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The Journal of Immunology, 1998, 161: 1677-1685.
Copyright © 1998 by The American Association of Immunologists

Assembly of MHC Class I Molecules with Biosynthesized Endoplasmic Reticulum-Targeted Peptides Is Inefficient in Insect Cells and Can Be Enhanced by Protease Inhibitors

Yuping Deng*, James Gibbs*, Igor Bacík*, Angel Porgador{dagger}, James Copeman{ddagger}, Paul Lehner{ddagger}, Bodo Ortmann{ddagger}, Peter Cresswell{ddagger}, Jack R. Bennink1,* and Jonathan W. Yewdell1,*

Laboratories of * Viral Diseases and {dagger} Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892; and {ddagger} Section of Immunobiology, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06510


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
To study the requirements for assembly of MHC class I molecules with antigenic peptides in the endoplasmic reticulum (ER), we studied Ag processing in insect cells. Insects lack a class I recognition system, and their cells therefore provide a "blank slate" for identifying the proteins that have evolved to facilitate assembly of class I molecules in vertebrate cells. H-2Kb heavy chain, mouse ß2-microglobulin, and an ER-targeted version of a peptide corresponding to Ova257–264 were expressed in insect cells using recombinant vaccinia viruses. Cell surface expression of Kb-OVA257–264 complexes was quantitated using a recently described complex-specific mAb (25-D1.16). Relative to TAP-deficient human cells, insect cells expressed comparable levels of native, peptide-receptive cell surface Kb molecules, but generated cell surface Kb-OVA257–264 complexes at least 20-fold less efficiently from ER-targeted peptides. The inefficient assembly of Kb-OVA257–264 complexes in the ER of insect cells cannot be attributed solely to a requirement for human tapasin, since first, human cells lacking tapasin expressed endogenously synthesized Kb-OVA257–264 complexes at levels comparable to tapasin-expressing cells, and second, vaccinia virus-mediated expression of human tapasin in insect cells did not detectably enhance the expression of Kb-OVA257–264 complexes. The assembly of Kb-OVA257–264 complexes could be greatly enhanced in insect but not human cells by a nonproteasomal protease inhibitor. These findings indicate that insect cells lack one or more factors required for the efficient assembly of class I-peptide complexes in vertebrate cells and are consistent with the idea that the missing component acts to protect antigenic peptides or their immediate precursors from degradation.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Major histocompatibility complex class I molecules function to display short peptides on the cell surface for inspection by CD8+ T cells. Peptides are usually between 8 and 11 residues in length and are derived largely from a cytosolic pool of polypeptides (1, 2, 3). The evolutionary value of the class I processing system derives from its ability to inform CD8+ T cells of the presence of intracellular parasites, including viruses, bacteria, and eukaryotic parasites (4).

Class I molecules consist of two noncovalently bound subunits. The molecule is anchored to the membrane by the {alpha}-chain glycoprotein, which forms the bulk of class I molecules, including the peptide-binding groove. The conformational stability of {alpha}-chains is greatly enhanced by their assembly with ß2-microglobulin (ß2m),2 a small, nonglycosylated soluble protein. Both {alpha}-chains and ß2m are cotranslationally inserted into the endoplasmic reticulum (ER), the site of folding and assembly of secreted and cell surface proteins. Like other ER-targeted proteins, {alpha}-chain folding is facilitated by ER-resident molecular chaperones. Nascent {alpha}-chains transiently associate with the ER-chaperone, calnexin (5, 6), and probably other chaperones as well (7, 8, 9). The association of {alpha}-chains with ß2m occurs while {alpha}-chains are bound to calnexin or surrogate chaperones. The folding of nascent ß2m has not been carefully studied, but probably also involves molecular chaperones, since following synthesis, its binding to {alpha}-chains is delayed by ~10 min. The association of class I molecules with ß2m is associated with the transfer of the complex to calreticulin, a molecular chaperone closely related to calnexin. The calreticulin class I-ß2m complex next binds to a recently discovered MHC gene product, termed tapasin (or TAP-A), which is required for the complex to subsequently bind to TAP (10, 11, 12). TAP functions to deliver cytosolic peptides to the ER (13), and it is presumed that such peptides preferentially associate with class I molecules bound to the TAP that transported the peptide. Peptide binding induces the release of class I molecules from TAP, although not necessarily from calnexin (14, 15), and the completed molecules are rapidly transported through the Golgi complex to the cell surface.

Despite the impressive gains in knowledge over the past decade regarding class I biosynthesis, it is uncertain whether all of the critical components of the ER machinery required for efficient assembly of class I heterotrimers have been identified. The most rigorous strategy for addressing this question is to reconstitute class I assembly in cells or cell-free systems derived from invertebrates, since all vertebrates possess a class I Ag processing system. It is obviously advantageous to use cells that express housekeeping proteins as similar as possible to mammalian cells.

Insects are phylogenetically close to vertebrates, and their cells are able to properly target to the ER, glycosylate, fold, and assemble most mammalian glycoproteins. Insect cells are often called on when large quantities of secreted mammalian proteins are needed. Jackson, Peterson, and their colleagues have pioneered the use of Drosophila cell lines expressing class I molecules from transfected genes (16, 17). This system has yielded important insights into Ag processing (as well as milligram quantities of peptide receptive class I molecules suitable for crystallization), but suffers from problems inherent to working with cloned cell lines and the difficulty of transfecting cells, particularly when it is necessary to express multiple genes. These problems can be avoided using viral vectors to transiently express components of the class I machinery, since it is possible to infect with multiple viruses to "mix and match" gene products of interest. Baculoviruses have been successfully used in this manner to express class I molecules (18, 19) and TAP in insect cells (20), but this vector is limited by its inability to infect mammalian cells.

In the present report, we show that recombinant vaccinia viruses (rVV) can be used to express class I molecules in insect cells. VV has the advantage of being the most commonly used viral vector for expressing foreign genes in mammalian cells, and has been the workhorse in identifying Ags recognized by CD8+ T cells and in dissecting class I Ag processing mechanisms (21). Using rVVs expressing class I molecules and ER-targeted peptides, we demonstrate that mosquito cells efficiently synthesize peptide-receptive class I molecules in the ER and export them to the cell surface at a rate similar to mammalian cells. When it comes to assembling class I molecules with endogenously synthesized peptides, however, only ~3% or less of cell surface class I molecules contain peptides as opposed to ~40 to 80% of class I molecules expressed by human cells.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Cells

Aedes albopictus clone C6/36 cells (obtained from American Type Culture Collection (ATCC), Manassas, VA) were cultured at 27°C in L-15 Leibovitz medium supplemented with FBS (10%), nonessential amino acids, penicillin, and streptomycin. The .220 cells and .220 cells expressing HLA-B8 from a transfected gene (.220/B8) were generously provided by Dr. Thomas Spies (Fred Hutchinson Cancer Research Center, Seattle, WA). These cells and B3Z cells (22) were cultured in RPMI with 7.5% FBS at 37°C in a 94%:6% air:CO2 atmosphere. T2 cells and L929 cells were cultured in DMEM with 10% FBS at 37°C in a 91%:9% air:CO2 atmosphere.

Viruses

rVVs encoding the Kb {alpha}-chain, Kd {alpha}-chain, ESOVA257–264, ESNP147–155, and CD54 have been described (23, 24). A rVV expressing human tapasin under the control of the p7.5 early/late promoter was produced by inserting the tapasin cDNA into a modified form of pSC11. rVVs coexpressing ß2m with Kb, Kd, ESOVA257–264, or ESNP147–155 were produced as described by Coupar et al. (25). Briefly, the mouse ß2mb gene under the control of the VV promoter P-F was inserted into plasmid TK-7.5A containing the herpes simplex virus type I thymidine kinase gene. This plasmid was inserted in rVVs by transfecting infected cells. Double recombinants were selected using aminopterin for thymidine kinase expression (26).

Infections

Aedes cells were dislodged from flasks by incubation with EDTA-containing PBS and vigorous tapping. After washing with Dulbecco’s modified PBS (DPBS), cells were suspended at 107/ml PBS supplemented with 0.1% BSA (w/v), incubated with rVVs (10 plaque forming units/cell) for 1 h with gentle rocking at 27°C. Cells were diluted to 106 cells/ml in normal culture medium and incubated at 27°C for an additional 6 to 8 h with gentle rocking.

Flow cytometric analyses

Infected cells were washed with PBS and incubated with primary Abs for 20 min at room temperature. Ab binding was detected either using rabbit anti-mouse IgG conjugated to fluorescein (Dako, Carpinteria, CA) or in a three-step method using anti-mouse IgG1 conjugated to biotin (Jackson Immunoresearch, West Grove, PA) followed by streptavidin conjugated to phycoerythrin (Jackson Immunoresearch).

[35S]Methionine labeling and immunoprecipitation

Aedes cells (107) were infected with rVVs as described above. Six hours postinfection, cells were washed twice with DPBS and once with methionine-free DMEM. Cells were resuspended and incubated in 4 ml methionine-free DMEM for 20 min at 27°C. The cells were resuspended in fresh methionine-free DMEM (250 µl), and 200 µCi of [35S]methionine was added. Cells were incubated for 10 min at 27°C, washed with PBS, and then incubated with lysis buffer as described (23). mAb-reactive molecules were collected from postnuclear supernatants and analyzed by SDS-PAGE as described (23).

Quantitating ß-galactosidase expression in B3Z cells

Aedes cells were infected with rVVs as described above. Six hours postinfection, cells were washed with PBS three times, and 1.25 x 105 cells were aliquoted into wells of 96-well plates. Equal numbers of B3Z cells were added and mixed well with the insect cells. Cells were cocultured for 8 to 12 h, washed with PBS, and incubated with phycoerythrin-conjugated anti-Thy1.2 Ab (PharMingen, San Diego, CA) to identify B3Z cells. After washing twice with DPBS, cells were suspended in 50 µl of PBS, and incubated at 37°C. Fifty microliters of chloromethylfluorescein di-ß-D-galactopyranoside (Molecular Probes, Portland, OR) in water (1 µg/ml) was then added to each well. Five minutes later, 175 µl of PBS was added. Cells were pelleted, washed, incubated for 10 min at 37°C, and analyzed cytofluorographically. Live and phycoerythrin-positive cells were selected for analysis of green fluorescence.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Expression of rVV encoded proteins by insect cells

We initially screened four insect cell lines available from the ATCC for their ability to express rVV-encoded early gene products. This was determined by immunofluorescence performed on live (for surface proteins) or fixed and permeabilized cells (for internal proteins). Live cells were analyzed by cytofluorography and fixed cells by microscopy. The highest levels of expression were obtained using A. albopictus clone c6/36a cells (termed Aedes cells throughout; additional ATCC-provided cell lines examined were CRL 1494, CRL 8003, and CRL 1711). Levels of expression varied considerably between individual cells, suggesting clonal variation in expression of VV-encoded genes. Subcloning of cells yielded a cell line that expressed rVV gene products more uniformly than the general population, and this clone was used throughout the studies described below.

Unlike mammalian cells, which are almost invariably killed by VV if they support expression of viral genes, the same Aedes cell line was previously shown by Franke and Hruby (27) to express early VV gene products and to remain viable. Late viral gene products are not expressed, and as cells continue to divide, viral proteins are eventually degraded. Monitoring the expression of recombinant cell surface proteins under the control of early and late promoters, we confirmed these findings. Expression of cell surface proteins under control of the p7.5 viral promoter was detected as early as 3 to 4 h postinfection, peaking at 6 to 8 h postinfection. We found that the levels of expression of rVV encoded genes are similar to that of mammalian cells when cell volume is factored in, and mammalian cells are infected at 27 to 28°C (higher temperatures induce a heat shock response in insect cells resulting in inhibition of VV gene expression).

Biosynthesis of Kb molecules in Aedes cells

To study the biosynthesis of H-2Kb molecules in Aedes cells, cells were infected with rVV-expressing Kb {alpha}-chains and tested for cell surface expression of Kb by indirect immunofluorescence using the Y3 mAb. Y3 binding requires Kb to be in a native or near native conformation (28). At the temperature used for infection (28°C), Kb molecules do not require a high affinity ligand for stable expression on the surface of mammalian cells (29). As seen in Figure 1Go, expression of Y3-reactive Kb at the cell surface was negligible in the absence of endogenously synthesized ß2m. Coinfection with a rVV-expressing human ß2m resulted in expression of Y3-reactive Kb. This confirms numerous reports that expression of native Kb at the cell surface requires coexpression of ß2m (30, 31, 32, 33). Coinfection with two rVVs to produce class I molecules resulted in expression in approximately half of the cells. To increase the efficiency of synthesizing Kbß2m heterodimers, we constructed a rVV that coexpresses Kb with mouse ß2m. Infection with this rVV resulted in both higher levels of Kb expression and expression in a greater percentage of cells (Fig. 1Go).



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  		FIGURE 1. Cytofluorography of rVV-infected Aedes cells. The expression of Kb molecules on the surface of viable Aedes cells infected with the indicated rVV at 28°C was determined by indirect immunofluorescence using the Y3 mAb. MCF, mean channel fluorescence.

 
We next biochemically examined the assembly and intracellular transport of Kb molecules. VV-Kbß2m-infected cells were pulse radiolabeled for 10 min and chased for up to 3 h. All incubations were performed at 28°C. Kb molecules in detergent lysates were collected with immobilized Y3 and analyzed by SDS-PAGE. This resulted in the recovery of Kb {alpha}-chains and ß2m. Two distinctly migrating forms of Kb were recovered at all time points. We are uncertain whether this represents posttranslational modifications exclusively occurring in one of the species that increases or decreases its mobility, or, perhaps more likely in view of the rapidity of the process, the premature termination of the more rapidly migrating species.

The export of Kb molecules from the ER was monitored by endoglycosidase H (endo H) digestion Kb collected with Y3. In insect cells, N-linked oligosaccharides associated with glycoproteins are trimmed to endo H-resistant forms in the Golgi complex, but are not sialylated. Acquisition of endo H resistance is accompanied by increased mobility of glycoproteins in SDS-PAGE due to oligosaccharide trimming in the Golgi complex (sialylation is responsible for the decreased mobility of endo H-resistant Kb molecules observed in mammalian cells). In Figure 2Go, it can be seen that Y3-reactive-Kb molecules acquire endo H resistance with a t1/2 of 30 min. This is approximately twice the rate observed in Drosophila cells (34) and similar to that observed in TAP-expressing mammalian cells (17, 35). This is especially notable because export from the ER of mammalian cells is slowed at 28°C. These findings demonstrate that the ER of Aedes cells is able to rapidly assemble and export class I molecules.



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  		FIGURE 2. SDS-PAGE analysis of Kb molecules biosynthesized by Aedes cells. Aedes cells infected with VV-Kbß2m were pulse radiolabeled with [35S]methionine and chased for up to 180 min. Molecules reactive with Y3 were incubated without (-) or with (+) endo H and analyzed by SDS-PAGE.

 
Also of interest in Figure 2Go is the behavior of ß2m. Immediately following pulse labeling, radiolabeled ß2m is detected at ~60% of the maximal level collected (attained at the 30 min chase). This finding contrasts with those in mammalian cells in which detection of labeled ß2m in class I complexes occurs more slowly (t1/2 of ~15 to 30 min postchase). This may reflect either a more rapid availability of newly synthesized ß2m in insect cells or the decreased size of the preexisting nonradiolabeled ß2m pool. It is also clear that radiolabeled ß2m dissociates from Y3-reactive Kb between 30 and 180 min following synthesis. ß2m also dissociates in mammalian cells and probably represents replacement of mouse ß2m at the cell surface with bovine ß2m present in the chase medium.

Kb molecules expressed by Aedes cells are peptide receptive

Cell surface Kb molecules lacking high affinity peptides denature at 37°C and no longer bind Y3 (28). As seen in Table IGo, incubation of VV-Kbß2m infected Aedes cells for 60 min resulted in the loss of ~90% of Y3-reactive cell surface molecules. The few remaining molecules were resistant to a further 2-h incubation at 37°C. To determine whether cell surface Kb molecules synthesized at 28°C were capable of binding exogenous peptides, cells were exposed to a synthetic peptide (SIINFEKL) corresponding to residues 257–264 from OVA (OVA257–264), known to bind Kb with high affinity (36, 37, 38). Peptide exposure occurred only at temperatures below 4°C, which prevents peptide from gaining access to intracellular Kb molecules (39). Approximately 65% of cell surface Kb molecules were stabilized by exposure to OVA257–264. These molecules remained stable for 3 h at 37°C, demonstrating that most Kb molecules expressed at the surface of Aedes cells at 28°C are capable of binding antigenic peptides. Together with the biochemical findings presented above, these findings demonstrate that insect cells are capable of rapidly exporting newly synthesized class I molecules that are properly conformed and capable of binding peptides.


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  		Table I. Expression of thermostable cell surface Kb molecules by Aedescells1

 
Loading of endogenous peptides in Aedes cells: T cell activation

We first examined the ability of rVV-infected Aedes cells to assemble endogenous Kb-OVA257–264 complexes using B3Z T hybridoma cells. B3Z cells express ß-galactosidase under control of the IL-2 promoter. Triggering of their TCR, specific for Kb-OVA257–264 complexes, induces expression of ß-galactosidase (22). B3Z cells were incubated with rVV-infected Aedes cells and their level of ß-galactosidase was determined cytofluorographically following incubation with a substrate that becomes fluorescent after cleavage by ß-galactosidase.

As seen in Figure 3Go, incubation of B3Z cells with Aedes cells infected with rVVs expressing a control glycoprotein (ICAM-1) were minimally stimulated whether the cells were incubated with a control peptide or with OVA257–264. Incubation with Aedes cells infected with VV-Kbß2m and pulsed with OVA257–264 resulted in the induction of ß-galactosidase. This finding establishes that Aedes cells are capable of stimulating T cell hybridomas, extending prior results demonstrating stimulation of T cells by Drosophila cells expressing class I molecules from transfected genes (16, 41).



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  		FIGURE 3. Activation of OVA-specific T hybridoma cells by rVV-infected Aedes cells. Activation of B3Z cells by rVV-infected Aedes cells was determined cytofluorographically following incubation of cells with a fluorogenic ß-galactosidase substrate. Similar findings were made in two additional experiments.

 
We next examined whether endogenously synthesized peptides could be loaded onto Kb molecules. To minimize the requirements for accessory Ag processing components, cells were coinfected with a rVV expressing OVA257–264 delivered to the ER by the NH2-terminal signal sequence of the adenovirus 2 E3/19K glycoprotein. This signal sequence has been shown to be active in insect cells (18). Mammalian cells efficiently load ER-targeted peptides (including OVA257–264) in the absence of TAP (41) and are capable of producing tens of thousands of cell surface Kb-OVA257–264 complexes (24). Cells coinfected with VV-Kbß2m and VV-ESOVA257–264 stimulated B3Z cells (Fig. 3GoB). The specificity of induction was shown by the similar lower levels of ß-galactosidase induction when B3Z cells were incubated with Aedes cells coinfected with VV-Kbß2m and VV-ESNP147–155 (an ER-targeted Kd-binding peptide) or with VV-ß2m and VV-NP. Incubation of B3Z cells with a mixture of Aedes cells infected with either VV-Kbß2m or VV-ESOVA257–264 resulted in stimulation at a level similar to observed with VV-Kbß2m alone (Fig. 3GoB, "Mix" bar). This indicates, first, that reinfection of B3Z cells by rVVs released from Aedes cells was not responsible for stimulation, and second, that association of Kb with OVA257–264 occurred intracellularly in Aedes cells and not at the cell surface following secretion of OVA257–264. As expected, stimulation of B3Z cells was not observed following coexpression of Kbß2m with either a cytosolic version of the peptide (the E3/19K leader sequence replaced by Met), or full length OVA (not shown).

Loading of Kb by endogenous ER-targeted OVA257–264: detection by 25-D1.16 mAb

The findings obtained using the B3Z cells indicated that Aedes cells could produce Kb-OVA257–264 complexes, but provided no idea as to the number of complexes expressed at the cell surface. That this number might be quite small was first indicated by Y3 mAb staining of cells coexpressing Kbß2m with ESOVA257–264. As seen in Table IGo, these cells failed to detectably express complexes stable at 37°C above control values obtained with cells coinfected with VV-Kbß2m and a rVV expressing an ER-targeted Kd-binding peptide. In both cases, Kb molecules could be stabilized by the addition of exogenous OVA257–264, demonstrating that native Kb molecules were produced in coinfected cells in this experiment.

To facilitate detection of low numbers of Kb-OVA257–264 complexes, we used the 25-D1.16 mAb. This mAb is similar to a TCR in demonstrating high specificity for Kb-OVA257–264 complexes (42), but has a higher affinity than typical TCRs, enabling it to detect complexes more sensitively than soluble TCRs. Indeed, its sensitivity for detecting class I peptide complexes approaches that of T cells (42). As seen in Figure 4Go (top panel), binding of the 25-D1.16 mAb to VV-Kbß2m-infected cells was easily detected following exposure of cells to OVA257–264, providing direct confirmation that peptide induced stabilization of Kb to thermal denaturation reflects the presence of bound peptide.



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  		FIGURE 4. Cytofluorography of rVV-infected Aedes cells. The expression of Kb-OVA257–264 complexes in Aedes cells infected with rVVs as indicated was determined cytofluorographically using the 25-D1.16 mAb in a triple sandwich method (top two panels). In the top panel, complexes were created by incubating cells with the OVA257–264 synthetic peptide in the cold. In the bottom panel, Kb expression was determined using the Y3 mAb in a standard double sandwich assay. The lower sensitivity of this method accounts for the detection of lower number of molecules than with 25-D1.16 in the middle panel. In the top panel, control cells (gray histogram) are VV-Kbß2m infected. In the bottom two panels, control cells are uninfected cells. In a large number of additional experiments, staining of uninfected cells was similar to staining of cells infected with control rVVs.

 
Following coinfection of cells at 28°C with VV-Kbß2m and VV-ESOVA257–264, a small number of complexes could be detected at the cell surface by 25-D1.16 if a sensitive triple staining procedure was used (Fig. 4Go, middle panel). The same cells expressed considerable amounts of Y3-reactive Kb molecules (bottom panel). Kb-OVA257–264 complexes were not detected following expression of either full length OVA or the cytosolic OVA257–264 peptide (not shown).

These data indicate that Aedes cells are capable of producing small numbers of endogenous peptide class I complexes from endogenous peptides, but only if the peptides are delivered to the ER by a signal sequence.

Loading of Kb by endogenous ER-targeted OVA257–264: comparison of Aedes and mammalian cells

The low numbers of Kb-OVA257–264 complexes expressed on the Aedes cell surface contrasts greatly with their expression in mouse cells (>50,000 copies per cell) (42). It was plausible that the poor efficiency of peptide loading in insect cells was due strictly to the absence of TAP. To examine this possibility, we compared the ability of TAP-deficient mammalian cells to express Kb-OVA257–264 complexes. Following coinfection with VV-ESOVA257–264 and VV-Kbß2m at 28°C, T2 cells expressed only 1.5-fold as much Y3-reactive Kb as Aedes cells (Table IIGo). By contrast, T2 cells expressed 30-fold more Kb-OVA257–264 complexes than Aedes cells. In other experiments, T2 cells were up to 100-fold more effective at assembling endogenous Kb-OVA257–264 complexes. This indicates that relative to mammalian cells, Aedes cells have a greatly reduced capacity for creating class I complexes from endogenously synthesized ER-targeted peptides.


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  		Table II. Expression of endogenous Kb-OVA257–264 complexes by human and Aedes cells1

 
Effect of tapasin expression on the production of Kb-OVA257–264 complexes

Tapasin, which is necessary for association of TAP with class I molecules, binds to class I molecules before TAP association (11) and also appears to bind antigenic peptides (10). This raises the possibility that tapasin is needed for the loading of TAP-independent peptides. Tapasin appears to be a dedicated component of the class I Ag processing pathway, since it is induced by IFN-{gamma} and, as such, is not expected to be expressed in insect cells. To investigate whether this could contribute to the poor assembly of Kb-OVA257–264 complexes, we examined the capacity of .220 cells to assemble such complexes.

.220 cells are radiation-induced mutant EBV-transformed lymphocytes with a compromised capacity to assemble class I molecules, stemming from their absence of tapasin (11, 43, 44). .220 cells and control, tapasin-expressing .45 cells were infected overnight with the appropriate rVVs at 28°C, and the expression of Kb and Kb-OVA257–264 complexes was determined by Y3 and 25-D1.16 staining, respectively (Table IIGo). Despite the absence of tapasin, .220 cells were able to assemble class I complexes at ~55% the efficiency of .45 cells. By comparison, in the same experiments, the efficiency of complex assembly in Aedes cells was 0 to 3% of the efficiency of .45 cells. Given the potential differences between the levels of TAP-transported peptides in .45 and .220 cells that will compete with binding of OVA257–264 to Kb, as well as the differences in the degrees of rVV coinfection of the two cell lines, we do not believe that the difference in assembly efficiency is significant. This conclusion is supported by the finding that the efficiency of complex formation in HeLa cells (described in the next section) is even slightly lower than in .220 cells.

To examine more directly the role of tapasin in the generation of Kb-OVA257–264 complexes, we inserted the human tapasin gene into VV. Following infection of human cells, this rVV produced a protein of the correct Mr in SDS-PAGE that coprecipitated with human TAP, using a mAb specific for TAP1 (the biochemical characterization of VV-expressed tapasin will be described in a future publication). As seen in Table IIIGo, infection of .220/B8 cells with VV-tapasin enhanced the cell surface expression of endogenous class I molecules 1.7-fold, as determined using FITC-conjugated Abs specific for ß2m. By contrast, infection with VV-ICAM-1 resulted in an 0.7-fold decrease in ß2m expression. Such decreased class I cell surface expression is frequently associated with VV infection. As seen in Table IIIGo, tapasin coexpression with Kbß2m and ES-OVA257–264 had only a marginal effect on expression of Y3-reactive Kb molecules (11% enhancement) or Kb-OVA257–264 complexes (14% enhancement) relative to a control rVV expressing influenza virus nucleoprotein (NP). These findings clearly demonstrate that tapasin is not required for the generation of Kb-OVA257–264 complexes in VV-infected human cells, and it has has only a slight effect in enhancing complex generation.


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  		Table III. Effect of tapasin expression on class I-peptide complex formation in tapasin deficient human cells1

 
We next examined the effect of tapasin expression on the Kb expression in Aedes cells (Table IVGo). Coinfection of cells with VV-tapasin had no effect on the generation of cell surface Y3-reactive Kb molecules and did not enhance the generation of Kb-OVA257–264 complexes, which were undetectable in this experiment. This demonstrates that tapasin is not the sole factor preventing insect cells from assembling Kb-OVA257–264 complexes with greater efficiency. Note, however, that it is plausible that other human gene products are required for the proper interaction of tapasin with class I molecules, a possibility that is presently under investigation.


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  		Table IV. 1

 
Generation of endogenous Kb-OVA257–264 complexes in insect cells: effects of protease inhibitors

Insect cells have not evolved with the requirement to present peptides to the immune system and may either possess proteases that rapidly destroy oligopeptides or lack chaperones to protect peptides from proteasomes or other proteases. To examine whether the OVA257–264 peptide or its precursor is destroyed by cellular proteases, we determined the effects of four protease inhibitors on Kb-OVA257–264 complex formation. cbz-LLL-CHO and cbz-LLF-CHO are peptide aldehydes that competitively inhibit proteasomes as well as other proteases (45). cbz-LL-CHO exhibits overlapping inhibition of cellular proteases with the tripeptide aldehydes, but does not block mammalian proteasomes at the concentrations used (46). Lactacystin is a microbial product that blocks proteasomes by covalently binding to the catalytic site (47). Inhibitors were used over a fourfold concentration range (Fig. 5Go). The effects of the inhibitors on complex generation were determined using the 25-D1.16 mAb to measure cell surface complexes. Effects of the inhibitors on Kb expression were determined using the Y3 mAb.



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  		FIGURE 5. Effect of protease inhibitors on expression of cell surface Kb-OVA257–264 complexes. The amounts of Kb molecules or Kb-OVA257–264 complexes present on the surface of viable Aedes cells (first five sets of columns) or HeLa cells infected with VV-Kbß2m and VV-ESOVA257–264 in the presence of protease inhibitors at the indicated concentrations was determined by three-step immunofluorescence using 25-D1.16 and Y3 mAbs. Background staining values against uninfected Aedes and HeLa cells are subtracted; these are, respectively, 25-D1.16, 49 and 20; Y3, 86 and 20.

 
In this experiment, Aedes cells failed to produce Kb-OVA257–264 complexes above background values obtained using the 25-D1.16 mAb against uninfected cells. It was particularly impressive, therefore, that complexes were easily detected in the presence of cbz-LeuLeu and, to a much lesser extent, cbz-LeuLeuLeu. Both inhibitors interfered with VV gene expression (this can be seen by the inhibition of Kb expression and was also observed for mouse CD54, influenza virus hemagglutinin, human CD4, and human CD26 (not shown)), which probably reduced the enhancing effects of these drugs on complex formation. Two findings indicate that these effects are not due to inhibition of blocking proteasome activity. First, neither cbz-LeuLeuLeu, a highly potent proteasome inhibitors, nor lactacystin, a highly specific proteasome inhibitor, had an effect on complex formation. Second, cbz-LeuLeu does not inhibit proteasomes at the concentrations used. cbz-LL-CHO is a potent inhibitor of calpains, but calpains are unlikely to be involved in peptide destruction, since we found no effect on expression of Kb-OVA257–264 complex using calpain inhibitors I and II (not shown).

Using HeLa cells in the same experiment, we confirmed the efficient loading of Kb molecules with OVA257–264 (as noted above, the value of 40% efficiency of loading is similar to that observed in .220 cells) and found that cbz-LL-CHO had only an inhibitory effect on expression of Kb-OVA257–264 complexes, again probably related to its inhibition of viral gene expression (Fig. 5Go). In another experiment, cbz-LL-CHO failed to enhance formation of Kb-OVA257–264 complexes in TAP-deficient human .174 cells (not shown). These data suggest that the effects of cbz-LL-CHO in insect cells are due to features of insect cells that are unfavorable to the generation or intracellular trafficking of antigenic peptides or their precursors.

cbz-LL-CHO could act either by enhancing the formation of complexes or by reducing destruction of cell surface complexes. To distinguish these possibilities, VV-Kbß2m-infected Aedes cells were pulsed with peptide and incubated at 28°C for up to 3.5 h in the presence or absence of cbz-LL-CHO. cbz-LL-CHO did not affect the stability of cell surface Kb-OVA257–264 complexes or Y3-reactive Kb molecules (Fig. 6Go), indicating that it acts by enhancing complex formation.



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  		FIGURE 6. Effect of cbz-LL-CHO on stability of Kb-OVA257–264 complexes. Aedes cells infected with VV-Kbß2m and incubated with OVA257–264 synthetic peptide to create Kb-OVA257–264 complexes were incubated for the indicated times at 28°C in the presence or absence of 10 µM cbz-LL-CHO. The amounts of Kb molecules or Kb-OVA257–264 complexes present on the surface of viable Aedes cells infected with VV-Kbß2m and VV-ESOVA257–264 were determined by standard two-step immunofluorescence using Y3 and 25-D1.16 mAbs, respectively.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our findings indicate that Aedes cells are capable of generating similar amounts of peptide-receptive cell surface rVV-encoded class I molecules as mammalian cells, but produce complexes from an endogenously produced ER-targeted peptide far less efficiently than mammalian cells. Due to the availability of the 25-D1.16 mAb, we have focused these studies on generation of the Kb-OVA257–264 complexes. The generality of the our findings, however, are supported by additional experiments (unpublished) in which we failed to observe the generation of thermostabile Kd molecules following the infection of Aedes cells with a rVV coexpressing Kd with mouse ß2m and a rVV expressing an ER-targeted, Kd-binding peptide (corresponding to residues 147 to 155 from influenza virus NP). We previously showed that coinfection of T2 cells with these rVVs results in expression of considerable numbers of thermostabile, cell surface Kd molecules (48).

The rapidity of class I assembly and export from the ER in Aedes cells (Fig. 2Go), which is similar to that observed in mammalian cells, contrasts to the findings obtained with Drosophila cells, where Y3-reactive Kb export from the ER occurred two to three times more slowly than mammalian cells (17). Expression of canine calnexin in Drosophila cells slowed ER export even further (17), although it had the salubrious effect of enhancing the efficiency of assembly (49). We have not examined the efficiency of Kb folding in Aedes cells, and it is possible that it is low relative to mammalian cells. Following coinfection of Aedes cells with a rVV-expressing canine calnexin, however, we have failed to detect any difference in Kb folding, trafficking, cell surface expression, or ability to assemble with ER-targeted OVA257–264 (unpublished findings). It is plausible that Aedes cells express ER chaperones that are better able to facilitate Kb folding than those present in Drosophila cells. Another possibility is that Drosophila chaperones are fully able to support Kb folding, but are present in limiting quantities and cannot cope with the possibly large amount of Kb produced from a strong promoter. Findings that class I molecules are efficiently assembled in mutant cells lacking either calnexin (9) or a glucosidase needed for calnexin function (8) indicate that calnexin is not required for assembly of class I molecules. More directly, we recently demonstrated that calnexin is not required for the assembly of Kb-OVA257–264 complexes (50). In any event, insect cells express both calnexin and calreticulin homologues (51, 52). Given that calnexin interacts with oligosaccharides present in all eukaryotic cells and its ability to interact with numerous proteins, it seems likely that insect calnexin/calreticulin can interact with class I {alpha}-chains.

The major finding in this study is that TAP-independent assembly of Kb molecules with an ER-targeted peptide occurs inefficiently in Aedes cells relative to mammalian cells and can be enhanced by protease inhibitors. There are several plausible mechanisms that could contribute to this finding.

1. The delivery of ESOVA257–264 to the ER may be less efficient in insect cells than in mammalian cells. This could be due to either decreased delivery to the ER from the cytosol or inefficient liberation of the peptide from the signal sequence by signal peptidase. The latter possibility is unlikely, since first, mammalian and insect signal peptidases are not known to differ in specificity; and second, ESNP147–155 is similarly inept at providing peptides capable of associating with Kd in the ER despite having a different junctional sequence. The former possibility cannot be as easily dismissed, and indeed, cbz-LL-CHO might enhance the generation of Kb-OVA257–264 complexes by prolonging the survival of ESOVA257–264 in the cytosol. Notably, ESOVA257–264 cannot be targeted cotranslationally to the ER by signal recognition particle, since its limited size (26 residues) precludes emergence of the signal sequence from the ribosome before translation termination (this requires ~40 residues). Thus, ESOVA257–264 is probably exposed to cytosolic proteases before its translocation, and it is plausible that Aedes cells more actively degrade cytosolic peptides than mammalian cells, due either to enhanced proteolysis or diminished chaperone-mediated protection (53).

2. Evolution may have altered the protease activity of the ER of vertebrate cells to minimize the destruction of class I binding peptides, while optimizing trimming of TAP-transported peptides, particularly those with NH2-terminal extensions (54).

3. The ER of mammalian cells may contain dedicated chaperones that protect antigenic peptides from proteolysis and ferry them to class I molecules. There would be a number of candidates for such an ER-localized peptide chaperone. By virtue of its ability to bind both class I molecules and peptides, tapasin was the prime candidate for the missing chaperone in insect cells. Our findings clearly indicate, however, that tapasin is dispensable for the formation of Kb-OVA257–264 complexes in human lymphoid cells. Srivastava and colleagues have shown that gp96, a resident ER chaperone, binds antigenic peptides and has the capacity to provide them to class I molecules for immune recognition (55). Other mammalian chaperones are known to bind antigenic peptides (53); indeed, TAP-transported peptides bind to numerous ER chaperones (56, 57). Chaperones are highly conserved among eukaryotes, and insect cells possess close homologues for all of the mammalian chaperones reported to bind antigenic peptides. Since antigenic peptides probably bind to regions in chaperones that recognize unfolded proteins, it is likely that insect chaperones bind antigenic peptides to an extent similar to mammalian chaperones. One or more vertebrate chaperones, however, may possess alterations that enable them to participate in Ag processing in a more directed manner than chaperones from lower eukaryotes.

In future studies, it should be possible to use the system we describe herein to systematically define the specialized components that enable mammalian cells to efficiently generate peptide-class I complexes in the ER.


   Acknowledgments
 
Bethany Buschling provided outstanding technical assistance. We thank Nilabh Shastri (University of California, Berkeley) for providing the B3Z cells, Marian Orlowski (Mt. Sinai School of Medicine, New York, NY) for the panel of protease inhibitors, Thomas Spies (Fred Hutchinson Cancer Institute, Seattle, WA) for .220 cells, and Robert DeMars (University of Wisconsin, Madison, WI) for .45 cells.


   Footnotes
 
1 Address correspondence and reprint requests to Dr. Jonathan W. Yewdell or Dr. Jack R. Bennink, Room 213, Building 4, National Institutes of Health Bethesda, MD. E-mail address: Back

2 Abbreviations used in this paper: ß2m, ß2-microglobulin; ER, endoplasmic reticulum; VV, vaccinia virus; NP, nucleoprotein; ES, adenovirus 2 gp19 signal sequence; DPBS, Dulbecco’s modified PBS; endo H, endoglycosidase H. Back

Received for publication December 16, 1997. Accepted for publication April 14, 1998.


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 Introduction
 Materials and Methods
 Results
 Discussion
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