10.1128/JVI.74.8.3918-3923.2000.

2000, 74(8):3918. DOI:J. Virol. Dexter T. K. Poon, Lori V. Coren and David E. Ott Requires Envelope Glycoprotein PackagingHuman Immunodeficiency Virus Type 1 Efficient Incorporation of HLA Class II onto

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JOURNAL OF VIROLOGY,0022-538X/00/$04.0010

Apr. 2000, p. 3918–3923 Vol. 74, No. 8

Copyright © 2000, American Society for Microbiology. All Rights Reserved.

Efficient Incorporation of HLA Class II onto HumanImmunodeficiency Virus Type 1 Requires Envelope

Glycoprotein PackagingDEXTER T. K. POON, LORI V. COREN, AND DAVID E. OTT*

AIDS Vaccine Program, SAIC Frederick, National Cancer Institute, Frederick Cancer Researchand Development Center, Frederick, Maryland 21702-1201

Received 9 July 1999/Accepted 25 January 2000

HLA class II DR is one of the most abundant cell surface proteins incorporated onto human immunodefi-ciency virus type 1 (HIV-1) during budding. The mechanism for HLA class II protein incorporation is notknown and may involve a viral protein. To determine whether Env affects HLA class II protein incorporation,HIV-1 virions, either with or without Env on their surface, were produced from HLA class II-expressing cellsand analyzed by whole-virus immunoprecipitation with antisera against HLA class II proteins. HLA class IIproteins were detected on virions only when wild-type Env was incorporated, while similar experiments showedthat HLA class I proteins were incorporated independent of Env packaging. Therefore, the packaging of HIV-1Env protein is required for the efficient incorporation of HLA class II but not class I proteins into the virion.Analysis of two Env mutants revealed that the presence of a 43-amino-acid sequence between amino acids 708and 750 in the gp41TM cytoplasmic tail was required for efficient incorporation of HLA class II proteins. Thesedata show that HIV-1 actively incorporates HLA class II proteins in a process that, either directly or indirectly,requires Env.

Human immunodeficiency virus type 1 (HIV-1) acquiressurface glycoproteins, including Env, by budding through theplasma membrane (reviewed in reference 35). Env is a com-plex made up of the surface glycoprotein gp120SU nonco-valently bound to the gp41TM transmembrane (TM) protein.This complex binds to CD4 and various coreceptors and allowsfor viral entry by mediating virus-cell membrane fusion (re-viewed in reference 19). Unlike type C retroviral TM proteins,gp41TM has a relatively long cytoplasmic C terminus (tail) thathas been shown to induce cytopathic effects and affect thepackaging of Env into the virion (9, 12, 21, 36, 37).

In addition to Env, many different cellular proteins havebeen detected on the surface of HIV-1 (reviewed in reference24). Human leukocyte antigen (HLA) class II DR is the mostcommonly detected and the most abundant host protein onHIV-1 propagated in cell culture and on virus isolated frompatient plasma (24, 33). Based on biochemical analysis, HLAclass II DR may be present at approximately 1.5 to 2 times thelevel of gp120SU on the surface of HIV-1 produced from theH9 human T-cell line (2). Although the mechanism by whichHIV-1 acquires HLA class II DR is not known, one possibilityis that it is merely incorporated by its proximity to the buddingvirion. However, many host proteins appear to be excludedfrom enveloped viruses in general (38). Interestingly, onlyHLA class II DR is appreciably incorporated into virions, evenwhen the other isotypes of HLA class II (DP and DQ) arepresent on the surface of cells (2, 8, 14, 34). While these resultsimply that HLA class II DR is specifically incorporated into thevirion, this possibility has not been demonstrated.

Expression of Env and HLA class II protein incorporation.One possible mechanism for HLA class II protein incorpora-tion is that the packaging of Env might cause the placement ofHLA class II proteins onto the virion. To test this hypothesis,the presence of HLA class II proteins on virions with andwithout Env was examined. Since microvesicles containingHLA class II proteins have been shown to contaminate evenpurified H9 and peripheral blood mononuclear cell (PBMC)virion preparations (4, 14), simply examining virus stocks forHLA class II proteins would detect protein from both virionsand microvesicles. Therefore, the presence of HLA class IIproteins on the virions was examined by a whole-virus immu-noprecipitation with HLA class II antiserum (i.e., withoutdetergent lysis of the particle), followed by detection of pre-cipitated virions by immunoblotting for Gag. To produce Env-deficient virions from HLA class II-expressing cells, these cellswere infected with VSV-G pseudotypes of wild-type or Envmutant viruses produced by cotransfection of 293T humankidney cells with a proviral DNA construct (pNL4-3 [1] or itsmutants) and a VSV-G expression plasmid, pCMVHg (6), byusing a calcium phosphate mammalian cell transfection kit (5Prime-3 Prime, Inc., Boulder, Colo.). These pseudotypedstocks were then used to infect HLA class II-expressing cells,either H9 cells or phytohemagglutinin-stimulated PBMCs, inthe presence of 2 mg of Polybrene per ml. The inoculating viruswas removed by washing at 4 h postinfection, and virus-con-taining clarified supernatants were harvested 48 to 72 h postin-fection.

Virions were produced from H9 cells by using wild-typeDNA and two NL4-3 mutant constructs that do not incorpo-rate Env (25): Env2 that produces no Env and the p6Gag

mutant Y36S-L41P that expresses but does not incorporateEnv. Immunoprecipitation was carried out with cell-free su-pernatants containing equal amounts of p24CA (typically 10 to30 ng, as determined by a p24CA enzyme-linked immunosor-bent assay (ELISA) kit [New England Nuclear, Boston,Mass.]) with 1% (vol/vol) rabbit anti-HLA class II b-chain

* Corresponding author. Mailing address: AIDS Vaccine Program,SAIC Frederick, NCI-Frederick Cancer Research and DevelopmentCtr., P.O. Box B, Bldg. 535, Rm. 433, Frederick, MD 21702-1201.Phone: (301) 846-5723. Fax: (301) 846-5588. E-mail: ott@avpvx1.ncifcrf.gov.

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serum (DJ-31681; AIDS Vaccine Program, National CancerInstitute-Frederick Cancer Research and Development Center[NCI-FCRDC]) and 2 mg (dry weight) of hydrated proteinA-Sepharose beads (Amersham Pharmacia Biotech, Inc., Pis-cataway, N.J.). After the antibody-bead complexes werewashed three times with phosphate-buffered saline (Life Tech-nologies Inc., Gaithersburg, Md.), the amount of virus precip-itated was visualized by immunoblot analysis (essentially asdescribed in reference 17) with a mouse monoclonal antibodyagainst p24CA (AIDS Vaccine Program, NCI-FCRDC) andthe Enhanced ChemiLuminescence procedure (AmershamPharmacia Biotech). To eliminate most of the signals producedby the secondary antibody binding to the light and heavy chainsof the precipitating antibody, the gels for these immunoblotswere run under nonreducing conditions to keep the antibodymolecules intact. Under these conditions, p24CA and Pr55Gag

migrated similarly under reducing conditions (data not shown),while the majority of the background from the precipitatingantibodies migrated at a higher molecular weight (;180 kDa).

Wild-type and mutant virions produced from H9 cells wereanalyzed by using this HLA class II immunoprecipitation assay.The p24CA immunoblot of the samples showed that only thelane containing the wild-type immunoprecipitate sample con-tained bands corresponding to p24CA and Pr55Gag, indicatingthat the wild-type HIV-1 was precipitated by the anti-HLAclass II serum (Fig. 1). A blot of a precipitation experimentwith an irrelevant antibody (rabbit anti-horse immunoglobulinG [IgG]; Sigma Chemical Co., St. Louis, Mo.) did not produceany Gag signal, demonstrating that precipitation was specificfor the HLA class II antibody. Virions produced from cells thatdo not express HLA class II proteins did not precipitate in thisassay (data not shown). These data indicate that HLA class IIproteins are present on the surface of the wild-type virion, asexpected. In addition to the viral bands in the immunoblot,other bands were present that did not appear to be viral andwere present in the mock control, possibly associated with theHLA class II-precipitating antibody. In contrast to the wild-type result, samples of mutant virions that either fail to expressthe Env complex, Env2, or express Env on the cell surface butfail to incorporate Env, Y36S-L41P, did not produce any de-tectable Gag signal in this assay. The inability of these mutantvirions to be precipitated indicates that HLA class II proteinswere not incorporated onto the virion surface in the absence ofEnv packaging.

HLA class II protein incorporation and gp41TM. Since theHIV-1 Env complex contains a long cytoplasmic tail that couldinteract with HLA class II proteins, two mutants were con-

structed that truncated the C terminus of gp41TM (summarizedin Fig. 2A) by the insertion of a nonsense codon at eitheramino acid (aa) 708 (C to T at nucleotide 8342 [1]), Cyt1, or aa751, Cyt2 (T to G at nucleotide 8472). The Cyt2 mutation didnot alter the overlapping rev reading frames. DNA fragmentsthat contained the mutation were produced by the PCR over-lap extension procedure (18) and then reinserted into thepNL4-3 molecular clone. All mutants were confirmed bydideoxy sequencing. Following transfection, the two mutantDNA constructs produced wild-type levels of particles, as mea-sured by reverse transcriptase and p24CA ELISA (Table 1).Virions were centrifuged through a 20% sucrose cushion (aspreviously described [25]), and equal amounts of particles (100ng by p24CA) were analyzed by immunoblot to examine theEnv proteins in the virions. Probing a blot with a gp41TM

antiserum (Fitzgerald International, Inc., Concord, Mass.) re-vealed that the wild-type samples contained a band at 43 kDa,corresponding to gp41TM, while those from the Cyt1 and Cyt2mutant virions contained bands at 29 and 37 kDa, respectively(Fig. 2B). These sizes were close to those predicted for thenonglycosylated sizes of the three species: 39.7 kDa forgp41TM, 22.8 kDa for Cyt1, and 27.5 kDa for Cyt2 (Fig. 2B).Since this antiserum also reacts with gp120SU, other higher-migrating bands were present in the immunoblot in addition tothe gp41TM bands (Fig. 2B). These same larger bands, corre-sponding to gp160Env and gp120SU, were present in an immu-noblot using a gp120SU monoclonal antibody (AIDS VaccineProgram), confirming that these mutants incorporated Env(Fig. 2B). In contrast, results of blots with either gp120SU orgp41TM antiserum showed that the Env2 mutant did not con-tain either protein as expected (Fig. 2B). Blotting with theAIDS patient serum (AIDS Vaccine Program) also detectedthe full-length and the truncated gp41TM proteins in the wild-type and mutant samples, respectively. Similar amounts of theother viral proteins (e.g., Gag and Pol) were present in boththe wild-type and the mutant samples (Fig. 2B).

To test for mutant Env function, the Cyt1 and Cyt2 mutantswere tested in an HCLZ single-round infectivity assay (longterminal repeat-driven LacZ complementation assay as previ-ously described [25]). Supernatants containing equivalentp24CA levels derived from two independent transfections weretested. The results showed that both the Cyt1 and Cyt2 mu-tants were infectious and that the truncated Env proteins werefunctional, though their infectivity was 100-fold lower than thewild-type virus (Table 1). As expected, the Env2 mutant wasnot significantly infectious since it lacks Env.

To determine if the truncations in the cytoplasmic tail ofgp41TM had an effect on HLA class II protein incorporation,wild-type and mutant viruses were produced from H9 cells orPBMCs and examined by the whole-virus anti-HLA class IIimmunoprecipitation-immunoblot protocol. The blots of thewild-type and the Cyt2 virion precipitates produced from ei-ther H9 cells (Fig. 3A) or from PBMCs (Fig. 3B) readilydetected both p24CA and Pr55Gag. In contrast, the blots of theEnv2 and Cyt1 samples did not detect any Gag proteins (Fig.3A and B). In addition, experiments with irrelevant antibodyalso failed to recover Gag signal from any of the samples (Fig.3). Therefore, the Cyt2 mutant was precipitated by anti-HLAclass II serum, indicating that this mutant incorporates HLAclass II proteins. In contrast, the Cyt1 mutant was not precip-itated, indicating that there were no detectable HLA class IIproteins on the virion surface of the Cyt1 mutant, even thoughthey were present in the pelleted supernatant, most likely as-sociated with microvesicles (data not shown). Therefore, thepresence of sequences between aa 708 and 750 within gp41TM

FIG. 1. Immunoblots of whole-virus immunoprecipitates. The p24CA blots ofvirions precipitated with either HLA class II or an irrelevant antibody (rabbitanti-horse IgG [Ir]) are presented. The precipitated samples are identified abovetheir respective lanes. A sample of wild-type virions is included for reference.

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are required for the efficient incorporation of HLA class IIproteins.

HLA class I protein incorporation is Env independent. HLAclass I proteins are also present on HIV-1 particles but at lowerlevels than HLA class II proteins (reviewed in reference 24).Based on our results, we examined whether Env influencesHLA class I protein incorporation onto the surface of thevirions. Supernatants containing virions (with equivalentp24CA amounts) produced from H9 cells or PBMCs were sub-jected to the whole-virus immunoprecipitation procedure withantiserum to HLA class I proteins (DJ-31679; AIDS VaccineProgram, NCI-FCRDC). Immunoblotting of the HLA class Iprecipitations detected p24CA and Pr55Gag in all of the virus-containing samples from H9 and PBMCs (Fig. 3). The samplesfrom the irrelevant antibody precipitation control contained noGag proteins, showing that the results were specific for HLA

class I proteins. The ability to precipitate virions with anti-HLA class I serum shows that HLA class I proteins werepresent on the wild-type virus as well as the Env mutant vi-ruses. Thus, incorporation of HLA class I proteins onto thesurface of HIV-1 is independent of the presence of Env.

Tyrosine endocytosis signal. Recent studies have shown thata YXXL endocytosis signal in gp41TM, YSPL at aa 710 to 713of NL4-3, interacts with the clathrin-mediated endocytosis sys-tem to internalize HIV-1 Env (3, 5, 13, 32). Additionally, thissignal is a determinant for polar budding of HIV-1 in lympho-cytes (11). Since the YSPL sequence is within the aa 708 to 750region identified above, the tyrosine at aa 710 of pNL4-3 waschanged to a serine (TMY710S) and mutant virions were an-alyzed. Consistent with the report of a similar mutation madein the HXB2 DNA clone (11), this virus incorporated its Env(Fig. 4A) and was infectious (data not shown). Both of theHLA class II and class I whole-virus immunoprecipitation ex-periments could readily detect p24CA in the blots from boththe wild-type and TMY710S virion samples produced from H9cells (Fig. 4B). Slight differences in signal intensities among theblots were not reproducible: in total, all of the samples pro-duced similar results. Therefore, this mutant contains HLAclass II proteins on the virion surface in the absence of theintact YSPL signal, showing that this motif is not required forefficient HLA class II protein incorporation.

Incorporation of HLA class II proteins and Env. The resultspresented above show that the packaging of Env is required forefficient incorporation of HLA class II proteins onto HIV-1 inboth cultured and primary cells. The phenomenon is specificfor this major histocompatibility complex protein as the incor-poration of HLA class I proteins was not appreciably affected

FIG. 2. Immunoblot analysis of gp41TM mutants. (A) Env truncation mutants are presented with mutations indicated above the altered regions. Vertical arrowindicates the Env cellular protease-processing site. (B) The gp41TM, gp120SU, and AIDS patient serum immunoblots of virions (100 ng of p24CA in each lane) producedfrom 293T cells are presented. Virion samples are labeled above their respective lanes. Positions of the molecular weight standards are presented at left, and those ofthe viral proteins are identified at the margins of the blots. p, gp41TM bands in the AIDS patient serum blot.

TABLE 1. Infectivity of Env mutants

Virus Concn of p24(ng/ml)a

RT activity(cpm/ml)b

No. of BCFUc

(U/ml)

Mock ,0.008 1.3 3 103 ,1NL4-3 28.7 8.1 3 106 1 3 104

Env2 13.9 5.4 3 106 ,1Cyt1 25.3 7.4 3 106 1 3 102

Cyt2 27.4 6.8 3 106 3 3 102

a p24CA content determined by enzyme-linked immunosorbent assay kit(Retro-Tek, Buffalo, N.Y.).

b RT, reverse transcriptase. RT activity assay was performed as previouslydescribed (16).

c BCFU, blue cell-forming units.

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by Env packaging. Incorporation, not just expression of Env, isrequired for this effect as the Y36S-L41P Gag mutant did notincorporate detectable amounts of HLA class II proteins. To-gether, these data show that efficient HLA class II proteinincorporation requires the packaging of Env. However, HLAclass II proteins are not required for Env packaging since Envincorporation is not affected when produced from cells thatlack HLA class II proteins, such as 293T cells (M. T. Esser,NCI-FCRDC, personal communication).

The requirement for Env-mediated HLA class II proteinincorporation suggests that Env brings HLA class II proteinsinto the virion. Our mutagenic analysis of gp41TM showed thatsequences in its cytoplasmic tail, between Env aa 708 and 750,were important for this effect. Our data also show that theYXXL endocytosis signal within this 43-aa region is not re-quired for efficient HLA class II protein incorporation. Addi-tionally, this region is upstream from the two amphipathichelices present in the gp41TM cytoplasmic region (Fig. 2A) thatare thought to be involved in the cytopathic effect of Env (21).Currently, it is not obvious what sequence or sequences in thisregion are required for HLA class II protein incorporation andwhether the absence of the 43-aa sequence disrupts the con-formation of other regions of Env. Additional experiments arebeing carried out to define the sequences within this 43-aaregion important for efficient HLA class II protein incorpora-tion as well as the portion of Env that is sufficient for this effect.

The results presented here demonstrate that HLA class IIproteins are actively incorporated into HIV-1 particles and notby random association with the budding virion. The findingthat a 43-aa stretch of gp41TM close to the inner leaflet of theplasma membrane is required for HLA class II protein incor-poration suggests that this occurs either by a direct or a third-

party interaction between the molecules. However, a directinteraction between HLA class II proteins and Env has notbeen demonstrated. It has been observed that HLA class IIproteins and Env share some antigenic properties (30), andthese could be a basis for interaction. However, at least someof these homologous regions are present in the portion of theC-terminal tail (15) that is dispensable for HLA class II proteinincorporation.

An alternative mechanism, that Env directs the budding ofHIV-1 to regions of the plasma membrane that contain HLAclass II proteins and thereby indirectly cause HLA class IIprotein incorporation, cannot be formally ruled out. HIV-1 hasbeen observed specifically budding from pseudopodia of acti-vated T cells (27). It has been shown that Env can cause HIV-1to bud basolaterally in polarized epithelium (26) and that thiseffect requires the same sequences in the cytoplasmic tail ofgp41TM that are deleted in our Cyt2 mutant (22). Since HLAclass II proteins were present on the Cyt2 mutant, it is unlikelythat specific HLA class II protein incorporation is due to thistype of polarized budding. Likewise, the YSPL signal ingp41TM that can also produce polarized budding of HIV-1 (11)is dispensable for HLA class II protein incorporation. How-ever, the presence of the first 10 membrane-proximal aminoacids (including YSPL) of the gp41TM cytoplasmic tail canredirect the localization of chimeric VSV-G proteins on thecell surface, suggesting that this sequence might also influenceEnv localization (20). However, it has not been determined ifsequences in this region other than YSPL also influence HIV-1budding; therefore, its relevance for HLA class II protein in-corporation is unclear. One study has shown that redirection ofEnv does not necessarily affect the site of Gag budding (29).Based on the current understanding of Env and viral budding,

FIG. 3. HLA incorporation of gp41TM mutants. The p24CA immunoblots of whole-virus precipitates of virions produced from H9 cells (A) or PBMCs (B) by usingrabbit anti-HLA class II antibodies, anti-horse IgG (Ir), or anti-HLA class I antibodies are presented. The precipitated samples are identified above their respectivelanes. A sample of wild-type virions is included for reference.

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HLA class II protein incorporation does not appear to be dueto Env redirecting the assembling virus to an HLA class II-richsite on the cell.

While HIV-1 Env is important for HLA class II proteinincorporation, it does not seem to affect HLA class I proteinincorporation. HLA class I and class II proteins follow differ-ent biosynthetic and transport pathways (23, 28): class I mol-ecules traffic through compartments of the secretory pathway,whereas class II molecules interact with the endocytic pathway.Even though both HLA class II molecules and HIV-1 Env areendocytosed, our finding that a gp41TM endocytosis signal isnot required for HLA class II protein uptake suggests that thisprocess is not important for its incorporation. However, Envpeptides are presented by HLA class II proteins in the absenceof endocytosis (32), possibly suggesting that Env and HLAclass II proteins could associate during transit to the cell sur-face.

The presence of HLA class II proteins on the virion surfacemight also influence HIV-1 pathogenesis. It has been shownthat the presence of HLA class II proteins on HIV-1 modestlyincreased viral infectivity in vitro by a factor of 1.6 to 2.3 (7).Given the number of rounds of replication in an infected in-dividual, even this modest increase could be compounded intoa very large difference in viral load over time (10). It has alsobeen proposed that antigen-bearing HLA class II proteins onHIV-1 could induce anergy and possibly apoptosis in theCD41 T cells (2, 31). HIV-1 might specifically incorporateHLA class II proteins to selectively eliminate virus-specificCD4 helper T cells that could mount immune responses in aninfected individual. This might explain why HIV-1 has a mech-

anism, either direct or indirect, to actively incorporate HLAclass II proteins. Further study of the mechanism of HLA classII incorporation might further our understanding of HIV-1pathology and disease.

This project has been funded in whole or in part with federal fundsfrom the National Cancer Institute, National Institutes of Health,under contract no. NO1-CO-56000.

We thank Jane Burns, University of California, San Diego MedicalCenter, for the kind gift of pCMVHg; David Waters, NCI-FCRDC, forthe kind gift of the HCLZ cells; Ray Sowder for help with computermolecular weight predictions; Mark Esser for sharing 293T data beforepublication; and Lou Henderson, Jeffrey Rossio, and Mark Esser forhelpful suggestions on the manuscript.

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FIG. 4. Endocytosis signal mutant. (A) Immunoblot analysis of theTMY710S mutant by using gp41TM antibody and p24CA anti-serum. Each blot islabeled with the immune reagent used, and the virion samples are identifiedabove their respective lanes. Lane sssDNA, the sample of a negative controltransfected with sheared salmon sperm DNA. (B) The p24CA immunoblots ofwhole virus precipitates of viruses produced from H9 cells by using rabbit anti-HLA class II antibodies, anti-horse IgG, or anti-HLA class I antibodies arepresented. The precipitated samples are identified above their respective lanes.A sample of wild-type virions is included for reference.

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