Entecavir (ETV) has been used for more than 2 decades in treating hepatitis B virus (HBV) infections. It has shown significant anti-HBV effect and has led to histological improvement in the liver of chronic hepatitis B (CHB) patients. In patients treated with ETV for over two years, reversal of cirrhosis to normal tissue has also been observed. However, the mechanisms of these tissue repairing or recovery processes are not yet clear. In order to determine the roles that bone marrow and liver stem/progenitor cells play in these processes, we evaluated the CD34⁺ and CD133⁺ stem/progenitor cells in peripheral blood from 292 patients and liver tissues from 43 patients who had received therapies with and without ETV. A significant increase in both CD34⁺ and CD133⁺ cells was found in CHB and cirrhosis patients compared to the healthy controls. In patients treated with ETV, CD34⁺ cells increased 2 and 4 fold in peripheral blood and liver tissues, respectively, while their CD4⁺ and CD8⁺ cells remained the same. On the other hand, CD133⁺ cells did not change or even slightly decreased with ETV treatment. Results from immunohistochemistry staining, real time RT-PCR, and the enzyme-linked immunosorbent assay also revealed the same level of CD34⁺ cell increase and CD133⁺ cell decrease (or no change) in ETV treated patients, compared to patients without ETV therapies. Liver functions in patients with ETV treatment improved in general, but one liver cirrhosis patient with high expression of CD133 in liver tissue developed hepatocellular carcinoma (HCC). In summary, ETV may have differential effects on various stem cell subtypes. ETV-activated stem cells in bone marrow and liver tissues may contribute to the recovery from injuries caused by HBV infection. They also contribute to the regeneration of normal tissue and the recovery of normal liver function. Meanwhile, ETV does not activate stem cells that may participate in the initiation of HCC.

We report here a lentiviral vector system for regulated transgene expression. We used the tetracycline repressor fused with a transcriptional suppression domain (tTS) to specifically suppress transgene expression. Human cells were first transduced with a tTS-expressing vector and subsequently transduced with a second lentiviral vector-containing transgene controlled by a regular promoter adjacent to a high-affinity tTS-binding site (tetO). After optimizing the location of the tetO site in the latter vector, we achieved a better inducible transgene expression than the previous lentiviral vectors using the tetracycline repressor systems. In this new system, the transgene transcription from a cellular promoter such as EF1 alpha or ubiquitin-C promoter is suppressed by the tTS bound to the nearby tetO site. In the presence of the tetracycline analog doxycycline (Dox), however, the tTS binding is released from the transgene vector and transcription from the promoter is restored. Thus, this system simply adds an extra level of regulation, suitable for any types of promoters (ubiquitous or cell-specific). We tested this tTSsuppressive, Dox-inducible system in 293T cells, human multipotent hematopoietic progenitor cells, and three human embryonic stem cell lines, using a dual-gene vector containing the green fluorescent protein reporter or a cellular gene. We observed a tight suppression in the uninduced state. However, the suppression is reversible, and transgene expression was restored at 5 ng/ml Dox. The lentiviral vectors containing the tTS-suppressive, Dox-inducible system offer a universal, inducible, and reversible transgene expression system in essentially any mammalian cell types, including human embryonic stem cells.