Immunogenic tumor variants induced by drug treatment of the L5178Y lymphoma: search for serologically defined antigens at the clonal level

A retroviral peptide encoded by mutated env p15E gene is recognized by specific CD8+ T lymphocytes on drug-treated murine mastocytoma P815

Highly immunogenic ("xenogenized") tumour variants appear after treatment of murine mastocytoma P815 with the triazene derivative DTIC, a phenomenon associated with the appearance of structurally abnormal p15E env proteins in the variant cells. In the present study, we have isolated and sequenced several p15E cDNA gene fragments amplified by means of polymerase chain reaction (PCR) from parental (P815) and xenogenized (P815/DTIC) tumour cells. Compared to known p15E sequences in parental cells, one p15E sequence from xenogenized cells presented three distinct nucleotide changes, one of which was apparently unique to P815/DTIC DNA and cDNA upon single-nucleotide primer extension assay. One major histocompatibility complex (MHC) class I-binding peptide, corresponding to a putative mutation in the p15E sequence, was tested in parallel with the parental peptide for recognition by P815/DTIC-specific cytotoxic T cells in vitro. The results suggested that the amino acid substitution at the relevant position of the p15E protein may produce an antigenic T cell epitope. By skin test assay of mice primed with either the synthetic peptide or P815/DTIC cells, evidence was obtained that the mutated peptide is immunogenic in vivo, and that the neoepitope is expressed by P815/DTIC cells. In accordance with previous data in the L5178Y/DTIC tumour model system, these findings reinforce the notion that xenogenization of tumour cells may result in the expression of class I-binding mutated peptides of retroviral origin.

In vitro combined effects of a triazene compound and interferon-beta on natural immunity against lymphoblastoid cells: studies at effector and target cell level

It was shown that Dacarbazine and other triazene compounds render murine leukemias highly immunogenic and susceptible to natural immunity (NI). In addition a pilot clinical study revealed that Dacarbazine can be cytotoxic for bone marrow blasts in patients with acute non-lymphoblastic leukemias through a mechanism that could be, at least in part, of immunological origin. However triazenes depress antigen-dependent responses and NI, whereas interferons, including interferon-beta (IFN), antagonize drug-induced impairment of NI. Therefore the complex interaction between triazenes and IFN on NI effector (i.e. NK) lymphocytes and human target lymphoblastoid cells has been investigated. The results show that: (a) IFN increases NK activity and antagonizes the depressive effects of methyl-triazene-benzoic acid (MTBA, an in vitro active triazene compound) on the NK function; (b) a lymphoblastoid cell line exposed to multiple in vitro treatments with MTBA, shows increased growth rate, augmented chemoresistance to MTBA, and higher susceptibility to NI than parental cells; (c) as expected IFN pretreatment down-regulates the susceptibility of lymphoblastoid cells to NK effectors; (d) however a net "therapeutic gain" was found if the overall influence of MTBA+IFN on target and effector cells is considered.

Drug-mediated changes in the susceptibility of human lymphoblastoid B cells to NK-mediated cytolysis: studies with a triazene compound

Previous studies have shown that treatment of leukemia-bearing mice with triazene compounds results in a profound alteration of the immunological properties of leukemic cells. These cells become highly immunogenic and susceptible to natural immunity (NI). Moreover, in a pilot clinical study, dacarbazine was found to suppress bone marrow blasts in patients with acute non-lymphoblastic leukemias. The cytotoxic mechanism involved could be of biochemical and immunological origin as well. Therefore experiments were carried out to test whether triazenes could influence the susceptibility of blast cells to human NI effector lymphocytes (represented, at least in part, by NK cells). The results obtained with target Epstein-Barr virus (EBV)-immortalized B cells and effector cells of different donors, showed that: (a) multiple in vitro treatments of lymphoblastoid cells with methyl-triazene-benzoic acid (MTBA, a triazene compound active in vitro), gave origin to lines that were more resistant than the parental lines to the antitumor effects of MTBA; (b) MTBA-treated lines were more susceptible (37.5% of cases), or less susceptible (31.2% of cases) to NI than parental cells. Effector lymphocytes of various donors recognized different changes in susceptibility to natural killer (NK)-mediated lysis; (c) treatment of parental or MTBA-treated lines with interferon-beta reduced target cell susceptibility to NK-mediated cytolysis, but increased NK activity and lymphoblast chemosensitivity to MTBA.

Untreated or drug-treated tumor cells are differentially recognized by allogeneic lymphocytes

Murine tumor cells treated with triazene compounds (TZC), in vivo or in vitro, are capable of eliciting specific transplantation resistance in syngeneic hosts, and T-cell-mediated proliferative and cytotoxic responses, directed against novel drug-induced antigen(s). Since this phenomenon, referred to as chemical xenogenization (CX) could open up new perspectives in the immunochemotherapy of human neoplasias, it was of interest to investigate whether CX could also occur in human tumors. However, established human tumor cell lines along with fully immunocompetent autologous lymphocytes, are seldom available. Therefore studies were carried out to test whether parental or TZC-treated tumor cells could be differentially recognized by allogeneic lymphocytes. Experiments were performed in both human and murine models, using a lung adenocarcinoma line treated in vitro with TZC, or an established xenogenized mouse lymphoma, respectively. The results indicate that allogeneic cytotoxic T-lymphocytes (CTL) recognize specifically murine TZC-treated tumor cells. This was supported by the finding that antisera directed against the drug-treated cells abrogated the generation and the cytolytic activity of allogeneic CTL reactive against the TZC-treated tumor. In addition it was found that changes of the antigenic pattern of cell membrane recognizable by cloned allogeneic CTL occur in the TZC-treated human carcinoma cell line.

Endogenous retroviral gp70 genes of the murine lymphoma L5178Y: analysis of restriction fragment polymorphism upon induction of drug-mediated immunogenicity

Highly immunogenic ("xenogenized") tumor variants appear after treatment of murine lymphoma L5178Y with the triazene derivative DTIC, this phenomenon being associated with the appearance of structurally abnormal gp70 env proteins in the cell variants. In the present study, we have isolated and sequenced several PCR-amplified gp70 cDNA genes from L5178Y cells. One of the resulting clones was used as a probe in Southern and Northern analysis of parental and xenogenized cells. The results indicated that xenogenization of tumor cells is associated with changes in retroviral env sequences detectable at the genomic level.

Immunogenic properties of retroviral protein P15E from drug-treated murine mastocytoma P815

A triazene-xenogenized tumor sub-line was derived from the mouse mastocytoma cell line P815 following several transplant generations in vivo on DTIC. The highly immunogenic P815/DTIC variant line expressed new CTL-defined antigens. Novel antigens were also detected by antibodies in immunoprecipitation and by Western blot analysis. Upon immunoprecipitation of metabolically labeled cells, one such variant-specific 20-kDa antigen was shown to be related to retroviral envelope protein p15E. When injected intrasplenically into recipient mice, the electroblotted nitrocellulose-bound 20-kDa antigen resulted in increased frequency in CTL precursors to P815/DTIC cells. In addition to previous data in the L5178Y/DTIC tumor-model system, these data suggest that expression of aberrant, retrovirus-related proteins may be a common finding in different parental tumors xenogenized by triazene treatment.

Cell-mediated immunity to chemically xenogenized tumors--VI. The effect of cell treatment with retroviral env antisense oligonucleotides

The occurrence of aberrant, retrovirus-related proteins is a common finding in immunogenic clones of the triazene-xenogenized L5178Y lymphoma line (L5178Y/DTIC). In clone D-cells, newly expressed 80 kDa antigens related to xenotropic murine leukemia virus env gene products induce specific humoral and cell-mediated responses and possess biologic activity in vivo. To further clarify the relationship between immunogenic properties of clone D and retroviral gene expression, tumor cells were treated in vitro with antisense oligonucleotides complementary to xenotropic and/or polytropic env sequences of murine leukemia virus. The cells were then assayed for expression of antigens recognized by humoral and cell-mediated responses with specificity for clone D. The results demonstrated that inhibition of env mRNA translation adversely affected the expression of immunogenic determinants in the xenogenized tumor cells.