Abrogation of simian virus 40 DNA-mediated transformation of primary C57BL/6 mouse embryo fibroblasts by exposure to a simian virus 40-specific cytotoxic T-lymphocyte clone

IQGAP1 modulates activation of B-Raf

IQGAP1 modulates several cellular functions, including cell-cell adhesion, transcription, cytoskeletal architecture, and selected signaling pathways. We previously documented that IQGAP1 binds ERK and MAPK kinase (MEK) and regulates EGF-stimulated MEK and ERK activity. Here we characterize the interaction between IQGAP1 and B-Raf, the molecule immediately upstream of MEK in the Ras/MAPK signaling cascade. B-Raf binds directly to IQGAP1 in vitro and coimmunoprecipitates with IQGAP1 from cell lysates. Importantly, IQGAP1 modulates B-Raf function. EGF is unable to stimulate B-Raf activity in IQGAP1-null cells and in cells transfected with an IQGAP1 mutant construct that is unable to bind B-Raf. Interestingly, binding to IQGAP1 significantly enhances B-Raf activity in vitro. Our data identify a previously unrecognized interaction between IQGAP1 and B-Raf and suggest that IQGAP1 is a scaffold necessary for activation of B-Raf by EGF.

Early Immunization Induces Persistent Tumor-Infiltrating CD8+T Cells against an Immunodominant Epitope and Promotes Lifelong Control of Pancreatic Tumor Progression in SV40 Tumor Antigen Transgenic Mice

The ability to recruit the host's CD8+ T lymphocytes (T(CD8)) against cancer is often limited by the development of peripheral tolerance toward the dominant tumor-associated Ags. Because multiple epitopes derived from a given tumor Ag (T Ag) can be targeted by T(CD8), vaccine approaches should be directed toward those T(CD8) that are more likely to survive under conditions of persistent Ag expression. In this study, we investigated the effect of peripheral tolerance on the endogenous T(CD8) response toward two epitopes, designated epitopes I and IV, from the SV40 large T Ag. Using rat insulin promoter (RIP) 1-Tag4 transgenic mice that express T Ag from the RIP and develop pancreatic insulinomas, we demonstrate that epitope IV- but not epitope I-specific T(CD8) are maintained long term in tumor-bearing RIP1-Tag4 mice. Even large numbers of TCR-transgenic T cells specific for epitope I were rapidly eliminated from RIP1-Tag4 mice after adoptive transfer and recognition of the endogenous T Ag. Importantly, immunization of RIP1-Tag4 mice at 5 wk of age against epitope IV resulted in complete protection from tumor progression over a 2-year period despite continued expression of T Ag in the pancreas. This extensive control of tumor progression was associated with the persistence of functional epitope IV-specific T(CD8) within the pancreas for the lifetime of the mice without the development of diabetes. This study indicates that an equilibrium is reached in which immune surveillance for spontaneous cancer can be achieved for the lifespan of the host while maintaining normal organ function.

Circulating anti-Tax cytotoxic T lymphocytes from human T-cell leukemia virus type I-infected people, with and without tropical spastic paraparesis, recognize multiple epitopes simultaneously

CD8+ T cells were freshly isolated from a human T-cell leukemia virus type I (HTLV-I)-infected patient with tropical spastic paraparesis. These cells, which were specific for HTLV-I Tax, simultaneously recognized a minimum of five, and possibly as many as seven, distinct peptide epitopes within the protein. A further Tax epitope was recognized after a short period of culture without exogenous peptide stimulation. All but one of these epitopes were clustered in the N-terminal third of Tax, and one of the epitopes was clearly immunodominant on two separate occasions of testing. Recognition of the immunodominant epitope was restricted by human leukocyte antigen (HLA) B15, and recognition of all the others was by HLA A2. Similar patterns of cytotoxic T lymphocyte recognition of the HLA A2-restricted Tax peptides in two healthy HTLV-I-seropositive individuals, each of whom carried the HLA A2 allele, were observed.

Cytotoxic T lymphocytes (CTL) against a transforming gene product select for transformed cells with point mutations within sequences encoding CTL recognition epitopes

The 94-kD large tumor (T) antigen specified by simian virus 40 (SV40) is sufficient to induce cell transformation. T antigen contains four H-2Db-restricted cytotoxic T lymphocyte (CTL) recognition epitopes that are targets for CTL clones Y-1, Y-2, Y-3, and Y-5. These epitopes have been mapped to T antigen amino acids 207-215 (site I), 223-231 (sites II and III), and 489-497 (site V), respectively. Antigenic site loss variant cells that had lost one or more CTL recognition epitopes were previously selected by coculturing SV40-transformed H-2Db cells with the site-specific Db-restricted CTL clones. The genetic bases for T antigen CTL recognition epitope loss from the variant cells were identified by DNA amplification and direct sequencing of epitope-coding regions from variant cell DNAs. Cells selected for resistance to CTL clone Y-1 (K-1; K-1,4,5; K-3,1) carry deleted SV40 genomes lacking site I, II, and III coding sequences. Point mutations present within the site II/III coding region of Y-2-/Y-3-resistant cell lines specify the substitution of asparagine for lysine as T antigen amino acid 228 (K-2) or phenylalanine for tyrosine at position 230 (K-3). Point mutations identified within independently selected Y-5 resistant populations (K-5 and K-1,4,5) direct the substitution of isoleucine for asparagine at position 496 (K-5) or the substitution of phenylalanine for isoleucine at position 491 (K-1,4,5) of T antigen. Each substitution causes loss of the relevant CTL recognition epitope, apparently by compromising CTL T cell receptor recognition. These experiments identify specific amino acid changes within a transforming protein that facilitate transformed cell escape from site-specific CTL clones while allowing maintenance of cellular transformation. This experimental model system provides unique opportunities for studying mechanisms of transformed cell escape from active immunosurveillance in vivo, and for analysis of differential host immune responses to wild-type and mutant cell-transforming proteins.

Localization of common cytotoxic T lymphocyte recognition epitopes on simian papovavirus SV40 and human papovavirus JC virus T antigens

Human papovavirus JC virus (JCV) and Simian virus 40 (SV40) tumor or T antigens demonstrate considerable sequence homology which is reflected by antibody cross-reactivity. This similarity raised the possibility that JCV and SV40 T antigen also might contain common cytotoxic T lymphocyte (CTL) recognition epitopes. In this study we identified and mapped such sites on the JCV T antigen. C57Bl/6 cell lines transformed by JCV/SV40 T antigen chimeras were generated and tested for susceptibility to lysis by five H-2b restricted SV40-specific CTL clones: Y-1, Y-2, Y-3, Y-4, and Y-5. These CTL clones recognize specific epitopes within amino acids 205-219 (site I), 220-233 (sites II and III), 369-511 (site IV), and 489-503 (site V) on SV40 T Ag, respectively. The results show that sites I, II, III, and IV (recognized by CTL clones Y-1, Y-2, Y-3, and Y-4, respectively) represent common epitopes on SV40 and JCV T antigens. CTL clone Y-5 failed to recognize JCV T antigen indicating that CTL can discriminate between the two antigenically related T antigens.

SV40 DNA in a carrier system of human glioblastoma cells

The state of the SV40 DNA in a stable carrier system of A172 human glioblastoma cells was examined by Southern blot hybridization analysis. At a sensitivity of 0.1 viral genome equivalents per cell, we detected only free, apparently nondefective, viral genomes. However, when we overexposed our autoradiograms or examined cloned cell populations, integrated viral sequences were observed. Furthermore, aberrant forms of free viral DNA were seen as well. Four clones, isolated at 15 weeks, produced T antigen and displayed enhanced saturation density and plating efficiency characteristic of SV40 transformation. None of these clones produced capsid proteins or infectious virus, even upon fusion with CV-1 cells, Viral DNA in the clones ranged from 0.5 to 50 equivalents per cell, on the average. Two of the Week-15 clones contained a similar (but not identical) predominant truncated SV40 sequence which was present both in a free state and integrated at a single major site in a reiterated head-to-tail array. These clones also contained other minor integrated sequences. Another Week-15 clone contained viral sequences integrated at two major sites as well as heterogeneous free DNA. Only free aberrant DNA was detected in the fourth Week-15 clone. Seven of eight clones isolated at 23 weeks produced no infectious virus or T antigen. No viral DNA was detected in those clones. The eighth clone did produce infectious virus and contained a predominance of free viral DNA. All of the clones were susceptible to superinfection with wild-type SV40, although less so than uninfected A172 cultures.

Recognition of simian virus 40 T antigen synthesized during viral lytic cycle in monkey kidney cells expressing mouse H-2Kb- and H-2Db-transfected genes by SV40-specific cytotoxic T lymphocytes leads to the abrogation of virus lytic cycle

Simian virus 40 (SV40)-encoded tumor or T antigen localizes in the membranes in addition to the nucleus of SV40-infected permissive monkey cells and SV40-transformed nonpermissive cells. The surface T antigen in SV40-transformed mouse cells provides a target for the cytotoxic T lymphocytes (CTL) which recognize SV40 T antigen in association with murine K/D, class I H-2 antigens. In order to demonstrate that SV40 T antigen synthesized in SV40-infected permissive monkey kidney cells (TC-7) may also function as a target for CTL, cloned murine H-2Db and H-2Kb genes were expressed in TC-7 cells by DNA transfection and TC-7 cell lines expressing high levels of either H-2Kb or H-Db antigens were established after cell sorting. SV40-infected TC-7/H-2Kb and TC-7/H-2Db cells became susceptible to lysis by SV40-specific H-2b restricted CTL. The susceptibility of these transfected SV40-infected monkey cells to anti-SV40 bulk culture CTL and SV40-specific H-2Db- and H-2Db-restricted CTL clones depended upon the synthesis of SV40 T antigen and the expression of the appropriate H-2Kb or H-2Db restriction elements. Treatment of SV40-infected TC-7/H-2Db and TC-7/H-2Kb with CTL clones abrogated the virus lytic cycle indicating that CTL may play an important role in limiting papovavirus infection in the natural host.

The cellular secretory pathway is not utilized for biosynthesis, modification, or intracellular transport of the simian virus 40 large tumor antigen

Unlike most proteins, which are localized within a single subcellular compartment in the eucaryotic cell, the simian virus 40 (SV40) large tumor antigen (T-ag) is associated with both the nucleus and the plasma membrane. Current knowledge of protein processing would predict a role for the secretory pathway in the biosynthesis and transport of at least a subpopulation of T-ag to account for certain of its chemical modifications and for its ability to reach the cell surface. We have examined this prediction by using in vitro translation and translocation experiments. Preliminary experiments established that translation of T-ag was detectable with as little as 0.1 microgram of the total cytoplasmic RNA from SV40-infected cells. Therefore, by using a 100-fold excess of this RNA, the sensitivity of the assays was above the limits necessary to detect the theoretical fraction of RNA equivalent to the subpopulation of plasma-membrane-associated T-ag (2 to 5% of total T-ag). In contrast to a control rotavirus glycoprotein, the electrophoretic mobility of T-ag was not changed by the addition of microsomal vesicles to the in vitro translation mixture. Furthermore, T-ag did not undergo translocation in the presence of microsomal vesicles, as evidenced by its sensitivity to trypsin treatment and its absence in the purified vesicles. Identical results were obtained with either cytoplasmic RNA from SV40-infected cells or SV40 early RNA transcribed in vitro from a recombinant plasmid containing the SP6 promoter. SV40 early mRNA in infected cells was detected in association with free, but not with membrane-bound, polyribosomes. Finally, monensin, an inhibitor of Golgi function, failed to specifically prevent either glycosylation or cell surface expression of T-ag, although it did depress overall protein synthesis in TC-7 cells. We conclude from these observations that the constituent organelles of the secretory pathway are not involved in the biosynthesis, modification, or intracellular transport of T-ag. The initial step in the pathway of T-ag biosynthesis appears to be translation on free cytoplasmic polyribosomes. With the exclusion of the secretory pathway, we suggest that T-ag glycosylation, palmitylation, and transport to the plasma membrane are accomplished by previously unrecognized cellular mechanisms.

Transformation of precrisis human cells by the simian virus 40 cytoplasmic-localization mutant pSVCT3 is accompanied by nuclear T antigen

pSVCT3 is a cytoplasmic-localization mutant of simian virus 40 (SV40) isolated from the SV40 adenovirus 7 hybrid virus (PARA) and cloned into plasmid PBR. The large T antigen of pSVCT3 accumulates in the cytoplasm of infected monkey cells instead of being transported to the nucleus. The sole change in CT3 large T antigen is amino acid residue 128 (Lys----Asn). Transformation of precrisis rodent cells by pSVCT3 is negligible, whereas the frequency of transformation of established rodent cell lines by pSVCT3 is comparable to that of wild-type SV40. According to the model, in which transformation of precrisis cells involves the combined oncogenic action of both nuclear and cytoplasmic gene products, we predicted that pSVCT3 would localize in the cytoplasm of human cells and would therefore at most only partially and rarely transform precrisis human cells. We have found that pSVCT3 is able to transform precrisis human cells at high frequency. Furthermore, pSVCT3-transformed human precrisis cells relocalized T antigen to their nuclei. The relocalization of large T antigen was not dependent on cell growth. Wild-type and pSVCT3-transformed human cell lines both have about five copies of integrated SV40 DNA. SV40 virus-specific proteins, including the 100,000-molecular-weight super large T antigen, were expressed in pSVCT3-transformed human cells. Our results suggest that molecules in precrisis human cells, but not cells of other species, are able to complement the cytoplasmic-localization defect of the CT3 mutant large T antigen.

Influence of amino acids encoded in the 3' open reading frame of the SV40 early region on transformation and antigenicity of large T antigen

The mutant dlA2414 bears a frame-shift deletion of nucleotides 2936-2927 in the coding sequence for the simian virus 40 (SV40) large T antigen. Based on its nucleotide sequence, this mutant should produce a T antigen containing the first 627 authentic large T antigen amino acids followed by 97 amino acids encoded in the alternate open reading frame at the 3' end of the early region. This protein resembles the hypothetical T* protein that would be translated from an early SV40 mRNA if it were spliced to permit utilization of the open reading frame. We show that stable mouse cell lines can be generated that express the T antigen produced by dlA2414 and that this T antigen has an altered carboxy terminus. In addition, the expected tryptic peptides were missing from the large T antigen and replaced by more hydrophobic peptides. The T*-like protein produced by dlA2414 was apparently less stable than wild-type T antigen and did not stably complex with the cellular phosphoprotein p53. This protein retained the ability to immunize mice against a challenge of syngeneic SV40-tumor cells. The dlA2414 T antigen was expressed at the surface of cells as shown by in vitro lymphocyte-mediated cytotoxicity assay. The results presented here also showed that the expression of a T*-like protein at the cell surface is not likely to be essential for tumorigenesis of cells transformed by SV40.