Immune Responses to AAV Capsid: Are Mice Not Humans After All?
Authors: Roland W Herzog
Journal: Molecular Therapy (2007)

See page 792Adeno-associated viruses (AAVs) have been favored for in vivo gene transfer of therapeutic genes because of their low immunogenicity, strong safety record, and high efficiency of transduction of a number of cell types in animal models.1Wu Z Asokan A Samulski RJ Adeno-associated virus serotypes: vector toolkit for human gene therapy.Mol Ther. 2006; 14: 316-327Abstract Full Text Full Text PDF PubMed Scopus (618) Google Scholar However, in a recent gene therapy trial involving delivery of human coagulation factor IX (F.IX) for the treatment of hemophilia, two patients developed a T-cell response to AAV-2 capsid, which was not predicted from preclinical studies in animals. Importantly, transgene expression in the two patients declined to pretreatment levels, suggesting that a cytotoxic T-lymphocyte (CTL) response to the capsid may have been responsible for a loss of transgene-expressing cells. In this issue, Li et al. now provide a detailed analysis of the effect of CTL responses to AAV capsid in mice.2Li H Murphy SL Giles-Davis W Edmonson S Xiang Z Li Y et al.Pre-existing AAV capsid-specific CD8+ T cells are unable to eliminate AAV-transduced hepatocytes.Mol Ther. 2007; 15: 792-800Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar The results highlight a problem facing the field: the lack of an appropriate and predictive animal model for immune-mediated loss of hepatocytes after transduction with AAV.Enthusiasm for the AAV vector system grew tremendously approximately a decade ago, when several laboratories demonstrated sustained expression of a bacterial LacZ gene without activation of CTLs after intramuscular injection of recombinant AAV (serotype 2).3Fisher KJ Jooss K Alston J Yang Y Haecker SE High K et al.Recombinant adeno-associated virus for muscle directed gene therapy.Nat Med. 1997; 3: 306-312Crossref PubMed Scopus (574) Google Scholar,4Kessler PD Podsakoff GM Chen X McQuiston SA Colosi PC Matelis LA et al.Gene delivery to skeletal muscle results in sustained expression and systemic delivery of a therapeutic protein.Proc Natl Acad Sci USA. 1996; 93: 14082-14087Crossref PubMed Scopus (533) Google Scholar,5Xiao X Li J Samulski RJ Efficient long-term gene transfer into muscle tissue of immunocompetent mice by adeno-associated virus vector.J Virol. 1996; 70: 8098-8108Crossref PubMed Google Scholar Similarly, T-cell responses and innate responses to AAV capsid were consistently low in animal studies. In other experiments, sustained expression of a F.IX gene over several years in animal models of the X-linked bleeding disorder, hemophilia B, has been documented for muscle- and liver-directed gene transfer.6Herzog RW Yang EY Couto LB Hagstrom JN Elwell D Fields PA et al.Long-term correction of canine hemophilia B by gene transfer of blood coagulation factor IX mediated by adeno-associated viral vector.Nat Med. 1999; 5: 56-63Crossref PubMed Scopus (468) Google Scholar,7Snyder RO Miao C Meuse L Tubb J Donahue BA Lin HF et al.Correction of hemophilia B in canine and murine models using recombinant adeno-associated viral vectors.Nat Med. 1999; 5: 64-70Crossref PubMed Scopus (314) Google Scholar These observations led to two phase I/II clinical trials. In the muscle-directed approach, gene transfer was long lasting and safe but subtherapeutic.8Manno CS Chew AJ Hutchison S Larson PJ Herzog RW Arruda VR et al.AAV-mediated factor IX gene transfer to skeletal muscle in patients with severe hemophilia B.Blood. 2003; 101: 2963-2972Crossref PubMed Scopus (593) Google ScholarA major concern in gene therapy for hemophilia is the risk of antibody formation to the coagulation factor transgene product. Therefore, subjects enrolled in the muscle trial were limited to those with F.IX missense mutations. Interestingly, liver-directed gene transfer showed higher efficacy than intramuscular injection, and the hepatic route was found to induce immune tolerance to F.IX and other transgene products.9Mingozzi F Liu YL Dobrzynski E Kaufhold A Liu JH Wang Y et al.Induction of immune tolerance to coagulation factor IX antigen by in vivo hepatic gene transfer.J Clin Invest. 2003; 111: 1347-1356Crossref PubMed Scopus (371) Google Scholar Consequently, hopes were high for the liver trial, and the only major immunological concern was that preexisting immunity in the form of neutralizing antibodies might prevent gene transfer, because most humans are seropositive for AAV-2. As predicted from studies in large animals, the first human subject with low preexisting neutralizing antibodies in the highest vector dose cohort exhibited therapeutic F.IX expression (>10%) after infusion of the AAV-2 vector into the hepatic artery.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar However, F.IX activity declined to pretreatment levels within 2 months, a result quite different from the years of expression observed in animals. A transient elevation of liver transaminase enzymes (transaminitis) was found to coincide with the loss of transgene expression.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar No antibody to F.IX was found in the affected patient, and causes for the transaminitis not related to the gene transfer were ruled out. In a subject treated subsequently with a slightly lower vector dose, transaminitis was again documented, and a detailed immunological analysis revealed a T-cell response to AAV-2 capsid. Computer software predicted the dominant epitope (initially identified by enzyme-linked immunosorbent spot) to represent a major histocompatibility complex (MHC) class I-restricted CD8+ T-cell epitope.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar Ironically, a CTL response apparently had limited expression from a vector praised earlier for lack of CTL induction.Although there is no direct demonstration of a CTL response to AAV capsid proteins leading to human hepatocyte death, the data supporting this interpretation are compelling. The temporal pattern of the T-cell response can be superimposed onto the transaminitis, and both subjects experienced similar time courses for the transient increase of liver enzyme concentrations.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar An AAV capsid-specific CD8+ T-cell clone was expanded from one of the subjects and showed high cytolytic activity against cells that displayed capsid antigen.11Maus MV Mingozzi F Sabatino DE Hui D Ragni MV High KA T cell responses to AAV vector capsid in normal donors and subjects who have undergone liver-directed AAV-mediated gene transfer.Mol Ther. 2006; 13: S282Abstract Full Text Full Text PDF Google Scholar At the same time, AAV capsid T-cell epitopes were mapped in mice with the aim of developing an animal model to study these CTL responses.12Chen J Wu Q Yang P Hsu HC Mountz JD Determination of specific CD4 and CD8 T cell epitopes after AAV2- and AAV8-hF.IX gene therapy.Mol Ther. 2006; 13: 260-269Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar,13Sabatino DE Mingozzi F Hui DJ Chen H Colosi P Ertl HC et al.Identification of mouse AAV capsid-specific CD8+ T cell epitopes.Mol Ther. 2005; 12: 1023-1033Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar CD8+ T-cell epitopes in humans and mice are highly conserved among many AAV serotypes.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar,11Maus MV Mingozzi F Sabatino DE Hui D Ragni MV High KA T cell responses to AAV vector capsid in normal donors and subjects who have undergone liver-directed AAV-mediated gene transfer.Mol Ther. 2006; 13: S282Abstract Full Text Full Text PDF Google Scholar,12Chen J Wu Q Yang P Hsu HC Mountz JD Determination of specific CD4 and CD8 T cell epitopes after AAV2- and AAV8-hF.IX gene therapy.Mol Ther. 2006; 13: 260-269Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar,13Sabatino DE Mingozzi F Hui DJ Chen H Colosi P Ertl HC et al.Identification of mouse AAV capsid-specific CD8+ T cell epitopes.Mol Ther. 2005; 12: 1023-1033Abstract Full Text Full Text PDF PubMed Scopus (74) Google ScholarThe main question at this point is obviously why the duration of transgene expression was not limited by CTL responses to AAV capsid in any of the animal models. One potential explanation is natural infection in humans with AAV, which is not the case for murine and canine models (but does, however, occur in nonhuman primates). Upon natural infection in the presence of a helper virus, AAV capsid-specific memory T cells may be generated14Mingozzi F Maus MV Sabatino DE Hui DJ Rasko JEJ Ragni MV et al.AAV-2 capsid-specific CD8+ T cells limit the duration of gene therapy in humans and cross-react with AAV-8 capsid.Blood. 2006; 108: 138aGoogle Scholar. The kinetics of the transaminitis-CTL response in the documented human cases (4-6 weeks after gene transfer) may then be explained by low frequency of memory T cells and/or inefficient processing of AAV capsid into MHC class I pathways (Figure 1).A second important question is how the vector could have elicited an MHC class I-restricted response, in that there are no coding sequences for expression of capsid in the vector genome. It is known that AAV vector particles escape the endosome so as to traffic to the nucleus via the cytoplasm. Some of the vector particles become ubiquitinated and thus targeted for degradation by proteasomes.15Yan Z Zak R Luxton GW Ritchie TC Bantel-Schaal U Engelhardt JF Ubiquitination of both adeno-associated virus type 2 and 5 capsid proteins affects the transduction efficiency of recombinant vectors.J Virol. 2002; 76: 2043-2053Crossref PubMed Scopus (179) Google Scholar Therefore, capsid-derived peptides would be expected to be presented by MHC class I molecules via this cross-presentation pathway. A recent study has shown that AAV-2 capsid, as a result of the presence of the heparin-binding site, is more efficiently cross-presented by dendritic cells than other serotypes such as AAV-8, thereby directing T-cell activation.16Vandenberghe LH Wang L Somanathan S Zhi Y Figueredo J Calcedo R et al.Heparin binding directs activation of T cells against adeno-associated virus serotype 2 capsid.Nat Med. 2006; 12: 967-971Crossref PubMed Scopus (173) Google Scholar One could now ask whether this pathway could lead to a primary T-cell response efficient enough to cause a CTL response in vivo without a need for reactivation of memory T cells. However, new data are emerging that humans harbor CD8+ memory T cells to AAV capsid and that these can be activated by AAV-8 capsid.14Mingozzi F Maus MV Sabatino DE Hui DJ Rasko JEJ Ragni MV et al.AAV-2 capsid-specific CD8+ T cells limit the duration of gene therapy in humans and cross-react with AAV-8 capsid.Blood. 2006; 108: 138aGoogle Scholar It may therefore not be possible to avoid the CTL response by switching serotypes, even if there are quantitative differences in the efficiency of cross-presentation between capsids of alternate serotypes.In the carefully designed new experiments, Li et al. generated capsid-specific CTLs in one set of animals and then adoptively transferred these to syngeneic mice that received AAV-F.IX gene transfer.2Li H Murphy SL Giles-Davis W Edmonson S Xiang Z Li Y et al.Pre-existing AAV capsid-specific CD8+ T cells are unable to eliminate AAV-transduced hepatocytes.Mol Ther. 2007; 15: 792-800Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar Much to everyone's surprise, these CTLs failed to destroy AAV-transduced hepatocytes. The study rules out active downregulation of MHC class I expression by AAV in hepatocytes, lack of functionality of transferred CTLs, and suppression by regulatory T cells as causes for failure to eliminate expression. Although not directly addressed in this study, the results suggest that it is unlikely that hepatocyte-synthesized capsid (which could possibly arise from vector-helper plasmid recombination during vector production) was responsible for loss of expression in humans, as has been speculated in the field (capsid-producing hepatocytes should have been targeted by transferred CTLs).If mice differ from humans in that they lack the ability to eliminate AAV vector-transduced hepatocytes by capsid-specific CTLs, the next step in unraveling this puzzle must be to study AAV capsid processing in human hepatocytes. The results by Li et al. raise questions about differences between human and murine hepatocytes in the ability to cross-present AAV capsid antigen. Certainly, the results of all the recent studies combined suggest that the T-cell response to AAV capsid is a serious hurdle for therapeutic gene transfer, and that more studies are required to understand how such responses are generated, how they may cause loss of expression, and how these responses can be prevented. Thus far, however, direct experimental evidence for a crucial step in the proposed model, namely the destruction of hepatocytes by capsid-specific T cells, remains elusive. See page 792 Adeno-associated viruses (AAVs) have been favored for in vivo gene transfer of therapeutic genes because of their low immunogenicity, strong safety record, and high efficiency of transduction of a number of cell types in animal models.1Wu Z Asokan A Samulski RJ Adeno-associated virus serotypes: vector toolkit for human gene therapy.Mol Ther. 2006; 14: 316-327Abstract Full Text Full Text PDF PubMed Scopus (618) Google Scholar However, in a recent gene therapy trial involving delivery of human coagulation factor IX (F.IX) for the treatment of hemophilia, two patients developed a T-cell response to AAV-2 capsid, which was not predicted from preclinical studies in animals. Importantly, transgene expression in the two patients declined to pretreatment levels, suggesting that a cytotoxic T-lymphocyte (CTL) response to the capsid may have been responsible for a loss of transgene-expressing cells. In this issue, Li et al. now provide a detailed analysis of the effect of CTL responses to AAV capsid in mice.2Li H Murphy SL Giles-Davis W Edmonson S Xiang Z Li Y et al.Pre-existing AAV capsid-specific CD8+ T cells are unable to eliminate AAV-transduced hepatocytes.Mol Ther. 2007; 15: 792-800Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar The results highlight a problem facing the field: the lack of an appropriate and predictive animal model for immune-mediated loss of hepatocytes after transduction with AAV. Enthusiasm for the AAV vector system grew tremendously approximately a decade ago, when several laboratories demonstrated sustained expression of a bacterial LacZ gene without activation of CTLs after intramuscular injection of recombinant AAV (serotype 2).3Fisher KJ Jooss K Alston J Yang Y Haecker SE High K et al.Recombinant adeno-associated virus for muscle directed gene therapy.Nat Med. 1997; 3: 306-312Crossref PubMed Scopus (574) Google Scholar,4Kessler PD Podsakoff GM Chen X McQuiston SA Colosi PC Matelis LA et al.Gene delivery to skeletal muscle results in sustained expression and systemic delivery of a therapeutic protein.Proc Natl Acad Sci USA. 1996; 93: 14082-14087Crossref PubMed Scopus (533) Google Scholar,5Xiao X Li J Samulski RJ Efficient long-term gene transfer into muscle tissue of immunocompetent mice by adeno-associated virus vector.J Virol. 1996; 70: 8098-8108Crossref PubMed Google Scholar Similarly, T-cell responses and innate responses to AAV capsid were consistently low in animal studies. In other experiments, sustained expression of a F.IX gene over several years in animal models of the X-linked bleeding disorder, hemophilia B, has been documented for muscle- and liver-directed gene transfer.6Herzog RW Yang EY Couto LB Hagstrom JN Elwell D Fields PA et al.Long-term correction of canine hemophilia B by gene transfer of blood coagulation factor IX mediated by adeno-associated viral vector.Nat Med. 1999; 5: 56-63Crossref PubMed Scopus (468) Google Scholar,7Snyder RO Miao C Meuse L Tubb J Donahue BA Lin HF et al.Correction of hemophilia B in canine and murine models using recombinant adeno-associated viral vectors.Nat Med. 1999; 5: 64-70Crossref PubMed Scopus (314) Google Scholar These observations led to two phase I/II clinical trials. In the muscle-directed approach, gene transfer was long lasting and safe but subtherapeutic.8Manno CS Chew AJ Hutchison S Larson PJ Herzog RW Arruda VR et al.AAV-mediated factor IX gene transfer to skeletal muscle in patients with severe hemophilia B.Blood. 2003; 101: 2963-2972Crossref PubMed Scopus (593) Google Scholar A major concern in gene therapy for hemophilia is the risk of antibody formation to the coagulation factor transgene product. Therefore, subjects enrolled in the muscle trial were limited to those with F.IX missense mutations. Interestingly, liver-directed gene transfer showed higher efficacy than intramuscular injection, and the hepatic route was found to induce immune tolerance to F.IX and other transgene products.9Mingozzi F Liu YL Dobrzynski E Kaufhold A Liu JH Wang Y et al.Induction of immune tolerance to coagulation factor IX antigen by in vivo hepatic gene transfer.J Clin Invest. 2003; 111: 1347-1356Crossref PubMed Scopus (371) Google Scholar Consequently, hopes were high for the liver trial, and the only major immunological concern was that preexisting immunity in the form of neutralizing antibodies might prevent gene transfer, because most humans are seropositive for AAV-2. As predicted from studies in large animals, the first human subject with low preexisting neutralizing antibodies in the highest vector dose cohort exhibited therapeutic F.IX expression (>10%) after infusion of the AAV-2 vector into the hepatic artery.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar However, F.IX activity declined to pretreatment levels within 2 months, a result quite different from the years of expression observed in animals. A transient elevation of liver transaminase enzymes (transaminitis) was found to coincide with the loss of transgene expression.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar No antibody to F.IX was found in the affected patient, and causes for the transaminitis not related to the gene transfer were ruled out. In a subject treated subsequently with a slightly lower vector dose, transaminitis was again documented, and a detailed immunological analysis revealed a T-cell response to AAV-2 capsid. Computer software predicted the dominant epitope (initially identified by enzyme-linked immunosorbent spot) to represent a major histocompatibility complex (MHC) class I-restricted CD8+ T-cell epitope.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar Ironically, a CTL response apparently had limited expression from a vector praised earlier for lack of CTL induction. Although there is no direct demonstration of a CTL response to AAV capsid proteins leading to human hepatocyte death, the data supporting this interpretation are compelling. The temporal pattern of the T-cell response can be superimposed onto the transaminitis, and both subjects experienced similar time courses for the transient increase of liver enzyme concentrations.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar An AAV capsid-specific CD8+ T-cell clone was expanded from one of the subjects and showed high cytolytic activity against cells that displayed capsid antigen.11Maus MV Mingozzi F Sabatino DE Hui D Ragni MV High KA T cell responses to AAV vector capsid in normal donors and subjects who have undergone liver-directed AAV-mediated gene transfer.Mol Ther. 2006; 13: S282Abstract Full Text Full Text PDF Google Scholar At the same time, AAV capsid T-cell epitopes were mapped in mice with the aim of developing an animal model to study these CTL responses.12Chen J Wu Q Yang P Hsu HC Mountz JD Determination of specific CD4 and CD8 T cell epitopes after AAV2- and AAV8-hF.IX gene therapy.Mol Ther. 2006; 13: 260-269Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar,13Sabatino DE Mingozzi F Hui DJ Chen H Colosi P Ertl HC et al.Identification of mouse AAV capsid-specific CD8+ T cell epitopes.Mol Ther. 2005; 12: 1023-1033Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar CD8+ T-cell epitopes in humans and mice are highly conserved among many AAV serotypes.10Manno CS Pierce GF Arruda VR Glader B Ragni M Rasko JJ et al.Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response.Nat Med. 2006; 12: 342-347Crossref PubMed Scopus (1534) Google Scholar,11Maus MV Mingozzi F Sabatino DE Hui D Ragni MV High KA T cell responses to AAV vector capsid in normal donors and subjects who have undergone liver-directed AAV-mediated gene transfer.Mol Ther. 2006; 13: S282Abstract Full Text Full Text PDF Google Scholar,12Chen J Wu Q Yang P Hsu HC Mountz JD Determination of specific CD4 and CD8 T cell epitopes after AAV2- and AAV8-hF.IX gene therapy.Mol Ther. 2006; 13: 260-269Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar,13Sabatino DE Mingozzi F Hui DJ Chen H Colosi P Ertl HC et al.Identification of mouse AAV capsid-specific CD8+ T cell epitopes.Mol Ther. 2005; 12: 1023-1033Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar The main question at this point is obviously why the duration of transgene expression was not limited by CTL responses to AAV capsid in any of the animal models. One potential explanation is natural infection in humans with AAV, which is not the case for murine and canine models (but does, however, occur in nonhuman primates). Upon natural infection in the presence of a helper virus, AAV capsid-specific memory T cells may be generated14Mingozzi F Maus MV Sabatino DE Hui DJ Rasko JEJ Ragni MV et al.AAV-2 capsid-specific CD8+ T cells limit the duration of gene therapy in humans and cross-react with AAV-8 capsid.Blood. 2006; 108: 138aGoogle Scholar. The kinetics of the transaminitis-CTL response in the documented human cases (4-6 weeks after gene transfer) may then be explained by low frequency of memory T cells and/or inefficient processing of AAV capsid into MHC class I pathways (Figure 1). A second important question is how the vector could have elicited an MHC class I-restricted response, in that there are no coding sequences for expression of capsid in the vector genome. It is known that AAV vector particles escape the endosome so as to traffic to the nucleus via the cytoplasm. Some of the vector particles become ubiquitinated and thus targeted for degradation by proteasomes.15Yan Z Zak R Luxton GW Ritchie TC Bantel-Schaal U Engelhardt JF Ubiquitination of both adeno-associated virus type 2 and 5 capsid proteins affects the transduction efficiency of recombinant vectors.J Virol. 2002; 76: 2043-2053Crossref PubMed Scopus (179) Google Scholar Therefore, capsid-derived peptides would be expected to be presented by MHC class I molecules via this cross-presentation pathway. A recent study has shown that AAV-2 capsid, as a result of the presence of the heparin-binding site, is more efficiently cross-presented by dendritic cells than other serotypes such as AAV-8, thereby directing T-cell activation.16Vandenberghe LH Wang L Somanathan S Zhi Y Figueredo J Calcedo R et al.Heparin binding directs activation of T cells against adeno-associated virus serotype 2 capsid.Nat Med. 2006; 12: 967-971Crossref PubMed Scopus (173) Google Scholar One could now ask whether this pathway could lead to a primary T-cell response efficient enough to cause a CTL response in vivo without a need for reactivation of memory T cells. However, new data are emerging that humans harbor CD8+ memory T cells to AAV capsid and that these can be activated by AAV-8 capsid.14Mingozzi F Maus MV Sabatino DE Hui DJ Rasko JEJ Ragni MV et al.AAV-2 capsid-specific CD8+ T cells limit the duration of gene therapy in humans and cross-react with AAV-8 capsid.Blood. 2006; 108: 138aGoogle Scholar It may therefore not be possible to avoid the CTL response by switching serotypes, even if there are quantitative differences in the efficiency of cross-presentation between capsids of alternate serotypes. In the carefully designed new experiments, Li et al. generated capsid-specific CTLs in one set of animals and then adoptively transferred these to syngeneic mice that received AAV-F.IX gene transfer.2Li H Murphy SL Giles-Davis W Edmonson S Xiang Z Li Y et al.Pre-existing AAV capsid-specific CD8+ T cells are unable to eliminate AAV-transduced hepatocytes.Mol Ther. 2007; 15: 792-800Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar Much to everyone's surprise, these CTLs failed to destroy AAV-transduced hepatocytes. The study rules out active downregulation of MHC class I expression by AAV in hepatocytes, lack of functionality of transferred CTLs, and suppression by regulatory T cells as causes for failure to eliminate expression. Although not directly addressed in this study, the results suggest that it is unlikely that hepatocyte-synthesized capsid (which could possibly arise from vector-helper plasmid recombination during vector production) was responsible for loss of expression in humans, as has been speculated in the field (capsid-producing hepatocytes should have been targeted by transferred CTLs). If mice differ from humans in that they lack the ability to eliminate AAV vector-transduced hepatocytes by capsid-specific CTLs, the next step in unraveling this puzzle must be to study AAV capsid processing in human hepatocytes. The results by Li et al. raise questions about differences between human and murine hepatocytes in the ability to cross-present AAV capsid antigen. Certainly, the results of all the recent studies combined suggest that the T-cell response to AAV capsid is a serious hurdle for therapeutic gene transfer, and that more studies are required to understand how such responses are generated, how they may cause loss of expression, and how these responses can be prevented. Thus far, however, direct experimental evidence for a crucial step in the proposed model, namely the destruction of hepatocytes by capsid-specific T cells, remains elusive.