Nukuzuma, unpublished data). Proliferation characteristics of COS-tat cells may provide important background information for studies using these cell lines. Thus, we first compared the cell proliferation of three COS-tat cell lines

with those of parental check details COS-7 cells. COS-7 cells (ATCC CRL 1651) and COS-tat cell clones (8) were cultivated in EMEM containing 10% FBS (hereafter called culture medium). Cell cultures were maintained at 37°C in a humidified incubator containing 5% CO2 in air. The relative number of live cells was determined by measuring mitochondrial succinate dehydrogenase activity using MTT assay. COS-7 cells and COS-tat cell clones were each plated in five wells of 96-well culture plates at a concentration of 2 × 103 cells/well in 100 μL culture medium and incubated at 37°C in a CO2 incubator. MTT assay was performed using a Cell Proliferation Kit I (MTT) (Roche, Penzberg, Germany) according to

the manufacturer’s instructions. After an incubation period of 5 days, 10 μL MTT solution was added to each well to a final concentration of 0.5 mg/mL, and the plates incubated for 4 hr. Then, click here 100 μL solubilization solution was added to each well, and the plates placed in an incubator overnight. The formazan products were solubilized, and spectrophotometric data were measured using an enzyme-linked immunosorbent assay reader (Bio-Rad, Hercules, CA, USA) at a wavelength of 550 nm with a reference wavelength of 650 nm. The significance of inter-group differences was statistically determined by Student’s t-test. As shown in Table 1, the enzyme activity of COS-tat7 and COS-tat15, and COS-tat22 cells was lower than that of parental COS-7 cells and this difference PI-1840 was statistically significant (P < 0.01). Of note, the enzyme activity of COS-tat22 cells was lower than that of COS-tat7 and COS-tat15 cells (P < 0.01). To measure the doubling time, COS-7 cells and COS-tat cell clones were plated in 6-well culture plates at a concentration of 4 × 104 cells/well in 2 mL culture medium. After an incubation period of 72 hr, cell numbers were counted. The

doubling time of COS-7, COS-tat7, COS-tat15, and COS-tat22 were 21.6, 24.6, 22.8, and 30.8 hr, respectively. The doubling time of COS-7 COS-tat cells were in agreement with the proliferation characteristics of the cells as judged by MTT assay. Taken together, these results indicate that stable expression of Tat leads to down-regulation of cell proliferation. We next compared the production of PML-type JCV in COS-tat cell clones with that in parental COS-7 cells. Since JCV capsids have the property of agglutinating human type O erythrocytes, HA assay has been traditionally employed to determine the virus titer (12). COS-7 and COS-tat cell clones were cultured in 35-mm dishes containing 2 mL culture medium until the cells were 50–80% confluent.