Antigen-specific tolerance of human alpha1-antitrypsin induced by helper-dependent adenovirus

Immunogenicity of Cas9 Protein

Clustered regularly interspaced short palindromic repeats (CRISPR) form the adaptive immune system in archaea and bacteria and have been modified for genome engineering in eukaryotic cells. CRISPR systems contain 2 components, a single-guide RNA, which is a short RNA composed of a 20 nucleotide sequence that targets specific sites in the genomic DNA and a scaffold necessary for its binding to the CRISPR-associated endonuclease (Cas9). Because of its high efficiency and accuracy, the CRISPR-Cas9 genome editing based therapies are poised to treat a multitude of human diseases with a promise to target previously "undruggable" proteins. As the first in-body clinical trial with CRISPR-Cas9 is embarked on, the risks associated with administering the genome editing machinery to patients become increasingly relevant. Recent studies have demonstrated an innate and adaptive cellular immune response to Cas9 in mouse models and the presence of anti-Cas9 antibodies and T-cells in human plasma. Pre-existing immunity against therapeutic Cas9 delivery could decrease its efficacy in vivo and may pose significant safety issues. This review focuses on the immunogenicity of the Cas9 protein and summarizes potential approaches to circumvent the problem of immune recognition.

Therapeutics: Gene Therapy for Alpha-1 Antitrypsin Deficiency

This review seeks to give an overview of alpha-1 antitrypsin deficiency, including the different disease phenotypes that it encompasses. We then describe the different therapeutic endeavors that have been undertaken to address these different phenotypes. Lastly we discuss future potential therapeutics, such as genome editing, and how they may play a role in treating alpha-1 antitrypsin deficiency.

Combinatorial treatment with oncolytic adenovirus and helper-dependent adenovirus augments adenoviral cancer gene therapy

Oncolytic adenoviruses (Onc.Ads) produce significant antitumor effects but as single agents they rarely eliminate tumors. Investigators have therefore incorporated sequences into these vectors that encode immunomodulatory molecules to enhance antitumor immunity. Successful implementation of this strategy requires multiple tumor immune inhibitory mechanisms to be overcome, and insertion of the corresponding multiple functional genes reduces the titer and replication of Onc.Ads, compromising their direct ant-tumor effects. By contrast, helper-dependent (HD) Ads are devoid of viral coding sequences, allowing inclusion of multiple transgenes. HDAds, however, lack replicative capacity. Since HDAds encode the adenoviral packaging signal, we hypothesized that the coadministration of Onc.Ad with HDAd would allow to be amplified and packaged during replication of Onc.Ad in transduced cancer cells. This combination could provide immunostimulation without losing oncolytic activity. We now show that coinfection of Onc.Ad with HDAd subsequently replicates HDAd vector DNA in trans in human cancer cell lines in vitro and in vivo, amplifying the transgenes the HDAd encode. This combinatorial treatment significantly suppresses the tumor growth compared to treatment with a single agent in an immunocompetent mouse model. Hence, combinatorial treatment of Onc.Ad with HDAd should overcome the inherent limitations of each agent and provide a highly immunogenic oncolytic therapy.

Challenges and Prospects for Helper-Dependent Adenoviral Vector-Mediated Gene Therapy

Helper-dependent adenoviral (HDAd) vectors that are devoid of all viral coding sequences are promising non-integrating vectors for gene therapy because they efficiently transduce a variety of cell types in vivo, have a large cloning capacity, and drive long-term transgene expression without chronic toxicity. The main obstacle preventing clinical applications of HDAd vectors is the host innate inflammatory response against the vector capsid proteins that occurs shortly after intravascular vector administration and result in acute toxicity, the severity of which is dose dependent. Intense efforts have been focused on elucidating adenoviral vector-host interactions and the factors involved in the acute toxicity. This review focuses on the recent acquisition of data on such interactions and on strategies investigated to improve the therapeutic index of HDAd vectors.

High-capacity adenoviral vectors circumvent the limitations of ΔE1 and ΔE1/ΔE3 adenovirus vectors to induce multispecific transgene product-directed CD8 T-cell responses

BACKGROUND

The ability to induce cytotoxic T lymphocyte (CTL) responses that are multispecific is considered to comprise an essential feature for an efficacious genetic vaccine against many pathogens including HIV and hepatitis C virus. ΔE1Ad vectors are promising vectored vaccines but have been shown to induce antigen-specific CTLs with only limited multispecificity. In the present study, we investigated the applicability of gene-deleted high-capacity adenovirus (HC-Ad) vectors and focused on the induction of multispecific CTL responses.

METHODS

We generated Δ E1 and HC-Ad vectors expressing hepatitis B virus small surface antigen (HBsAg). We comparatively analyzed the CTL profiles against various transgene product- and vector-derived epitopes in several mouse strains and HBsAg- and vector-directed antibody responses.

RESULTS

HC-Ad vectors efficiently induced multispecific HBsAg-directed CTLs. By contrast, ΔE1Ad vectors mainly primed CTLs against one immunodominant epitope of HBsAg. This absence of multispecific CTL responses correlated with the induction of CTLs against viral epitopes generated by de novo expression of Ad genes from the ΔE1Ad vector. However, Ad-specific CTLs induced in trans did not impair HC-AdS-induced multispecific CTL responses against HBsAg. Finally, HC-Ad vectors also induced higher HBsAg antibody titers compared to ΔE1Ad vectors.

CONCLUSIONS

De novo expression of viral genes from ΔE1Ad vector genomes restricts the multispecificity of transgene product-specific CTLs by immunodominance effects. HC-Ad vectors devoid of Ad genes are favorable for the induction of both multispecific CD8 T-cell responses and high antibody responses. Our results suggest the deletion of Ad genes as an important means for developing potent Ad-based vectored vaccines.

Differential type I interferon-dependent transgene silencing of helper-dependent adenoviral vs. adeno-associated viral vectors in vivo

We previously dissected the components of the innate immune response to Helper-dependent adenoviral vectors (HDAds) using genetic models, and demonstrated that multiple pattern recognition receptor signaling pathways contribute to this host response to HDAds in vivo. Based on analysis of cytokine expression profiles, type I interferon (IFN) mRNA is induced in host mouse livers at 1 hour post-injection. This type I IFN signaling amplifies cytokine expression in liver independent of the nature of vector DNA sequences after 3 hours post-injection. This type I IFN signaling in response to HDAds administration contributes to transcriptional silencing of both HDAd prokaryotic and eukaryotic DNA in liver. This silencing occurs early and is mediated by epigenetic modification as shown by in vivo chromatin immunoprecipitation (ChIP) with anti-histone deacetylase (HDAC) and promyelocytic leukemia protein (PML). In contrast, self-complementary adeno-associated viral vectors (scAAVs) showed significantly lower induction of type I IFN mRNA in liver compared to HDAds at both early and late time points. These results show that the type I IFN signaling dependent transgene silencing differs between AAV and HDAd vectors after liver-directed gene transfer.

NOD2 signaling contributes to the innate immune response against helper-dependent adenovirus vectors independently of MyD88 in vivo

We previously demonstrated that Toll-like receptor/myeloid differentiation primary response gene 88 (MyD88) signaling is required for maximal innate and acquired [T helper cell type 1 (Th1)] immune responses following systemic administration of helper-dependent adenoviral vectors (HDAds). However, MyD88-deficient mice injected with HDAdLacZ exhibited only partial reduction of innate immune cytokine expression compared with wild-type mice, suggesting MyD88-independent pathways also respond to HDAds. We now show that NOD2, a nucleotide-binding and oligomerization domain (NOD)-like receptor known to detect muramyl dipeptides in bacterial peptidoglycans, also contributes to innate responses to HDAds, but not to humoral or Th1 immune responses. We established NOD2/MyD88 double-deficient mice that, when challenged with HDAds, showed a significant reduction of the innate response compared with mice deficient for either gene singly, suggesting that NOD2 signaling contributes to the innate response independently of MyD88 signaling following systemic administration of HDAds. In addition, NOD2-deficient mice exhibited significantly higher transgene expression than did wild-type mice at an early time point (before development of an acquired response), but not at a later time point (after development of an acquired response). These results indicate that the intracellular sensor NOD2 is required for innate responses to HDAds and can limit transgene expression during early phases of infection.

Helper-Dependent Adenoviral Vectors

Helper-dependent adenoviral vectors are devoid of all viral coding sequences, possess a large cloning capacity, and can efficiently transduce a wide variety of cell types from various species independent of the cell cycle to mediate long-term transgene expression without chronic toxicity. These non-integrating vectors hold tremendous potential for a variety of gene transfer and gene therapy applications. Here, we review the production technologies, applications, obstacles to clinical translation and their potential resolutions, and the future challenges and unanswered questions regarding this promising gene transfer technology.

Improved induction of immune tolerance to factor IX by hepatic AAV-8 gene transfer

Gene therapy for hemophilia B has been shown to result in long-term expression and immune tolerance to factor IX (F.IX) after in vivo transduction of hepatocytes with adeno-associated viral (AAV-2) vectors in experimental animals. An optimized protocol was effective in several strains of mice with a factor 9 gene deletion (F9(-/-)). However, immune responses against F.IX were repeatedly observed in C3H/HeJ F9(-/-) mice. We sought to establish a gene transfer protocol that results in sustained expression without a requirement for additional manipulation of the immune system. Compared with AAV-2, AAV-8 was more efficient in transgene expression and induction of tolerance to F.IX in three different strains of wild-type mice. At equal vector doses, AAV-8 induced transgene product-specific regulatory CD4(+)CD25(+)FoxP3(+) T cells at significantly higher frequency. Moreover, sustained correction of hemophilia B in C3H/HeJ F9(-/-) mice without antibody formation was documented in all animals treated with > or =4 x 10(11) vector genomes (VG)/kg and in 80% of mice treated with 8 x 10(10) VG/kg. Therefore, it is possible to develop a gene transfer protocol that reliably induces tolerance to F.IX largely independent of genetic factors. A comparison with other studies suggests that additional parameters besides plateau levels of F.IX expression contributed to the improved success rate of tolerance induction.

Hepatic gene transfer as a means of tolerance induction to transgene products

The liver is a preferred target organ for gene therapy not only for liver-specific diseases but also for disorders that require systemic delivery of a protein. Diseases that could benefit from hepatic gene transfer include hemophilia, metabolic disorders, lysosomal storage disorders, and others. For a successful delivery of the transgene and sustained expression, the protocol must avoid immune responses in order to be efficacious. A growing number of studies have demonstrated that liver-directed transfer can induce transgene product-specific immune tolerance. Tolerance obtained via this route requires optimal engineering of the vector to eliminate transgene expression in antigen presenting cells while restricting high levels of therapeutic expression to hepatocytes. Innate immune responses may prevent tolerance induction, cause toxicity, and have to be minimized. Discussed in our review is the crucial role of CD4(+)CD25(+) regulatory T cells in tolerance to the hepatocyte-derived gene product, the immunobiology of the liver and our current understanding of its tolerogenic properties, current and proposed research as to the mechanisms behind the liver's unique cellular environment, as well as development of the tools for tolerance induction such as advanced vector systems.

Induction of immune tolerance to FIX following muscular AAV gene transfer is AAV-dose/FIX-level dependent

Direct intramuscular injection (IM) of adeno-associated virus (AAV) has been proven a safe and potentially efficient procedure for gene therapy of many genetic diseases including hemophilia B. It is, however, contentious whether high antigen level induces tolerance or immunity to coagulation factor IX (FIX) following IM of AAV. We recently reported induction of FIX-specific immune tolerance by IM of AAV serotype one (AAV1) vector in mice. We hypothesize that the expression of high levels of FIX is critical to induction of FIX tolerance. In this study, we investigated the correlation among AAV dose, FIX expression, and tolerance induction. We observed that induction of immune tolerance or immunity to FIX was dependent on the dose of AAV1-human FIX (hFIX) given and the level of FIX antigen expressed in both normal and hemophilia mice. We then defined the minimum AAV1-hFIX dose and the lowest level of FIX needed for FIX tolerance. Different from hepatic AAV-hFIX gene transfer, we found that FIX tolerance induced by IM of AAV1 was not driven by regulatory T cells. These results provided further insight into the mechanism(s) of FIX tolerance, contributing to development of hemophilia gene therapy, and optimization of FIX tolerance induction protocols.

Antigen-specific humoral tolerance or immune augmentation induced by intramuscular delivery of adeno-associated viruses encoding CTLA4-Ig-antigen fusion molecules

This study initially sought to investigate the immunostimulatory properties of recombinant adeno-associated virus (rAAV) with a view to developing a genetic vaccine for malaria using muscle as a target tissue. To augment humoral immunity, the AAV-encoded antigen was genetically fused with CTLA4-Ig, a recombinant molecule that binds B7 costimulatory molecules. At 10(9) vg, CTLA4-Ig fusion promoted the humoral immune response 100-fold and was dependent on CTLA4-Ig binding with B7 costimulatory molecules, confirming plasmid DNA models using this strategy. In distinct contrast, 10(12)-10(13) vg of rAAV1 specifically induced long-lived humoral tolerance through a mechanism that is independent of CTLA4-Ig binding with B7. This finding was unexpected, as rAAV delivery to muscle, unlike liver, has shown that this tissue provides a highly immunogenic environment for induction of humoral immunity against rAAV transgene products. An additional unpredicted consequence of antigen fusion with CTLA4-Ig was the enhancement of antigen expression by approximately one log, an effect mapped to the hinge and Fc domain of IgG(1,) but not involving antigen dimerization or the neonatal Fc receptor. Collectively, these findings significantly advance the potential of rAAV both as a vaccine or immunotherapeutic platform for the induction of antigen-specific humoral immunity or tolerance and as a gene therapeutic delivery system.

Coaxing the liver into preventing autoimmune disease in the brain

The liver has several unique immunological properties that affect T cell activation and immune regulation. Recent studies have uncovered opportunities for the treatment of genetic disease by directing expression of the functional therapeutic protein to hepatocytes. In a new study in this issue of the JCI, Lüth and colleagues demonstrate that hepatic expression of a brain protein is protective against neuroinflammatory disease in a mouse model of human MS (see the related article beginning on page 3403). Suppression of autoimmunity was dependent on transgene expression in the liver and was mediated by induction of antigen-specific CD4+CD25+Foxp3+ Tregs. These findings suggest that the introduction of antigens to the liver may have potential as a preventative or therapeutic intervention for autoimmune disease.