Hepatitis C Virus RNA Levels Following Virologic Failure With Direct-acting Antivirals: Implications for Lower Sensitivity Diagnostic Assays.

We analyzed post-treatment hepatitis C virus (HCV) RNA levels from 330 subjects who experienced virologic failure in clinical trials of direct-acting antivirals. We demonstrated that 97% had post-treatment Week 12 HCV RNA >10 000 IU/mL, above reported sensitivity limits of novel diagnostic assays being considered for simplified HCV treatment monitoring.

Benefit-risk assessment for sofosbuvir/velpatasvir/voxilaprevir based on patient population and hepatitis C virus genotype: U. S. Food and Drug Administration's evaluation.

On July 18, 2017, the U.S. Food and Drug Administration (FDA) approved sofosbuvir/velpatasvir/voxilaprevir (SOF/VEL/VOX) (Vosevi) fixed-dose combination (FDC), an interferon-free, complete regimen for adult patients with chronic hepatitis C virus (HCV) infection without cirrhosis or with compensated cirrhosis (Child-Pugh A) who have: • genotype 1, 2, 3, 4, 5, or 6 infection and have previously been treated with an HCV regimen containing a nonstructural protein 5A (NS5A) inhibitor; and • genotype 1a or 3 infection and have previously been treated with an HCV regimen containing sofosbuvir without an NS5A inhibitor. Approval was based on an acceptable safety profile and high sustained virological response rates 12 weeks after the end of treatment (SVR12) in two phase 3 clinical trials in subjects previously treated with a direct-acting antiviral (DAA) regimen. In POLARIS-1, 96% of SOF/VEL/VOX-treated subjects achieved SVR12. In POLARIS-4, 98% of SOF/VEL/VOX-treated subjects achieved SVR12. A key and challenging question in evaluating the data was determining the contribution of VOX to SOF/VEL and how this differed depending on the genotype and patient population. In this article, we provide our perspective on the issues considered in making these determinations, especially regarding the POLARIS-4 data in subjects who have previously been treated with a chronic HCV regimen containing sofosbuvir without an NS5A inhibitor. Conclusion: We seek to provide context as to why a broad indication was given for NS5A inhibitor-experienced patients (HCV genotypes 1-6) while the indication for NS5A inhibitor- naïve patients was limited to HCV genotypes 1a and 3 only. (Hepatology 2018;67:482-491).

Impact of hepatitis C virus polymorphisms on direct-acting antiviral treatment efficacy: Regulatory analyses and perspectives.

Several highly effective, interferon-free, direct-acting antiviral (DAA)-based regimens are available for the treatment of chronic hepatitis C virus (HCV) infection. Despite impressive efficacy overall, a small proportion of patients in registrational trials experienced treatment failure, which in some cases was associated with the detection of HCV resistance-associated substitutions (RASs) at baseline. In this article, we describe methods and key findings from independent regulatory analyses investigating the impact of baseline nonstructural (NS) 3 Q80K and NS5A RASs on the efficacy of current United States Food and Drug Administration (FDA)-approved regimens for patients with HCV genotype (GT) 1 or GT3 infection. These analyses focused on clinical trials that included patients who were previously naïve to the DAA class(es) in their investigational regimen and characterized the impact of baseline RASs that were enriched in the viral population as natural or transmitted polymorphisms (i.e., not drug-selected RASs). We used a consistent approach to optimize comparability of results across different DAA regimens and patient populations, including the use of a 15% sensitivity cutoff for next-generation sequencing results and standardized lists of NS5A RASs. These analyses confirmed that detection of NS3 Q80K or NS5A baseline RASs was associated with reduced treatment efficacy for multiple DAA regimens, but their impact was often minimized with the use of an intensified treatment regimen, such as a longer treatment duration and/or addition of ribavirin. We discuss the drug resistance-related considerations that contributed to pretreatment resistance testing and treatment recommendations in drug labeling for FDA-approved DAA regimens. CONCLUSION: Independent regulatory analyses confirmed that baseline HCV RASs can reduce the efficacy of certain DAA-based regimens in selected patient groups. However, highly effective treatment options are available for patients with or without baseline RASs. (Hepatology 2018;67:2430-2448).

Regulatory Analysis of Effects of Hepatitis C Virus NS5A Polymorphisms on Efficacy of Elbasvir and Grazoprevir.

BACKGROUND & AIMS: Elbasvir (an NS5A inhibitor) and grazoprevir (an NS3/4A protease inhibitor) are direct-acting antiviral agents recently approved in the United States for treatment of chronic hepatitis C virus (HCV) genotype 1 and 4 infections, as a fixed-dose combination. Trials of elbasvir and grazoprevir, with or without ribavirin, demonstrated high rates of sustained virologic response 12 weeks after treatment ended (SVR12). However, 12 weeks of treatment with elbasvir and grazoprevir failed in a small proportion of patients with HCV genotype 1 infection. We summarize findings from independent US Food and Drug Administration analyses of drug resistance data from trials of elbasvir and grazoprevir, with and without ribavirin. METHODS: We independently analyzed HCV drug resistance and HCV RNA measurement results that were submitted to the US Food and Drug Administration to support the regulatory approval of elbasvir and grazoprevir. These data were reported from selected phase 2 and 3 clinical trials of elbasvir and grazoprevir, with and without ribavirin. Genotypic resistance analyses were conducted using Sanger population nucleotide sequencing data derived from blood samples from study patients. RESULTS: In 56 of 506 (11%) patients with HCV genotype 1a infection who received elbasvir and grazoprevir for 12 weeks, baseline HCV genetic variants encoding amino acid polymorphisms in NS5A (M28, Q30, L31, or Y93) reduced treatment efficacy; rates of SVR12 were 70% and 98% for patients with or without NS5A polymorphisms, respectively (P < .0001). Most patients with treatment failure acquired resistance-associated substitutions in NS3 and/or NS5A. Based on data from a small number of patients (n = 6), an intensified 16-week regimen of elbasvir and grazoprevir plus ribavirin could increase efficacy in patients with HCV genotype 1a infection with NS5A polymorphisms. Among patients with HCV genotype 4a or 4d infections with NS5A polymorphisms, all 26 who received the elbasvir and grazoprevir regimens recommended in prescribing information achieved an SVR12. CONCLUSIONS: The combination of elbasvir and grazoprevir, with or without ribavirin is safe and effective for patients with HCV genotype 1 or 4 infections. In patients with HCV genotype 1a infection, polymorphisms in NS5A at baseline (before treatment) can affect the efficacy of this direct-acting antiviral regimen, and pretreatment resistance analyses can optimize treatment selection.

Effect of dolutegravir functional monotherapy on HIV-1 virological response in integrase strand transfer inhibitor resistant patients.

BACKGROUND: VIKING-4 assessed the safety and efficacy of dolutegravir in heavily antiretroviral treatment-experienced patients who had documented integrase strand transfer inhibitor (INSTI) resistance-associated substitutions in their HIV. VIKING-4 had a placebo-controlled 7-day dolutegravir functional monotherapy phase followed by dolutegravir plus an optimized background regimen for 48 weeks. METHODS: Independent resistance analyses evaluated week 48 virological responses in the VIKING-4 trial based on the presence of baseline INSTI resistance-associated substitutions and baseline dolutegravir phenotypic susceptibility. Response rates at week 48 based on baseline dolutegravir resistance subgroups were compared for the 7-day dolutegravir functional monotherapy arm and placebo-control arm. Additionally, genotypic and phenotypic resistance at day 8 and time of failure was analysed for the virological failures from both arms. RESULTS: Week 48 response rates for VIKING-4 were 23% (3/13) in the 7-day dolutegravir functional monotherapy arm compared with 60% (9/15) in the 7-day placebo arm. Response rates were consistently lower in the dolutegravir functional monotherapy arm across baseline INSTI genotypic and phenotypic subgroups. There was a higher proportion of virological failures in the 7-day dolutegravir functional monotherapy arm (n=6/13; 46%) compared with the 7-day placebo arm (n=3/15; 20%). Additionally, five virological failures in the dolutegravir arm had virus expressing emergent INSTI resistance-associated substitutions compared with two in the placebo arm. CONCLUSIONS: Analysis of response rates and resistance emergence in VIKING-4 suggests careful consideration should be given to the duration of functional monotherapy in future studies of highly treatment-experienced patients to reduce the risk of resistance and virological failure.

Clinical evidence and bioinformatics characterization of potential hepatitis C virus resistance pathways for sofosbuvir.

UNLABELLED: Sofosbuvir (Sovaldi, SOF) is a nucleotide analog prodrug that targets the hepatitis C virus (HCV) nonstructural protein 5B (NS5B) polymerase and inhibits viral replication. High sustained virological response rates are achieved when SOF is used in combination with ribavirin with or without pegylated interferon in subjects with chronic HCV infection. Potential mechanisms of HCV resistance to SOF and other nucleos(t)ide analog NS5B polymerase inhibitors are not well understood. SOF was the first U.S. Food and Drug Administration (FDA)-approved antiviral drug for which genotypic resistance analyses were based almost entirely on next-generation sequencing (NGS), an emerging technology that lacks a standard data analysis pipeline. The FDA Division of Antiviral Products developed an NGS analysis pipeline and performed independent analyses of NGS data from five SOF clinical trials. Additionally, structural bioinformatics approaches were used to characterize potential resistance-associated substitutions. Using protocols we developed, independent analyses of the NGS data reproduced results that were comparable to those reported by Gilead Sciences, Inc. Low-frequency, treatment-emergent substitutions occurring at conserved NS5B amino acid positions in subjects who experienced virological failure were also noted and further evaluated. The NS5B substitutions, L159F (sometimes in combination with L320F or C316N) and V321A, emerged in 2.2%-4.4% of subjects who failed SOF treatment across clinical trials. Moreover, baseline polymorphisms at position 316 were potentially associated with reduced response rates in HCV genotype 1b subjects. Analyses of these variants modeled in NS5B crystal structures indicated that all four substitutions could feasibly affect SOF anti-HCV activity. CONCLUSION: SOF has a high barrier to resistance; however, low-frequency NS5B substitutions associated with treatment failure were identified that may contribute to resistance of this important drug for chronic HCV infection.

Resistance of human cytomegalovirus to ganciclovir/valganciclovir: a comprehensive review of putative resistance pathways.

Human cytomegalovirus (HCMV) is a pathogen that can be life-threatening in immunocompromised individuals. Valganciclovir and its parent drug ganciclovir are currently the principle drugs used for the treatment or prevention of HCMV disease. The development of HCMV resistance to ganciclovir/valganciclovir has been documented in treated patients and is associated with the emergence of amino acid substitutions in the viral proteins pUL97, pUL54 or both. Generally, single amino acid substitutions associated with clinical resistance that alone do not confer decreased ganciclovir susceptibility in cell culture have been disregarded as causative or clinically significant. This review focuses on the analysis and mechanisms of antiviral drug resistance to HCMV. We also conducted a review of publicly available clinical and nonclinical data to construct a comprehensive list of pUL97 and pUL54 amino acid substitutions that are associated with a poor clinical response to the first line therapies ganciclovir and valganciclovir, or associated with reduced HCMV ganciclovir susceptibility in cell culture. Over 40 putative ganciclovir/valganciclovir resistance-associated substitutions were identified in this analysis. These include the commonly reported substitutions M460I/V and C592G in pUL97. There were additional substitutions that are not widely considered as ganciclovir/valganciclovir resistance-associated substitutions, including V466M in pUL97 and E315D in pUL54. Some of these ganciclovir/valganciclovir resistance-associated substitutions may confer cross-resistance to other HCMV therapies, such as cidofovir and foscarnet. Based on this review, we propose that there are more potential HCMV ganciclovir/valganciclovir resistance pathways than generally appreciated. The resulting comprehensive list of putative ganciclovir/valganciclovir resistance-associated substitutions provides a foundation for future investigations to characterize the role of specific substitutions or combinations of substitutions, which will enhance our understanding of HCMV mechanisms of ganciclovir/valganciclovir resistance and also provide insight regarding the potential for cross-resistance to other HCMV therapies.

Clinical relevance of detectable but not quantifiable hepatitis C virus RNA during boceprevir or telaprevir treatment.

UNLABELLED: Boceprevir- and telaprevir-based treatments for chronic hepatitis C virus (HCV) infection use specific response-guided therapy (RGT) guidelines. Eligibility for shortened treatment duration is based on achieving undetectable HCV RNA early during treatment. It is unclear whether a detected HCV RNA level that is below the assay lower limit of quantitation (detectable/BLOQ) is comparable to an undetectable HCV RNA level for RGT decision making. We analyzed data from boceprevir and telaprevir clinical trials to obtain a comprehensive understanding of the frequency and clinical relevance of detectable/BLOQ HCV RNA measurements. In Phase 3 trials P05216 (boceprevir), C216 (telaprevir), and 108 (telaprevir), detectable/BLOQ levels were reported for approximately 10%-20% of all on-treatment HCV RNA measurements. In P05216 and C216, subjects with detectable/BLOQ HCV RNA, on average, had a reduced sustained virologic response (SVR) rate compared with subjects with undetectable HCV RNA at the same on-treatment timepoint. At key RGT timepoints (week 8 for boceprevir, week 4 for telaprevir), subjects with detectable/BLOQ HCV RNA had an approximately 20% lower SVR rate compared with subjects with undetectable HCV RNA, and this difference widened for later on-treatment timepoints. A similar trend was observed for Study 108, but the differences in SVR rates were modest, potentially explained by a higher frequency of reported detectable/BLOQ results. Analyses of Phase 2 boceprevir and telaprevir trials indicated subjects with detectable/BLOQ HCV RNA at RGT timepoints benefited from extended treatment duration. CONCLUSION: During boceprevir- and telaprevir-based treatment, subjects with detectable/BLOQ HCV RNA had a reduced virologic response compared with subjects with undetectable HCV RNA. Eligibility for shortened treatment duration should be based on achieving undetectable HCV RNA (i.e., HCV RNA not detected) at RGT decision timepoints.

Running a tightrope: regulatory challenges in the development of antiretrovirals.

Since the approval of Retrovir, (zidovudine, AZT) in 1987 by the Food and Drug Administration, a number of regulatory initiatives were codified into regulation which contributed to the rapid development of new treatments for HIV-1 infection. These initiatives are a testament to the efforts of AIDS activists and regulators to improve access to drugs for serious and life-threatening diseases. Currently, 28 antiretroviral drugs and combinations of antiretrovirals are available to treat HIV-1 infection. The broadening armamentarium of approved antiretroviral drugs provides new options and more choices for physicians and HIV patients. Importantly, the introduction of these newly approved HIV drugs has shown that the majority of HIV-1-infected treatment-naïve and treatment-experienced patients can achieve maximal virologic suppression (less than 50 copies/mL HIV-1 RNA). This article describes the past and current regulatory challenges in the development of new HIV treatments and provides an overview of the drug regulations that were required for the approval of HIV drugs. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Food and Drug Administration analysis of tipranavir clinical resistance in HIV-1-infected treatment-experienced patients.

OBJECTIVE: To assess the resistance profile of tipranavir. METHODS: Resistance analyses were performed on Boëhringer Ingelheim-sponsored studies examining the safety and efficacy of tipranavir in highly treatment-experienced individuals at 24 weeks. Virologic response rates based on the presence of baseline primary protease inhibitor mutations and based on baseline tipranavir susceptibility were evaluated, and the development of protease mutations during treatment with tipranavir was analyzed. RESULTS: Virologic response rates in tipranavir-treated individuals were reduced when isolates with substitutions at amino acid positions I13, V32, M36, I47, Q58, D60 V82 or I84 were present at baseline. In addition, virologic response rates to tipranavir decreased when the number of baseline protease inhibitor (PI) mutations was five or more. Individuals who received tipranavir without concomitant enfurvitide and had five or more baseline PI mutations group began to lose antiviral response between weeks 4 and 8. However, individuals taking enfuvirtide with tipranavir were able to achieve greater than 1.5 log10 reductions in viral load from baseline out to 24 weeks even if they had five or more baseline PI mutations. Virologic response rates to tipranavir decreased when the baseline phenotype for tipranavir had a greater than three-fold shift in the 50% effective concentration (EC50) from reference. The most common protease mutations that developed in tipranavir-treated individuals who experienced virologic failure were L10I/V/S, I13V, L33V/I/F, M36V/I/L V82T, V82L, and I84V. The resistance profile in treatment-naive individuals was not characterized. CONCLUSIONS: Baseline genotypic and phenotypic data provide valuable information on the likelihood of a virologic response to tipranavir.

Effect of baseline protease genotype and phenotype on HIV response to atazanavir/ritonavir in treatment-experienced patients.

OBJECTIVES: To assess the virologic response rates of atazanavir/ritonavir and lopinavir/ritonavir based on baseline genotype and phenotype. METHODS: Resistance analyses were performed on a Bristol-Myers Squibb-sponsored study comparing the safety and efficacy of atazanavir/ritonavir to lopinavir/ritonavir in treatment-experienced subjects at 48 weeks. Analyses evaluated virologic response based on the presence of baseline primary protease inhibitor mutations and baseline susceptibility. RESULTS: Less than 30% of atazanavir/ritonavir-treated patients were responders if substitutions at positions M46, G73, I84 or L90 were present in their HIV at baseline. In comparison, lopinavir/ritonavir response rates were less than 30% when protease substitutions at M46, I54, or I84 were present at baseline. The response rates were similar between atazanavir/ritonavir and lopinavir/ritonavir-treated subjects with zero to four baseline protease inhibitor mutations, but response rates were reduced if five or more baseline mutations were present: 0% for atazanavir/ritonavir compared with 28% for lopinavir/ritonavir. Baseline phenotype results showed that response rates were similar between atazanavir/ritonavir and lopinavir/ritonavir if shifts in susceptibility were zero to five, but response rates were lower if shifts were greater than five; 11% for atazanavir/ritonavir compared with 27% for lopinavir/ritonavir. CONCLUSIONS: Both type and number of baseline protease inhibitor mutations affected virologic response to atazanavir/ritonavir and lopinavir/ritonavir in treatment-experienced subjects. In addition, baseline phenotypic susceptibility could differentiate virologic response rates to the two drugs. These resistance analyses provide information on the likelihood of a virologic response to antiretroviral drugs based on baseline genotypic and phenotypic data, which is valuable to physicians and patients when choosing antiretroviral regimens.

Structures of HIV-1 RT-DNA complexes before and after incorporation of the anti-AIDS drug tenofovir.

Tenofovir, also known as PMPA, R-9-(2-(phosphonomethoxypropyl)adenine, is a nucleotide reverse transcriptase (RT) inhibitor. We have determined the crystal structures of two related complexes of HIV-1 RT with template primer and tenofovir: (i) a ternary complex at a resolution of 3.0 A of RT crosslinked to a dideoxy-terminated DNA with tenofovir-diphosphate bound as the incoming substrate; and (ii) a RT-DNA complex at a resolution of 3.1 A with tenofovir at the 3' primer terminus. The tenofovir nucleotide in the tenofovir-terminated structure seems to adopt multiple conformations. Some nucleoside reverse transcriptase inhibitors, including 3TC and AZT, have elements ('handles') that project beyond the corresponding elements on normal dNTPs (the 'substrate envelope'). HIV-1 RT resistance mechanisms to AZT and 3TC take advantage of these handles; tenofovir's structure lacks handles that could protrude through the substrate envelope to cause resistance.

Molecular mechanisms of tenofovir resistance conferred by human immunodeficiency virus type 1 reverse transcriptase containing a diserine insertion after residue 69 and multiple thymidine analog-associated mutations.

Two amino acids inserted between residues 69 and 70 of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) are rare mutations that may develop in viruses containing multiple thymidine analog (zidovudine [AZT], stavudine)-associated mutations and that confer high-level resistance to all currently approved chain-terminating nucleoside and nucleotide RT inhibitors (NRTIs). The two known mechanisms of resistance to NRTIs are decreased incorporation and increased excision. The mechanism used by RT insertion mutants has not been described for tenofovir (TFV), a recently approved agent in this class. A patient-derived HIV-1 strain (strain FS-SSS) that contained an insertion mutation in a background of additional resistance mutations M41L, L74V, L210W, and T215Y was obtained. A second virus (strain FS) was derived from FS-SSS. In strain FS the insertion and T69S were reverted but the other resistance mutations were retained. The FS virus showed strong resistance to AZT but low-level changes in susceptibilities to other NRTIs and TFV. The FS-SSS virus showed reduced susceptibilities to all NRTIs including TFV. Steady-state kinetics demonstrated that the relative binding or incorporation of TFV was slightly decreased for FS-SSS RT compared to those for wild-type RT. However, significant ATP-mediated excision of TFV was detected for both mutant RT enzymes and followed the order FS-SSS RT > FS RT > wild-type RT. The presence of physiological concentrations of the +1 nucleotide inhibited TFV excision by the wild-type RT and slightly inhibited excision by the FS RT, whereas the level of excision by the FS-SSS RT remained high. Computer modeling suggests that the increased mobility of the beta3-beta4 loop may contribute to the high-level and broad NRTI resistance caused by the T69 insertion mutation.

Distinct requirements for the naturally occurring splice forms Stat4alpha and Stat4beta in IL-12 responses.

Signal transducer and activator of transcription (Stat)4 is a signaling molecule required for normal responses to interleukin-12 (IL-12) and is critically involved in inflammatory responses. We have isolated an alternatively spliced isoform of Stat4, termed Stat4beta, which lacks 44 amino acids at the C-terminus, encompassing the putative transcriptional activation domain. To assess the in vivo roles of these Stat4 isoforms, we generated transgenic Stat4-deficient mice expressing Stat4alpha or Stat4beta. Our results indicate that T-cell-specific expression of Stat4alpha or Stat4beta can mediate many aspects of IL-12 signaling including the differentiation of Th1 cells. However, Stat4alpha is required for normal levels of IL-12-induced interferon-gamma production from Th1 cells. Microarray analysis identified 98 genes induced by both Stat4 isoforms, 32 genes induced only by Stat4alpha and 29 genes induced only by Stat4beta. Some induced genes correlate with specific functions including the ability of Stat4beta, but not Stat4alpha, to mediate IL-12-stimulated proliferation. Thus, Stat4alpha and Stat4beta have distinct roles in mediating responses to IL-12.

Molecular mechanisms of resistance to human immunodeficiency virus type 1 with reverse transcriptase mutations K65R and K65R+M184V and their effects on enzyme function and viral replication capacity.

Human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) resistance mutations K65R and M184V result in changes in susceptibility to several nucleoside and nucleotide RT inhibitors. K65R-containing viruses showed decreases in susceptibility to tenofovir, didanosine (ddI), abacavir, and (-)-beta-D-dioxolane guanosine (DXG; the active metabolite of amdoxovir) but appeared to be fully susceptible to zidovudine and stavudine in vitro. Viruses containing the K65R and M184V mutations showed further decreases in susceptibility to ddI and abacavir but increased susceptibility to tenofovir compared to the susceptibilities of viruses with the K65R mutation. Enzymatic and viral replication analyses were undertaken to elucidate the mechanisms of altered drug susceptibilities and potential fitness defects for the K65R and K65R+M184V mutants. The relative inhibitory capacities (K(i)/K(m)) of the active metabolites of tenofovir, ddI, and DXG were increased for the RT containing the K65R mutation compared to that for the wild-type RT, but the relative inhibitory capacity of abacavir was only minimally increased. For the mutant viruses with the K65R and M184V mutations, the increase in tenofovir susceptibility compared to that of the mutants with K65R correlated with a decrease in the tenofovir inhibitory capacity that was mediated primarily by an increased K(m) of dATP. The decrease in susceptibility to ddI by mutants with the K65R and M184V mutations correlated with an increase in the inhibitory capacity mediated by an increased K(i). ATP-mediated removal of carbovir as well as small increases in the inhibitory capacity of carbovir appear to contribute to the resistance of mutants with the K65R mutation and the mutants with the K65R and M184V mutations to abacavir. Finally, both the HIV-1 K65R mutant and, more notably, the HIV-1 K65R+M184V double mutant showed reduced replication capacities and reduced RT processivities in vitro, consistent with a potential fitness defect in vivo and the low prevalence of the K65R mutation among isolates from antiretroviral agent-experienced patients.

Th1 cells regulate hematopoietic progenitor cell homeostasis by production of oncostatin M.

Regulation of hematopoietic progenitor cell homeostasis is crucial for maintenance of innate immunity and the ability of the body to respond to injury and infection. In this report, we demonstrate that progenitor cell numbers and cycling status in vivo are dramatically increased in mice deficient in Stat6 and decreased in mice deficient in Stat4, targeted mutations which also alter T helper cell polarization. Experiments using mice that have T cell restricted transgenic expression of Stat4 or Stat6 or have been in vivo depleted of T cell subsets demonstrate that CD4(+) T cells regulate progenitor cell activity. Injection of the Th1 cytokine Oncostatin M but not other cytokines into Stat4-deficient mice recovers progenitor cell activity to wild-type levels. Thus, T helper cells actively regulate hematopoietic progenitor cell homeostasis.