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1From the Department of Ophthalmology, the 2Heart Institute (InCor), and the 4Division of Clinical Immunology and Allergy, University of São Paulo School of Medicine, São Paulo, Brazil; the 3Institute for Investigation in Immunology, Millennium Institutes, São Paulo, Brazil; and the 5Department of Genomic and Information Sciences, Hoffmann-La Roche, Inc., Nutley, New Jersey.
| Abstract |
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METHODS. Peripheral blood mononuclear cells (PBMC) proliferation and Th1 (IFN-
) and Th2 (IL-4 and IL-5) cytokine production were analyzed in HLA-DRB1*0405-positive (n = 12) and -negative (n = 22) patients with VKH and HLA-DRB1*0405-positive (n = 9) and -negative (n = 8) control subjects in response to human melanoma cell line lysate (HMCLL) and 28 synthetic peptides derived from the human melanocyte differentiation proteins TYR, TRP1, TRP2, and Pmel17. The peptides were selected using the TEPITOPE algorithm, based on their predicted binding to HLA-DRB1*0405 and to the nondisease-related HLA-DRB1*15.
RESULTS. HMCLL was recognized exclusively by the patients PBMC (44%) but not by those of the control subjects (P < 0.01). PBMC from patients with VKH recognized an increased breadth of melanocyte-derived peptides at lower peptide concentrations than in the control subjects (68% vs. 25%; P < 0.01, at 1 µM) and did not produce the Th2 cytokine IL-4 in response to disease-specific peptides (0% vs. 50%, P < 0.001). Five peptides were exclusively recognized in patients bearing HLA-DRB1*0405. Furthermore, HLA-DRB1*0405-bearing patients, but not those with HLA-DRB1*15, recognized an increased breadth of melanocyte epitopes in comparison to HLA-matched control subjects (60% vs. 28%; P < 0.05).
CONCLUSIONS. These data indicate that patients with VKH are sensitized to melanocyte epitopes and display a peptide-specific Th1 cytokine response. In addition, the data indicate that patients bearing HLA-DRB1*0405 recognize a broader melanocyte-derived peptide repertoire, reinforcing the importance of this allele in susceptibility to the development of VKH disease.
Findings in several studies have revealed the autoimmune nature of the disease.5 6 It has been shown that uveal pigment inhibits leukocyte migration of peripheral blood mononuclear cells (PBMC) from patients with VKH,7 and that both CD4+ and CD8+ T lymphocytes are cytotoxic against melanocytes in vitro.8 Immunohistochemical studies in eye globes of patients with VKH have also demonstrated the aberrant expression of major histocompatibility complex (MHC) class II molecules on the surface of choroidal melanocytes, pointing out the local production of inflammatory cytokines.9 As suggested by these studies, the immune response is aimed at melanocytes, and thus it is important to pursue its autoantigenic target protein and peptides. Melanocyte-specific proteins, shown to have a major role in differentiation, such as tyrosinase (TYR), tyrosinase-related protein 1 (TRP1) and -2, MART-1/Melan A and Pmel17/gp100,10 11 12 are also expressed in human melanoma cell lines and are recognized by T lymphocytes of patients with melanoma.13 PBMCs from patients with VKH recognize peptides derived from the tyrosinase family of proteins involved in melanin synthesis,14 15 and peptides derived from TYR, TRP1, and TRP2 induce an autoimmune disease in rats that resembles VKH,16 making these melanocyte proteins candidate autoantigens for VKH.
Immunogenetic studies have revealed that HLA-DRB1*0405 and DRB1*0410 are strongly associated with VKH in the Japanese population (relative risk [RR] = 100).17 Recently, our group has shown that HLA-DRB1*0405 is the predominant allele associated with VKH, with a RR of 12 in Brazilian patients.18 This cohort of patients was an ethnically heterogeneous population, with Japanese descent in only three patients. The coincident finding of disease association with HLA-DRB1*0405 in ethnically different populations suggests that this HLA class II allele, and not another closely linked HLA-encoded factor, is the major susceptibility determinant. Given the role of the HLA molecule in antigen presentation to T cells, it has been hypothesized that HLA-DRB1*0405 plays a selective role in presenting the target epitopes.18 However, the actual mechanism of this association is unknown.
In contrast to Japanese patients with VKH, the fact that a significant number of individuals do not carry the susceptibility allele in the Brazilian VKH population18 provides a unique opportunity to search for differences in the cellular immune responses against melanocyte peptides between patients carrying the HLA susceptibility allele and those carrying other alleles, consistent with a selective presentation of target epitopes. In the present study, we sought to analyze the role of HLA-DRB1*0405 in melanocyte-derived peptide presentation, by measuring PBMC proliferation and cytokine secretion by DRB1*0405-positive and -negative patients with VKH and HLA-matched normal control subjects, in response to a human melanoma cell line lysate and 28 synthetic peptides selected from human melanocyte differentiation proteins.
| Methods |
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Seventeen healthy individuals were included in a control group (2852 years old; 8 men and 9 women). Ethnic distribution was 82% white, 6% black, and 12% Asian.
All 34 patients and 17 control subjects were typed for HLA-DRB with low-resolution polymerase chain reactionsequence specific primer (PCR-SSP).20 In HLA-DR4-positive samples, PCR was followed by hybridization with sequence-specific oligonucleotide probes with DR4-specific primers for exon-2 amplification, according to 12th International Histocompatibility Workshop protocols.21 Patients and control subjects were divided into different groups according to their HLA-DRB1 types: carriers of disease-associated HLA-DRB1*0405 (7 patients and 9 control subjects), carriers of nondisease-related HLA-DRB1*1501 (8 patients and 8 control subjects), carriers of both alleles HLA-DRB1*0405 and DRB1*1501 (5 patients), and carriers of other HLA alleles (14 patients).
Written, informed consent has been given by every participant of the study, which was approved by the Institutional Review Board of the University of São Paulo School of Medicine. The research followed the tenets of the Declaration of Helsinki.
Peptide Selection
The amino acid sequences of TYR (M27160), TRP1 (AF001295), TRP2 (D17547), Pmel17 (M77348), and MART1 (U06654), all human melanocyte differentiation proteins known to be recognized by T cells,13 were scanned by using the TEPITOPE algorithm.22 This algorithm can predict, after scanning all nonamer windows starting with a hydrophobic residue on a protein sequence, those that have the potential to bind to one or more of 25 different HLA-DR molecules, by using 25 virtual matrices that cover most of the HLA class II peptide binding profile in the white population.23 Briefly, the algorithm incorporates, for each of the 25 HLA-DR molecules, a matrix of values for each amino acid residue at each position, p2 to p9. Position- and residue-specific matrix values were assembled from empiric HLA-DR peptide-binding assays.24 The algorithm provides a scorethe algebraic sum of the matrix values for each peptide positionto each of the nonamer windows along the scanned sequence. Nonamers attaining a score above the threshold for a given HLA-DR molecule (e.g., a 5% threshold selects sequences with HLA-binding scores equal to or higher than those 5% of the sequences with highest binding scores to a given HLA-DR molecule in the TEPITOPE database) are selected by the software. The algorithm also allows the selection of sequences predicted to bind simultaneouslythus, promiscuouslyto several HLA-DR molecules. Peptides were selected based on their chance of binding to the disease susceptibility HLA allele HLA-DRB1*0405 and/or to the nondisease-related HLA allele HLA-DRB1*15, with a threshold of 5%, with the exception of two peptides predicted to bind to HLA-DRB1*0405 above the 8% threshold.
Among peptides selected by the TEPITOPE algorithm, 28 peptides with different chances of binding to the respective HLA allele were synthesized: 11 predicted to bind to the HLA-DRB1*0405 molecule and not to HLA-DRB1*15, 13 predicted to bind to HLA-DRB1*15 and not to HLA-DRB1*0405, and 4 predicted to bind to both HLA molecules, as shown in Table 1 . The peptide distribution among the melanocyte-derived proteins was: TYR, 5 peptides; TRP1, 7 peptides; TRP2, 6 peptides, and Pmel17, 10 peptides.
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Melanoma Cell Line Lysate
Human melanoma cell line lysate was prepared from MMAc, a human melanoma cell line whose donor carried the HLA-DRB1*0405 and HLA-DRB1*1502 alleles (kindly donated by Manabu Mochizuki, Kurume University, Japan). Melanoma cells were lysed through 10 sessions of freezing and thawing on liquid nitrogen and a water bath at 37°C.26 Melanoma cell lysate concentration was adjusted to 1 mg/mL and was tested in PBMC proliferation assays at five concentrations (10, 20, 50, 100, and 200 µg/mL).
PBMC Proliferation Assay
PBMC were separated by density gradient centrifugation (d = 1.077 g/L). Cells were incubated in RPMI 1640 medium (Invitrogen-Gibco, Grand Island, NY) supplemented with 10% heat-inactivated normal human serum, 2 mM L-glutamine, 1 mM sodium pyruvate, 50 µg/mL gentamicin, and 10 mM HEPES buffer (complete medium), in triplicate on 96-well U-bottomed culture plates (105 cells/well; final volume, 0.2 mL) in the presence of melanocyte-derived peptide (1, 10, and 50 µM), phytohemagglutinin (PHA, 2.5 µg/mL; Difco Laboratories, Detroit, MI), purified protein derivative (PPD, 5 µg/mL; kindly donated by Yujiro Fujino, University of Tokyo, Japan) or complete medium alone. Plates were incubated in 5% CO2 at 37°C for 5 days and cultures were pulsed with [3H]thymidine at 0.5 µCi/well for the last 18 hours before the end of culturing. Cells were harvested, and the uptake of the [3H]thymidine (Amersham Pharmacia Biotech, Piscataway, NJ) was measured by a ß-counter (1205 BS Betaplate; Wallac, Turku, Finland). Data are presented as the stimulation index (SI), calculated by dividing mean counts per minute in cultures with antigen by mean counts per minute in cultures without antigen. A stimulation index
2.0 was considered positive.27 28
Cytokine Detection in Culture Supernatants
PBMC were isolated and cultured with or without antigen (10 µM peptide, 2.5 µg/mL PHA, or 5 µg/mL PPD) as just described, and supernatants were collected 48 hours after stimulation. Cytokine quantitation was performed with anti-human cytokine antibody pairs for IFN-
(Pierce-Endogen; Rockford, IL) and IL-4 (BD-Pharmingen, San Diego, CA). IL-5 detection was performed by human IL-5 colorimetric ELISA kit (Pierce-Endogen). Specific cytokine production was expressed as: (mean values from duplicate experiments in stimulated wells) (mean values from duplicate control wells). The detection limit of each assay was as follows: IFN-
, 4 to 30 pg/mL; IL-4, 10 to 40 pg/mL; and IL-5, 4 to 10 pg/mL.
Statistical Analysis
The nonparametric Mann-Whitney rank sum test was used to compare SIs and cytokine production for each antigen among clinical groups. The
2 or Fisher exact test was used to compare frequencies of responding and nonresponding individuals.
| Results |
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PBMC Recognition Profile of the Melanocyte-Derived Peptides
Taking into account responses to all three peptide concentrations tested, the overall frequency of responders and the number of peptides recognized were similarly high in both patients and control subjects, targeting several peptides from several proteins. In contrast, the analysis of the lymphoproliferative responses to peptides at different concentrations showed that, at the lowest concentration (1 µM), PBMC from the patients as a group recognized a more diverse array of peptides (68%, 19/28 peptides) than those from the control group (25%, 7/28 peptides; P = 0.003; Fig. 1 ).
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producers and the number of peptides able to induce IFN-
secretion were similarly high in patients and control subjects, targeting several peptides from several proteins (Fig. 2 , Table 2 ). In contrast, it was striking that, whereas PBMC from 50% of the control subjects secreted IL-4 in response to any peptide (43 ± 20 pg/mL), none of the PBMC from patients secreted peptide-induced IL-4 (P < 0.001; Fig. 2 , Table 2 ). Peptide-induced IL-5 secretion was obtained from PBMC of only one patient (Table 2) .
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production: TYR496-510 (35% and 40%, respectively), TRP1167-181 (21% and 43%), and TRP1308-322 (32% and 45%). In our sample, we did not find any correlation between disease stage, systemic therapy, and immune responses against melanocyte-derived peptides in patients with VKH (data not shown).
Recognition Profile of Patients with VKH Bearing the Major Susceptibility Allele HLA-DRB1*0405
The peptide-specific lymphoproliferative and cytokine responses were similar between HLA-DRB1*0405-positive and -negative patients and control subjects, except for peptide-induced IL-4, as previously mentioned (Table 2) . Peptide-induced IL-4 was not detected in any patient with VKH irrespective of HLA typing, whereas it was detected in 50% (DRB1*0405) and in 50% (DRB1*15) of control subjects (Table 2) .
Data were also analyzed based on the number of peptides recognized in lymphoproliferative assays. As a group and at any concentration, DRB1*0405-positive PMBC of patients recognized a higher number of peptides (including peptides predicted to bind to HLA-DRB*15 but not to HLA-DRB1*0405) than those of HLA-matched control subjects (17/28 vs. 8/28; P = 0.031), whereas DRB1*15 patients PMBC recognized a lower number of peptides than those of the HLA-matched control subjects, including peptides predicted to bind to HLA-DRB*0405 but not to HLA-DRB1*15 (16/28 vs. 25/28; P = 0.014; Figs. 3 4 ). The average number of melanocyte-derived peptides individually recognized by each subject in the four groups were: DRB1*0405 VKH, 3.4 (range, 017); DRB1*0405 control, 1.4 (range, 04); DRB1*15 VKH, 3.6 (range, 110); and DRB1*15 control, 6.5 (range, 118).
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(TRP2274-288 and TYR442-456) detection assays (data not shown). In contrast, we found two peptides exclusively recognized by cells of DRB1*15-bearing patients: TRP2277-291 and TRP2139-153 (data not shown). Furthermore, cells of DRB1*0405-bearing patients as a group, but not those of DRB1*15 patients, had a tendency to recognize a more diverse array of peptides at 1 µM than did cells of HLA-matched control subjects (11/28 vs. 4/28 peptides; P = 0.068; data not shown); this tendency disappeared at 10 and 50 µM.
| Discussion |
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The clear cut difference in response against the human melanoma cell line lysate observed among patients with VKH and control subjects is in line with previous observations of T-cell cytotoxicity against melanocyte and melanoma cell lines in patients with VKH.29 30 Furthermore, it reinforces the importance of the melanocyte as the target of autoimmunity in this disease5 6 9 and the validity of pursuing the responses against peptides derived from melanocyte-derived proteins. The finding that cells from patients with VKH recognize a broader range of peptides at low concentration (1 µM) than do those from control subjects in proliferative responses (Fig. 1) is consistent with epitope spreading secondary to a sensitization by a still undefined self-antigen.31 The increased breadth of peptide recognition at the lower concentration in patients compared with control subjects indicates that, for most peptides, patients show a lower threshold for T-cell activation, in line with previous in vivo sensitization with the tested melanocyte-derived peptides. This suggests that T cells from patients with VKH recognize melanocyte-derived peptides with high affinity.32 Conversely, T cells from control subjects show low-affinity responses to epitopes from proteins without prior exposure, in line with the expected behavior of a cross-reactive T-cell response.33 The property of cross-reactivity seems to be inherent in T-cell recognition.34
In addition, our data have demonstrated for the first time that PBMC from patients with VKH do not produce an anti-Th1 cytokine, such as IL-4, against disease-specific peptides (Fig. 2 , Table 2 ). A similar Th1 cytokine profile has been described in PBMC from patients with VKH after an unspecific anti-CD3 stimulation.35 36 It is noteworthy that most human organ-specific autoimmune diseases,37 as well as murine models,38 are reported to be mediated by autoreactive T1-type T cells, and rodent strains that are prone to the induction of T-celldependent autoimmune disease often have a genetic predisposition to a predominantly T1-type cytokine response.39 40 Significantly, there is clinical and laboratory evidence of viral infection preceding the acute ocular VKH symptoms (Usui M, et al. IOVS 1991;32:ARVO Abstract 807),4 41 which suggest the participation of infection that could activate Toll-like receptors and induce a T1 response.42
The identification of three novel epitopes derived from TYR, TRP1, and TRP2 that are exclusively or preferentially recognized by PMBC of patients bearing HLA-DRB1*0405, further supports the role of HLA-DRB1*0405 in the presentation of melanocyte-derived peptides. Yamaki et al.15 have shown that the peptides TYR442-456 (SKDLGYDYSYLQDSD), and TRP-1263-277 (DDLMGSRSNFDSTLI), also recognized by cells in our study, were preferentially recognized by T-cell clones from patients with acute, nontreated VKH. The finding of melanocyte-derived peptides that are specifically recognized by PMBC of HLA-DRB1*0405-positive patients with VKH may have a bearing on the association of HLA-DRB1*0405/VKH with the pathogenesis of the disease.
The fact that only DRB1*0405-positive patients with VKH showed an increased breadth of recognition of melanocyte epitopes in comparison to HLA-matched control subjects (Fig. 3) is in line with the phenomenon of epitope spreading found in other autoimmune diseases43 and supports melanocytes as the target of the autoimmune response in DRB1*0405-bearing patients with VKH. However, the reduced breadth of peptide recognition by DRB1*15-positive patients with VKH in comparison to HLA-matched control subjects (Fig. 3) is puzzling. A possible explanation would be the existence of other target proteins not analyzed in this study, preferentially recognized by patients carrying HLA-DRB1*15 or other non-HLA-DRB1*0405 alleles.
Even though the peptide selection method has been successfully used for depicting immunodominant T-cell epitopes in immune-mediated diseases,44 45 there is still no guarantee that all selected peptides are indeed naturally produced. However, the high peptide concentrations used in the study, similar to other epitope mapping studies,14 15 could be responsible for the increased frequency of positive lymphoproliferative responses against melanocyte-derived peptides in comparison to melanoma lysate. Such synthetic peptide concentrations would be well in excess of the peptide concentration attainable in the melanoma lysate.
In summary, we have shown that patients with VKH are sensitized to melanocyte antigens, displaying a melanocyte peptide-specific Th1 cytokine response. We also found that HLA-DRB1*0405-positive patients are sensitized to a significant number of melanocyte-differentiation, protein-derived epitopes. Future studies could further elucidate the underlying reasons for the predominant Th1 cytokine profile and the increased breadth of melanocyte peptide recognition by HLA-DRB1*0405-bearing patients with VKH and the implications in their susceptibility to VKH.
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Submitted for publication October 30, 2004; revised January 6, and February 3 and 22, 2005; accepted March 6, 2005.
Disclosure: F.M. Damico, None; E. Cunha-Neto, None; A.C. Goldberg, None; L.K. Iwai, None; M.L. Marin, None; J. Hammer, (P); J. Kalil, None; J.H. Yamamoto, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Joyce Hisae Yamamoto, Heart Institute (InCor), University of São Paulo School of Medicine, Av. Dr. Enéas de Carvalho Aguiar, 44 Bl 2, 9° andar, São Paulo, SP, 05403-000, Brazil; joycehy{at}uol.com.br.
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