Establishment of productively infected walleye dermal sarcoma explant cells

Walleye dermal sarcoma virus (WDSV) is a complex retrovirus associated with dermal sarcomas in walleye fish. Virus expression is tightly regulated and limited to accessory gene transcripts throughout tumour development. During tumour regression, this regulation is lost and the replication of virus is greatly enhanced. Cultured walleye fibroblasts infected in vitro do not produce significant quantities of infectious virus. Tissue culture cells established by explantation of tumour cells were found to harbour WDSV provirus and to express accessory and structural proteins. The sequence of the provirus showed little variation from a previous WDSV isolate. Retroviral particles were isolated from supernatants from these cells and were able to transfer infection to uninfected walleye fibroblasts. In addition to the virus present in supernatants, much of the virus was cell associated and liberated only by sonication. This virus was found at internal cellular membranes, including mitochondria, and was infectious.

Adenovirus E1A, not human papillomavirus E7, sensitizes tumor cells to lysis by macrophages through nitric oxide- and TNF-alpha-dependent mechanisms despite up-regulation of 70-kDa heat shock protein

Expression of adenovirus (Ad) serotype 2 or 5 (Ad2/5) E1A or human papillomavirus (HPV)16 E7 reportedly sensitizes cells to lysis by macrophages. Macrophages possess several mechanisms to kill tumor cells including TNF-alpha, NO, reactive oxygen intermediates (ROI), and Fas ligand (FasL). E1A sensitizes cells to apoptosis by TNF-alpha, and macrophages kill E1A-expressing cells, in part through the elaboration of TNF-alpha. However, E1A also up-regulates the expression of 70-kDa heat shock protein, a protein that inhibits killing by TNF-alpha and NO, thereby protecting cells from lysis by macrophages. Unlike E1A, E7 does not sensitize cells to killing by TNF-alpha, and the effector mechanism(s) used by macrophages to kill E7-expressing cells remain undefined. The purpose of this study was to further define the capacity of and the effector mechanisms used by macrophages to kill tumor cells that express Ad5 E1A or HPV16 E7. We found that Ad5 E1A, but not HPV16 E7, sensitized tumor cells to lysis by macrophages. Using macrophages derived from mice unable to make TNF-alpha, NO, ROI, or FasL, we determined that macrophages used NO, and to a lesser extent TNF-alpha, but not FasL or ROI, to kill E1A-expressing cells. Through the use of S-nitroso-N-acetylpenicillamine, which releases NO upon exposure to an aqueous environment, E1A was shown to directly sensitize tumor cells to NO-induced death. E1A sensitized tumor cells to lysis by macrophages despite up-regulating the expression of 70-kDa heat shock protein. In summary, E1A, but not E7, sensitized tumor cells to lysis by macrophages. Macrophages killed E1A-expressing cells through NO- and TNF-alpha-dependent mechanisms.

Activity of the vascular targeting agent combretastatin A-4 disodium phosphate in a xenograft model of AIDS-associated Kaposi's sarcoma

Combretastatin A4 disodium phosphate (CA4DP) was evaluated in a xenograft model of AIDS-KS. KS xenografts were highly vascular, showing brisk mitotic activity, focal areas of necrosis, and intervening fibrovascular septae. Neoplastic cells were large or spindle-shaped, with vesicular nuclei and modest pleomorphism. Multiple junctions, microvillous-like projections, abortive lumina and rare Weibel Palade bodies were revealed by electron microscopy. Treatment with CA4DP (100 mg/kg) resulted in rapid onset of vascular effects that within 4 h resulted in an almost complete vascular shutdown in these tumors. Histological evaluation showed morphological damage within a few hours after treatment, followed by extensive necrosis which increased to approximately 90% by 24 h. At this time, viable tumor cells were evident only at the periphery of the tumor. These findings demonstrate not only the marked susceptibility of the KS model to CA4DP but also its potential application in studies related to the pathogenesis and therapy of AIDS-KS.

CpG island protects Rous sarcoma virus-derived vectors integrated into nonpermissive cells from DNA methylation and transcriptional suppression

CpG islands are important in the protection of adjacent housekeeping genes from de novo DNA methylation and for keeping them in a transcriptionally active state. However, little is known about their capacity to protect heterologous genes and assure position-independent transcription of adjacent transgenes or retroviral vectors. To tackle this question, we have used the mouse aprt CpG island to flank a Rous sarcoma virus (RSV)-derived reporter vector and followed the transcriptional activity of integrated vectors. RSV is an avian retrovirus which does not replicate in mammalian cells because of several blocks at all levels of the replication cycle. Here we show that our RSV-derived reporter proviruses linked to the mouse aprt gene CpG island remain undermethylated and keep their transcriptional activity after stable transfection into both avian and nonpermissive mammalian cells. This effect is most likely caused by the protection from de novo methylation provided by the CpG island and not by enhancement of the promoter strength. Our results are consistent with previous finding of CpG islands in proximity to active but not inactive proviruses and support further investigation of the protection of the gene transfer vectors from DNA methylation.

Adenovirus-mediated p53 gene therapy inhibits human sarcoma tumorigenicity

Mutations of the p53 tumor-suppressor gene are the most frequent genetic abnormality in soft tissue sarcomas. Because these rare tumors also respond poorly to standard chemotherapy and bear a 50% 5-year mortality rate, we investigated the possible therapeutic benefits of p53 gene restoration in sarcomas. We constructed Ad5p53, which is an E1A-deleted, replication-deficient adenovirus expressing a cytomegalovirus promoter-driven wild-type p53 cDNA with a Flag sequence tag. SKLMS-1 human leiomyosarcoma cells containing a mis-sense p53 point mutation were effectively transduced with Ad5p53. Increasing levels of Flag-p53 protein, as well as dose-dependent p21Cip1 induction, were observed through a dose range of 10-500 plaque-forming units (PFU)/cell. In vitro administration of Ad5p53 as a single 100 PFU/cell dose caused 40-60% growth inhibition of SKLMS-1 cells at posttreatment days 4, 6, and 8 compared with untreated or viral control treated-cells (P < .05, Student's t test). Relative to these same controls, in vivo treatment of SKLMS-1-bearing severe combined immunodeficient mice with 6 x 10(9) PFU of Ad5p53 by intratumoral injection resulted in a 35-day tumor growth delay and complete tumor regression in 40% of mice (P < .05, Student's t test). The expression of virally derived p53 mRNA in Ad5p53-treated tumor tissues was detected in treated tumor specimens by reverse transcriptase polymerase chain reaction. Reduced intratumoral cellularity and the presence of p53 staining in adjacent normal tissue, consistent with delivery of exogenous p53 to the tumor target, were evident only in Ad5p53-treated tumors after immunohistochemical staining for p53. These results indicate that wild-type p53 gene restoration in sarcomas retards tumor growth and may come to be usefully applied to the clinical treatment of this disease as a single regimen or in combination with conventional therapies.

Investigations on the aminoacid content of tumor associated antigens of rat sarcoma cells induced by virus

In our previous work (Alexandrov et al., 1996) was reported that the rat sarcoma cells induced by SR-RSV express two tumor associated antigens (TAA). The one TAA has a molecular weight of 52 kD and is detected by the help of a monoclonal antibody 2C2 only on the outer side of the plasma membrane of the sarcoma cells. The other antigen, with molecular weight of 28 kD, is expressed on the outher and inner side of the membrane. The antigens were isolated as a pure fraction by polyacrylamide electrophoresis and prepared for aminoacid analysis after that. The consisting 16 bound aminoacids were in different amounts. Both antigens are rich in glycine and poor in aromatic and sulphur-containing aminoacids. The presence of glucosamine and galactosamine in the antigens proves their glycoprotein nature. The received data show that the both TAA-s differ not only in molecular weights, place of expression and functional activity, but also in the amount of the bound aminoacids which constitute their proteins.

Haploid male germ cells show no susceptibility to transformation by simian virus 40 large tumour antigen in transgenic mice

Cell-type specific tumorigenesis can be induced in transgenic mice by the directed expression of simian virus 40 (SV 40) large tumour antigen (TAg). In an attempt to determine the susceptibility of haploid male germ cells to neoplastic transformation by this oncogene, transgenic mice were generated that harboured a chimeric gene composed of the SV40 T antigen genes fused to the 2.3-kb 5' flanking sequences of the rat proacrosin gene. It was previously shown that this regulatory sequence is able specifically to direct the expression of CAT reporter gene in male germ cells with the onset of translation in early haploid male germ cells. The transgene showed regulated expression in male germ cells. Although T antigen immunostaining was detected specifically in spermatids, no testicular pathology was observed. This indicates that spermatids show no susceptibility to transformation by oncogene TAg. However, in about 10% of animals of two independent transgenic lines, we could find non-testicular tumours in abdomen with a sarcoma-like structure in advanced age which showed SV40 TAg expression.

Studies on the pathology, especially brain lesions, induced by R7, a spontaneous mutant of Moloney murine sarcoma virus 124

We have recently isolated R7, a spontaneous Moloney murine sarcoma virus (MoMuSV) 124 variant. Molecular cloning and sequence analysis showed that, relative to MoMuSV 124, R7 has an extra repeat in each enhancer and a truncated mos gene in frame with the truncated gag coding sequence. This report presents a detailed study on the pathology induced by R7. R7 induced not only sarcomas with well developed angiomatous components but also brain lesions. Brain lesions were observed in all less-than-48-hour-old BALB/c mice inoculated with greater than 2 x 10(5) R7 focus-forming units (FFUs). R7 was detected in all brains examined by day 9 after inoculation, and brain lesions were observed in two of four mice examined by day 14 after inoculation. Light microscopy of brains revealed that approximately 15% of the lesions were unenclosed blood pools of varying sizes containing red blood cells and inflammatory cells spreading into surrounding brain tissues. The remainder of the brain lesions had tumor cells. These lesions ranged from a few enlarged vascular endothelial cells intermixed with blood cells to large circumscribed lesions consisting of well developed tangled masses of vessels surrounded by blood pools. Activated astrocytes surrounded and infiltrated the tumors. In addition, the thymus of R7-infected mice regressed significantly and precipitously due to apoptosis (especially of cortical thymocytes) at the end stage of the disease.

Transcriptional analysis of walleye dermal sarcoma virus (WDSV)

Walleye dermal sarcoma virus (WDSV) is a complex retrovirus associated with dermal sarcomas of walleye that develop and regress on a seasonal basis. WDSV contains, in addition to gag, pol, and env, three open reading frames (ORFs) designated ORF A, ORF B, and ORF C. The polymerase chain reaction technique was used to amplify and clone cDNAs representing subgenomic viral mRNAs isolated from developing (fall) and regressing (spring) tumors. Nine different singly or multiply spliced viral transcripts were identified and all were found to utilize a common 5' leader sequence. This leader sequence is spliced to the pol/env junction or downstream of env to generate singly spliced transcripts. Multiply spliced transcripts contain the 5' leader, the pol/env junction, and sequences derived from the 3' end of the genome. One multiply spliced transcript was isolated with the potential to encode the full-length ORF A protein. In addition, WDSV produced mRNAs that utilize alternative splice acceptor sites which would allow synthesis of five variant forms of the ORF A protein. In contrast, the ORF B protein is postulated to arise from a singly spliced transcript with the potential to encode the entire open reading frame. Spliced subgenomic transcripts representing ORF C mRNAs were not identified, suggesting that ORF C may be encoded from the full-length viral genomic transcript. We estimate that at least a 100-fold lower amount of the accessory/regulatory subgenomic transcripts exists in developing vs regressing tumors. These results demonstrate that WDSV undergoes an elaborate pattern of mRNA splicing similar to that of other complex retroviruses.

Role of p53 mutation in polyomavirus-induced tumorigenesis

Small DNA tumour viruses, such as simian virus 40 (SV40), papilloma viruses and adenoviruses, encode proteins that form complexes with and inactivate the p53 and retinoblastoma (RB) proteins. This convergent evolution reflects the common need of these viruses to inactivate these two important regulators of cell cycle progression and cell survival. Polyomavirus, a close relative of SV40, is different. Its large T protein complexes only with RB, not with p53. We have examined whether this is compensated by the frequent appearance of p53 mutations in polyomavirus-induced tumours. We tested the p53 status of 15 polyomavirus-induced sarcomas. Two sarcomas were p53-negative while six carried mutant p53. Another six sarcomas expressed low levels of wild-type p53. One tumour expressed high levels of wild-type p53 protein as shown by DNA sequencing and immunofluorescence staining. MDM2 amplification was not detected in any of the tumours, but Northern blotting showed that MDM2 was overexpressed in at least two tumours that expressed wild-type p53 and in one tumour that expressed both wild-type and mutant p53. Treatment with the DNA-damaging agent mitomycin C caused p53 protein accumulation followed by induction of MDM2 and WAF1/p21 mRNA in four of the tumours expressing wild-type p53, indicating that p53-mediated transcriptional activation was unaltered in these tumours. However, p53-mediated transactivation of WAF1/p21 was impaired in the wild-type p53-expressing tumours that expressed elevated levels of MDM2. These results demonstrate that p53 mutation and inactivation are frequently but not invariably involved in polyomavirus-induced tumorigenesis.

SRR-SB3, a disulfide-containing macrolide that inhibits a late stage of the replicative cycle of human immunodeficiency virus

From a series of macrocyclic diamides possessing the disulfide linkage, only SRR-SB3, a compound that complexes with zinc, was found to inhibit human immunodeficiency virus type 1 (HIV-1; strain IIIB) replication at a concentration of 1.8 to 6.5 micrograms/ml in MT-4, CEM, and peripheral blood mononuclear cells. SRR-SB3 was toxic to MT-4 cells at a concentration of 15.9 micrograms/ml, resulting in a selectivity index of 9 in these cells. This macrolide was also effective against various other HIV-1 strains, including clinical isolates and HIV-1 strains resistant to protease inhibitors and nucleoside and nonnucleoside reverse transcriptase inhibitors. It was also active against various HIV-2 strains, simian immunodeficiency virus (strain MAC251), and Moloney murine sarcoma virus, but not against viruses other than retroviruses. In addition, the compound was found to inhibit chronic HIV-1 infections in vitro. The compound in combination with other antiviral agents, such as zidovudine, zalcitabine, and stavudine, showed an effect that was between additive and synergistic. Time-of-addition experiments indicated that SRR-SB3 acts at a late stage of the HIV-1 replicative cycle.

Neutral metoclopramide induces tumor cytotoxicity and sensitizes ionizing radiation of a human lung adenocarcinoma and virus induced sarcoma in mice

Radiation induced cytotoxicity was potentiated by neutralized metoclopramide (nMCA; Neu-Sensamide, Oxigene Inc) when a human lung adenocarcinoma (H2981) transplanted into scid mice and an adeno-type 12 virus induced mouse sarcoma (A12B3) inoculated into CBA mice were exposed in vivo to low dose radiation at single doses of 1 and 2 Gy respectively. However, when the radiation dose was increased to 6, 10 or 18 Gy (single dose) and combined with a single dose nMCA (2 mg/kg), tumor cytotoxicity was not sensitized by the combination treatment. A fractionated dose of ionizing radiation (3 x 1 Gy) in combination with nMCA at a repeated dose of 3 x 10 mg/kg body weight (1 dose/day, i.m.) significantly increased cytotoxicity in H2981 compared with radiation given alone. nMCA alone also had a statistically significant dose dependent cytotoxic effect on H2981 growth when it was administered as repeated doses (8 doses) at 2 mg/kg or 10 mg/kg (1 dose every second day), and a similar result was achieved at 20 mg/kg but not at 2 and 10 mg/kg in the A12B3 tumor. In addition, the tumor volume at the start of treatment was important for the anti-tumor effect of nMCA (i.e. the larger initial tumor volume gave less effect on tumor growth). Taken together, our data propose that the mode of action of nMCA is different from radiation, and hence the two mechanisms are at least additive when in combination with lower radiation doses. The data further suggest that the cytotoxic mechanism is consistent with potentiating apoptosis because low and repeated doses of radiation (1-2 Gy), which are known to increase cytotoxicity by apoptosis, are sensitized by nMCA but not high doses and nMCA has more potent anti-tumor effects against H2981 tumors which have a higher constitutive apoptotic fraction of cells than A12B3.

The susceptibility to cytotoxic T lymphocyte mediated lysis of chemically induced sarcomas from immunodeficient and normal mice

A panel of sarcomas induced with 3-methylcholanthrene in normal and immunodeficient mice was studied for their capacity to present antigen by the endogenous, MHC class I restricted pathway. Lymphocytic choriomeningitis virus was used to infect cultured tumour cells, and the infected cells were tested for susceptibility to cytolysis by virus specific cytotoxic T cells. Tumour cells originating from tumours induced in immunocompetent C.B.-17 mice presented virus antigen more efficiently than tumour cells from immunodeficient SCID mice. No significant difference in virus antigen presentation was found between tumours from nude and nu/+ BALB/c mice. The sensitivity of target cells from the individual tumours to cytotoxic T lymphocyte (CTL) mediated lysis correlated negatively with their sensitivity to natural killer (NK) cell mediated lysis. There was a positive correlation between the sensitivity to CTL mediated lysis and surface expression of the MHC class I molecule Ld of the tumour cells. Tumour cells incapable of in vitro presentation of viral antigen to specific cytotoxic T cells originated from tumours known from previous experiments to be readily accepted after transplantation to immunocompetent, histocompatible recipients.

BK virus-induced osteosarcoma (Os515) as a model of human osteosarcoma

BK virus was isolated by Gardner et al in 1971 from the urine of an immunosuppressed patient with a kidney allograft. Antibodies to this virus are ubiquitous in the general populations worldwide, but the oncogenic capacity of BKV in humans had not been reported. The virus transformed in vitro permissive human and non-permissive animal cells, and the transformed cells had the T antigen. Intracerebral and intravenous inoculation of BKV in newborn hamsters induced malignant tumours (mainly ependymomas, malignant insulinomas, and osteosarcomas). Subcutaneous and intraperitoneal routes were also effective. The virus was only rescued from a few tumours by fusion with human embryonic cells or Vero cells. Brain tumours appeared earlier and osteosarcomas developed in animals which survived for more than 6 months. Many of the osteosarcomas were bony and grew slowly with frequent lung metastases, and a few osteosarcomas were soft and grew rapidly without lung metastases. Experimental targeting chemotherapy with doxorubicin (DX)-containing immunoliposomes was performed against Os515 osteosarcoma. In in vitro experiments, DX-Lip-MoAb29 showed a more significant inhibitory effect on cultured Os515 cells than free Dx and DX-Lip. DX-Lip DNP had less effect. In in vivo experiments, DX-Lip-MoAb29 suppressed the growth of Os515 tumour isografts in hamsters and prolonged the survival of recipients more significantly than free DX.

Anti-L-selectin monoclonal antibody treatment in mice enhances tumor growth by preventing CTL sensitization in peripheral lymph nodes draining the tumor area

To examine the in vivo contribution of L-selectin in the sensitization of tumor-specific CTL, we investigated the effects of treatment with the anti-L-selectin monoclonal antibody (MAb) MEL-14 on the immune response to Moloney-murine sarcoma virus (M-MSV)-induced tumors, which exhibit spontaneous regression following generation of a strong virus-specific CTL response. Daily systemic administration of MEL-14 for 10 days to M-MSV-injected mice gave rise to larger sarcomas that persisted for a longer time, compared with those arising in control mice injected with virus only. The enhanced tumor growth could not be attributed to cytotoxic activity on leukocytes by MEL-14 since no reduction in the total cell number was detected in peripheral blood and spleen of MAb-treated mice. Evaluation of the immunological response in MAb-treated animals revealed a strong reduction in the generation of virus-specific CTL precursors (CTLp) in tumor-draining peripheral lymph nodes (PLN) 10 and 15 days after M-MSV injection, while in spleen, where lymphocyte localization is independent of L-selectin expression, CTLp generation was only delayed. By day 20, when tumors had begun to regress, the CTLp number showed a marked increase in both spleen and local PLN, where naive recirculating CTL could now enter because L-selectin was no longer down-regulated or blocked by the injected MAb. Our findings indicate that functional inactivation of L-selectin by MEL-14 treatment prevented migration of naive L-selectin+CTL through high endothelial venules (HEV) and their accumulation in PLN draining the tumor area, thereby precluding the initiation of a tumor-specific CTL response that takes place primarily at this site.

Mouse parvovirus infection potentiates rejection of tumor allografts and modulates T cell effector functions

Lymphocytotropic mouse parvoviruses can perturb immune responses. For example the recently identified mouse parvovirus designated MPV-1 persistently infects lymphoid tissues and interferes with the ability of cloned T cells to proliferate. As a consequence of these findings the present studies were undertaken to characterize further the immunomodulatory effects of MPV-1 on T cell-mediated immune responses in vivo and in vitro. To evaluate the effect of MPV-1 on CD8+ T cell-mediated responses sarcoma I (SaI) cells, devoid of class II major histocompatibility (MHC) antigens, were administered to MPV-1-infected adult BALB/c mice. MPV-1 infection accelerated tumor allograft rejection. Immunofluorescence staining and in situ hybridization studies of tumors suggested that direct infection of the tumor cells was not responsible for accelerated rejection. Furthermore, compared with uninfected mice, T cells from infected mice that had rejected SaI tumors had a diminished cytolytic capacity. Taken together these results suggest that MPV-1 may induce "bystander help." To examine the in vivo effect of MPV-1 on CD4+ T cell mediated responses adult mice were primed with ovalbumin (OVA) and infected with MPV-1. Spleen and popliteal lymph node cells from OVA-primed mice 3 or 7 days after MPV-1 inoculation had reduced proliferation responses, whereas the proliferative capacity of mesenteric lymph node cells from these mice was increased. Similarly, MPV-1 reduced cytokine-induced proliferation of allospecific CD8+ cloned L3 T cells and OVA-reactive CD4+ T cells without effecting cell viability. Since parvoviruses are widespread among laboratory rodents, these findings emphasize the importance of identifying and excluding parvovirus infections in mice used for transplantation studies and in cultures of mouse T lymphocytes.

V-onc mutation associated with host cell growth in retroviral tumors

In sarcomagenesis in rats infected neonatally with feline sarcoma virus (ST-FeSV), v-fes product (P85) was previously shown by us to be a predictive and preventive determinant. In order to explore the part played by P85 in tumor suppression, DNA was extracted from precancerous granulomas and from slow or rapid growing sarcomas induced by neonatal injection of the virus. The v-fes signal from extracted DNA was analyzed by PCR-SSCP. The prototype v-fes gene signal was detected in most lesions and found to be generally amplified in rapid growing sarcomas and in some granulomas. Several v-fes homologs showing varying mobilities in gel were seen in most sarcomas and some granulomas with or without the prototype v-fes signal. In slow growing sarcomas and granulomas induced in hosts that were immunized with ST-FeSV induced syngeneic sarcoma and proved to carry IgG antibody to P85, the prototype v-fes gene was found to be down-regulated and v-fes homologs were found to be reduced in number or eliminated. These results suggest that the development of v-fes mutations is associated with the growth potential of cells carrying the v-fes gene, and that host immunity to v-onc product influences the development of virogene rearrangements and results in slow and suppressed growth of tumors caused by neonatal infection with retrovirus.

Akv murine leukemia virus enhances bone tumorigenesis in hMT-c-fos-LTR transgenic mice

hMt-c-fos-LTR transgenic mice (U. Rüther, D. Komitowski, F. R. Schubert, and E. F. Wagner. Oncogene 4, 861-865, 1989) developed bone sarcomas in 20% (3/15) of females at 448 +/- 25 days and in 8% (1/12) of males at 523 days. After infection of newborns with Akv, an infectious retrovirus derived from the ecotropic provirus of the AKR mouse, 69% (20/28) of female animals and 83% (24/29) of males developed malignant fibrous-osseous tumors. The tumors in infected transgenics developed with higher frequency and a 200-days shorter mean tumor latency period. The hMt-c-fos-LTR transgene was expressed in all the fibrous-osseous tumors. They also showed newly integrated Akv proviruses, but in most tumors Akv was detected and expressed in only a small number of the tumor cells. Wild-type C3H mice infected with Akv developed benign osteomas with an incidence of 33% and a latency period of 474 days. The data indicate that Akv exerts distinct pathogenic effects on the skeleton. In hMt-c-fos-LTR transgenic mice, predisposed to bone sarcomagenesis, Akv acts synergistically with the fos transgene, resulting in the development of fibrous-osseous tumors.

[Expression of a cross-reactive antigen on the surface of human epidermoid carcinomas overproducing EGF-receptor shared with RSV-induced tumors]

Analysis based on the complement-dependent cytotoxicity (CDC) assays using syngeneic antiserum against a Rous sarcoma virus(RSV)-induced mouse tumor(CSA1M) showed that a cross-reactive antigen with a common tumor-associated cell surface antigen(TASA) of RSV-induced mouse tumors was shared with two human tumors A431 and MDA-468 overexpressing epidermal growth factor receptor(EGFR). The TASA, however, was not expressed on four human choriocarcinomas, a human lung cancer A2182, and human embryo fibroblasts HFF. Immunofluorescent studies demonstrated that A431 does not express a src gene product detected by anti-pp60src monoclonal antibody(MoAb). Two variant clones derived from A431 reducing number of ERFR (cl-15 and cl-16) have almost same growth rate and expression of transferrin receptor(Tf-R) in comparison with parental A431 cells. These clones, however, decreased the expression of TASA. Furthermore, CDC assays and enzyme-linked immunosorbent assay(ELISA) revealed that A431 reduced the expression of the TASA by pretreatment of EGF, but not with insulin. All these findings indicate a close association between a cross reactive antigen with the TASA of RSV-induced tumors and EGFR.