Tumor-specific Transplantation Antigen Research Articles

Clonal Origin of Tumors Induced by Ultraviolet Radiation<xref ref-type="fn" rid="FN2">2</xref>

Skin tumors were induced in (C3H/HeN X C3H/HeN-PGK-1a)F1 female mice, heterozygous at the X-linked phosphoglycerate kinase-1 (PGK-1) locus, by exposure to the carcinogenic influence of ultraviolet radiation (UVR), 3-methylcholanthrene [(MCA) CAS: 56-49-5], or benz[a]pyrene [(BP) CAS: 50-32-8]. An assessment of the clonal origin of these tumors was accomplished through an analysis of the PGK-1 enzyme phenotype expressed by the transformed cells. In vitro culture was employed as a means of depleting nontransformed cells of host origin from the induced tumors. Cultured lines derived from tumors induced by each of the above agents were found to express only one of the two enzyme forms encoded by the host genotype, consistent with the probability that all the UVR-, MCA-, and BP-induced tumors examined in this study were monoclonal in origin. For further substantiation of the monoclonality of UVR-induced tumors, 2 UVR-induced tumors were enzymatically dissociated immediately following excision from the primary hosts, and the resulting cell suspensions were cloned in soft agar. Upon analysis, each set of clones selected in soft agar expressed only a single PGK-1 enzyme form. To rule out the possibility that the apparent monoclonality of culture-adapted tumor lines was due to in vitro selection, tumors that arose in UVR-treated PGK-1a/b female heterozygote mice were transplanted into PGK-1a and PGK-1b homozygous recipients. These transplanted tumors expressed a single PGK-1 allozyme following growth in recipients that were genetically homozygous for the major PGK-1 enzyme form expressed by the tumor prior to transplantation. These data strongly support the concept that most, if not all, UVR-induced tumors are derived from the progeny of a single transformed cell. This observation is important to the understanding of the nature of tumor-specific transplantation antigens expressed by individual UVR-induced tumors and indicates that such antigens also are clone specific. In addition, the results of this study indicate that polyclonality does not play a significant role in the generation of cellular and phenotypic heterogeneity known to be present within individual UVR-induced tumors.

Polyoma virus-induced hemangiomas in grafts of visceral yolk sac and of embryos

The development of hemangiomas in grafts of visceral yolk sac and embryos after in vitro infection with polyoma virus is described. These hemangiomas were shown to be positive for the polyoma tumor-specific transplantation antigen(s) and to develop as well in recipient virgin as in pregnant rats. In grafts of fetal skin or gut infected with polyoma virus neoplastic transformation was never observed.

Antigen-specific therapy of experimental metastases.

In a methylcholanthrene-induced tumor (MCA-F) model in C3H/HeJ mice, curative resection of a progressive tumor promotes artificial lung metastases following intravenous injection of a highly metastatic cell variant designated clone 9-4. The number of metastatic lung colonies depends upon the status of host immunity at the tumor resection: mice resected 7 or 14 days, but not 28 days after tumor inoculation display significantly retarded postoperative, experimentally induced lung metastases. The number of lung colonies in mice that had tumors resected at 14 days was three times greater than in mice that had 28-day neoplasms resected. Neither therapy with weekly injection of 50 micrograms tumor-specific transplantation antigen, which had been extracted using a single phase of 2.5% 1-butanol (CBE), nor 20 mg/kg cyclophosphamide (CY) alone prevented lung colonization. Assessment of helper-suppressor ratios in the spleen from mice after tumor surgery showed that CBE administration decreased the ratio in mice after resection of 14-day tumors, but not after resection of 28-day tumors. Combined therapy with specific tumor antigen and an antisuppressor cell agent reduced metastases, regardless of the tumor size.

The effect of purification on the immunogenicity of tumor-specific transplantation antigens.

Immunization with the tumor-specific transplantation antigens (TSTA) of experimental, chemically induced sarcomas engenders specific host resistance to challenge with viable, homotypic neoplastic cells. The strength of tumor resistance depends upon the physical state of the TSTA used for immunization. Treatment with 10(5)-10(6) irradiated tumor cells, a 2-log dose range, induces complete rejection of neoplastic challenges, while immunization within a 1-log dose range with crude 3 M KCl or with 2.5% butanol extracts containing TSTA evokes a weak state of resistance characterized by decreased outgrowth of tumor challenges, but not neoplastic regression. The reduced immunogenicity may be due to either contamination with substances that antagonize host resistance, for example by induction of suppressor cells, or an intrinsic limitation by virtue of the molecular properties of extracted compared with cell-surface TSTA. MCA-F and MCA-D, two noncross-reactive fibrosarcomas induced in C3H/HeJ mice with 3-methylcholanthrene, were employed to compare the relative immunogenic activity of intact tumor cells, 2.5% butanol extracts, and materials sequentially purified by preparative isoelectric focusing (pIEF), preparative isotachophoresis (pITP), and high performance gel permeation chromatography (HPGPC). Immunoprotective TSTA activity purified 50,000-fold by this protocol extended the effective dose range by four to five logs: 15 pg to 1.5 micrograms MCA-F or 1 pg to 10 ng MCA-D antigen-induced specific host resistance. However, despite the appreciable purification of TSTA, immunization with extracted materials only delayed neoplastic outgrowth. They induced neither immediate rejection nor only temporary progression of transplanted tumor cells. Thus, purified TSTA preparations by themselves lack the immunogenic properties of intact cells that result in maximal induction of tumor resistance.

Blocking (suppressor) factors, immune complexes, and extracorporeal immunoadsorption in tumor immunity.

Some animal neoplasms, particularly many of those induced by chemical carcinogens, have tumor-specific transplantation antigens. These are detected by their ability to induce the rejection of tumors grafted onto properly immunized syngeneic hosts (Prehn and Main, 1957; Old and Boyse, 1964; Sjogren, 1965; K. E. Hellstrom and Brown, 1979), and the tumors expressing the antigens are commonly referred to as immunogenic. It is being debated whether “spontaneous” tumors, like most human tumors, have antigens that can be immunogenic in the native host (Klein and Klein, 1977; Hewitt et al., 1976; Hellstrom et al., 1983).

Alloantigen-activated lysis of syngenic tumor augmented by allogenic lymphocytes as cold targets.

In the past, most work in tumor immunology involved attempts to demonstrate tumor-specific transplantation antigens; however, recent in vitro work has shown that it is not necessary to sensitize to a specific tumor antigen to achieve lysis of syngenic tumor cells. Responder spleen cells from B6AF1 mice (H-2a,b) were sensitized in mixed lymphocyte cultures (MLC) to alloantigens on spleen cells from B10.D2 (H-2d) mice. In cell-mediated lympholysis (CML) assays the relevant allogenic P815 (H-2d) tumor targets and B10.D2 lymphoblast targets were lysed. In addition, the semisyngenic tumor target EL-4 (H-2b) was lysed but RDM-4 (H-2k) was not. B10.BR (H-2k), C57BL/6 (H-2b) and B6AF1 lymphoblasts were not lysed. If lymphocytes were added as cold targets to the CML assay, B10.D2 lymphocytes completely absorbed lytic activity when B10.D2 lymphoblasts were the target. If B10.D2 lymphocytes were added when P815 was the target, lysis was reduced but could not be abolished. When B10.D2 lymphocytes were added with EL-4 as a target, lysis doubled. These experiments show that the neoplastic determinant on tumor cells recognized by alloantigen-activated lymphocytes does not cross-react with stimulator cell alloantigen and is not an alien histocompatibility antigen. These observations should be considered when in vivo attempts are made to control tumor with in vitro activated lymphocytes; transfer of not only in vitro activated cells but also allogenic stimulator cells to the tumor site may be necessary for maximal tumor destruction.

Retardation of Postsurgical Metastases With the Use of Extracted Tumor-Specific Transplantation Antigens and Cyclophosphamide<xref ref-type="fn" rid="FN1">2</xref><xref ref-type="fn" rid="FN3">3</xref>

In a 3-methylcholanthrene [(MCA) CAS: 56-49-5]-induced tumor model of C3H/HeJ mice excision of a growing primary tumor decreased concomitant immunity and facilitated experimental lung metastases. Administration of tumor-specific transplantation antigens extracted from viable MCA-F cells with the use of single-phase (2.5%) 1-butanol [crude butanol extract (CBE)] augmented immunity after resection of the primary MCA-F tumor. Two weeks after footpad inoculation of 2 X 10(5) MCA-F cells, the tumor-bearing limbs were amputated and the mice were challenged subsequently with 5 X 10(4) cells of clone-9-4, a metastatic variant of MCA-F, via the tail vein. Whereas treatment with either 50 micrograms CBE sc or 20 mg cyclophosphamide (CY)/kg ip failed to retard lung colonization, combination therapy with the two agents reduced the incidence of lung colonies by 69.8% (26.5 vs. 8; P less than .001) compared with the incidence in the surgery-alone group and by 55.5% (18 vs. 8; P less than .001) compared with the incidence in the group treated with surgery and CY. Furthermore, the combined effects of CBE and CY were immunologically specific: The combined therapy with the non-cross-reactive MCA-D-CBE did not protect against iv challenge with clone-9-4. Treatment with antigenic extracts induced a 21.4% (4.2 vs. 3.3%) decrease in the ratio of Lyt 1+:Lyt 2+ cells in the spleens of tumor-resected mice, which suggested restoration to normal levels. Therefore, in the combined regimen, antigen may induce specific activation of helper lymphocytes, while CY inhibits activation of suppressor cells.

Studies on the polyoma-virus-induced tumor-specific transplantation antigen (TSTA)--does middle or large T-antigen play a role?

Mice and rats could be immunized against the polyoma-virus-induced tumor-specific transplantation antigen (TSTA) by repeated inoculation of frozen or irradiated cells of an MT-cDNA-transformed rat cell line (2.8) that contains only the polyoma middle T-antigen, or by cells that carried a host range mutant and expressed a full-length large T-antigen, but only non-functional N-terminal fragments of small and middle T. This shows that neither large T nor an intact middle T is necessary to elicit a polyoma tumor-specific graft rejection response. Either one of them is sufficient by itself.

Tumor-specific immunity induced by somatic hybrids: IV. Relationship between immunogenicity and expression of surface tumor-associated antigens

Semi-allogeneic hybrid clones were derived by fusion of the TEPC-15 plasmacytoma (H-2 d) and mouse L cells of C3H (H-2 k) origin. Three representative clones were chosen to study the relationship between the expression of different membrane antigens and their immunogenicities leading to protection of recipient mice from the parent TEPC-15 plasmacytoma. The level of surface tumor-specific transplantation antigens (TSTA) was measured by radioimmunoprecipitation with syngeneic anti-TSTA and by inhibition of anti-TSTA binding to the TEPC-15 tumor cells. The most immunogenic hybrid clone (LTC-1) expressed the highest level of TSTA and the weakly immunogenic (LTC-2) and nonimmunogenic (LTC-4) hybrid clones exhibited relatively low levels of TSTA on the surface. Moreover, the strongly immunogenic LTC-1 hybrid cells, but not the parent tumor cells, were effective in priming recipient spleen cells to generate TEPC-15 tumor-specific cytotoxic cells upon subsequent in vitro exposure to the TSTA-bearing cells. Therefore, the level of TSTA on the semi-allogeneic hybrid clones may play an important role in enhancing the immunogenicity of TSTA.

Detection of Tumour‐Associated Transplantation Antigens by Syngeneic Cytotoxic T Lymphocytes after Fusion of Tumour Cell Membrane Vesicles to Recipient Cells

Sendai virus envelope-mediated fusion was used to transfer plasma membrane vesicles of ESb lymphoma cells (H-2d) to syngeneic or allogeneic recipient cells. After fusion, ESb-specific cell surface determinants were detectable on the recipient cells, as revealed by indirect immunofluorescence and cytofluorographic analysis. Fused cells were also tested as targets in 51Cr-release assays, using syngeneic DBA/2 anti-ESb- or anti-H-2d-specific T lymphocytes (CTL). Fused recipient cells became susceptible to lysis by both types of CTL, indicating that both tumour-specific and H-2d determinants were inserted on fusion. The experimental approach presented could be of help as an assay system for the molecular identification of tumour-specific transplantation antigens.

Selective extraction by 1-butanol of surface glycoprotein antigens from human melanoma cells.

The butanol extraction method has previously been used to achieve selective release of tumor-specific transplantation antigens from mouse sarcoma cells. In this study we investigated the feasibility of this method for extracting four surface glycoprotein antigens (87K, 95-150K, HLA-DR, and HLA-A,B,C) from cultured human melanoma cells. Of the four antigens examined, only 95-150K and HLA-DR antigens could readily be detected in material extracted by 2%, 3%, or 5% butanol. The 3% butanol was found to be most effective in releasing these two antigens. Treatment of melanoma cells with less than or equal to 3% butanol did not decrease the viability of extracted cells as judged by either Trypan Blue dye exclusion or plating efficiency. Thus the noncytolytic butanol extraction method offers a promising approach to the isolation of certain glycoproteins such as 95-150K and HLA-DR from viable human melanoma cells for further purification and structural analysis.

Suppressor T cell recognition of major and minor histocompatibility alloantigens: selected suppression of MHC-directed responses by minor alloantigen Ts.

The induction of suppression by i.v. administered alloantigens in the murine host was analyzed as a model of the possible effects of blood transfusion on transplant survival. The results indicated that suppressor T cells (Ts) specific for minor histocompatibility alloantigens could be readily induced by the i.v. presentation of minor alloantigen-disparate spleen cells. In contrast, similar priming with cells differing solely at the H-2 major histocompatibility complex stimulated only positive T cell immunity, with no evidence of suppression. The induction of H-2 directed Ts activity could be accomplished only by i.v. priming with major plus minor incompatible donor cells, suggesting that suppressor cell recognition of minor alloantigens may have facilitated the generation of Ts against H-2-encoded major transplantation antigens. A role for minor histocompatibility antigens in the regulation of H-2-specific immunity at the effector level was also indicated. Ts induced by i.v. pretreatment with minor antigen-disparate donor cells not only suppressed the delayed-type hypersensitivity (DTH) response to the relevant minor alloantigens, but also inhibited DTH against unrelated H-2 alloantigens introduced during subsequent intradermal immunization. Suppression of H-2-directed T cell reactivity was specific in that the presence of the Ts-inducing minor alloantigens was also required and occurred only when the minor and unrelated major alloantigens were presented within the same inoculum, if not on the same cell surface. The capacity of Lyt-2+Ts or Ts-derived suppressive factors specific for one set of cell surface molecules to modulate responses to an unrelated group of surface antigens does not appear to represent a general phenomenon, because similar suppression of immunity to unrelated tumor-specific transplantation antigens by minor-specific Ts was not observed. These results are discussed with respect to the possible mechanism of H-2-directed suppression and the role of the I region in Ts recognition of antigen.

Properties of a herpesvirus-transformed hamster cell line: immunogenicity of sublines of high and low metastatic potential.

The immunogenicity of a herpesvirus hominis type-2-transformed hamster cell line (HSV-2-333-2-26) of low spontaneous metastatic ability was compared with that of its two in vivo-derived sublines of increased metastatic potential. The parent (HSV-2-333-2-26) tumour was immunogenic as assessed by protection against tumour challenge afforded by implantation of irradiated tumour cells or tissue. In contrast, the two metastatic sublines, designated Met A and Met B, were non-immunogenic as defined by the above critera . However, the parent Met A and Met B tumours were shown to possess a common antigen(s), since immunization with irradiated parent tumour cells afforded protection to challenge with Met A or Met B. Immunization with the metastatic sublines, however, gave no protection to homologous or heterologous tumour challenge. Bacillus Calmette-Guérin (BCG) inoculated in admixture with irradiated tumour cells and followed 7 days later by one immunization with X-irradiated tumour cells alone, increased host immunocompetence to subsequent homologous or cross-tumour cell challenge with parent, Met A or Met B cells. The immunity raised by using BCG plus irradiated tumour cells was shown to be specific to antigens expressed on the HSV-2 parent cell line and its metastatic sublines. In addition, BCG admixed with live inocula of parent, Met A or Met B cells induced contact suppression of in vivo tumour growth of the parent cells, but not of Met A or Met B cells. It is suggested from these studies that the parent tumour possesses a tumour-specific transplantation antigen(s) ( TSTAs ) which is not functionally active on its metastatically derived sublines. Common antigens, shared between the parent and Met A and Met B cells, are detectable by cross-challenge experiments, but they themselves appear not to be immunologically offensive. The loss of immunogenicity is discussed as a possible mechanism for the in vivo selection of sublines with increased metastatic potential.

Cell lines derived from tumors induced in syngeneic rats by FR 3T3 SV40 transformants no longer synthesize the early viral proteins

Several SV40-transformed FR 3T3 rat cell lines formed tumors upon inoculation to syngeneic immunocompetent Fisher rats. These tumors, which appeared only after a long latency period, showed a fast rate of growth. Tumor-derived (TD) cell lines were established in culture from several tumors induced by independent transformants, and their properties were analyzed. Though TD cells were highly tumorigenic, their level of transformation in culture was similar to that of the original transformants. They did not synthesize detectable amounts of the two early viral gene products, the large-T and small-T polypeptides. However, the transformation-associated cellular p53 protein was detected in all of them by [ 35S]methionine labeling and immune precipitation with monoclonal antibodies directed against the mouse p53. Growth in the animal apparently counterselected the cells expressing the early viral proteins, and hence, possibly, the tumor-specific transplantation antigen. This selection was mediated at least in part by the T-cell immune response, as the tumors induced by the same transformants in nu/nu mice still expressed the nuclear T-antigen. Absence of expression of the early viral region was frequently correlated with the loss of the integrated SV40 DNA. Some tumors, however, still contained early viral DNA sequences, which were, in even fewer cases, transcribed into RNA. These results altogether suggest that tumor formation by the FR 3T3-SV40 transformed cells in immunocompetent rats requires two events, the selection for the acquisition of a high tumorigenic potential, and against the expression of the early viral genes. Only the first of these two events was observed upon tumor formation in nude mice.

Differential Extraction of Tumor-Specific Transplantation Antigen and Embryonic Antigen From Simian Virus 40-and Adenovirus 7-Induced Sarcoma Cells of Hamsters With 1-Butanol and 3 M Potassium Chloride2

Simian virus 40 (SV40)-induced sarcomas and adenovirus 7-induced sarcomas (Adv-7) exhibit both specific tumor-specific transplantation antigens (TSTA) and cross-protective embryonic antigens at the cell surface in the LAK:LVG(SYR) strain of Syrian golden hamsters. Specific SV40 TSTA could be released from the surfaces of living hamster sarcoma cells in a 2.5% crude 1-butanol extract (CBE) and served as immunogen to protect syngeneic recipients against subsequent homologous but not heterologous tumor cell challenge. The CBE-extracted SV40-induced TSTA (tumor-specific) was observed to be free of detectable, cross-protective embryonic antigens (EA) by tumor transplantation assays. The induction of cytotoxic lymphocyte-mediated immunity with the CBE-released TSTA was dependent on the administration of a single sensitizing injection of 12-20 micrograms antigen protein. Higher concentrations (50-1,000 micrograms) of the CBE tumor cell extract, given in a single injection, enhanced tumor growth as did two injections of 12.5 micrograms CBE-extracted SV40-induced TSTA at 1-week intervals. A cross-protective antigen(s), not detected in the CBE tumor extracts, was retained in the intact, 1-butanol-extracted SV40 and Adv-7-induced tumor cell lines after completion of the CBE extraction procedure and in similarly extracted 10-day hamster fetal cells. Some alterations in the normal immunogenicity of EA extracted with CBE followed by KCI from SV40-induced sarcoma cells could be detected in the transplantation assays and lymphocyte transformation assays, whereas EA extracted from CBE-KCI-treated Adv-7 cells or 10-day hamster fetal cells retained normal immunogenicity in vivo and in vitro. These procedures provide a means for successful separation of immunogenic SV40- and Adv-7-induced TSTA from detectable, biologically active, cross-protective EA from the surfaces of these sarcoma cells.

Purification of immunoprotective tumor antigens by preparative isotachophoresis.

Tumor-specific transplantation antigens (TSTA) were purified from 3 M KCl and butanol extracts of C3H/HeJ 3-methylcholanthrene-induced fibrosarcomas by preparative isotachophoresis (pITP). Fractions from pITP which reacted with antisera to TSTA preparations in an enzyme-linked immunospecific assay were tested in vivo for induction of resistance to the growth of transplanted tumor cells. Isotachophoresis of crude 3 M KCl extracts yielded TSTA that was immunogenic at doses between 17 and 124 micrograms. Isotachophoresis of TSTA partially purified from crude 3 M KCl or butanol extracts by preparative isoelectric focusing (pIEF) of 3 M KCl and butanol extracts yielded TSTA that was immunogenic over a two-fold log dose range. As little as 10 ng purified TSTA reduced tumor growth by 50%. Tumor growth reduction was specific, and immunized animals survived longer than non-immunized controls. A purification of 10,000-fold over crude 3 M KCl extracts and 2,500-fold over crude butanol extracts was obtained. These results suggest that TSTA from murine tumor cells is preparatively purified by following extraction with pIEF and then pITP.

Tightly associated lipids may anchor SV40 large T antigen in plasma membrane

Simian virus 40 (SV40) large T antigen, a multifunctional protein necessary for lytic growth and cell transformation, is located mainly in the nucleus and in small amounts on the cell surface (surface T). Surface T may have a passive role in SV40 tumour rejection by cytotoxic T cells as a component of SV40-TSTA (tumour-specific transplantation antigen). The unusual induction of this immune response by immunizing mice with soluble T antigen led us to investigate the in vitro binding of T antigen to the surface of living cells in more detail. Our results show that native surface T and a minor subset of large T antigen having a high cell surface binding affinity in vitro, behave like integral membrane proteins. Several viral proteins including SV40 T antigen and cellular proteins seem to be linked to fatty acids (acylation). To analyse whether this mechanism is involved in the stable attachment of in vitro-bound T antigen to the plasma membrane of living target cells, we determined the degree of labelling of this molecule by using target cells prelabelled with 3H-fatty acid. Here we report that T antigen extracted from unlabelled SV40-transformed cells (SV80) becomes 3H-labelled after in vitro binding to the cell surface of 3H-palmitate-prelabelled HeLa cells. These results suggest that T antigen attached externally to living cells, may be anchored by tightly linked lipids.

Tumour specific transplantation antigen (TSTA) and viral induced antigens of a chemically induced murine fibrosarcoma : M.A. Pierotti, G. Carbone, M.P. Colombo, A. Cernuschi, M. Boiocchi and G. Parmiani, Istituto Nazionale Tumori, Milano, Italy

Tumour specific transplantation antigen (TSTA) and viral induced antigens of a chemically induced murine fibrosarcoma : M.A. Pierotti, G. Carbone, M.P. Colombo, A. Cernuschi, M. Boiocchi and G. Parmiani, Istituto Nazionale Tumori, Milano, Italy

Promotion of Syngeneic Intraocular Tumor Growth in Mice by Anterior Chamber-Associated Immune Deviation<xref ref-type="fn" rid="FN2">2</xref><xref ref-type="fn" rid="FN3">3</xref><xref ref-type="fn" rid="FN4">4</xref>

Regressor tumors (P91 and UV-5C25), known to express potent tumor-specific transplantation antigens, were briskly rejected when transplanted either sc or ip into syngeneic mice (DBA/2 and BALB/c, respectively). These tumors demonstrated markedly different behavior following transplantation into the anterior chamber of syngeneic mice. Tumors grew to significant masses and survived for prolonged periods within the anterior chamber of the eye. The extended growth and survival of regressor tumors in the anterior chamber (i.e. immune privilege) were abrogated by splenectomy and thus resemble anterior chamber-associated immune deviation. However, this immune privilege proved to be temporary and was superseded by complete tumor resolution. Studies of mice immunosuppressed with UV light or X-ray irradiation demonstrated that spontaneous intraocular resolution of regressor tumors was due to specific systemic immunity that not only led to intraocular tumor resolution but also prevented the spread of the primary intraocular tumor to distant organs and rendered the hosts highly resistant to secondary challenge with sc tumor inocula. The present findings were relevant to understanding human retinoblastoma, an intraocular neoplasm demonstrating a high incidence of spontaneous resolution, and will hopefully form a foundation for designing, immunotherapeutic strategies for treating human intraocular neoplasms.

Characterization of the surface proteins of SV40-transformed mouse and human cells: absence of SV40-specific proteins.

The proteins of a number of SV40- and spontaneously transformed mouse and human cell lines were compared in an effort to identify a surface protein which would correspond to the SV40 tumor-specific transplantation antigen (TSTA). Analysis of the one- and two-dimensional electrophoretic patterns of 35S-methionine-labelled total proteins and 125I-labelled surface proteins of several of these cell lines failed to reveal the presence of proteins specific to transformation by SV40. Antisera were prepared against SV40- and spontaneously transformed mouse cells in syngeneic mice. In serological assays, these antisera reacted with surface antigens common to both SV40- and spontaneously transformed mouse cell lines. Electrophoretic analysis of the 125I-surface-labelled proteins which these antisera immunoprecipitated from extracts of SV40- and spontaneously transformed mouse and human cells identified a set of common surface proteins with apparent molecular weights of 15, 46, 50, 72, 77, 105, 150 and 230kdal. No SV40-specific surface proteins were detected. Two of the transformed cell surface proteins (105 and 150kdal) were present as well in membrane fractions of 35S-methionine-labelled primary mouse kidney cultures. The proteins of the primary cultures could not be iodinated by lactoperoxidase suggesting that these proteins were present at a "cryptic" location at the surface of normal cells. We were not able to obtain serological or immunochemical evidence for the presence of SV40 large T-antigen at the surface of any of the SV40-transformed cell lines tested using either hamster anti-SV40 tumor sera, a rabbit antiserum against SDS-denatured gel-purified large T-antigen or antisera against SV40-transformed mouse cells. In conjunction with the report that large T-antigen released from disrupted SV40-transformed cells will bind to cell surfaces (Lange-Mutschler and Henning, 1982), we consider the possibility that the specific rejection of SV40-induced tumors by sensitized animals is the result of immunological reactions against both common transformation-related surface antigens and SV40 T-antigen from disrupted cells that has bound to the surface of other tumor cells.