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CUTTING EDGE |


* Department of Microbiology and Immunology and
William K. Warren Medical Research Institute, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104
| Abstract |
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| Introduction |
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Although it has been demonstrated that MHC molecules sample a vast array of endogenous proteins during the normal cellular lifecycle, characterization of host-protein-derived peptides after HIV infection has not been performed. Therefore, a fundamental question arises: what host-encoded peptides are uniquely presented on the surface of infected cells? Based upon the observation that HIV produces and interacts with multiple host-encoded proteins inside the cell (including Tsg101 (4) and RNA polymerase II (5)), we hypothesized that host-protein-derived peptides are uniquely presented during infection.
We previously described a bioreactor-HLA-protein production method and a mass-spectrometric-ion-mapping system for comparatively screening class I-eluted peptide ligands (6, 7). In this study, we extend this approach to test the hypothesis that HIV infection alters the presentation of host-encoded peptides. Peptides eluted from HLA-B*0702 molecules produced in HIV-infected or uninfected cells were directly compared using mass spectrometry. Comparative mapping of HIV-infected and uninfected peptides results in the identification of 15 host-derived peptides uniquely presented on HIV-infected cells.
| Materials and Methods |
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Soluble HLA-B*0702 transfectants were produced as described (6) using Sup-T1 cells (7). Transfectants were cultured in a Unisyn CP2500 bioreactor unit (Biovest International, Minneapolis, MN) for 2 mo with continuous peptide collection. Approximately 30 mg of soluble HLA (sHLA)4 were collected from either uninfected or infected cells, supplemented with 1% Triton X-100, and stored at 4°C.
HIV infection
HIV-1 strain MN was propagated in Sup-T1 transfectants and monitored by p24 ELISA (Zeptometrix, Buffalo, NY). For cell pharm infection, 3 x 109 cells were infected at a multiplicity of infection (MOI) of 0.5. For time course protein analysis, 1 x 109 cells were infected at an MOI of 4.5 for 2 h, washed once, and replaced in RPMI 1640 + 20% FBS.
Peptide purification
B*0702 molecules were affinity purified over a W6/32 affinity column. Peptides were eluted with 0.2 N acetic acid, brought to 10% acetic acid concentration, and heated to 78°C for 10 min. Fractions were purified in a stirred cell with a 3-kDa molecular mass cutoff cellulose membrane (Millipore, Bedford, MA). Peptides were reversed-phase-HPLC fractionated using a standard gradient of acetonitrile. Separate but identical peptide purifications were done from uninfected and infected cells.
Mass spectrometric analysis
Fractionated peptides were mapped by mass spectrometry as described (7). Peptides were nanoelectrosprayed (Protana, Odense, Denmark) into a Q-Star QTOF mass spectrometer (PerSeptive Sciex, Foster City, CA). Spectra from the same fraction in uninfected/infected cells were aligned to the same mass range and visually assessed for the presence of differences that were selected for manual and automated sequence assignment using the programs BioMultiview (PerSeptive Sciex) and MASCOT (Matrix Science, London, U.K.) (8). Synthetic peptides corresponding to each putative sequence were produced and subjected to mass spectroscopy MS/MS under identical collision conditions as the naturally occurring peptide and overlaid to confirm sequence identity.
Peptide binding assay
IC50 values were determined using the HLA-B*0702 PolyScreen kit (Pure Protein, Oklahoma City, OK) according to the manufacturers instructions. Fluorescently labeled control peptide and sHLA were incubated with each test peptide until equilibration of peptide replacement was reached as read on an Analyst AD plate reader (Molecular Devices, Sunnyvale, CA). IC50 values were calculated using a dose-response curve.
Real-time PCR and Western blots
For Western blots, cells were lysed in electrophoresis buffer and total protein was quantified using the BCA Protein Analysis kit (Pierce, Rockford, IL). Proteins were transferred onto nitrocellulose membranes (Osmonics, Westborough, MA) before probing and detecting with commercially available Abs. For real-time PCR, total RNA was isolated using the Total RNA Isolation kit (Ambion, Austin, TX) and cDNA synthesized using the Retroscript kit. PCR was performed on a PE-7700 Light Cycler (Applied Biosystems, Foster City, CA) using primer pairs designed using Primer Express software.
-Actin was used as an internal standard. Relative transcript levels were calculated using the
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cycle threshold method and normalized to zero in the uninfected cells.
Apoptosis analysis
Infected cells were treated with the Apo-Direct kit (BD Biosciences, Mountain View, CA) according to manufacturers instructions and followed by analysis on a FACSCalibur. Poly(ADP-ribose) polymerase (PARP) cleavage was detected from the same Western blot lysates as above with an anti-PARP Ab (BD Biosciences).
| Results and Discussion |
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Following the harvest of B*0702/peptide complexes from both infected and uninfected cells, peptide ligands were eluted, fractionated, and each fraction was comparatively mapped using MS. Multiple peptides unique to HIV-infected cells were identified in these MS spectra (Fig. 1A). For example, comparison of spectra produced with peptides eluted from infected and uninfected cells identified a peak unique to the infected cells at 484.7 atomic mass units of fraction 16 (Fig. 1B). Peptide peaks unique to or up-regulated on HIV-infected cells were analyzed with tandem MS (Fig. 1, C and E). De novo sequence identification from MS/MS fragmentation patterns was performed on each peak unique to or up-regulated on infected cells (Fig. 1, D and E). Putatively identified ligands were analyzed for their predicted tandem MS fragmentation pattern (Fig. 1D) and compared with the spectra produced from uninfected cells (Fig. 1C). As a final confirmation of sequence integrity, peptides corresponding to the putative ligand sequences were synthesized and fragmented (Fig. 1E). Only peptides with identical experimental and control MS/MS fragmentation patterns were selected for further analyses.
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Comparison of mass spectra from peptides eluted from infected and uninfected cells yielded 15 self protein-derived peptides showing altered expression on HIV-infected cells. These host-encoded peptides could be categorized by primary cellular function (Table II). Several of the peptides derived from proteins involved in RNA transcription or translation; peptides NPNQNKNVAL and SPNQARAQAAL are both fragments of mRNA binding proteins, while the source protein for APKRPPSAF, high-mobility group protein (HMG) 1, facilitates the binding of transcription factors to DNA sequences (9). Another ligand in this category (AARPATSTL) derived from eukaryotic translation initiation factor (eIF) 4GI, a protein that plays a key role in cap-dependent mRNA translation initiation (10).
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Unique peptides were derived from host proteins with altered protein levels during infection
Unique presentation of peptides during infection could be the result of multiple alterations inside the infected cell. Because HIV has been shown to transcriptionally up-regulate cellular genes (14) and gene overexpression can result in presentation by class I molecules (15), transcriptional up-regulation of genes representing unique peptides could be occurring before peptide presentation. To test this interpretation, real-time PCR was performed on mRNA transcripts from infected cells (Fig. 2A). Because the bioreactor system used for the harvest of sHLA proteins from infected cells represented a mixture of cells at different points in infection, T cells were infected with HIV and fed biweekly with uninfected cells. Once syncitia were evident visually, cells were pelleted, lysed, and real-time PCR was performed on extracted mRNA. Little change in the transcriptional level of the proteins examined was found. Thus, there was no pattern of transcriptional activation that would account for an overabundance of particular MHC-bound peptides on the infected cells.
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Peptide changes occur early in infection
To determine the timing of protein changes, we performed a time course infection with HIV. Sup-T1 T cells were infected with HIV-1 MN at an MOI of 4.5 and cells and supernatants were sampled at 24-h intervals postinfection. Primary indicators of HIV infection were apparent early in the time course; almost 100% of cells exhibited down-regulation of CD4 by day 2 (Fig. 3A), while p24 release from infected cells began on day 4 (Fig. 3B). Flow cytometric measurement of cell viability and apoptosis (Fig. 3A) showed that a majority of the cells remained viable throughout the infection while TUNEL staining indicated that only one-fourth of the cells were undergoing apoptosis by day 8. As a secondary measurement of apoptosis, PARP cleavage was detected on day 7 (Fig. 3C). Interestingly, secretion of class I from infected cells precipitously dropped at day 5 (Fig. 3B), before the onset of apoptosis. These data indicate that cell death and apoptosis occur at late time points during infection of Sup-T1 T cells with HIV strain MN and that class I secretion predominates at early time points.
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, and PTB. Western blots further showed that HSP27, Hu R, and HMG 1/2 were up-regulated 12 days postinfection before dropping to lower levels later in the infection. The viral proteins envelope (gp120) and p24 (by ELISA) became apparent at day 4 of infection. As demonstrated in the mixed bioreactor infection, decreased levels of source proteins eIF4G, TCP-1
, and PTB during HIV infection were not a result of decreased mRNA production (measured by real-time PCR at days 14, data not shown). Taken together, these data indicate that HLA-B*0702 presentation of unique peptides occurs as a result of protein changes occurring early in HIV infection (Fig. 3E). In summary, the analysis of the HLA-B*0702 repertoire after HIV infection reveals a series of host-protein-derived peptides presented uniquely by infected cells. The host peptides most likely are presented as the result of protein level fluctuations that occur early during HIV infection. The consequence of these overabundant self peptides during infection is currently unknown but fits well into the paradigm of autoimmunity; autoimmune reactions are often present in individuals suffering from AIDS (16). Immune recognition of virus-induced host epitopes such as those reported in this study could function in the induction of autoimmune responses directly through increasing the concentration of self peptides on the cell surface. Indeed, this mechanism is supported by a recent study demonstrating autoreactivity following measles virus-induced up-regulation of self peptides (17). Irrespective of possible function, these host-derived peptide ligands provide an expanded view of peptide presentation to the immune system following viral infection.
| Acknowledgments |
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| Footnotes |
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2 Current address: Department of Microbiology and Immunology, Texas Tech University, Lubbock, TX 79430. ![]()
3 Address correspondence and reprint requests to Dr. William H. Hildebrand, Department of Microbiology and Immunology, University of Oklahoma Health Sciences Center, 975 NE 10th Street, Oklahoma City, OK 73104. E-mail address: william-hildebrand{at}ouhsc.edu ![]()
4 Abbreviations used in this paper: sHLA, soluble HLA; MOI, multiplicity of infection; PARP, poly(ADP-ribose) polymerase; MS, mass spectroscopy; HMG, high-mobility group protein; eIF, eukaryotic translation initiation factor; USP3, ubiquitin-specific protease 3; E6BP, E6-binding protein; HSP27, heat shock protein 27; TCP, tailless-complex protein; PTB, polypyrimidine tract-binding protein; Hu R, Hu Ag R. ![]()
Received for publication March 28, 2003. Accepted for publication May 1, 2003.
| References |
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