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Center for Cancer Research, National Cancer Institute, Institutes of Health, Bethesda, MD 20892
| Abstract |
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| Introduction |
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Most tumor Ags have been identified using lymphocytes obtained from tumor-infiltrating lymphocyte cultures or from mixed lymphocyte tumor interactions. The use of tumor-infiltrating lymphocytes associated with tumor regression when adoptively transferred into cancer patients has improved the likelihood that the Ags identified may be suitable targets for immunotherapy (2). In the current study, we have taken a different approach to identify Ags that might be particularly important for use in active immunization. We have identified patients with metastatic melanoma that have experienced complete and durable regressions following immunotherapy. Extensive cloning of fresh, uncultured circulating PBLs from these patients has been performed using OKT3 plus IL-2 to attempt to identify circulating lymphocytes reactive with tumor without any bias introduced by artificial tumor stimulation or selective growth conditions by prior culture in vitro.
In the present paper, we describe studies of the circulating lymphocytes of a patient who received active immunization with a recombinant adenovirus and underwent complete regression of biopsy-proven s.c. and mediastinal metastases from melanoma, and remains continuously disease-free >5 years later (3). By directly cloning circulating lymphocytes from this patient, we have identified reactivity against a new shared melanoma Ag translated from a very short alternative open reading frame of a gene in the extended region of the class II MHC.
| Materials and Methods |
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In June 1989, patient TC underwent excision of a 0.9-mm-thick primary melanoma from the left thigh. A palpable lymph node containing melanoma was excised in November 1991, and she was disease-free until July 1995 when recurrent melanomas developed in the left breast and left arm. Both masses were excised, and in November 1995 new left breast and mediastinal masses appeared, and she was referred to the Surgery Branch, National Cancer Institute. Fine needle aspiration confirmed the diagnosis of metastatic melanoma. In December 1995, the patient was entered into a clinical protocol approved by the National Cancer Institute Institutional Review Board, and on December 22, 1995, and January 1, 1996, she received the s.c. injection of 108 PFU of a recombinant adenovirus encoding the melanoma Ag recognized by T cells 1 (MART-1)2 (3). On follow-up 1 mo after the second injection, there was partial regression of both the mediastinal and breast melanoma metastases, and she received two additional injections of the same recombinant adenovirus on February 15, 1996, and on March 13, 1996. Shortly after the last injection, all of her disease regressed and she has remained continuously disease-free as of October 2001. The PBLs used for the present studies were obtained on January 17, 1996, 26 days after her first injection of recombinant adenovirus. This patient was the only one of 16 patients similarly treated with recombinant adenovirus who experienced cancer regression.
Cloning lymphocytes with antitumor activity
Lymphocytes were thawed into medium and cloned in round-bottom
96-well plates using techniques similar to those previously described
(4). Twenty-five plates each contained 30 cells/well or
100 cells/well (total of 312,000 cells cultured). Each well also
contained 5 x 104 irradiated (5000 cGy)
PBMCs combined from three non-HLA-A2 donors, 30 ng/ml OKT3
(Orthoclone, Ortho-biotech, Raritan, NJ) and 300 IU IL-2 (Chiron,
Emeryville, CA) per milliliter in a total volume of 0.2 ml/well. The
medium was changed at 7 days and approximately every 3 days thereafter.
By 3 wk after initiating the culture, 120 wells (5.0%) plated at 30
cells/well and 250 wells (10.4%) plated at 100 cells/well exhibited
growth. Two-thirds of the cells in each of these 370 wells were
transferred to a separate plate and tested for IFN-
release when
incubated overnight with 5 x 104, 624.38
mel, or 624.28 mel. These two culture lines were clones derived in our
laboratory from the parental 624 line. However, the 624.28 mel line had
selectively lost HLA-A2 Ag expression (data not shown). Six of these
370 clones exhibited reactivity against 624.38 mel, and not 624.28 mel
cells. These six individual clones were expanded in a rapid expansion
protocol using OKT3 and 300 IU IL-2/ml, as previously described
(5). Following the expansion, retesting of the six clones
revealed that three recognized the HLA-A*0201-restricted MART-1:2735
peptide from the known MART-1 melanoma Ag (6, 7), one
recognized the HLA-A*0201-restricted NY-ESO-1:157165 peptide
from the NY-ESO-1 melanoma Ag (8, 9), and one had no
recognition of melanoma cell lines. The sixth clone reacted with 624.38
mel cells but had no reactivity against known Ags. It was recloned at
one and five cells/well (three plates each) and 16 (0.2%) of the wells
exhibited growth. A total of 7 of the 12 wells growing at five
cells/well exhibited selective reactivity with 624.38 mel, and not
624.28 mel, and after expansion exhibited no reactivity against the
COS-A2 cell line transfected with the MART-1,
gp100, tyrosinase, tyrosinase-related
protein (TRP)1, TRP2,
MAGE-1, or NY-ESO-1 genes. One of these
subclones, termed 10-B5, recognized several
HLA-A2+, but not HLA-A2-
melanoma cell lines, and was used to screen a cDNA library from the 624
mel cell line.
Screening and testing of cDNA library
A cDNA expression library from the 624 mel cell line was
prepared by techniques previously described (9). In brief,
total RNA was extracted, poly(A) RNA was purified and converted to cDNA
using an oligo(dT) primer, ligated into the pEAK 8 vector (Edge
Biosystems, Gaithersburg, MD), and electroporated into DH10B cells.
Pools containing
100 cDNA clones were prepared from bacteria and the
plasmid DNA transfected into 293-A2 cells. Following transfection, the
plates were incubated for 24 h and 5 x
104 10-B5 lymphocytes were added to each well.
The supernatant from each well was harvested after 24 h and
IFN-
was measured using an ELISA.
Peptide synthesis and testing
Peptides were synthesized using a solid-phase method based on
standard F-moc chemistry on a multiple peptide synthesizer (Gilson,
Worthington, OH). The identities of the peptides were verified by mass
spectrometry (Biosynthesis, Lewisville, TX). Lyophilized peptides were
solubilized in DMSO and 1 µM peptide was added to T2 cells and used
to stimulate the 10-B5 T cell clone in an overnight coculture assay
containing 5 x 104 cells of each type.
IFN-
secretion was measured by ELISA.
| Results |
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The specificity of clone 10-B5 was tested against a variety of
normal and malignant HLA-A2+ and
HLA-A2- cell lines shown in Table I
. Reactivity was seen against the three
HLA-A2+ melanomas tested, but not the
HLA-A2- melanomas, nor against multiple
HLA-A2+ EBV-B lines, fibroblasts, human umbilical
vein endothelial cell lines, or cultures of breast, colon, or prostate
cancer. Thus, clone 10-B5 appeared to recognize an HLA-A2-restricted Ag
expressed by melanoma cells, but not normal cells, nor cells from other
nonmelanoma tumors tested.
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Transfectants of the 293-A2 cell line with 384 cDNA
pools, including
38,400 cDNA clones from the 624
library, were screened using the lymphocyte clone 10-B5, and
four positive pools were identified. A total of 2 of 192 subclones from
the most positive well stimulated cytokine release from the 10-B5
lymphocyte following transfection into 293-A2 cells, and one of these
clones was then sequenced.
The cDNA clone contained 1162 bp that represented a partial cDNA
encoding the last 330 aa of the BING-4 gene (GenBank accession number
BC000388; Refs. 10, 11, 12, 13), although presumably only the last
288 aa were translated from an initiating methionine encoded within
this cDNA clone. The full-length product was then cloned using primers
based on the known sequence of BING-4. Although several bands were
found using 624 mel RNA, probably representing alternatively spliced
variants, the full-length BING-4 transcript (
2 kb) was identical to
the BING-4 sequence in GenBank. The first nucleotide in the clone we
isolated represented nt 839 with respect to the start site of the
full-length BING-4 protein. BING-4 is a 610 aa protein of unknown
function present in a gene-rich segment of chromosome six immediately
centromeric to the classical class II MHC that contains many genes
involved in Ag processing (Fig. 1
; Refs.
10, 11, 12). A database search revealed the presence of
expressed sequence tags encoding BING-4 in infant brain,
retinoblastoma, melanoma, choriocarcinoma, prostate cancer, liver
cancer, bone marrow, and tonsillar B cell cDNA libraries.
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Using algorithms that predict the binding of peptides to HLA-A2
within the last 288 aa of the BING-4 protein (the segment in our
original cDNA clone), a total of 88 peptides (35 9-mers and 43 10-mers)
were synthesized. These peptides were individually pulsed onto the
HLA-A2+ T2 cell line and none were recognized by
lymphocyte clone 10-B5 (data not shown). Because of the inability to
detect a reactive peptide in the normal open reading frame of the
BING-4 protein, alternative open reading frames of the cDNA clone that
started with a methionine codon and that encoded a minimum of 9 aa were
then evaluated. This cDNA encoded nine potential open reading frames
(Table II
) that varied between 9 aa and
the 288 aa, which represented the normal BING-4 open reading frame.
Twenty-five peptides conforming to an extended HLA-A2 Ag-binding motif
from all of the remaining open reading frames were tested for
recognition by the 10-B5 lymphocyte clone. A single 9-aa peptide,
CQWGRLWQL, from a 10 aa open reading frame conferred a high degree of
reactivity against lymphocyte 10-B5 (Table III
). No other peptide tested within any
of the open reading frames showed reactivity against the lymphocyte
clone with the exception of the overlapping 10-mer, MCQWGRLWQL, which
was similar to the 9-mer peptide in its ability to stimulate clone
10-B5 (Fig. 2
).
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Expression of BING-4 mRNA on multiple cell lines and tissues
Although the 10-B5 lymphocyte clone that selectively recognized
the BING-4 peptide recognized HLA-A2+ melanomas
and not other HLA-A2+ normal cells or other tumor
cell lines and because Abs against BING-4 are not available, the
presence of BING-4 transcripts in expressed sequence tag libraries led
us to more extensively explore the expression of BING-4 mRNA. Only low
levels of BING-4 expression were detected when RNA derived from
multiple normal human tissues were tested using a sensitive RT-PCR
assay (data not shown). A real-time semiquantitative RT-PCR assay was
then performed on multiple melanoma cell lines as well as lines from
other tumors and normal tissues to determine the number of BING-4 mRNA
copies relative to mRNA encoding the GAPDH housekeeping gene (Fig. 3
). Many melanoma cell lines expressed
relatively high levels of BING-4 mRNA, whereas all
nonmelanoma tumors as well as EBV-B cells, fibroblast,
andmelanocyte-cultured cells had relatively low levels of BING-4
expression.
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To estimate the relative immunogenicity of the BING-4 peptide,
lymphocytes from 10 patients were in vitro sensitized against both the
native BING-4 9-mer peptide, CQWGRLWQL, or the BING-4 peptide with
methionine substituted for glutamine at the second position using
techniques similar to those previously described (14).
Peptide (10 µM) was added to 3 x 106
preimmune PBMC in 2 ml containing 500 mg/ml CD40 ligand. Four
weekly restimulations were performed using PBMC pulsed with BING-4
peptide. On the day after each restimulation, 300 IU IL-2/ml was added
to each culture. After four restimulations, PMBC derived from 7 of 10
patients that were sensitized in vitro to the native peptide developed
antipeptide reactivity and in three of these patients specific
reactivity was seen against HLA-A2+ BING-4
expressing tumor cells as well as 293-A2 cells transfected with the
full-length BING-4 gene (Table IV
).
Following in vitro sensitization against the modified peptide, the PBMC
from six patients also developed antipeptide reactivity, although none
developed reactivity against melanoma lines (data not shown).
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| Discussion |
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To avoid bias resulting from prolonged in vitro culture of lymphocytes, we used an approach to identify melanoma-reactive T cells generated by limiting dilution analysis of circulating lymphocytes that were stimulated with OKT3 and IL-2. Thus, we were able to grow lymphocyte clones without a specific antigenic stimulus. Because the patients autologous tumor was not available, screening of the clones was performed using the 624.38 and 624.28 mel cell lines both derived from the parental 624 mel cell line. These lines differed only in the loss of expression of HLA-A2 from the 624.28 cell line. A total of 312,000 cells were cultured, and thus any reactivities that were detected would likely be present in high frequency. The starting lymphocyte population used in this study was derived 26 days after the first injection of recombinant adenovirus at a time just before any obvious tumor regression was noted.
Reactivity against three Ags was detected in the circulating
lymphocytes from this patient. Reactivity against the MART-1 melanoma
Ag may have been the result of immunization with the recombinant
adenovirus encoding MART-1. As we have reported previously, many
melanoma patients have preexisting reactivity against the MART-1
immunodominant peptide and reactivity does not appear to correlate with
cancer regression (15). Reactivity was also found against
the NY-ESO-1:157165 peptide. This patient also had circulating Ab
reactive with the NY-ESO-1 protein, and we were able to generate
HLA-DP-restricted T cells from the lymphocytes of this patient by in
vitro sensitization against the HLA class II-restricted
NY-ESO-1:161180 peptide (16). Patient TC had not been
immunized against the NY-ESO-1 Ag, and thus, this reactivity was either
preexisting or was somehow stimulated by the adenoviral immunization.
The secretion of IFN-
by immune lymphocytes reactive to MART-1
and/or NY-ESO-1 Ags may have led to up-regulation of BING-4 Ag
expression and the development of reactivity to this previously
unrecognized tumor Ag.
There are several unique aspects to the detection of BING-4 as a tumor
Ag in this patient. BING-4 is a protein of unknown function containing
610 aa encoded by 15 exons within a gene-rich region of chromosome six
considered to represent an extended part of the class II MHC (Fig. 1
;
Refs. 10, 11, 12). This region contains a variety of genes
including tapasin, a gene required for presentation of Ag by MHC class
I molecules, DAXX, which encodes an effector of fas that
stimulates apoptosis through the Jun kinase pathway as well as other
novel genes. Particularly unusual is the presence of the BING-5 gene
within intron 11 of the BING-4 gene. BING-5 is transcribed from the
opposite strand to that encoding BING-4 (11). The function
of BING-4 is unknown, although there are numerous nuclear localization
signals situated near both the amino and carboxyl termini of the
protein suggesting its localization to the nucleus. Proteins sharing
conserved regions with BING-4 have been found in a variety of other
species including the mouse, rat, Saccharomyces,
Caenorhabditis, and others (11, 13). The
location of BING-4 in this region and its close proximity to genes
involved in Ag presentation suggest that it may be involved in
immunologic phenomena.
Other tumor Ags such as TRP1 and NY-ESO-1 contain epitopes
translated from alternative open reading frames (9, 17).
However, the immunoreactive peptide presented on the surface of
melanoma cells from the BING-4 protein is translated from an
alternative open reading frame only 10 aa long. This finding emphasizes
the need to explore all open reading frames when searching for
presented peptides within putative tumor Ags (Table II
).
Expression of BING-4 mRNA was found in virtually all normal tissues.
Real-time semiquantitative RT-PCR analysis revealed that the levels of
expression of the BING-4 transcript in normal tissues, including
melanocytes, as well as in nonmelanoma tumors, were low relative to the
levels found in melanomas. The correlation between high levels of
expression of the BING-4 mRNA and recognition of tumors by
BING-4-reactive T cells strongly suggests that a threshold level of
BING-4 expression is necessary to mediate tumor recognition (Figs. 3
and 4
). The high level of BING-4 expression on melanomas compared with
other cancer types is unexplained.
It is not known whether the BING-4 Ag played a role in the tumor destruction in this patient. To test the immunogenicity of BING-4, we performed in vitro sensitizations against the BING-4 peptide using PBLs from 10 nonimmunized HLA-A2+ melanoma patients. A total of 7 of 10 patients were successfully sensitized in vitro to the native peptide, and in three of these patients a high enough level of reactivity was developed to recognize HLA-A2+ BING-4-positive tumor cells. This suggests that immunization with the BING-4 immunodominant peptide may be an effective means of immunizing patients with melanoma.
The present study was undertaken to obtain clues to explain the factors important in patients undergoing regression of metastatic cancer following immunotherapy. The reactivity of patient TC to the NY-ESO-1 Ag as well as to the new BING-4 tumor Ag in the absence of any deliberate immunization to these Ags suggests that these reactivities may have played a role in the tumor destruction. The mechanism of in vivo tumor destruction by immune lymphocytes is unclear, however, and other lymphocyte functions such as lysis and secretion of additional cytokines by BING-4-reactive cells in patients need to be evaluated. Similar analyses of other patients who have undergone significant durable cancer regressions are underway to determine whether common factors will emerge in these successfully treated patients.
| Footnotes |
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2 Abbreviations used in this paper: MART-1, melanoma Ag recognized by T cells 1; TRP, tyrosinase-related protein. ![]()
Received for publication October 11, 2001. Accepted for publication December 31, 2001.
| References |
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