Half-life Of Virus Research Articles

Evidence for a relation between the viral load and genotype and hepatitis C virus-specific T cell responses

Background/Aims The reason why patients with hepatitis C virus (HCV) genotype non-1 infection respond better to antiviral therapy than patients with genotype 1 infection is not known. The aim of this study is to explore the relation between the viral genotype, viral load, and the endogenous T cell response. Methods The viral genotype, the viral load, and the endogenous proliferative T cell response to the non-structural 3 protein (NS3) was analysed using serum and peripheral blood mononuclear cells from 103 patients with chronic HCV infection. Results Among 71 nontreated patients a T cell response was more common among those infected by genotype 3, as compared to those infected with genotype 1 ( P<0.05). Among 32 patients undergoing antiviral therapy, presence of a T cell response was more common in genotype non-1 infected patients than in those infected by genotype 1 ( P<0.01). Presence of a T cell response was related to a more rapid viral clearance ( P<0,05), a negative HCV RNA test at week 12 ( P<0.05), and a shorter viral half-life ( P<0.05). Conclusions The presence of an NS3-specific T cell response is related to the viral genotype and to a more rapid clearance of HCV RNA during antiviral therapy.

Analysis of hepatitis B viral load decline under potent therapy: Complex decay profiles observed

We used a new real-time polymerase chain reaction (PCR)-based assay that is sensitive, has a wide dynamic linear range, and is highly reproducible to quantify hepatitis B virus (HBV) DNA in the serum of infected individuals undergoing potent antiviral therapy. In addition, we made frequent measurements of viral load after initiation of treatment and maintained follow-up to about 12 weeks. To analyze the data we used a new model of HBV decay, which takes into account that existing drug treatments do not completely block de novo infection and the possibility of noncytolytic loss of infected cells. On initiation of therapy, there was a mean delay of 1.6 days followed by a biphasic or muliphasic decay of plasma HBV DNA. The slope of the first phase varied considerably, with one individual having rapid decay, corresponding to a virion half-life of 1 hour, but others showing half-lives of up to 92 hours. Individuals either had a slow second-phase decline (t12 = 7.2 ± 1.2 days) or a flat second phase. Some individuals exhibited a complex “staircase pattern” of decay, with further phases of viral DNA decline and phases with little change in viral load. (HEPATOLOGY 2001;34:1012-1020.)

Detection of hepatitis B virus resistance to antivirals.

The development of new nucleoside analogs, that inhibit the HBV reverse transcriptase activity, such as lamivudine, famciclovir and others, has provided recently an alternative to interferon therapy for chronic hepatitis B. However, due to the kinetics of viral replication with a high rate of virus production, a relatively long half-life of virus (CCC) DNA in the nucleus of infected hepatocytes, long-term antiviral therapy with a reverse transcriptase inhibitor is required to eradicate viral infection. Recently, it has been reported that lamivudine therapy for chronic hepatitis B in immune competent patients may be associated with the selection of resistant strains in aproximately 20% of the patients after 12 months of therapy. Sequence analysis of the reverse transcriptase domain of resistant viral strains, at the time of viral breakthrough, revealed the occurrence of mutations located in the YMDD motif within the C domain of the viral enzyme with a methionine to valine (M552V) or to isoleucine (M5521) change. Recent reports on larger series of patients pointed that other mutations residing outside of the C domain but mainly in the B domain of the viral polymerase (L528M) could be associated with these mutations in the YMDD motif. The lamivudine resistant mutants, selected in vivo, can be classified in 2 main groups: group I with a double mutation L528M and M552V, and group II with a single mutation M5521. In vitro studies performed in cell culture showed that these mutants have a decreased replication capacity and are indeed resistant to lamivudine. With the development of new antiviral options, genotyping assays and quantitative determination of viremia with highly sensitive assay are clearly warranted for an optimal monitoring of antiviral therapy of chronic hepatitis B. In view of the experimental and clinical data, the capacity of new antiviral strategies based on combination of new inhibitors, including adefavir and entecavir, with immune modulators needs to be further evaluated in animal models and clinical trials to prevent the emergence of resistant viral strains.

Serum levels of HCV RNA and core protein before and after incubation at 37°C for 24 h

The kinetics of HCV during interferon (IFN) therapy have recently been described and the estimated virion half-life is an average of 2.7 h, suggesting that HCV infection is highly dynamic. The aim of this study was to evaluate serum levels of HCV-RNA and HCV core protein (HCV-Ag) before and after incubation at 37°C for 24 h. We also evaluated the viral kinetics during IFN treatment by determining their serum levels at 0, 24 and 48 h, and day 8 after the start of treatment. The decay slope was calculated as the logarithm of the ratio of HCV-RNA levels at 0 and 24 h of incubation: log(virus load) 24 h-log(virus load) 0 h and the estimated half-life was also calculated. The decay slope was −1.66±0.75 (−4.12 to −0.18) (mean±S.D. (range)) and the estimated virion half-life was 6.2±6.9 h (1.8–39.3). The HCV-RNA level was rapidly decreased to 6.8±13.1% of the initial load after incubation independently of the serotype. In contrast, the HCV-Ag level after incubation for 24 h was 98.7±12.2% of the initial level. The synthesized naked HCV-RNA (equivalent to 10 7 copy/ml) was not detected after 1-min incubation. These data suggested that HCV virions are very unstable and collapsed rapidly and that HCV-RNA, existing outside of virions, is immediately degraded in serum, whereas HCV-Ag remains stable. IFN treatment caused a rapid decrease in the levels of both HCV-RNA and HCV-Ag. The HCV-RNA decay slope was −1.95±0.96 (range: −3.48 to −0.50) and was similar to that seen in the incubation study. Our result suggested the significance of measuring HCV-Ag during clinical management independently of HCV-RNA, especially because of its high stability.

Decline of hepatitis C virus load in serum during the first 24 h after administration of interferon-beta as a predictor of the efficacy of therapy

Background/Aims : Hepatitis C virus (HCV) kinetics during interferon (IFN)-alpha treatment have been evaluated recently, however, little is known about the resultant viral kinetics in IFN-beta treatment. In this study, we evaluated HCV kinetics during the first 24 h of IFN-beta treatment, and also assessed their relationship to therapeutic outcomes. Methods : We measured HCV RNA levels at 0 and 24 h after the initiation of IFN-beta treatment, and we calculated the decay slope, viral half-life, and viral production and clearance. Then we analyzed these factors as they related to therapeutic responses with IFN-beta as well as to clinical variables, i.e. genotype, diversity of hyper variable region, and histological findings. Results : Patients with sustained responses (SR) displayed steeper decay slopes of the viral load than those without SR (2.87±1.41 vs. 1.82±1.66, P =0.031). On the other hand, the decay slope was not affected by the clinical variables. The values of viral half-life and viral production and clearance showed no significant correlation to the response and the clinical variables. Conclusion : This study demonstrated that the decay slope of the viral load during the first 24 h is related to the virological response to IFN-beta treatment.

Treatment of Chronic Hepatitis C: Lessons from Human Immunodeficiency Virus Dynamics

Treatment of chronic hepatitis C is a major problem. A sustained viral response to interferon alpha monotherapy occurs in <20% of patients. Using a combination therapy of interferon alpha and ribavirin, the sustained response rate in naive hepatitis C patients has increased to 31%—0;47%. The success of therapy for chronic hepatitis C depends on both virus- and host-related factors, such as age, histology, duration of hepatitis C virus (HCV) carriage and biochemical parameters. During the last 5 years, insight into the dynamics of human immunodeficiency virus (HIV) has been obtained by analysing the changes in viral load after starting antiviral treatment. By using a mathematical model of HIV kinetics as an example, an exponentially rapid decline in serum HCV RNA level was seen after the first dose of interferon alpha, followed by a slower exponential decline: a so-called biphasic pattern. The estimated virion half-life varies between 2.7 and 16.8 h. The high virion turnover allows the generation of a heterogeneous quasi-species population of HCVs. It is therefore supposed that initial aggressive treatment can be helpful to prevent the development of mutations that make the virus more defensible for the interferon alpha treatment. Various trials are now being conducted based on this principle of high induction antiviral therapy.

Blood clearance rates of adenovirus type 5 in mice.

Persistence of adenovirus type 5 in blood has implications for the pathogenicity of the virus infection and for the use of this virus in oncolysis and gene therapy. In this study, the kinetics of adenovirus clearance from blood in mice has been evaluated. After a single inoculation of concentrated virus into the vena cava, virus half-life was less than 2 min. Depletion of Kupffer cells (KC) resulted in increased viraemia. After tail-vein injection, virus and latex beads co-localized within KC. An important factor in clearance by KC is the negative charge of particles. Deletion of the hexon hypervariable region 1 acidic stretch decreased the negative charge of the virion but it did not increase blood persistence. Coating with PEG ('PEGylation') reduced the clearance rate but also reduced infectivity.

A competition model for viral inhibition of host cell proliferation

Some viruses encode proteins that promote cell proliferation while others, such as the human immunodeficiency virus (HIV), encode proteins that prevent cell division. It has been hypothesized that the selective advantage determining which strategy evolves depends on the ability of the virus to induce a cellular environment which maximizes both virus production and cell life span. In HIV, the protein that causes cell cycle arrest is Vpr. In this paper, we develop a mathematical model, based on difference equations, to study the competition between two genotypes of HIV – one that encodes this protein (Vpr+) and one that does not (Vpr−). In particular, we are interested in parameters that could be different between the in vitro condition, where the Vpr− genotype dominates, and the in vivo condition, where the Vpr+ genotype dominates. Our model indicates that the infected cell death-rate, the viral half-life, and the dynamics of the target cell population all effect the competition dynamics between the Vpr+ and Vpr− viral genotypes. Perturbing any of these parameters from the in vitro estimates while holding the others fixed has no affect on the competition outcome, i.e., the Vpr− genotype dominates. Perturbing the infected cell death-rate and the target cell source causes a switch in competitive outcome, although not necessarily at values, which represent the in vivo condition. Adding a perturbation in the viral half-life from in vitro to in vivo condition results in a switch of the competitive advantage from the Vpr− genotype to the Vpr+ genotype with parameters for all three mechanisms set to estimated in vivo values.

Intrahepatic induction of alpha/beta interferon eliminates viral RNA-containing capsids in hepatitis B virus transgenic mice.

We have previously shown that hepatitis B virus (HBV) replication is abolished in the liver of HBV transgenic mice by stimuli that induce alpha/beta interferon (IFN-alpha/beta) in the liver. The present study was done to identify the step(s) in HBV replication that is affected by this cytokine in transgenic mice treated with the IFN-alpha/beta inducer polyinosinic-polycytidylic acid [poly(I-C)]. Here we show that the pool of cytoplasmic HBV pregenomic RNA (pgRNA)-containing capsids is reduced 10-fold within 9 h after poly(I-C) administration, while there is no change in the abundance of HBV mRNA or in the translational status of cytoplasmic HBV transcripts. In addition, we show that the pool of HBV DNA-containing capsids is not reduced to the same degree until at least 15 h posttreatment, and we show that virus export is not accelerated and the half-life of virions in the serum is unchanged. These results indicate that IFN-alpha/beta triggers intracellular events that either inhibit the assembly of pgRNA-containing capsids or accelerate their degradation, and that maturation and secretion of virus is responsible for clearance of HBV capsids and their cargo of replicative intermediates from the cytoplasm of the hepatocyte.

Hepatitis C Kinetics: Mathematical Modeling of Viral Response to Therapy

Mathematical models have been used to study the dynamics of HIV. Using these same principles, the dynamics of hepatitis C virus (HCV) are reviewed during interferon (IFN) therapy. After initiating IFN treatment, there is an IFN dose-dependent exponential decline in viral RNA levels within the first 48 hours. This rapid 1.0 to 2.0 log decline was best explained by an effect of IFN in inhibiting viral production with a varying degree of effectiveness. By applying mathematical principles, viral serum half-life was estimated to be 3.0 hours and viral production rat was calculated to be 1.0 x 10(12) virions per day. After this rapid first-phase decline there was a slower second phase decline in viral levels that was highly variable between subjects. This phase was dependent on the rate of elimination of HCV-infected liver cells. The rapidity of the second phase proved to be the best predictor of early viral clearance. The use of these models to understand the life cycle of viruses and their response to therapy is reviewed.

Percutaneous adenoviral gene transfer into porcine coronary arteries: is catheter-based gene delivery adapted to coronary circulation?

Recombinant adenoviral (Ad) vectors represent an efficient gene transfer system for targeting the cardiovascular system. Phenotypic modulation of coronary vascular cells in vivo is, however, critically dependent on the efficacy of local delivery devices. Four local drug delivery catheters were tested for intracoronary gene transfer efficiency: the Infiltrator (INF, n = 10), the Crescendo (CRE, n = 10), the Infusasleeve (SLE, n = 8), and the Remedy balloon (channel balloon [CHA], n = 8). After balloon injury of the LAD, Ad vector containing the firefly luciferase cDNA (AdCMVluc, 1.5 x 10(10) plaque-forming units) was administered at the site of injury. On day 4, tissue samples from different regions in the heart and from the liver were assayed for luciferase activity to evaluate local and systemic gene transfer. INF, CRE, and SLE catheters showed higher transduction levels of the target LAD segment than did the CHA catheter (median luciferase activity = 4.2 x 10(6), 11 x 10(6), and 1.3 x 10(6) light units [LU]/vessel versus 0.09 x 10(6) LU/vessel, respectively, p < 0.05). Luciferase activity was occasionally observed in nontarget tissues (right and left ventricular free wall, distal LAD, and liver) and was not significantly different between groups. The viral circulatory half-life was similar for the four groups (<1 min). Gene transfer efficiency was positively correlated with the degree of injury for the intralumenal catheters (CRE, SLE, and CHA) but was independent of the vessel wall injury for the intramural INF. Local drug delivery catheters enable efficient vascular gene transfer in balloon-injured coronary arteries, a prerequisite for further development of intracoronary gene therapy for restenosis.

The effect of the cellular stress response on human T-lymphotropic virus type I envelope protein expression.

In this report the influence of the cellular stress response in mediating changes in human T-lymphotropic virus type I (HTLV-I) viral envelope (Env) protein metabolism is determined. Previously, we reported that induction of the cellular stress response enhanced HTLV-I-mediated syncytia formation following induction of the cellular stress response in persistently infected lymphocytes. In this study, we show that the increase in HTLV-I-mediated syncytia formation following stress response induction is a result of increased cell surface expression of viral Env protein (gp46). Cellular stress in MT 2.6 cells did not alter the turnover of intracellular Env protein (gp68) as no changes in viral protein half-life were demonstrated as compared to non-stressed cells. However, Env expression in stressed cells treated with a protein synthesis inhibitor (cycloheximide) indicates the effect is mediated through increased translation of viral Env protein.

Influence of delayed viral production on viral dynamics in HIV-1 infected patients.

We present and analyze a model for the interaction of human immunodeficiency virus type 1 (HIV-1) with target cells that includes a time delay between initial infection and the formation of productively infected cells. Assuming that the variation among cells with respect to this 'intracellular' delay can be approximated by a gamma distribution, a high flexible distribution that can mimic a variety of biologically plausible delays, we provide analytical solutions for the expected decline in plasma virus concentration after the initiation of antiretroviral therapy with one or more protease inhibitors. We then use the model to investigate whether the parameters that characterize viral dynamics can be identified from biological data. Using non-linear least-squares regression to fit the model to simulated data in which the delays conform to a gamma distribution, we show that good estimates for free viral clearance rates, infected cell death rates, and parameters characterizing the gamma distribution can be obtained. For simulated data sets in which the delays were generated using other biologically plausible distributions, reasonably good estimates for viral clearance rates, infected cell death rates, and mean delay times can be obtained using the gamma-delay model. For simulated data sets that include added simulated noise, viral clearance rate estimates are not as reliable. If the mean intracellular delay is known, however, we show that reasonable estimates for the viral clearance rate can be obtained by taking the harmonic mean of viral clearance rate estimates from a group of patients. These results demonstrate that it is possible to incorporate distributed intracellular delays into existing models for HIV dynamics and to use these refined models to estimate the half-life of free virus from data on the decline in HIV-1 RNA following treatment.

Dynamics of hepatitis C viremia following interferon-alpha administration.

Knowledge of the dynamics of hepatitis C virus (HCV) in vivo is important for elucidation of its pathogenesis and the establishment of therapeutic guidelines. The aim of this study was to obtain kinetic information about virus load following interferon-alpha (IFN-alpha) administration. Serial serum HCV core protein and HCV RNA levels were measured. IFN-alpha exponentially reduced serum HCV levels. The mean (+/-SD) viral half-life was 7.0 +/- 2.6 h in HCV core protein assay and 7.2 +/- 3.1 h in HCV RNA assay on the first day of therapy. This initial rapid decrease was followed by a slower decrease in serum HCV levels thereafter. Thus, the biphasic reduction in virus load during IFN-alpha therapy was demonstrated.

Rous sarcoma virus direct repeat cis elements exert effects at several points in the virus life cycle.

Two approximately 135-nucleotide (nt) direct repeats flank the Rous sarcoma virus (RSV) oncogene src and are composed of two smaller repeats, dr1 (approximately 100 nt) and dr2 (approximately 36 nt). These sequences have been reported to contain cis-acting signals necessary for RNA packaging and elements that allow cytoplasmic accumulation of unspliced RNA (cytoplasmic transport elements). In this report, we show that avian fibroblasts infected with the Prague A strain of RSV with precise deletions of both dr1 elements express src and are transformed by this mutant virus but production of virus particles is very low and virus spread throughout the culture requires several weeks. We show that the replication defect is due to complex effects on viral RNA transport, viral RNA half-life, and virus particle assembly. The dr1 elements may contain binding sites for a permissive cell-specific factor(s) that facilitates efficient nuclear-cytoplasmic transport, RNA stability, and cytoplasmic utilization of unspliced viral RNA. The implications of these results for understanding the defects of nonpermissive virus infections in mammalian cells are discussed.

Dose-dependent acute clearance of hepatitis C genotype 1 virus with interferon alfa

To determine if the clearance of hepatitis C genotype 1 virus (HCV) is dependent on the dose of interferon alfa-2b (IFN-α2b), the acute clearance of HCV after a single dose of either 3, 5, or 10 mIU of IFN-α was compared in patients with chronic hepatitis C. HCV-RNA levels following IFN-α administration were measured. At 24 hours, mean percentage serum viral reduction was 41.4%, 63.7%, and 85.5% for 3, 5, and 10 mIU, respectively (P < .001). At 48 hours, the mean viral reduction was consistently less than the reduction at 24 hours, averaging 22.9%, 61.9%, and 74.3%, respectively (P < .001), indicating that the drug effect diminishes before 48 hours. Regression analysis showed a positive correlation between dose and percent reduction of HCV- RNA levels (r = .6; P < .001). A mathematical model showed that such dose dependence is expected if IFN-α partially blocks viral production. Minimum clearance and production rates of HCV were estimated from measurements of HCV-RNA levels after the 10-mIU dose. HCV decay followed an exponential decline with a minimum estimate of the viral clearance rate constant of 2.8 per day, corresponding to a virion half-life of 0.3 days or less. A minimal estimate of the daily HCV production and clearance is 3.7 x 1011 virions per day, indicating a high rate of replication and turnover. These results indicate that there is a dose-dependent effect of IFN-α in clearance of HCV genotype 1. Because the virion production rate is very rapid and because the current recommended dose of IFN-α (3 mIU) is often ineffective, larger doses should be considered to treat genotype 1-infected patients. (Hepatology 1997 Jul;26(1):226-31)

Dose-dependent acute clearance of hepatitis C genotype 1 virus with interferon alfa.

To determine if the clearance of hepatitis C genotype 1 virus (HCV) is dependent on the dose of interferon alfa-2b (IFN-alpha2b), the acute clearance of HCV after a single dose of either 3, 5, or 10 mIU of IFN-alpha was compared in patients with chronic hepatitis C. HCV-RNA levels following IFN-alpha administration were measured. At 24 hours, mean percentage serum viral reduction was 41.4%, 63.7%, and 85.5% for 3, 5, and 10 mIU, respectively (P < .001). At 48 hours, the mean viral reduction was consistently less than the reduction at 24 hours, averaging 22.9%, 61.9%, and 74.3%, respectively (P < .001), indicating that the drug effect diminishes before 48 hours. Regression analysis showed a positive correlation between dose and percent reduction of HCV-RNA levels (r = .6; P < .001). A mathematical model showed that such dose dependence is expected if IFN-alpha partially blocks viral production. Minimum clearance and production rates of HCV were estimated from measurements of HCV-RNA levels after the 10-mIU dose. HCV decay followed an exponential decline with a minimum estimate of the viral clearance rate constant of 2.8 per day, corresponding to a virion half-life of 0.3 days or less. A minimal estimate of the daily HCV production and clearance is 3.7 x 10(11) virions per day, indicating a high rate of replication and turnover. These results indicate that there is a dose-dependent effect of IFN-alpha in clearance of HCV genotype 1. Because the virion production rate is very rapid and because the current recommended dose of IFN-alpha (3 mIU) is often ineffective, larger doses should be considered to treat genotype 1-infected patients.

Viral dynamics of hepatitis C early after orthotopic liver transplantation: Evidence for rapid turnover of serum virions

The pathogenesis of hepatitis C virus (HCV) infection is likely to be associated with viral replication in vivo, but little is known concerning the dynamics of HCV turnover. We performed serial measurements of serum HCV-RNA levels following orthotopic liver transplantation (OLT) in nine patients with HCV-positive cirrhosis. Serum HCV-RNA levels were determined by quantitative polymerase chain reaction before, immediately after, and for up to 1 month after OLT. There was a rapid decline in HCV-RNA levels from 3.1 +/- 1.3 x 105 copies/ mL (mean +/- SEM) preoperatively to 0.15 +/- 0.6 x 105 copies/mL on the first and 0.16 +/- 0.6 x 105 copies/mL on the second postoperative day (mean viral half-life, 4.0 +/- 0.5 h). Thereafter, HCV-RNA levels increased in all but one patient, and by postoperative day 8 reached 3.6 +/- 1.3 x 105 copies/mL, exceeding the preoperative levels irrespective of the use or not of rescue immunosuppressive therapy including steroid bolus administration. In most patients, serum virions continued to increase averaging 11.6 +/- 2.8 x 105 copies/mL on the 30th postoperative day. These findings indicate that the half- life of HCV is quite short, and that extrahepatic viral replication contributes little to the total virus pool in serum. Furthermore, the marked HCV replication beginning as early as the third postoperative day strongly suggests that the liver graft is rapidly reinfected by HCV after OLT. (Hepatology 1996 Dec;24(6):1351-4)

Viral dynamics of hepatitis C early after orthotopic liver transplantation: evidence for rapid turnover of serum virions.

The pathogenesis of hepatitis C virus (HCV) infection is likely to be associated with viral replication in vivo, but little is known concerning the dynamics of HCV turnover. We performed serial measurements of serum HCV-RNA levels following orthotopic liver transplantation (OLT) in nine patients with HCV-positive cirrhosis. Serum HCV-RNA levels were determined by quantitative polymerase chain reaction before, immediately after, and for up to 1 month after OLT. There was a rapid decline in HCV-RNA levels from 3.1 +/- 1.3 x 10(5) copies/ mL (mean +/- SEM) preoperatively to 0.15 +/- 0.6 x 10(5) copies/mL on the first and 0.16 +/- 0.6 x 10(5) copies/mL on the second postoperative day (mean viral half-life, 4.0 +/- 0.5 h). Thereafter, HCV-RNA levels increased in all but one patient, and by postoperative day 8 reached 3.6 +/- 1.3 x 10(5) copies/mL, exceeding the preoperative levels irrespective of the use or not of rescue immunosuppressive therapy including steroid bolus administration. In most patients, serum virions continued to increase averaging 11.6 +/- 2.8 x 10(5) copies/mL on the 30th postoperative day. These findings indicate that the half-life of HCV is quite short, and that extrahepatic viral replication contributes little to the total virus pool in serum. Furthermore, the marked HCV replication beginning as early as the third postoperative day strongly suggests that the liver graft is rapidly reinfected by HCV after OLT.

Kinetics of Retrovirus Mediated Gene Transfer: The Importance of Intracellular Half-Life of Retroviruses

Gene therapy is a new therapeutic modality that holds vast potential for the treatment of genetic disorders. Retroviruses are used as a vehicle for the transfer of genes into mammalian cells. The process of gene transfer has been shown to depend on the cell cycle status of target cells. We constructed a mathematical model that integrates the kinetics of gene transfer with cell cycle kinetics. We define three cell populations: uninfected cells, cells with the virus in their cytoplasm, but not integrated, and infected cells, in which the viral DNA has integrated in their genome. Our model predicts that the stability of the viral particles after internalization in the cellular cytoplasm, limits the process of gene transfer. Intracellular viral half-life also limits the usefulness of synchronization experiments, used to detect cell cycle dependence of the gene transfer process. We use the predictions of our model to propose a new experimental method for the detection of cell cycle dependence of retrovirus mediated gene transfer. It is based on the maturity distributions of the infected cells, and it is independent of viral intracellular stability. Despite the importance of the viral intracellular half-life this quantity still remains unknown. An extended version of the model is used to simulate a novel experimental method that measures the intracellular retroviral half-life. Analytical solutions of a simplified model confirm our numerical results and reveal the key dimensionless groups that characterize the process of gene transfer. Knowledge of the intracellular half-life of retroviral vectors is of particular importance for the design of new vectors, especially for slowly growing target cells, such as the stem cells of the haematopoietic system.