsin Institutional Animal Care and Use Committee.

Microsatellite analysis. Multiplex PCR assays were developed for 18 microsatellite loci spanning the MHC region (Table 1). Four of these loci were adapted for cynomolgus macaques from a previous study with rhesus macaques (31). Additional microsatellites were identified by screening human MHC primer pairs (15) for specificity against the rhesus MHC genomic sequence (9) with BLASTn (2). Three human primer pairs were used directly, while another 10 microsatellite primer sequences were modified to reflect differences in the rhesus genomic MHC sequence. Finally, the 9268 locus was identified by searching for novel microsatellites in the rhesus genomic MHC sequence using ETANDEM (EMBOSS suite of software) (34), and primers were designed with Primer3 software (http://frodo.wi.mit.edu).

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TABLE 1. MHC microsatellite primers and multiplex PCR amplification conditionsa

Microsatellite PCRs were carried out with PTC-225 thermocyclers (MJ Research) as 10-µl reactions containing 1x Phusion master mix (New England BioLabs, Ipswich, MA), 10 ng genomic DNA, and 0.08 to 0.3 µM primers (Table 1). The following touchdown PCR program was used: 98°C for 30 s; 3 cycles of 98°C for 5 s, 64°C for 5 s, 72°C for 20 s; 3 cycles of 98°C for 5 s, 62°C for 5 s, 72°C for 20 s; 3 cycles of 98°C for 5 s, 60°C for 5 s, 72°C for 20 s; 6 cycles of 98°C for 5 s, 58°C for 5 s, 72°C for 20 s; 25 cycles of 98°C for 5 s, 50°C for 5 s, 72°C for 20 s; and a final extension at 72°C for 5 min. Fragment analysis of PCR products was performed with an ABI 3730 DNA analyzer (Applied Biosystems, Foster City, CA). One microliter of PCR product and 0.4 µl of ROX-ET550 DNA ladder (GE Health Care, Piscataway, NJ) were diluted in 8.6 µl HiDi formamide (Applied Biosystems) and denatured for 1 min at 98°C. Samples were electrokinetically injected at 2.5 kV for 15 s and run at 15 kV for 2,000 s using POP7 polymer (Applied Biosystems). Data were analyzed using DAx data acquisition analysis software (Van Mierlo Software Consultancy, Eindhoven, The Netherlands).

MHC class I RSCA. Transcribed MHC class I alleles were genotyped by reference strand conformation analysis (RSCA) of cDNA heteroduplexes from peripheral blood mononuclear cells or whole blood essentially as described previously (20) with the following modification. A 304-bp amplicon was amplified by PCR from cDNA in order to scan additional polymorphic sites in the highly variable peptide binding domains encoded by exons 2 and 3 using the 5' phosphate (Phos)-modified primer 5'P-Refstrand (5'-[Phos]GCTACGTGGACGACACGC-3') and Short3'RSCA (5'-TTCAGGGCGATGTAATCC-3'). The reference strand providing optimal resolution of MCM heteroduplexes was Mamu-B*-07. A Mamu-B*07 clone was amplified using the dye-labeled primer 6FAM-5'-Refstrand (5'-6-carboxyfluorescein [FAM]CTACGTGGACGACACGC-3') and the 5' phosphate-modified primer Short3'RSCA-P (5'-[Phos]TTCAGGGCGATGTAATCC-3'). Heteroduplex mobilities were determined relative to a ROX-ET900 size standard (GE Health Care) using DAx data acquisition analysis software (Van Mierlo Software Consultancy).

MHC class I allele cloning and sequencing. MHC class I cDNAs were amplified by PCR using a high-fidelity polymerase (Phusion; New England BioLabs), cloned into pCR-Blunt (Invitrogen, Carlsbad, CA), and sequenced essentially as previously described (20). In order to obtain sequences containing complete predicted open reading frames, cDNAs were amplified using PCR primers optimized for known rhesus macaque MHC class I sequences. Each cDNA pool was amplified with consensus primers (5'MHC UTR, 5'-GGACTCAGAATCTCCCCAGACGCCGAG-3'; and 3'MHC UTR A, 5'-CAGGAACAYAGACACATTCAGG-3', or an alternate reverse primer 3'MHC UTR B, 5'-GTCTCTCCACCTCCTCAC-3'). Sequences were compiled for a minimum of 192 cDNA clones from a representative homozygote of each MCM haplotype. Sequences were analyzed using Aligner software (CodonCode Corp.).

DRB genotyping. Sequence-specific PCR assays for 14 DRB alleles identified in MCM by Leuchte et al. (23) were optimized using the following conditions: 1x Phusion master mix, 0.05 µM concentration of each forward and reverse primer, and 10 ng of genomic DNA. Samples were amplified on MJ Research PTC-225 thermocyclers at 98°C for 30 s; 35 cycles of 98°C for 5 s, 62°C to 72°C for 5 s, 72°C for 20 s; and a final extension at 72°C for 5 min (specific annealing temperatures are available upon request). PCR products were then resolved on a 2.5% agarose gel and visualized with ethidium bromide and UV light.

Plasma virus analysis. The plasma virus concentration was determined using a modification of methods described previously (41). Viral RNA was reverse transcribed and amplified using a SuperScript III Platinum one-step quantitative reverse transcription-PCR system (Invitrogen, Carlsbad, CA) in a LightCycler 1.2 (Roche Diagnostics, Indianapolis, IN). The final reactions (20 µl) contained 0.2 mM each deoxynucleoside triphosphate, 3 mM MgSO₄, 0.015% bovine serum albumin, 150 ng random hexamers (Promega, Madison, WI), 0.8 µl SuperScript III reverse transcriptase and Platinum Taq DNA polymerase in a single enzyme mix, 600 nM each amplification primer (5'-GTCTGCGTCATCTGGTGCATTC-3' and 5'-CACTAGCTGTCTCTGCACTATGTGTTTTG-3'), and 100 nM probe (5'-[FAM] CTTCCTCAGTGTGTTTCACTTTCTCTTCTGCG-3'). The reverse transcriptase reaction was performed at 37°C for 15 min and then 50°C for 30 min. An activation temperature of 95°C for 2 min was followed by 50 amplification cycles of 95°C for 2 min and 62°C for 1 min, with ramp times set to 3 degrees per second. Serial dilutions of an SIV gag in vitro transcript were used to generate a standard curve for each run. Copy numbers were determined by interpolation onto the standard curve with the LightCycler software, version 4.0.

IFN- ELISPOT analysis. Peripheral blood mononuclear cells were isolated by Ficoll-Hypaque gradient centrifugation. A total of 1 x 10⁵ to 2 x 10⁵ cells were incubated in duplicate or triplicate overnight with pools of overlapping 15-mer peptides in gamma interferon (IFN-) enzyme-linked immunospot (ELISPOT) plates (Mabtech, Columbus, OH). Plates were developed per the manufacturer's instructions. Spots were imaged with an ELISPOT reader (AID, Strassberg, Germany) and counted by an ELISPOT reader, version 3.1.1, to limit bias. The mean number of spot-forming units (SFU) of background wells (without peptide) was subtracted from the mean of the sample wells. Responses were considered positive if the difference between the sample and background wells was above 2 standard deviations at two or more time points.

Viral sequence analysis. Cell-free plasma was obtained by Ficoll density gradient centrifugation of EDTA anticoagulated whole blood, and viral RNA was isolated as for measurements of plasma virus concentration. Amplification of viral sequences was performed using a QIAGEN one-step reverse transcription-PCR kit (QIAGEN, Valencia, CA). For time points with plasma virus concentrations of >10³ viral RNA copies/ml of plasma, amplicons of 500 to 1,000 base pairs were generated throughout the SIV genome as previously described (28). For time points with plasma virus concentrations of <10³ viral RNA copies/ml of plasma, amplicons of 150 to 200 base pairs were generated around targeted sites of interest within the SIV genome. Primer sequences and PCR conditions are available upon request. Amplicons were purified using a QIAquick gel extraction kit (QIAGEN, Valencia, CA) and then directly sequenced using a DYEnamic ET Terminator cycle sequencing kit (GE Health Care). Sequencing reactions were resolved on an ABI 3730 (Applied Biosystems, Foster City, CA) and edited using CodonCode Aligner software (CodonCode Corp., Dedham, MA).

Nucleotide sequence accession numbers. Novel MHC class I sequences were deposited in GenBank (accession numbers DQ979878 to DQ979886).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Microsatellite analysis of Mauritian cynomolgus macaques. In order to define MHC haplotypes in MCM, we identified a panel of 18 microsatellite markers spanning the entire 5-Mb MHC region of macaques; 10 of these loci lie within the MHC class I region (Fig. 1). The majority of the primers used to amplify these markers were adapted to reflect rhesus genomic sequences (Table 1) (9, 15, 31). First, we used these markers to genotype DNA from five MCM previously shown to possess the MHC class I alleles Mafa-B*430101, Mafa-B*440101, and Mafa-B*460101 (20). One of these animals, A4M, was homozygous at all 18 microsatellite loci (Fig. 1 and 2), while the other four animals had one copy of the same putative haplotype that we termed H1. These results demonstrate that the Mafa-B*430101, Mafa-B*440101, and Mafa-B*460101 cluster is a component of an expansive, well-conserved haplotype that encompasses the entire 5-Mb MHC region.

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FIG. 1. Localization and properties of microsatellite markers in the MHC region of cynomolgus macaques. The schematic map is extrapolated from the MHC genomic sequence of rhesus macaques (9). Approximate positions of microsatellites and shaded boxes for the class IA, IB, and II gene clusters are given on a kilobase scale, oriented with the telomere at the top. Microsatellite properties, including observed heterozygosity [Het (obs)], nucleotides comprising the repeat unit, and number of alleles observed, are given to the right of each marker.

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FIG. 2. Microsatellite analysis of cynomolgus macaques (A1M, A2M, A4M, A5M, and A6M) carrying high-frequency MHC class I alleles. Microsatellite allele sizes (in base pairs) are shown for each animal. The shaded box indicates the common haplotype (H1) that is shared between each of these MCM.

We then extended our MHC microsatellite analysis to a cohort of 112 additional feral MCM. Allele frequencies for these 18 loci in MCM are given in Table 2. The number of allele sizes per locus ranged from two (C4_2_25) to nine (D6S2691). Overall, 46/117 (39%) of this cohort carried at least one complete copy of the H1 haplotype (Fig. 3 and 4), and 9/117 (8%) animals were homozygous for H1. Additionally, we identified 14 more MCM that were homozygous at all 18 microsatellite loci (Fig. 4). From these 14 animals, we defined an additional five haplotypes (H2 to H6) (Fig. 3). Taken together, two-thirds of the 234 chromosomes examined bore microsatellite signatures of one of these six common haplotypes (Fig. 3), and simple recombination events could generally account for the remaining haplotypes (Fig. 4). This extensive sharing of MHC haplotypes is unprecedented among macaques (6, 29, 31).

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TABLE 2. Microsatellite allele frequencies and observed and expected heterozygosities in MCMa

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FIG. 3. Microsatellite haplotypes for the MHC region of Mauritian cynomolgus macaques. (a) Microsatellite allele sizes (in base pairs) characteristic for each microsatellite locus were associated with each MHC haplotype. The six common haplotypes were designated H1 to H6 and assigned colors (H1, black; H2, red; H3, blue; H4, green; H5, yellow; and H6, gray) for illustrative purposes throughout the figures. In several instances, multiple microsatellite alleles for a specific locus were associated with an MHC haplotype, e.g., the D6S2691 locus for H2 and H3. null, undetectable amplification due to primer mismatch or absence of target locus. (b) Microsatellite analysis was used to determine the frequency of common and recombinant MHC haplotypes (n = 234 chromosomes).

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FIG.4. Microsatellite MHC haplotypes of Mauritian cynomolgus macaques. Six common MHC haplotypes were inferred based on microsatellite analysis of 117 feral MCM obtained from Charles River BRF. Solid colored bars indicate intact MHC haplotypes, while mixed colors represent recombinant chromosomes. For example A3M and A4M are homozygous for H4 (green) and H1 (black), respectively, while A6M is a simple heterozygote for the H1 and H6 haplotypes. In contrast, A8M carries H4 plus a recombinant haplotype with the H2 class IA region and H6 for the rest of the MHC region. Hatched boxes define ambiguous regions resulting from identical microsatellite allele sizes between neighboring haplotype blocks. Individual boxes indicate variant microsatellite allele sizes relative to the expected common haplotype; these rare variants generally differ by the addition or loss of a single repeat unit.

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FIG. 4— Continued.

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FIG. 4— Continued.

RSCA of selected MCM confirms MHC haplotypes. To verify that the haplotypes inferred from microsatellite mapping are linked with specific MHC alleles, we performed MHC class I RSCA. RSCA is a modified heteroduplex assay that is particularly well suited for characterizing complex gene families, such as MHC class I and class II (3, 18, 20). After heteroduplex formation with a fluorescently labeled reference strand, individual alleles are distinguished from one another on a nondenaturing polyacrylamide gel. As expected, RSCA from representative homozygotes resulted in distinct peak profiles, reflecting the differing MHC class I allele repertoires on each haplotype (Fig. 5). Moreover, each of the homozygous haplotype profiles was additive in the respective heterozygous animals (Fig. 5).

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FIG. 5. RSCA of transcribed MHC class I alleles. RSCA was performed with a Mamu-B*07 reference strand and cDNA PCR products from homozygous and heterozygous animals representing H1 through H5. RSCA assesses differences in electrophoretic mobility that result from the unique heteroduplex conformations that form between sequence-mismatched MHC alleles and a fluorescently labeled reference strand. These profiles are characteristic for each homozygous haplotype, with three to six peaks per haplotype that correspond to individual class I alleles. The hatched heteroduplex peaks with an apparent mobility of 525 bp are the Mafa-A*25 variant alleles that are shared between H1 through H3 (see Table 3). Several samples contain a residual Mamu-B*07 homoduplex that migrates just before 300 bp.

In addition, MHC class I RSCA was used to test our hypothesis that almost all MCM MHC haplotypes either are intact or result from simple recombination events between H1 through H6. In animals where the predicted recombination region is distal to the MHC class I loci, transcribed allele patterns matched the relevant haplotype in the class I region. When the putative recombination breakpoint occurred within the MHC class I loci, chimeric allele profiles were observed (data not shown).

Identification of transcribed MHC class I alleles for each common haplotype. Next, we identified the specific transcribed MHC class I alleles associated with the H1 through H6 haplotypes. Cloning and sequencing of PCR-amplified cDNAs from representative homozygous animals unambiguously linked specific MHC class I alleles with each of the six haplotypes (Table 3). The H1 haplotype carries Mafa-B*430101, Mafa-B*440101, and Mafa-B*460101, a result that confirmed our previous speculation that these alleles are inherited on a common haplotype (20). This haplotype also carries two MHC class IA alleles, Mafa-A*290101 and Mafa-A*250301. Surprisingly, these MHC class IA alleles are conserved between the three most common haplotypes, H1, H2, and H3. All three haplotypes carry identical Mafa-A*290101 alleles and either Mafa-A*250201 or Mafa-A*250301, which differ by only a single amino acid in the signal peptide. Therefore, more than 90% of MCM are predicted to possess these class IA alleles (Fig. 4). In contrast, "high-frequency" MHC class I alleles in Indian rhesus macaques, such as Mamu-A*01, are rarely found in more than 25% of captive-bred monkeys.

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TABLE 3. MHC class I and II alleles detected for six common MCM haplotypesa

Common MHC haplotypes extend through the DRB locus in MCM. To verify that the microsatellite haplotype signatures associate with discrete MHC class II genotypes, we examined the highly polymorphic MHC class II-DRB locus (4, 23, 31). We used allele-specific PCR with representative homozygous MCM genomic DNAs to assign 11 of 15 known Mauritian DRB alleles to haplotypes H1 through H6 (Table 3). Both the allelic composition and relative frequencies of our microsatellite-based MHC haplotypes are consistent with short-range (150 kb) DRB haplotypes defined previously in two independent cohorts of MCM (4, 23). High-resolution cloning and sequencing from homozygous MCM will be necessary to more rigorously define the complete gene content of the MHC class II region.

High frequency of MHC class I- and MHC class II-identical MCM. As illustrated in Fig. 6, more than one-quarter of this feral MCM cohort (32/117) comprises clusters of 7 or more MHC-identical individuals. If the MHC class I region is considered alone, 72/117 (62%) MCM have one or more fully matched individuals distributed among 14 distinct homozygous and heterozygous haplotype combinations (Fig. 4 and 6). This unique population of animals provides opportunities to perform a wide variety of studies in which genetic control over the MHC of the subjects might be important and that have been previously unattainable with nonhuman primates.

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FIG. 6. Complete MHC identity among a cohort of 117 Mauritian cynomolgus macaques. Only instances where seven or more homozygotes or simple heterozygotes with identity across the entire 5-Mb MHC region were observed are illustrated. Our cohort contained 11 additional animals representing five distinct haplotype combinations with complete MHC identity. The physical order of loci within each gene cluster is unknown.

SIVmac239 challenge of MCM. We infected two MHC class I-identical MCM with SIVmac239 to examine the predictability and reproducibility of SIV pathogenesis and cellular immunity in animals with identical MHC genetics. This pair of MCM was originally selected based on RSCA that demonstrated that they share identical profiles of transcribed MHC class I alleles (data not shown). As illustrated in Fig. 7A, microsatellite analysis revealed that CY0113 carries an H2/H3 recombinant haplotype with a Mafa-A*250301 allele that differs by only a single residue in the signal peptide compared to the H3 haplotype in CY0111. After challenge with SIVmac239, these animals exhibited very similar plasma virus levels for the first 16 weeks of infection (Fig. 7B) before beginning to diverge. After 40 weeks of infection, plasma virus concentrations differed by approximately 2 log units.

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FIG. 7. SIVmac239 infection of MHC class I-identical Mauritian cynomolgus macaques. (a) Microsatellite haplotypes were determined and used to infer the complete complement of class I and class II-DRB transcribed alleles for CY0111 and CY0113. (b) Plasma virus concentrations for each animal were measured at various time points throughout infection. The plasma virus concentrations are quantified as the number of copies of viral RNA per milliliter of plasma. (c) IFN- ELISPOT analysis was used to measure SIV-specific CTL responses in the chronic phase. Either single peptides or peptide pools spanning the indicated amino acid sequences of the specific SIV proteins were used. A + indicates 50 to 99 SFU per million cells, ++ indicates 100 to 499 SFU per million cells, and +++ indicates 500 SFU per million cells; a – indicates <50 SFU. (d) Viral sequences in four specific regions of the SIV genome were analyzed to determine whether similar mutation patterns occurred in both animals. The SIV proteins and the wild-type amino acid sequences are indicated. Dots represent identity with the wild-type sequence. Amino acid replacements that resulted from a mixed population of nucleotides are indicated with a lowercase letter of the variant amino acid. Amino acid replacements that resulted from a complete nucleotide replacement are indicated with an uppercase letter of the variant amino acid. "ND" indicates that the sequence was not determined; wk, weeks.

We predicted that these two MHC-identical animals would exhibit similar CD8⁺-T-lymphocyte (CTL) responses and that these responses, in turn, would select similar viral escape variants (28). CTL responses were measured in this pair of animals by IFN- ELISPOT. During the chronic phase of infection, we consistently detected 11 CTL responses against regions of Rev, Nef, Gag, Tat, Env, and Pol in CY0113 (Fig. 7C). Six of these responses were also detected in CY0111, though the magnitudes of these responses were often lower. Interestingly, CY0111 did not mount any unique responses that were not also detected in CY0113. Unfortunately, sample limitations precluded whole-proteome analyses of acute-phase cellular immune responses in these animals.

Given the similarities in immunological responses in the two animals, we hypothesized that their immune responses would select similar viral variants. The higher plasma virus concentrations (greater than 1,000 copies/ml) in CY0113 allowed analyses of a majority of the viral genome at multiple time points spanning the course of infection. In CY0111, the low chronic-phase plasma virus concentrations (fewer than 1,000 copies/ml) precluded sequencing of the entire SIV genome. Therefore, we used our analysis of viral sequences from CY0113 to focus on a subset of regions for examination in virus isolated from CY0111 (Fig. 7D). We designed small amplicons (150 to 200 bp) to specifically amplify and sequence these targeted regions. Four regions of the genome with viral variation consistent with CTL escape were identified in CY0113 and subsequently evaluated in CY0111 (Fig. 7D). Remarkably, both MHC-identical animals exhibited mutations in these regions, though the affected amino acids were distinct in each animal. With the exception of the Tat_26-36 region, strong CTL responses were detected at 3 weeks postinfection in at least one of the two animals (data not shown), strongly suggesting that the shared variability results from immune escape.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
In this study, we discovered that six haplotypes account for almost all of the MHC diversity in feral Mauritian cynomolgus macaques. Combining genetic mapping with polymorphic microsatellite markers, MHC class I RSCA, high-throughput cloning and sequencing, and MHC class II-DRB allele-specific PCR, we were able to infer the entire MHC class I and class II-DRB genotypes of more than 100 animals. Sizable groups of completely MHC-identical animals, including a cluster homozygous for the most frequent MHC haplotype, were identified. We also successfully infected a pair of MCM with identical MHC class I and class II genetics with SIVmac239. To our knowledge, this is the first study to show successful SIVmac239 infection of Mauritian cynomolgus macaques, though the susceptibility of these animals is not surprising in light of recent data showing susceptibility to other pathogenic SIVs, including SIVmac251 and SHIV89.6P (32).

Our initial results suggest that SIV-specific cellular immune responses are generally uniform in specificity in MHC-identical animals. This study mirrored two recent evaluations of monozygotic twins infected at the same time with the same stock of HIV (13, 44). We identified four regions of SIV that accumulated variation by 16 weeks postinfection. CTL against all four regions were detected during infection, suggesting that the variation results from CTL selective pressure. The pattern of chronic-phase epitope recognition in our animals was very similar to the twins monitored by Yang and colleagues (44). Animal CY0113 mounted 11 CTL responses. Six of the same responses were detected in CY0111, though the magnitude of the responses was lower. Lower plasma virus concentrations in CY0111 may account for the weaker CTL responses, a phenomenon which was also noted in the twins monitored by Yang et al. (44). Despite the similar CTL responses in these animals, plasma viremia became substantially higher in CY0113 after the first 16 weeks of infection. The different clinical outcomes in these two animals could result from subtle differences in epitope specificity not resolved with IFN- ELISPOT, antibody responses, innate immune responses, stochastic differences in T-cell receptor utilization, and different patterns of viral evolution. Unlike the two studies that relied on the serendipitous identification of twins infected with the same strain of HIV, it should be possible to infect additional MHC-identical MCM with SIVmac239 to study why animals with identical MHC genetics and similar CTL responses nonetheless exhibit differences in SIV pathogenesis.

The genetic simplicity of the MCM MHC is unprecedented among macaques and will fundamentally expand the scope of SIV studies that can be undertaken with nonhuman primates. MCM that share identical MHC haplotypes (are MHC haploidentical) or that carry completely distinct MHC haplotypes can be easily identified using polymorphic microsatellite mapping and selected for further studies. Adoptive lymphocyte transfer studies, such as those with inbred strains of mice that have defined the correlates of protective immunity in Friend retrovirus infections, will be possible with MCM that are completely matched for both MHC haplotypes (11, 25). For the first time, it may be possible to study the in vivo correlates of protective cellular immunity by transferring SIV-specific lymphocytes from a donor animal into naive recipients immediately prior to SIV challenge. These studies could directly test the hypothesis that the failure of cellular immunity to control SIV infection results from an inability of CTL to mobilize to sites of viral replication early during infection (33). Additionally, in vitro data suggest that certain CTL specificities suppress SIV and HIV replication far more effectively than others (24, 43). The use of MCM for adoptive transfers of individual CTL specificities could provide a useful method for both identifying and characterizing the shared biological attributes of effective CTL response.

MCM with defined MHC haplotypes may also be very useful for vaccine studies that seek to elicit cellular immunity. Mamu-A*01-positive Indian rhesus macaques are often used in SIV vaccine research, primarily because these animals consistently mount an immunodominant Gag_181-189CM9 CTL response that provides a convenient biomarker for assessing the induction of cellular immune responses. The magnitude of Gag_181-189CM9 responses varies approximately 10-fold between Mamu-A*01-positive animals receiving identical vaccine formulations (1, 8) and SIV challenges (26). The magnitude of Gag_181-189CM9 responses may be indirectly modified by alleles other than Mamu-A*01, since competition between expressed class I alleles could lead to differential Mamu-A*01 cell surface expression (37, 38). MCM that possess completely identical MHC genes eliminate this source of variability and therefore may improve the consistency of vaccine-elicited cellular immune responses.

The evolutionary basis for the MHC genetic simplicity of MCM is unclear. The limited MHC repertoire of MCM may reflect selective advantages of these haplotypes for the Mauritian environment. It appears more likely that the limited MHC diversity described here is the result of a classic population bottleneck or founder effect (22, 30, 36; A. J. Tosi and C. S. Coke, submitted for publication). Consequently, there is little reason to assume that the relative genetic homogeneity of MCM is restricted to the MHC. Given the excitement surrounding gene mapping with isolated human populations (5, 35), MCM may provide an outstanding resource for mapping and identifying non-MHC loci associated with differences in SIV pathogenesis. Studies with HIV-infected individuals have revealed a number of such polymorphic non-MHC loci associated with AIDS restriction (27).

Fortunately, the population of MCM available for research and the selection of genetically defined macaques are relatively abundant. In 2005 alone, 1,670 MCM were imported to the United States by a single distributor (Tami Lass, Charles River BRF, personal communication). Based on our results, approximately 130 H1 homozygous animals should be available annually. Likewise, when all simple homozygotes and heterozygotes for the common haplotypes are included, the number of MCM estimated to populate MHC-identical clusters exceeds 600 per year. These numbers could likely be increased significantly with only a modest effort at selective breeding using MHC microsatellite markers such as those described here.

In conclusion, the high frequency of identical MHC haplotypes in MCM is extraordinary among nonhuman primates used in experimental biology. MCM represent an exceptional source of MHC-identical nonhuman primates with broad applications for AIDS vaccine and pathogenesis investigations.

 
This work was supported by NIAID contract number HHSN266200400088C/N01-AI-40088 and NIH grant 1R21AI068488-01A2. This publication was made possible in part by grant number P51 RR000167 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH), to the Wisconsin National Primate Research Center, University of Wisconsin—Madison. This research was conducted in part at a facility constructed with support from Research Facilities Improvement Program grant numbers RR15459-01 and RR020141-01.

This publication's contents are solely the responsibility of the authors and do not necessarily represent the official views of NCRR or NIH.

We thank Eva Rakasz, Shari Piaskowski, Jessica Furlott, Kim Weisgrau, Gemma May, and Robert DeMars for helpful discussions. We also thank Jody Hegeland, Amy Schara, Eric Peterson, Mike Dobbert, Casey Fitz, and staff at the Wisconsin National Primate Research Center for technical assistance and veterinary care.

 
* Corresponding author. Mailing address: University of Wisconsin—Madison, 555 Science Drive, Madison, WI 53711. Phone: (608) 890-0845. Fax: (608) 265-8084. E-mail: doconnor{at}primate.wisc.edu.

Published ahead of print on 11 October 2006.

R.W.W. and J.A.W. contributed equally to this study.

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Journal of Virology, January 2007, p. 349-361, Vol. 81, No. 1
0022-538X/07/$08.00+0     doi:10.1128/JVI.01841-06
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