Many human solid tumors express MHC class II (MHC-II) molecules, and proteins normally localized to melanosomes give rise to MHC-II-restricted epitopes in melanoma. However, the pathways by which this response occurs have not been defined. We analyzed the processing of one such epitope, gp10044–59, derived from gp100/Pmel17. In melanomas that have down-regulated components of the melanosomal pathway, but constitutively express HLA-DR*0401, the majority of gp100 is sorted to LAMP-1high/MHC-II+ late endosomes. Using mutant gp100 molecules with altered intracellular trafficking, we demonstrate that endosomal localization is necessary for gp10044–59 presentation. By depletion of the AP-2 adaptor protein using small interfering RNA, we demonstrate that gp100 protein internalized from the plasma membrane to such endosomes is a major source for gp10044–59 epitope production. The gp100 trapped in early endosomes gives rise to epitopes that are indistinguishable from those produced in late endosomes but their production is less sensitive to inhibition of lysosomal proteases. In melanomas containing melanosomes, gp100 is underrepresented in late endosomes, and accumulates in stage II melanosomes devoid of MHC-II molecules. The gp10044–59 presentation is dramatically reduced, and processing occurs entirely in early endosomes or stage I melanosomes. This occurrence suggests that melanosomes are inefficient Ag-processing compartments. Thus, melanoma de-differentiation may be accompanied by increased presentation of MHC-II restricted epitopes from gp100 and other melanosome-localized proteins, leading to enhanced immune recognition.
Melanoma immunotherapy strategies developed over the last several years have been focused on cytotoxic CD8 T cells that recognize defined peptide Ags presented by MHC class I molecules on tumor cells and widely recognized by T cells from patients (1, 2). Melanocyte differentiation proteins (MDPs)5 represent a majority of these shared melanoma Ags. Several groups have also identified MHC class II (MHC-II)-associated peptide epitopes derived from MDPs (3, 4, 5, 6, 7, 8, 9), and some of these have more recently been included in vaccination regimens (1, 10). Although many melanoma cells express MHC-II molecules constitutively or in response to IFN-γ induction (11, 12, 13), the extent to which they present MHC-II-associated epitopes derived from endogenously synthesized MDPs and the pathways leading to this presentation are not well understood. IFN-γ treatment of melanoma cells results in down-regulation of MDP expression (14 and J. Fortini and M. S. Marks, unpublished data) and there appears to be an inverse correlation between expression of MHC-II molecules and MDPs (15). It remains unclear whether this correlation also prevents presentation of MDP-derived epitopes in MHC-II+ melanoma cells. Alternatively, if presentation occurs, the common intracellular compartment where MHC-II molecules and melanosomal proteins meet for Ag processing remains to be identified.
Pigmented melanoma cells contain conventional endosomes as well as the melanosomes that synthesize and store the pigment melanin (16). In contrast to melanosomes, which are distinct organelles, the compartments in which MHC-II processing occurs correspond to conventional endosomes and lysosomes, modified by expression of MHC-II, invariant (Ii) chain, and HLA-DM (17). Despite the fact that melanosomes share a number of characteristics with conventional endosomes (18), their MHC-II processing abilities have not been investigated. In addition, many melanomas lose their ability to synthesize pigment and no longer contain identifiable melanosomes (19, 20). These changes are partially due to down-regulation of expression of MDPs and other components of the intracellular machinery involved in maintaining the identity of melanosomes within the endocytic pathway (21, 22). Given that MDPs expressed in nonmelanocytic cells localize to conventional late endosomes (23, 24, 25), it is conceivable that their intracellular localization will also be perturbed in melanoma cells that display this de-differentiated phenotype.
Protein gp100 (also called Pmel17 or Silver) is an MDP that plays a critical role in melanosome formation (26), and is also a tumor Ag expressed by more than 75% of human melanomas (27). The gp100 protein expressed endogenously in both melanoma and nonmelanoma cells is processed for presentation of multiple epitopes by MHC-II molecules (7, 28). This activity suggests that gp100 is targeted to MHC-II processing compartments via an intracellular pathway. In melanocytes and pigmented melanoma, gp100 is localized to the most immature (stage I and stage II) melanosomes and poorly represented in late endosomes and lysosomes (16). During transit through stage I melanosomes, the lumenal domain of gp100 is cleaved by a proprotein convertase into 26- and 70-kDa fragments (24, 29). The latter can be further processed to a 34- to 38-kDa molecule (30), and forms the fibrillar scaffolding characteristic of stage II melanosomes and on which melanin is deposited (29). In contrast, gp100 accumulates in conventional late endosomes and lysosomes when expressed in nonmelanocytic cells (24). This targeting is mediated by sequences encoded in the gp100 lumenal domain, the removal of which results in predominant retention of the protein in early endosomes (31, 32). Two alternative mechanisms for gp100 intracellular sorting have been proposed: a mechanism directly from the trans-Golgi network, and the other indirectly via endocytosis from the plasma membrane (33, 34, 35). Thus, the cell biology of this system offers a unique opportunity to investigate the compartments with class II processing capabilities in melanomas and the impact of subcellular trafficking of an intact membrane protein on its processing for Ag presentation.
Two distinct Ag processing pathways have been defined that enable presentation of a broad range of peptides derived from exogenous proteins entering endocytic compartments. In the classical pathway, epitopes require the proteolytic processing capacity of the highly acidic late endocytic compartments, where nascent class II molecules exchange CLIP for antigenic peptide (36). The presentation of such epitopes is sensitive to depletion of newly synthesized MHC-II molecules through the use of protein synthesis inhibitors and ablation of Ii chain and HLA-DM functions (37, 38). In the alternative pathway, epitopes are generated within mildly proteolytic conditions of early endosomes and are loaded on mature MHC-II molecules recycling through this compartment, independently of Ii chain and HLA-DM (37, 39, 40, 41, 42). Which of these pathways is involved in processing of endogenously synthesized gp100 protein leading to MHC-II presentation in melanoma cells has not been fully investigated.
In the current work we investigated the mechanisms for endogenous presentation of the HLA-DR*0401- restricted gp10044–59 epitope. We used melanoma cells that express MHC-II molecules constitutively and gp100 mutants targeted to early or late endosomes to identify the endosomal compartments with roles in generating this epitope and the processing requirements for its presentation. By expressing MHC-II molecules in pigmented melanoma cells, we also investigated the processing capabilities of melanosomes. Finally, we established that pigmented melanoma cells displayed lower levels of gp100 epitope compared with their de-differentiated counterparts that do not contain melanosomes. Our results emphasize that the presentation of gp100 epitopes by MHC-II molecules is influenced by its unique cell biology, and that the alteration of this biology during malignant transformation modulates its recognition by the immune system.
Materials and Methods
Melanoma cell lines and transfectants
The nonpigmented melanoma cell lines DM331 (gp100-negative) (43) and 1102mel (gp100-positive), a gift from S. L. Topalian (National Cancer Institute, Bethesda, MD), both express HLA-DR*0401 constitutively. The pigmented melanoma cell lines 1011mel and MNT-1 (16) expresses all MDPs but not MHC-II molecules. DM331, 1011, and MNT-1 all fail to express significant amounts of the invariant chain. DM331 cells transfected to express tyrosinase (DM331-tyrosinase) have been previously described (44). Melanoma cells were grown in RPMI 1640 (Invitrogen) supplemented with 5% heat-inactivated FBS (Valley Biologicals) and 2 mM glutamine.
The wild-type gp100 gene and the DEL, ΔPKD, and ΔRPT gp100 mutants (31) were subcloned in pcDNA3.0 (Invitrogen) and transfected in DM331 melanoma using Fugene6 (Roche Diagnostics) or a Nucleofection system (Amaxa Biosystems). The high efficiency of the latter system facilitated experiments with long timeframes or requiring cotransfection of multiple gene constructs. A bulk stable line expressing wild-type gp100 (DM331-GP) or short-term transfected lines were obtained by culturing with 300 μg/ml Zeocin (Invitrogen). HLA-DRB1*0401 α-chain and β-chain expression constructs, a gift from J. Gorski (Medical College of Wisconsin, Milwaukee, WI) were used for transient transfection of the HLA-DR-negative melanomas 1011mel and MNT-1. Expression of gp100 was measured by intracellular staining after permeabilization with BD Perm/Wash (BD Pharmingen) using the gp100-specific Abs HMB45 or HMB50 (Lab Vision) and PE-conjugated goat anti-mouse IgG (Jackson ImmunoResearch Laboratories). Expression of HLA-DRB1*0401 was measured by surface staining using the mAb L243 (45) or the HLA-DR4-specific Ab NFLD.D10 (46). Data were acquired on a flow cytometer and analyzed using CellQuest software (BD Pharmingen).
HLA-DR*0401+ gp10044–59-specific CD4 T cell lines
Human CD4 T cell lines specific for HLA-DR*0401+ gp10044–59 (6) were stimulated with a combination of irradiated (10,000 rad) HLA-DR*0401+ BLCL, gp10044–59-expressing tumor cells (1102mel), and nonmatched PBMC in T cell culture medium. Cells were fed after 5 days using fresh medium supplemented with 20 U/ml IL-2 and used in Ag presentation assays after another 2–3 days.
Murine CD4 T cells specific for HLA-DR*0401+ human gp10044–59 were generated in DR4-IE transgenic mice as previously described (6). For Ag presentation assays, T cells were used 6 or 7 days after restimulation.
Cells were grown on coverslips, washed with PBS, fixed for 20 min at room temperature in 4% paraformaldehyde in PBS, and permeabilized with 200 μl BD Perm/Wash (BD Pharmingen) on ice. Cells were incubated for 1 h with Abs specific for gp100 (HMB45 or HMB50), HLA-DR (L243) or lysosome-associated membrane protein 1 (LAMP-1) (BD Pharmingen). These primary Abs were detected with Alexa Fluor 488 anti-mouse IgG1 and Alexa Fluor 594 anti-mouse IgG2 conjugates (Molecular Probes). In some experiments, late endosomes or lysosomes were stained by incubating cells for 60 min at 37°C with 60 μg/ml Lysotracker-Alexa Fluor 594 (Molecular Probes) resuspended in phenol red-free HBSS. Early endocytic compartments were labeled by incubation with 40 μg/ml transferrin-Alexa Fluor 594 (Molecular Probes) for 10 min at 37°C. Cells were washed with PBS containing 5% serum, and fixed, permeabilized, and stained with gp100-specific Abs as described.
To examine internalization of gp100, cells were incubated with HMB50 Ab for 30 min at 4°C, and then shifted to 37°C for 90 min in the presence of Alexa Fluor 594-conjugated Lysotracker. After fixation and permeabilization, cells were stained with Alexa Fluor 488 donkey anti-mouse secondary Ab. To investigate the intracellular accumulation of newly formed peptide-MHC-II complexes, DM331 cells transfected with mock or AP-2-specific small interfering RNA (siRNA) 3 days earlier were exposed to brefeldin A at a final concentration of 3 μg/ml in complete medium for 12 h. Cells were washed three times in PBS and chased for 0 or 4.5 h at 37°C before fixation in 4% paraformaldehyde. Mature MHC-II molecules were detected with L243 mAb.
Samples were mounted on glass slides with Vectashield (Vector Laboratories), visualized using an Olympus confocal microscope, and processed with Adobe Photoshop 7.0.
Expression of the μ2-subunit of the AP-2 adaptor protein was blocked by transfecting cells twice at 4- or 72-h intervals with the siRNA duplexes (Qiagen) to the sequence GUGGAUGCCUUUCGGGUCA using Oligofectamine (Invitrogen) at 20 nM siRNA/transfection. An irrelevant siRNA duplex was used as control. Cells were analyzed for depletion of AP-2 at 48–72 h after the second round of transfection using flow cytometry or immunofluorescence microscopy.
T cell assays
CD4 T cells (5 × 104) were incubated with 5 × 104 melanoma cells in U-bottom 96-well plates (Costar) for 16 h. Culture supernatants were assayed using ELISA kits for murine IFN-γ or GM-CSF (eBioscience) or human IFN-γ (Endogen). The data presented are average values of duplicate wells, with error bars indicating SDs. To assess the ability of inhibitors to block endogenous Ag presentation, cell surface HLA-DR molecules were denatured by incubating melanoma cells in mild acid buffer (47) at room temperature for 3 min. Acid-stripped cells were incubated with drugs (100 μM chloroquine, 0.5 mM leupeptin, 100 μM primaquine) for 16 h, fixed for 10 min on ice with 1% paraformaldehyde, and then incubated with CD4 T cells for 16 h before analysis of cell supernatants by ELISA. Doses of inhibitors were chosen because they showed maximal inhibition of presentation without affecting cell viability.
Western blot analysis
Cell pellets were solubilized in 10 mM Tris-HCl (pH 7.5), 0.5% deoxycholate, 1% Igepal, 5 mM EDTA, 4 mM PMSF, 10 μg/ml aprotinin, 10 μM pepstatin A, 10 μg/ml leupeptin, and 100 μM iodoacetamide. Where indicated, melanoma cells were lysed using premixed M-PER (Pierce), which allows recovery of gp100 from stage II melanosomes. After centrifugation at 21,000 × g for 30 min, supernatants were separated by SDS-PAGE on 8–16% Tris-glycine gels (Invitrogen) and transferred to Immobilon-P membranes (Millipore). Blots were blocked in 10% (w/v) nonfat dry milk in PBS with 0.05% Tween 20 and incubated with first Abs diluted in the same buffer plus 2% nonfat dry milk for 2 h at room temperature (αPep13h) or overnight at 4°C (HMB45). Blots were incubated with HRP-linked anti-rabbit (αPep13h) or anti-mouse (HMB45) whole Abs (The Jackson Laboratory) and detected by ECL (Amersham Biosciences).
gp100 accumulates in melanosomes of pigmented melanoma and in conventional endosomes of melanoma that do not express other MDPs
We investigated the intracellular localization of gp100 and MHC-II molecules in melanoma cells displaying different pigmentation phenotypes. The nonpigmented melanoma cell line 1102mel expresses gp100 but not Tyrp1, and tyrosinase in these cells is retained in the endoplasmic reticulum due to a genetic polymorphism (V. Robila and V. H. Engelhard, unpublished data). The nonpigmented DM331 fails to express tyrosinase, gp100, Tyrp1, or MART-1 (43). Both of these cell lines constitutively express HLA-DRB1*0401. The pigmented melanoma 1011mel expresses all of these MDPs but not MHC-II molecules. To analyze the relationship between MHC-II molecules, gp100, and melanosomes, 1011mel was transfected to express HLA-DR*0401 (1011mel-DR4), whereas DM331 was stably transfected with a plasmid encoding wild-type gp100 (DM331-GP). Using laser scanning confocal microscopy and HMB50, an Ab that detects gp100 in both stage I and stage II melanosomes, we found a marked segregation between gp100 and LAMP-1, a marker of endosomes and lysosomes in 1011mel-DR4 cells (Fig. 1). This is consistent with previous work (16, 48) that has demonstrated in pigmented melanomas, gp100 is principally localized to stage I and stage II melanosomes and excluded from conventional endosomes. Using HMB45, an Ab that recognizes a proteolyzed fragment of gp100 characteristic of stage II melanosomes (30), we observed a similar segregation between gp100-positive and MHC-II+ vesicles. In contrast, there was significant colocalization of gp100-positive with both LAMP-1 and MHC-II in intracellular vesicles in nonpigmented DM331 and 1102mel melanomas (Fig. 1). This indicates that, in these de-differentiated cells, gp100 molecules are routed to conventional late endosomal compartments, including those where MHC-II molecules also reside.
Access of gp100 to the endosomal pathway is required for human gp10044–59 epitope production and MHC-II presentation
MHC-II-restricted epitopes from different endogenous proteins have been shown to arise from proteolytic processing either by the proteasome in the cytoplasm or by proteases in endocytic compartments (49, 50, 51). To test which of these mechanisms was involved in processing of the HLA-DR*0401-restricted gp100 epitope gp10044–59, surface MHC molecules of nonpigmented 1102mel and DM331-GP cells were first denatured by mild acid buffer treatment. Then new MHC-peptide complexes were allowed to form for 6 h in the presence of lactacystin, a specific proteasome inhibitor, or chloroquine, which neutralizes endosomal pH and inhibits resident cathepsins. Lactacystin did not inhibit gp10044–59 presentation to gp10044–59-specific CD4 T cells (Fig. 2, A and B), although it stabilized two gp100 degradation intermediates in the cytosol (data not shown) and, as expected, abrogated presentation of Tyr369, a proteasome-dependent (52, 53) HLA-A*0201-restricted epitope derived from tyrosinase (Fig. 2 C). In contrast, whereas presentation of Tyr369 by HLA-A*0201 was not affected by chloroquine, this inhibitor blocked gp10044–59 epitope presentation, indicating that its formation required the functions of acidified endosomes.
To test whether gp100 localization to endosomal compartments was essential for MHC-II presentation of gp10044–59, we used a gp100 mutant, DEL, in which both the transmembrane and cytoplasmic domains were deleted. DEL expressed in DM331 cells showed no vesicular distribution and failed to localize to LAMP-1+ compartments (Fig. 3,A). The range of expression of DEL was very broad in these transient transfectants, and the mean intensity fluorescence was approximately two-thirds that of cells expressing wild-type gp100 (Fig. 3,B). However, these cells failed to present the gp10044–59 epitope to any significant extent (Fig. 3 C). Thus, the presentation of gp10044–59 strongly correlates with the presence of full-length gp100 protein in late endosomes and lysosomes.
The gp10044–59 epitope can be produced in both early and late endosomal compartments
Distinct MHC-II-restricted epitopes are produced in either early or late endosomes based on differences in protease content, loading on mature or newly synthesized or mature MHC-II molecules or the presence or absence of HLA-DM (37, 41, 54, 55). To investigate whether gp10044–59 epitope could be produced in early endosomes, we used another gp100 mutant, ΔPKD, which lacks aa 243–293 in the lumenal domain and accumulates in early endosomes (31). In DM331 transfectants, wild-type gp100 colocalized with Lysotracker, a marker of late endosomes, but not with transferrin internalized for 10 min, an early endosomal marker (Fig. 4,A). Conversely, the ΔPKD mutant colocalized partially with internalized transferrin, and was largely excluded from late endosomes. Nonetheless, DM331 cells expressing ΔPKD were efficiently recognized by gp10044–59-specific T cells (Fig. 4 B).
To investigate whether localization of gp100 in early or late endosomes correlated with epitope production in that compartment, we treated cells with leupeptin, which inhibits late endosomal serine and cysteine proteases, or chloroquine, to deacidify these compartments (51, 56). Treatment with these agents did not affect the expression of MHC-II or transfected gp100 molecules (Fig. 4,C). DM331 also does not express the invariant chain (see Fig. 7A), and thus, leupeptin cannot alter epitope expression by blocking invariant chain degradation. As with wild-type gp100, chloroquine inhibited gp10044–59 production from the ΔPKD mutant, indicating that it also required intracellular processing in acidified endosomes (Fig. 4,B). However, gp10044–59 presentation from wild-type gp100 was inhibited by leupeptin, whereas presentation of this epitope from the ΔPKD mutant was not inhibited. This difference might have been due to the large deletion in ΔPKD, which could render gp100 more susceptible to proteolytic degradation and thus less sensitive to inhibition. To further investigate this possibility, we analyzed epitope presentation from a second gp100 mutant, ΔRPT, which has a deletion encompassing aa 314–424, but displays the same intracellular localization as wild-type gp100 (31). When expressed in DM331, ΔRPT colocalized with Lysotracker but not with internalized transferrin, indicating its distribution in late but not early endosomes (Fig. 4,A). As with GP, presentation of gp10044–59 from ΔRPT was inhibited by both chloroquine and leupeptin (Fig. 4 B). These results indicate that gp10044–59 can be generated in either early or late endosomes depending on the primary intracellular localization of the source protein.
Wild-type gp100 is processed both in early and late endosomal compartments
Despite prevalent localization of wild-type gp100 to late endosomes, gp10044–59 presentation was only ∼55–60% reduced after treatment with leupeptin. This result, together with the demonstration that gp10044–59 is produced in early endosomes from ΔPKD gp100, led us to hypothesize that wild-type gp100 is also processed in this compartment. Primaquine prevents re-expression of internalized recycling MHC-II molecules and inhibits presentation of epitopes produced in early endosomes (42). Treatment of ΔPKD-expressing cells with primaquine inhibited gp10044–59 presentation to CD4 T cells to a similar extent as chloroquine (Fig. 5, top left), whereas leupeptin had no effect either alone or in combination with primaquine. Primaquine did not reduce surface MHC-II expression (data not shown), HLA-A*0201 presentation of the Tyr369 epitope (Fig. 5, bottom left), or presentation of exogenous gp10044–59 peptide pulsed on DM331 (Fig. 5, bottom right). In keeping with the localization of the ΔPKD mutant protein and its insensitivity to leupeptin, this result suggests that the gp10044–59 derived from this molecule is presented by MHC-II molecules recycling through early endosomes. In four independent experiments, primaquine treatment also reduced gp10044–59 presentation by DM331 cells expressing wild-type gp100 by 30–50% relative to chloroquine (Fig. 5, top right and data not shown). The effects of leupeptin and primaquine on presentation by these cells were additive, and equivalent to treatment with chloroquine. These results suggest that gp10044–59 is also produced from wild-type gp100 in both late and early endosomes.
The major source protein for gp10044–59 presented by HLA-DR*0401 is internalized from the plasma membrane by AP-2
The protein gp100 might reach endosomal compartments for processing either directly from the Golgi or by transiting to the cell surface followed by internalization (16, 34, 57). The clathrin coat component AP-2 has been implicated in endosomal targeting of newly synthesized proteins by endocytosis from the plasma membrane (58, 59, 60, 61), and previous work was consistent with the possibility that gp100 was an AP-2 cargo protein (33). To test this idea directly, we ablated AP-2 expression in DM331-GP using siRNA oligonucleotides directed to the μ2 subunit of the complex (59). In keeping with earlier work (58, 60), AP-2 depletion led to a 5-fold increase in surface expression of LAMP-1, whereas the expression of mature MHC-II molecules was unchanged (Fig. 6,A). AP-2 depletion also led to a significant increase in cell surface gp100 while having no effect on total cellular gp100 content (Fig. 6,A). By prelabeling cell surface gp100 molecules with specific Ab, we also found that their subsequent internalization into endosomal compartments was substantially inhibited in AP-2-depleted cells (Fig. 6,B). In AP-2-depleted cells, gp10044–59 presentation was substantially inhibited (Fig. 6 C), suggesting that gp100 internalized from the plasma membrane is a major source of protein for epitope production.
Although AP-2 deficiency did not alter the overall level of MHC-II expression (Fig. 6,A), it has been shown to direct internalization of newly synthesized MHC-II molecules to endosomes by interaction with the Ii chain (58). Thus, impaired gp10044–59 presentation in AP-2-deficient cells could be due to limited availability of newly synthesized HLA-DR*0401 molecules in endosomes. However, DM331 melanoma fails to express significant amounts of Ii chain (Fig. 7,A). To directly test whether AP-2 knockdown decreased endosomal availability of newly synthesized HLA-DR*0401 molecules in DM331, we treated control and AP-2-deficient cells with brefeldin A for 12 h to trap newly synthesized MHC-II complexes in the cis-Golgi or medial-Golgi and clear late endosomal compartments of mature MHC-II molecules (detected with L243 mAb) (Fig. 7,B, left). When brefeldin A was washed out and the cells incubated for 4.5 h, mature MHC-II molecules were detected in endosomal compartments of both control and AP-2-depleted cells (Fig. 7 B, right). Thus, AP-2 is not essential for endosomal trafficking of newly synthesized HLA-DR*0401 molecules and formation of new peptide-MHC-II complexes in DM331 melanoma, presumably because of the absence of Ii chain. We conclude that the reduction of gp10044–59 epitope presentation after AP-2 knockdown is a consequence of preventing access of cell surface gp100 to MHC-II processing compartments.
Epitope gp10044–59 presentation is reduced in pigmented melanomas that contain melanosomes in addition to conventional endosomes
We next investigated whether the segregation of gp100 into melanosomes in pigmented cells reduced presentation of gp10044–59 relative to depigmented melanoma cells in which gp100 localizes to MHC-II+ endosomes. For this evaluation, we used 1011mel and a second pigmented melanoma cell line, MNT-1. As with DM331, neither of these cells expresses Ii chain (Fig. 7,A and data not shown). Both were transfected to express HLA-DR*0401 and sorted by flow cytometry to select cells expressing levels of surface MHC-II molecules similar to DM331-GP (Fig. 8,A). The gp10044–59 epitope presentation was markedly reduced in both 1011mel-DR4 and MNT-1-DR4, requiring approximately eight times as many cells as DM331-GP to achieve T cell comparable activation (Fig. 8,B). Although both of these cells expressed lower levels of HLA-A2 than DM331-GP (Fig. 8,C), 1011mel presented Tyr369 to specific CD8 T cells comparably to DM331-GP, whereas MNT-1 was about half as effective (Fig. 8 D). Thus, the substantial differences in presentation of gp10044–59 were not a consequence of general differences in the ability to stimulate T cells.
Western blot analysis using the C-terminal specific Ab αPep13h showed that all three melanoma lines express similar levels of full-length gp100 and a 26-kDa fragment that can be produced by proteolysis in either endosomes or early stage melanosomes (24, 29) (Fig. 8,E). However, both MNT-1 and 1011mel have much higher levels of the mature 34- to 38-kDa gp100 fragment produced only in melanosomes and detected by mAb HMB45 (30), as compared with DM331-GP (Fig. 8 F and data not shown). Although this gp100 fragment does not encompass the gp10044–59 peptide, its prevalence indicates that the relative paucity of gp10044–59 presentation is also not due to a lack of source protein expression. Instead, it suggests that this limitation is a consequence of source protein localization.
The gp10044–59 epitope presentation is completely abolished in AP-2-depleted 1011mel-DR4 (Fig. 9,A), demonstrating that gp100 internalized from plasma membrane is the only source protein for production of the gp10044–59 epitope in pigmented melanoma. To test whether gp10044–59 is produced in conventional early or late endosomes, despite the under-representation of gp100 localization in LAMP-1high compartments, we analyzed the effect of primaquine or leupeptin treatment on epitope presentation. 1011mel were transfected with DRB1*0401 and expression allowed to occur in the presence of inhibitors, then fixed and incubated with murine CD4 T cells specific for gp10044–59/DRB1*0401. Leupeptin had no effect on gp10044–59 presentation, whereas primaquine treatment led to a substantial reduction (Fig. 9 B). These results indicate that, in pigmented melanomas, gp10044–59 is generated entirely within early endosomes or stage I melanosomes and not in late endosomal compartments.
In the current work, we investigated the mechanism of presentation of an MHC-II-restricted epitope from gp100 (Pmel17), a cellular protein with an endosomal or melanosomal localization. Using melanoma cells that have down-regulated components of the melanosomal pathway, but constitutively express HLA-DR*0401 molecules, we established that the majority of gp100 is sorted to LAMP-1high/MHC-II+ conventional endosomes. In these cells, presentation of the gp10044–59 epitope required localization of gp100 source protein to endosomes and processing by acidic proteases. We also found that similar antigenic epitopes were produced in both early and late endosomes, despite differences in processing requirements. Epitope presentation depends in large part on gp100 trafficking to the plasma membrane and internalization mediated by the AP-2 adaptor protein. We also established that the presentation of this epitope was dramatically reduced in pigmented melanomas that contain melanosomes in addition to conventional endosomes, consistent with the localization of gp100 to melanosomes and not to LAMP-1high MHC-II+ late endosomes. In these cells, gp10044–59 presentation still depends on protein internalization from the cell surface via AP-2, but processing occurs entirely in early endosomes or stage I melanosomes and not late endosomes. Collectively, our results emphasize that: processing of gp100 epitopes is directly tied to intracellular targeting of the source protein; melanosomes are relatively poor compartments for MHC-II processing, despite their endosomal origin; and de-differentiation of melanoma cells, resulting in loss of melanosomes, augments presentation of epitopes from those MDP that continue to be expressed due to alterations in their endosomal targeting.
Previous studies demonstrated that endogenous cytoplasmic (49, 62) and transmembrane (50) proteins can be processed by the proteasome for presentation by MHC-II molecules. The resulting peptides reach endosomal compartments through TAP (51) or LAMP-2 (63). However, high and sustained levels of cytosolic source protein may be required compared with class I-mediated recognition (64, 65). In this study, we show that inhibition of proteasomes by lactacystin had no effect on presentation of gp10044–59, although it stabilized gp100 fragments in both membrane and cytosol. This indicates that defective ribosomal products or poorly folded forms of the protein that were retro-translocated into the cytosol are inefficient substrates for MHC-II presentation, compared with full-length protein directly targeted to endosomes, even though the latter are fully folded and thus presumably more resistant to proteolytic degradation.
Using mutant forms of gp100 that localize to different compartments, we established that antigenically indistinguishable epitopes are produced in both early and late endosomes. Thus, gp10044–59 is among a limited number of epitopes for which presentation via recycling MHC-II molecules in early endosomes has been demonstrated and the only one so far described to originate from an endogenous protein with an endosomal or melanosomal localization; other endosomal recycling-dependent epitopes have been derived from exogenous (37, 40, 41, 42) or cytosolic (51) proteins. However, the involvement of AP-2-mediated internalization of gp100 from the plasma membrane suggests that endocytosis is a common mechanism to deliver epitope source proteins, either endogenous or exogenous, to early endosomes for presentation by recycling MHC-II molecules.
Other studies have demonstrated that distinct MHC-II-restricted peptides are produced in early and late endosomal compartments. Epitopes produced in late endosomes require more substantial source protein denaturation, whereas those produced in early endosomes are produced by mild proteolysis, and may be destroyed in the harsher environment of late endosomes (37, 42). In this study, we demonstrated that gp10044–59 is generated by leupeptin-sensitive proteases in late endosomes, and by leupeptin-insensitive proteases in early endosomes. This may be facilitated by the fact that the protein itself is a normal resident of endosomes, and thus able to resist denaturation. In contrast, the N-terminal region of gp100, which contains this epitope, seems to be dissociated or degraded during the maturation of melanosomes (35, 57), suggesting that it is accessible to endosomal proteases. Although the epitopes generated in these two compartments are antigenically indistinguishable, our data do not exclude the possibility that structurally different peptides containing the epitope are generated with greater or lesser efficiency in either compartment.
The inhibition of epitope presentation by siRNA-mediated depletion of AP-2 suggests that the major forms of gp100 that serve as substrates for gp10044–59 epitope presentation were internalized from the plasma membrane. This observation supports and extends previous work demonstrating a correlation between gp100 cell surface expression and presentation of a DR*0701- restricted epitope, which led to the suggestion that gp100 accessed MHC-II+ processing compartments by endocytosis from the plasma membrane (66). AP-2 directs cargo internalization by recognition of tyrosine- or di-leucine-based sorting signals located in cytoplasmic domains (61, 67), and a di-leucine-based signal in the cytoplasmic domain of gp100, which is deleted in the natural Silver mutation of mouse Pmel17, has been shown to be required for its efficient internalization and accumulation in melanosome precursors (34). These data suggest that AP-2 facilitates internalization by interaction with the di-leucine-based motif. Interestingly, LePage and Lapointe (66) reported that deletion of the gp100 cytoplasmic domain reduced presentation of the DRB1*0701-restricted epitope, whereas deletion of the di-leucine motif did not, despite increased mutant protein localization to cell surface. However, that work used gp100 transfectants to assess this issue, whereas we have used cell expressing the endogenous gp100 gene. Overexpression of gp100 minimizes the influence of the endocytosis signal on efficient localization into endosomes (31, 34), perhaps because of AP-2-independent internalization or direct transport from the trans-Golgi network, and may explain the apparent discrepancy of our results and theirs. Our data do not exclude the possibility that a cohort of gp100 molecules traverses an alternative transport pathway from the trans-Golgi network directly to stage I melanosomes as has been proposed (33), but suggests that this is a minor pathway at best. Thus, our results establish that an endosomal protein reaches MHC-II processing compartments by endocytosis mechanisms resembling those responsible for internalization of exogenous proteins from the plasma membrane.
Our results represent one of the first analyses of MHC-II processing and presentation capabilities of melanosomes. Previous studies showed that melanosomes share a number of characteristics with conventional endosomes, including low luminal pH and the presence of lysosomal proteases and membrane proteins (18). In contrast, we found that MHC-II molecules accumulated in conventional LAMP-1high late endosomes, and were largely excluded from stage II melanosomes. In accordance with previous reports (16), the majority of gp100 was localized in stage II melanosomes. In keeping with our results demonstrating that gp100 targeting to endosomes was the primary determinant of the presentation of gp10044–59 by class II molecules, these cells presented this epitope inefficiently, and the processing was confined to early endosomes. These data indicate that the MHC-II molecules and MDP intersect at the early endosome or stage I melanosome, but are segregated thereafter, presumably via specific sorting signals. It will be interesting to determine whether melanosomal targeting also diminishes the presentation of epitopes from other MDPs such as tyrosinase or Tyrp1, particularly if these epitopes require the more robust processing environment of late endosomal compartments. Conversely, it will also be interesting to determine whether mutations that interfere with transit of these proteins to melanosomes (68, 69, 70, 71) enhance presentation of epitopes from these proteins.
An inverse correlation between expression of MHC-II molecules and of MDPs has been previously reported (15). However, melanocytes and some melanomas express MHC-II after induction with IFN-γ (11, 12, 13). It remains to be determined whether IFN-γ stimulation also alters the localization of MDP from melanosomes to late endosomes. Conversely, the limited ability of pigmented melanoma cells to present MHC-II epitopes derived from MDPs is likely to make presentation of MDP-derived epitopes more reliant on uptake by professional APC. Thus, the contribution of epitope presentation directly by tumor or mediated by professional APC may vary among melanomas and may correlate with the degree of pigmentation.
During the process of malignant transformation and progression, melanocytes or early stage melanomas develop defects in melanosome biogenesis and pigment synthesis (20, 72). These may be due to alterations in mechanisms that segregate melanosomes from conventional endosomes (21, 22). Our results demonstrate that this de-differentiation results in the mistargeting of gp100 to conventional late endosomes. This shift results in a significantly higher presentation of gp10044–59. Thus, melanoma de-differentiation may be accompanied by increased efficiency of presentation of gp100-derived MHC-II-restricted epitopes, leading to enhanced immune recognition.
The authors have no financial conflict of interest.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
This work was supported by Grants AI20963 and AI33134 from the U.S. Public Health Service (to V.H.E.), Grants AR041855 and EY015625 (to M.S.M.), and Grant ROP-38369 from Canadian Institutes for Health Research (to S.D.).
Abbreviations used in this paper: MDP, melanocyte differentiation protein; MHC-II, MHC class II; siRNA, small interfering RNA; LAMP, lysosome-associated membrane protein; Ii, invariant chain.