1 Ludwig Institute for Cancer Research, Brussels
Branch, 74 avenue Hippocrate UCL 7459, B-1200 Brussels, Belgium
and Cellular Genetics Unit, University of Louvain, 74 avenue Hippocrate
UCL 7459, B-1200 Brussels.
The existence of specific tumor rejection antigens was first demonstrated
with chemically induced mouse sarcomas: each tumor was found to
express a different antigen [1 ]. Similar findings were made with
ultraviolet-induced tumors [2]. Later, the generality of the existence
of tumor rejection antigens was questioned when spontaneous mouse
tumors were found to be completely incapable of eliciting an immune
rejection response [3]. However, further experiments demonstrated
that even these tumors express weak transplantation antigens that
are potential targets for immune rejection by the syngeneic host
[4]. But what is the molecular nature of tumor rejection antigens?
And what is the relation between their appearance and the tumoral
transformation process? These questions are still unanswered because
these antigens, which elicit strong T -cell mediated immune responses,
do not stimulate B cells to produce antibodies. It has therefore
been impossible to isolate the antigenic molecules by immunoprecipitation.
Recently, we have developed a gene transfection approach aimed at
identifying directly the genes that code for this type of antigen.
It was applied to "tum- " transplantation antigens, which arise
on mouse tumor cells when they are treated with mutagenic agents,
and to a tumor rejection antigen present on mouse mastocytoma P
815.
Tum- Antigens
In vitro mutagen treatment of mouse tumor cells generates at high
frequency stable immunogenic variants that are rejected by syngeneic
mice [5]. Because of their failure to form tumors, these variants
were named "turn-" as opposed to the original "turn + " cell, which
produces progressive tumors. This phenomenon has been observed on
a large number of mouse tumor cell lines of various types [6]. Most
turn variants express new transplantation antigens not found on
the original turn+ cell. The existence of these tum- antigens was
first demonstrated by transplantation experiments [7] We have studied
a series of tumvariants derived from mastocytoma P815, a tumor induced
in a DBA/2 mouse with methylcholanthrene. From clonal turn + line
P 1, we obtained more than 30 different tum- variants, which rarely
produced progressive tumors even when they were injected at very
large doses. When restimulated in vitro, spleen cells of mice that
had rejected these variants produced cytolytic T cells (CTL) that
lysed preferentially the immunizing turn variant [8]. From these
lymphocytes, we were able to isolate stable CTL clones [9]. Some
of these appeared to be directed against a tumor rejection antigen
of P815: they lysed P1 and all P815-derived cells but not syngeneic
control tumors. Others lysed the immunizing tum- variant, but neither
the original turn+ cell nor the other tum- variants. They therefore
defined new tum- antigens specific for each variant (Fig. 1 ). These
antigens displayed considerable diversity: no antigen was found
twice among 15 tumvariants that were analyzed. By in vitro immunoselection
with anti-tum- CTL clones it was possible to demonstrate that some
tum- variants carry several turn antigens [10]. These experiments
also demonstrated that the tum- antigens defined by CTL are relevant
to the rejection of the variants, as shown by the correlation between
the loss of these antigens and the reversal of the tum- phenotype
[10, 11]. To find an explanation that could reconcile the remarkably
high frequency of tum- variants with their stability and to understand
the source of their diversity, it appeared essential to identify
the antigenic molecules. We failed in our attempts to obtain antibodies
directed against tum- antigens. Therefore, we undertook to clone
directly the relevant genes on the basis of their ability to produce
the antigens recognized by the anti-tum- CTL.
Fig. 1. Tumor rejection antigens and turn antigen
present on the original P815 line P1 and on tum- variant P198. Lysis
by CTL clones directed against a tumor rejection antigen (anti-P815)
or a tum- antigen (anti-P198)
Cloning of Genes Encoding Tum- Antigens
The procedure that we developed for the cloning of the gene coding
for tumantigen P91 A is based on gene transfection. It involves
the use of a highly transfectable P 815 cell line called P 1. HTR
[12] and the detection of antigenexpressing transfectants by their
ability to stimulate CTLs [13]. By transfecting Pl. HTR cells with
a cosmid library prepared with the DNA of a cell expressing turn
antigen P91 A, we obtained transfectants expressing this antigen
at a frequency of 1 per 28 000 [14]. By direct encapsidation of
the DNA of these transfectants into lambda phage heads, we obtained
a cosmid capable of transferring the expression of the antigen.
An 800-bp restriction fragment from this cosmid was found to transfer
the expression of the antigen. This fragment was then used to identify
cosmids containing either the normal or the antigenic allele of
the entire P91A gene as well as complementary (c)DNA clones of the
homologous messenger RNA. The procedure that led to the isolation
of tum- gene P91 A was applied with success to the cloning of tum-
genes P35B and P198, which encode antigens expressed by other tum-
variants derived from P815 [15, 16].
Fig.2. Structure of genes P91 A, P35B, P198, and P 1 A
and antigenic peptides. Dark regions represent exons. The exon containing
the turn mutation is marked by an asterisk. Sections of the proteins
located around the mutated amino acid are indicated. Synthetic peptides
corresponding to the mutant and normal sequences of the genes are
represented by boxes. They were tested for their ability to render
P 1. HTR cells susceptible to lysis by anti-turn CTL. The concentration
indicated to the right of each peptide provided 50% of the lysis
obtained at saturating concentration of peptide. For P 1 A, the
box indicates the subgenic fragment capable of transferring the
expression of antigens P 1 A and P 1 B. The antigenic peptides for
P 1 A and P 1 Bare not yet identified
Tum -Mutations
Northern blots probed with the 800-bp fragment of gene P91 A revealed
a single messenger RNA species of 2.2 kb. The band was of equal
intensity for tum variant P91 and for P1, which does not express
the antigen. The expression of antigen P91 A is therefore not due
to the activation of a silent gene. The structure of gene P91 A
is shown in Fig. 2. It comprises 12 exons spread over 14 kb [17].
It does not show any similarity with Ig, T cell receptor or MHC
genes. The complete sequence was obtained. It is unrelated to any
sequence presently recorded in the main data banks. A sequence comparison
of the normal and tum- alleles of gene P91 A indicated that they
differ by a point mutation in the exon which is present in the transfecting
800-bp fragment (Fig. 2). This tum- mutation is a G to A transition
that changes an arginine into a histidine in the main open reading
frame of the gene [14]. This mutation appears to be the only difference
distinguishing the normal from the antigenic allele. The study of
the tum- alleles of genes P35B and P198 also revealed that they
differ from the normal alleles by a point mutation in an exon (Fig.
2). The general structures and the sequences of the three tum- genes
isolated so far are completely unrelated.
Antigenic Peptides
The main open reading frame of gene P 91 A encodes a protein of
60 kDa, which does not have atypical N-terminal signal sequence
[17]. In vitro translation experiments suggest that the two potential
Nglycosylation sites present in the sequence are unused (Godelaine,
Amar-Costesec, De Plaen, Beaufay, unpublished results). Antigen
P91 A is therefore unlikely to be borne by a membrane protein. This
is however hardly surprising, considering the recent demonstration
that CTL can recognize influenza antigens corresponding to endogenous
proteins remaining inside the cell and considering the observation
that CTL recognize small pep tides that bind to surface class I
MHC molecules [18-20]. On the basis of this evidence, we examined
whether we could also identify a small peptide that would trigger
the lysis of P815 cells by antiP91 A CTL. In our search for this
peptide we were guided by the location of the tum- mutation. A short
peptide (Fig. 2) corresponding to the mutant sequence induced the
lysis of P 1 by anti-P91 A CTL. Transfection and peptide studies
with H-2k fibroblasts, which expressed also either Kd, Dd or Ld,
demonstrated that antigen P91 A is associative with Ld. Antigenic
peptides corresponding to the sequence surrounding the turn -mutation
were also obtained for genes P35B and P198. They associate with
Dd and Kd respectively. Studies with P91 A peptides enabled us to
understand the role of the turn mutation. A priori, the mutation
could influence either the production of the antigenic peptide or
its ability to associate with the Ld molecule (i.e., the aggretope)
or also the epitope presented to T cells by the peptide-MHC complex.
Having the antigenic P91 A peptide, we prepared the homologous peptide
corresponding to the normal allele of the gene. This normal peptide
did not induce lysis by anti-P91A CTL, nor did it compete with the
mutant peptide. Moreover, we found that the mutant peptide competed
effectively to prevent a cytomegalovirusderived peptide from inducing
lysis by CTL directed against a Ld-associative cytomegalovirus antigen.
The normal peptide did not compete [17] and we concluded that it
does not bind to Ld. This indicates that the P 91 A turn mutation
generates the aggretope of the antigen, but does not exclude that
it also influences the epitope. For antigen P 198, the effect of
the mutation appears to be different: here anew epitope is introduced
on a normal peptide that is already capable of binding to the Kd
presenting molecule.
Cloning of the Gene Encoding a Mouse Tumor Rejection Antigen
We have applied the same cloning procedure to the isolation of
the gene coding for a tumor rejection antigen expressed by tumor
P815 [21]. As opposed to the tum- antigens, these antigens are present
on all P815 cells, whether they are mutagenized or not. The study
of antigen-loss variants enabled us to identify four distinct antigens
recognized by different syngeneic CTL clones. They were called P1A,
B, C, and D (Fig.1) [22]. Antigens P 1 A and P 1 B thus defined
in vitro are relevant in vivo, because P815 tumor cells that progressed
in mice after nearly complete initial rejection were found to have
lost the expression of one or both these antigens. Antigens P1 A
and P1 B showed linkage, since several antigenloss variants for
P1A were found to have lost P1 B concurrently. For the transfection
of antigen P1A, we used as recipient cell a P 1 A- B antigen-loss
variant selected from line P 1. HTR with an anti-P1 A CTL clone.
Transfectants expressing both antigens P1A and P1B were obtained
with the genomic DNA ofP1. HTR. This confirmed the close link between
these two antigens. Transfectants were then obtained with a cosmid
library made with the DNA of a genomic transfectant. By directly
packaging the DNA of one of these cosmid transfectants, we obtained
a cosmid that was able to transfect both antigens P 1 A and P 1
B. The structure and the complete sequence of gene P 1 A were then
obtained (Fig. 2). They proved completely different from those of
the tum- genes and of any known gene reported in data banks. Transfection
studies in H2-k fibroblasts previously transfected with either Kd,
Dd, or Ld demonstrated that both P 1 A and P 1 B were presented
to the CTL by the Ld molecule. We compared the sequence of this
gene, cloned from tumor cells, to the sequence of the equivalent
gene cloned from normal cells of the same mouse strain. From a genomic
library made with the DNA of normal DBA/2 mouse kidney we isolated
the gene homologous to gene P 1 A. The analysis of this gene revealed
that its sequence was identical to the sequence of the tumoral gene.
To confirm this, we transfected this normal gene and found that
it transferred the expression of antigens P 1 A and P 1 B as efficiently
as the gene cloned from P815 cells (Fig. 3). The antigenicity is
therefore not the result of a mutation in the tumoral gene, and
P 1 A and P 1 B are presumably two different peptides derived from
the same protein. The tumor specificity of antigens P 1 A and P
1 B can nevertheless be partially explained by the pattern of expression
of the gene. Northern blot analysis revealed
Fig. 3. Transfection of the P 1 A gene isolated from normal
cells. The P 1 A gene isolated from a genomic library from normal
DBA/2 mouse kidney was transfected in PO. HTR cells. The population
of drug-resistant transfectants was tested with the anti-P1A and
anti-P1B CTL clones
that the gene was silent in most normal tissues. However, one mast
cell line (L 138.8 A) was found to strongly express messenger (m)RNA
for P 1 A. This cell line, derived by Hültner et al. [28] from bone
marrow of BALB/c mice, is cultivated in medium supplemented with
interleukin 3. It grew as a permanent line and became spontaneously
tumorigenic. Because BALB/c mice and DBA/2 mice express the same
H2 haplotype, we were able to confirm the expression of gene P1A
in L138.8A cells by lysis with the anti-P 1 A and anti-P 1 B CTL
clones: we observed a significant lysis. Other nontransformed mast
cell lines on the other hand were negative for P 1 A expression,
so that we do not known whether the expression of the gene is associated
to the mast cell lineage at a given stage of its differentiation,
or whether it is related to the tumoral transformation. We failed
to identify other tumor cell lines expressing mRNA for P1A.
Immune Surveillance, Tolerance, and Tumor Rejection Antigens
A first conclusion based on the results obtained with the tum-
antigens is that mutations throughout all the mammalian genome generate
at high efficiency antigenic peptides recognized by T cells, and
that this mechanism could account for the presence of specific tumor
rejection antigens on carcinogen-induced tumors. However, the study
of antigen P 1 A clearly showed that gene P 1 A is identical to
its normal counterpart. The apparent tumor specificity of antigen
P 1 A seems to be due to a specific regulation of the transcription
of the gene rather than to a mutation generating an antigenic peptide.
We now have to understand how the immune system may be sensitized
against normal peptides to which is should be tolerant, and this
without generating an obvious autoimmune pathology. Several hypotheses
can be suggested. If the gene encodes an embryonic or oncofetal
protein, then the antigen might have disappeared before the establishment
of tolerance. If it codes for a differentiation or activation antigen,
we can imagine that it is expressed very transiently by a small
number of cells, so that tolerance does not develop and that an
immune reaction directed against this antigen does not impair normal
differentiation or activation. Lastly, if tolerance is actually
present for P 1 A, then it must have been broken, and the simultaneous
presence on the P815 tumor of other antigens like C and D may be
important in that respect. These antigens could indeed be the result
of a mutation and therefore be strongly immunogenic like tum antigens.
They could possibly trigger an immune response that would facilitate
a response against P1A [4].
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