Folgori A, Capone S, Ruggeri L, Meola A, Sporeno E, Ercole BB, Pezzanera M, Tafi R, Arcuri M, Fattori E, Lahm A, Luzzago A, Vitelli A, Colloca S, Cortese R, Nicosia A. (Istituto di Recherche di Biologica Molecolare “P. Angeletti,” Pomezia, Rome, Italy). A T-cell vaccine eliciting effective immunity against heterologous virus challenge in chimpanzees. Nat Med 2006;12:190–197. Hepatitis C virus (HCV) infection affects an estimated 3% of the world’s population and is an important risk factor for end-stage liver disease and hepatocellular carcinoma. Recent improvements in transmission prevention and optimization of therapeutic parameters should eventually decrease the incidence of HCV infection, but it is questionable whether these improvements alone will be able to fully contain the epidemic. Notably, certain high-risk groups such as injection drug users will continue to be at risk for new infections while simultaneously representing difficult targets for therapeutic interventions (especially those requiring strict long-term adherence to a drug regimen). Additionally, the epidemic is likely to continue in areas of the world with limited access to health care resources, where hygienic practices are and will remain out of reach. It is also unlikely that today’s costly antiviral therapies will be available for the millions of people in resource-poor settings already infected with HCV. Thus, the development of a low-cost vaccine that has the potential to prevent chronic HCV infection, if not prevent infection altogether, remains paramount to effectively reining in the epidemic. Equally desirable would be a simple immunotherapeutic intervention able to prevent or slow the development of complications of chronic HCV infection. It is well established that both humans and chimpanzees can be repeatedly infected with HCV, indicating that sterilizing immunity might be an elusive goal. This was best demonstrated by a large study of drug users, in whom acute HCV infection was frequently detected in subjects who had previously tested HCV Ab+, RNA−, indicating that they had spontaneously resolved a first infection with HCV (Lancet 2002;359:1478–1483). At the same time, this study also highlighted that preexisting immunity could prevent progression to chronic infection; subjects who had signs of a previously cleared infection had a high likelihood of clearing subsequent infection. When combined with data suggesting a critical role for HCV-specific T cells in controlling acute HCV infection (Science 2003;302:659–662; J Exp Med 2003;197:1645–1655; J Exp Med 2000;191:1499–1512; Gastroenterology 1999;117:933–941), these findings provide adequate rationale for an HCV T-cell vaccine design. A vaccine capable of inducing HCV-specific T cells in uninfected chimpanzees with comparable vigor and breadth of the T-cell response previously observed during natural acute HCV infection was recently described by Folgori et al (Nat Med 2006;2:190–197). The authors designed a highly sophisticated vaccination scheme in which they repeatedly delivered the same HCV genotype 1b sequence, coding for the nonstructural HCV proteins NS3 to NS5B. In the first 3 rounds of immunization, they delivered the HCV immunogens embedded in replication-defective adenoviral vectors, followed by 3 rounds of boosting with the same sequence as part of plasmid DNA. The full vaccination course was applied over 39 weeks, followed by a challenge with an HCV genotype 1a inoculum at week 49. In parallel, an age- and gender-matched group of chimpanzees that had received the same vaccination protocol coding for an HIV-1 sequence was challenged with the same HCV strain. The authors found that all animals in the vaccine group developed strong CD4+ and CD8+ T-cell responses against HCV in the blood and liver after immunization. The strength of the responses was impressive, resembling the typical response profile of a spontaneous resolver during acute hepatitis. Folgori et al also measured the breadth of the CD8+ T-cell response, with more variable results, as between 1 and 10 epitopes were targeted. Importantly, all but one animal had measurable CD8 responses that recognized not only the sequence of the vaccine strain, but also the sequence of the HCV challenge strain. After inoculation with the challenge virus, a rapid and vigorous expansion of T cells was observed in the vaccine group within 4 weeks. In the control animals, robust T-cell responses could only be detected after 10 weeks, and responses in that group were maintained for a shorter time. Although no significant difference in the clearance rate of HCV could be detected between the 2 groups of animals, with 4 of 5 eliminating HCV in the vaccine group and 3 of 5 in the control arm, there was, however, a clear difference in serum ALT and γ-glutamyltransferase levels between the 2 groups. None of the vaccinated animals experienced significant or prolonged elevation of liver associated laboratory chemistries. In contrast, all control chimpanzees had elevated liver-associated laboratory chemistries by 3 and 7 weeks, consistent with acute hepatitis. Generation of an effective HCV vaccine has proven an elusive goal and it seems likely that vaccine-induced sterilizing immunity against HCV will remain unattainable. Preventing evolution of chronic HCV infection may be more achievable and should be equally effective in preventing the further spread of infection and liver disease. The study by Folgori et al takes a significant step in this direction with the introduction of a vaccine approach that induces vigorous and broadly directed T-cell responses reminiscent of those seen during acute HCV infection. Being able to reliably induce such a response represents the first milestone toward an effective T-cell vaccine. The partial success of the immunization described by Folgori et al raises several important questions. The first and most relevant is the question of how effective the approach will be in humans. Even if the immunization scheme were to succeed in eliciting a multispecific and vigorous T-cell response in humans, we still do not know whether this response will reliably control HCV viremia. In this regard, it remains undefined what type of T-cell response, or actually any type of immune response, against HCV is required to clear or at least control the virus. Recent studies in humans have found extremely broad and vigorous CD8+ T-cell responses in humans with acute infection, irrespective of clinical outcome, highlighting that the mere presence of such responses does not suffice for viral control (Eur J Immunol 2000;30:2479–2487; Hepatology 2005;42:104–112). Based on other studies in humans and chimpanzees, it appears critical that HCV-specific CD4 responses are also induced (Science 2003;302:659–662; Gastroenterology 1999;117:933–941). In this respect, it is promising that the immunization approach discussed here was able to induce robust CD4 as well as CD8 responses. Based on these observations, it is essential that any approach that successfully elicits T-cell responses in the chimpanzee be tested as quickly as possible in humans. Executing these human studies will present its own set of challenges, because efficacy can only reliably be demonstrated in those cohorts at a high risk for infection. The only persons fulfilling these criteria would appear to be subjects who recently started to use IV drugs, but they are notoriously difficult to follow. It will be a significant challenge to apply an immunization schedule comprised of 6 injections over almost a full year in any cohort of humans and one can only hope that a simplified protocol will be available in the future. Even if a vaccine can successfully control a specific viral challenge, there are still no assurances that protection can be provided against multiple circulating viral strains. In this regard, the study by Folgori et al offers cautious reassurance. The authors challenged the animals with a HCV strain distinct from the one used for immunization (13% difference at the amino acid level), indicating that protection should reach beyond strains that are very closely related. However, at the same time, the immunization failed in 1 of 5 animals, and the reason for this is not fully clear. Overall, an animal study in which all animals must be challenged with the identical HCV inoculum, combined with the limited genetic diversity of a small chimpanzee cohort, does not allow us to predict how broadly protective a vaccine might be in humans. In the real world, challenges can be presented by a myriad of HCV variants, with different degrees, but also different areas of divergence between the vaccine strain and the infecting strain. One of the important questions to address in the near future will be whether artificial HCV sequences such as consensus or ancestral sequences will offer better protection when compared to prototype strains, such as the HCV genotype 1b strain DK used here. It will also be important to see whether the vaccine will be more effective in subjects expressing certain HLA alleles, analogous to what is seen in the natural course of HCV (Hepatology 2004;40:108–114) and HIV-1 infection (Nat Med 1996;2:405–411). If the vaccine proves to be protective against a broad range of HCV strains, then one would hope that this same approach would be helpful in the treatment of chronic infection. Indeed, T-cell vaccines are theoretically better suited than antibody-inducing vaccines for treating established infection because they target infected cells, rather than free virus. For this situation, the very impressive immunogenicity of the vaccine approach introduced by Folgori et al could be an important asset. As chronic HCV infection is associated with a crippled T-cell response (especially by CD4+ T cells) (Gastroenterology 2004;127:924–936; J Virol 2002;76:12584–12595), resurrecting these critical responses will be more similar to rescuing a sunken ship rather than just protecting it in stormy weather. However, an important limitation of this approach is the potential risk of decompensation in those persons with advanced fibrosis. In summary, the study by Folgori et al takes an important step toward development of a protective, and potentially therapeutic, vaccine for HCV infection. The ability to induce robust and especially broad T-cell responses provides a path forward for targeting this elusive virus, and one hopes that this approach will move swiftly into studies in humans.