Evaluating virus-transformed cell tumorigenicity

Characterization of in vivo tumorigenicity tests using severe immunodeficient NOD/Shi-scid IL2Rγnull mice for detection of tumorigenic cellular impurities in human cell-processed therapeutic products

The contamination of human cell-processed therapeutic products (hCTPs) with tumorigenic cells is one of the major concerns in the manufacturing and quality control of hCTPs. However, no quantitative method for detecting the tumorigenic cellular impurities is currently standardized. NOD/Shi-scid IL2Rγnull (NOG) mice have shown high xeno-engraftment potential compared with other well-known immunodeficient strains, e.g. nude mice. Hypothesizing that tumorigenicity test using NOG mice could be a sensitive and quantitative method to detect a small amount of tumorigenic cells in hCTPs, we examined tumor formation after subcutaneous transplantation of HeLa cells, as a model of tumorigenic cells, in NOG mice and nude mice. Sixteen weeks after inoculation, the 50% tumor-producing dose (TPD50) values of HeLa cells were stable at 1.3 × 104 and 4.0 × 105 cells in NOG and nude mice, respectively, indicating a 30-fold higher sensitivity of NOG mice compared to that of nude mice. Transplanting HeLa cells embedded with Matrigel in NOG mice further decreased the TPD50 value to 7.9 × 10 cells, leading to a 5000-fold higher sensitivity, compared with that of nude mice. Additionally, when HeLa cells were mixed with 106 or 107 human mesenchymal stem cells as well as Matrigel, the TPD50 values in NOG mice were comparable to those of HeLa cells alone with Matrigel. These results suggest that the in vivo tumorigenicity test using NOG mice with Matrigel is a highly sensitive and quantitative method to detect a trace amount of tumorigenic cellular impurities in human somatic cells, which can be useful in the quality assessment of hCTPs.

A mouse strain defective in both T cells and NK cells has enhanced sensitivity to tumor induction by plasmid DNA expressing both activated H-Ras and c-Myc

As part of safety studies to evaluate the risk of residual cellular DNA in vaccines manufactured in tumorigenic cells, we have been developing in vivo assays to detect and quantify the oncogenic activity of DNA. We generated a plasmid expressing both an activated human H-ras gene and murine c-myc gene and showed that 1 µg of this plasmid, pMSV-T24-H-ras/MSV-c-myc, was capable of inducing tumors in newborn NIH Swiss mice. However, to be able to detect the oncogenicity of dominant activated oncogenes in cellular DNA, a more sensitive system was needed. In this paper, we demonstrate that the newborn CD3 epsilon transgenic mouse, which is defective in both T-cell and NK-cell functions, can detect the oncogenic activity of 25 ng of the circular form of pMSV-T24-H-ras/MSV-c-myc. When this plasmid was inoculated as linear DNA, amounts of DNA as low as 800 pg were capable of inducing tumors. Animals were found that had multiple tumors, and these tumors were independent and likely clonal. These results demonstrate that the newborn CD3 epsilon mouse is highly sensitive for the detection of oncogenic activity of DNA. To determine whether it can detect the oncogenic activity of cellular DNA derived from four human tumor-cell lines (HeLa, A549, HT-1080, and CEM), DNA (100 µg) was inoculated into newborn CD3 epsilon mice both in the presence of 1 µg of linear pMSV-T24-H-ras/MSV-c-myc as positive control and in its absence. While tumors were induced in 100% of mice with the positive-control plasmid, no tumors were induced in mice receiving any of the tumor DNAs alone. These results demonstrate that detection of oncogenes in cellular DNA derived from four human tumor-derived cell lines in this mouse system was not possible; the results also show the importance of including a positive-control plasmid to detect inhibitory effects of the cellular DNA.

SV40 lymphomagenesis in Syrian golden hamsters

Simian virus 40 (SV40) isolates differ in oncogenic potential in Syrian golden hamsters following intraperitoneal inoculation. Here we describe the effect of intravenous exposure on tumor induction by SV40. Strains SVCPC (simple regulatory region) and VA45-54(2E) (complex regulatory region) were highly oncogenic following intravenous inoculation, producing a spectrum of tumor types. Three lymphoma cell lines were established; all expressed SV40 T-antigen, were immortalized for growth in culture, and were tumorigenic following transplantation in vivo. New monoclonal antibodies directed against hamster lymphocyte surface antigens are described. The cell lines expressed MHC class II and macrophage markers and were highly phagocytic, indicating a histiocytic origin. Many hamsters that remained tumor-free developed SV40 T-antigen antibodies, suggesting that viral replication occurred. This study shows that route of exposure influences the pathogenesis of SV40-mediated carcinogenesis, that SV40 strain VA45-54(2E) is lymphomagenic in hamsters, that hamster lymphoid cells of histiocytic origin can be transformed in vivo and established in culture, and that reagents to hamster leukocyte differentiation molecules are now available.

Nucleotide biosynthesis is critical for growth of bacteria in human blood

Proliferation of bacterial pathogens in blood represents one of the most dangerous stages of infection. Growth in blood serum depends on the ability of a pathogen to adjust metabolism to match the availability of nutrients. Although certain nutrients are scarce in blood and need to be de novo synthesized by proliferating bacteria, it is unclear which metabolic pathways are critical for bacterial growth in blood. In this study, we identified metabolic functions that are essential specifically for bacterial growth in the bloodstream. We used two principally different but complementing techniques to comprehensively identify genes that are required for the growth of Escherichia coli in human serum. A microarray-based and a dye-based mutant screening approach were independently used to screen a library of 3,985 single-gene deletion mutants in all non-essential genes of E. coli (Keio collection). A majority of the mutants identified consistently by both approaches carried a deletion of a gene involved in either the purine or pyrimidine nucleotide biosynthetic pathway and showed a 20- to 1,000-fold drop in viable cell counts as compared to wild-type E. coli after 24 h of growth in human serum. This suggests that the scarcity of nucleotide precursors, but not other nutrients, is the key limitation for bacterial growth in serum. Inactivation of nucleotide biosynthesis genes in another Gram-negative pathogen, Salmonella enterica, and in the Gram-positive pathogen Bacillus anthracis, prevented their growth in human serum. The growth of the mutants could be rescued by genetic complementation or by addition of appropriate nucleotide bases to human serum. Furthermore, the virulence of the B. anthracis purE mutant, defective in purine biosynthesis, was dramatically attenuated in a murine model of bacteremia. Our data indicate that de novo nucleotide biosynthesis represents the single most critical metabolic function for bacterial growth in blood and reveal the corresponding enzymes as putative antibiotic targets for the treatment of bloodstream infections.

Bacterial growth in the bloodstream is a common manifestation of a number of bacterial infections. When growing in blood, bacteria not only have to evade the host's immune response, but also adjust their metabolism to suit availability of nutrients. Although the concentrations of various metabolites in human blood are known, it is difficult to predict which nutrients are abundant and which are scarce. To proliferate in human blood, bacteria need to synthesize metabolites that are present in the limiting concentrations. For that, they need to produce specific enzymes that are, thus, critical for the bacterial growth in the bloodstream. We carried out a comprehensive, genome-wide search for Escherichia coli genes that are essential for growth in human serum. We found that inactivation of nucleotide biosynthesis genes leads to a significant growth defect in human serum not only for E. coli but also for two other pathogens, Salmonella Typhimurium and Bacillus anthracis. The results of this study demonstrate that the limiting amounts of the nucleotide bases in human serum force invading pathogens to rely on de novo nucleotide biosynthesis. Hence, our findings reveal nucleotide biosynthesis enzymes as a possible target for the treatment of bloodstream infections.

Assessing the tumorigenic phenotype of VERO cells in adult and newborn nude mice

VERO cell lines are important substrates for viral vaccine manufacture. The mechanism by which these cells became neoplastically transformed is unknown. During tissue-culture passage, VERO cells can develop the capacity to form tumors. Although at the passage levels (around p140) currently used for vaccine manufacture, VERO cells are non-tumorigenic, questions have been raised about safety issues that might be associated with this capacity to acquire a tumorigenic phenotype. To begin to address these issues, the tumorigenicity of VERO cell lines, derived at different passage levels under different growth conditions, were evaluated in 365-day assays in adult and newborn nude mice. High passage (p>200) VERO cell lines established by random passaging in tissue culture produced tumors in adult (10 out of 27) mice and newborn (21 out of 30) mice, respectively. In contrast, a high passage (p>250) cell line established by passage at sub-confluence produced tumors only in newborn mice (16 out of 30). Progressively growing tumors began forming at 36 days in newborns and at 69 days in adults. Higher tumor incidences and shorter tumor latencies suggest that newborn nude mice may be more sensitive than adults in detecting the expression of a tumorigenic phenotype by some VERO cell lines.

Safety characterization of HeLa-based cell substrates used in the manufacture of a recombinant adeno-associated virus-HIV vaccine

The use of transformed cell substrates for prophylactic vaccine manufacturing is widely debated. Extensive characterization is required to address the suitability of neoplastic cell substrates for vaccine manufacture. The HeLa-based cell substrate used in the manufacture of a prophylactic rAAV-HIV vaccine, AAV2-gagPR delta RT (tgAAC09) was tested in vivo for its tumor-forming potential, the oncogenic potential of its high molecular weight DNA and the potential presence of occult oncogenic adventitious agents. This data from these in vivo studies, in conjunction with prion gene and protein characterization, cell and viral clearance studies and quantity of residual host-cell DNA levels in the purified tgAAC09 vaccine, were used to establish what we believe to be an acceptable safety profile for the vaccine manufacturing process. The tumor-producing dose in 50% of the animals was consistent with that in a published report from FDA staff for HeLa cells. High molecular weight cellular DNA was not oncogenic and no occult oncogenic agents were detected by testing in nude mice and newborn rodent models, respectively. Endogenous prion protein was also normal and genomic sequence analysis detected no mutations associated with increased risk of prion disease. In addition, the purification process used to produce this vaccine candidate removed all detectable cells (clearance of greater than 22 log10), viral clearance study showed 6-17 log10 clearance of three model viruses and host-cell DNA in the bulk product was less than 100pg host-cell DNA per dose of 3 x 10(11) DNase resistant particles (DRP) of the vaccine. Taken together, the data from the in vivo and in vitro tests that were performed to characterize the HeLa based producer cell line (T3B12-5B) and HeLa S3 cells support the use of these cells as substrates for the manufacture of a purified rAAV-HIV vaccine candidate. The data also supports the ability of the process, employing the HeLa cell substrate, used to manufacture the rAAV-HIV vaccine to produce a product as free of adventitious agents as current testing procedures can document. Safety of the rAAV-HIV vaccine is currently being assessed in a Phase I clinical trial.