Gene Therapy (2003) 10, 941–945 & 2003 Nature Publishing Group All rights reserved 0969-7128/03 $25.00 www.nature.com/gt REVIEW Immune responses to replication-defective HSV-1 type vectors within the CNS: implications for gene therapy WJ Bowers1,4, JA Olschowka2 and HJ Federoff1,3,4 1Department of Neurology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA; 2Department of Neurobiology and Anatomy, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA; 3Department of Microbiology and Immunology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA; and 4the Center for Aging and Developmental Biology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA Herpes simplex virus (HSV) is a naturally occurring double- detailed HSV vector-engendered immune responses and stranded DNA virus that has been adapted into an efficient subsequent resolution events primarily within the confines of vector for in vivo gene transfer. HSV-based vectors exhibit the central nervous system. Herein, we describe the wide tropism, large transgene size capacity, and moderately immunobiology of HSV and its derived vector platforms, thus prolonged transgene expression profiles. Clinical implemen- providing an initiation point from where to propose requisite tation of HSV vector-based gene therapy for prevention and/ experimental investigation and potential approaches to or amelioration of human diseases eventually will be prevent and/or counter adverse antivector immune re- realized, but inherently this goal presents a series of sponses. significant challenges, one of which relates to issues of Gene Therapy (2003) 10, 941–945. doi:10.1038/sj.gt.3302047 immune system involvement. Few experimental reports have Introduction ment of HSV-based gene transfer vectors for diseases of the central nervous system (CNS).5,6 Viewed, albeit Herpes simplex virus type 1 (HSV-1) is a naturally mistakenly, as a relatively immunopriviledged site, the neurotropic DNA virus proficient in establishing latent brain appeared to be a highly compatible target for HSV infection within neurons, but moreover possesses the vector-mediated gene transfer. However, as the numbers ability to infect a wide range of cell/tissue types. The of CNS studies involving viral vectors (including HSV- cellular receptors (or appropriate species-specific homo- based types) augmented, the effects of CNS immune logues) responsible for virion docking and uptake have responses on the safety and efficiency of gene transfer been cloned, including the herpesvirus entry mediator A became increasingly apparent. These effects range from (HveA; formerly HVEM) and nectin-1 (formerly HveC), immune cell-mediated removal of transduced target cells and, not surprisingly, have been shown to be nearly and inflammation to detrimental influences on transgene ubiquitously expressed.1–4 Intracellular transport of the expression duration. It has therefore become imperative viral genome to the nucleus leads to a highly coordinated to detail the profiles of HSV vector-elicited CNS immune cascade of viral gene expression. Based upon cellular/ responses. In the following discourse, we will outline the molecular signals that gauge the status of the host cell manner in which wild-type HSV stimulates the host environment, HSV infection can progress via two distinct immunological responses and contrast these mechanisms paths: one that involves active viral gene expression to with those that are most applicable for the two major produce new virions (lytic phase) or another that types of HSV-derived vectors: the recombinant and the involves an abolition of a majority of viral gene amplicon. Understanding how the host’s immune system expression (latent phase). Its double-stranded 150-kb responds to each vector platform will likely provide DNA genome encodes for approximately 80 polypep- insights into devising HSV-based vectors that exhibit tides that play a role in establishing the two phases. safer and more effective in vivo profiles. Mutational analyses of the HSV-1 genome have determined that a number of the viral open reading frames (ORFs) are not essential to viral propagation. This finding combined with the inherent ability of this virus Wild-type HSV and the immune system to gain entry into neurons initially led to the develop- Up to 90% of human adults possess circulating anti- bodies against herpes simplex virus.7 These are gener- Correspondence: Dr HJ Federoff, Department of Neurology, Box 645, ated against the viral envelope-bound glycoproteins, University of Rochester School of Medicine and Dentistry, 601 Elmwood predominantly glycoproteins B, C, and D (gB, gC, and Ave., Rochester, NY 14642, USA gD).8 These antibodies bind to viral particles, and if in Immune responses to HSV vectors within CNS WJ Bowers et al 942 sufficiently high circulating levels, have the potential to of transporter associated with antigen presentation (TAP) inhibit viral adhesion and entry and participate in through a high-affinity interaction that ultimately results inducing antibody-dependent, cell-mediated cytotoxi- in diminished translocation of processed peptide into the city.9 Despite existing humoral immunity, however, ER.21 Working in concert, the ICP47 and vhs proteins are wild-type HSV is proficient in evading the immune responsible for why anti-HSV MHC Class I CTL activity system.10 In fact, the natural HSV life cycle typically is rarely substantial in infected hosts. It is important to involves long periods of latency, indicating that the virus note that anti-HSV MHC Class II T cells have been necessitates a series of elaborate and highly efficient reported in a majority of infected individuals, but their mechanisms to avoid detection and elimination by role in controlling HSV-1 infections is not fully eluci- immune cells.11 The unique ability of HSV to successfully dated.22 avoid immune surveillance is also evident from numer- Latency plays an indirect role in immune evasion. ous, unsuccessful investigations into the development of During this phase of the life cycle, low-level transcription anti-HSV vaccines.12 of a restricted number of viral sequences called the ‘latency-associated transcripts’ (LATs) occurs.23,24 The function of these transcripts has been a point of Immune system evasion mechanisms utilized contention within the HSV field, arguments that range by wild-type HSV from the LATs acting as antisense RNAs to prevent exit from latency to these transcripts encoding proteins One mechanism by which HSV avoids immune detection important for viral reactivation.25 Regardless of how is via inhibition of the complement cascades and in the LATs function, the state of latency is a time of essence, antibody-dependent cell lysis. Although HSV-1 minimal viral gene expression, thus diminishing the glycoproteins E (gE) and I (gI) are considered nones- likelihood that viral peptides are presented via MHC I sential for viral propagation, these constituents of the molecules to circulating immune cells. viral envelope as part of a complex appear to serve an important role in immune evasion. The gE–gI complex 13 has been shown to bind the Fc domain of IgG. It has HSV-1-derived vector platforms been hypothesized that anti-HSV antibodies via their Fab domains recognize gB, gC, and/or gD but that simulta- These highly evolved and intricate mechanisms of neous Fc binding by the gE–gI complex may result in a immune system evasion appear to be an indication that masking effect, termed ‘antibody bipolar bridging.’14 host immune responses would not impose a major This would lead to an inability of the bound IgG to burden on the in vivo employment of HSV-based gene activate the classical complement cascade and would transfer vectors. However, depending upon the iteration, result in failure to induce phagocyte function.15,16 HSV-based vectors have not only lost many of these Glycoprotein C of HSV-1 acts via a different mechanism virally encoded immunomodulatory activities, but can in that it possesses the capacity to bind the C3 factor produce cellular toxicity at the site of administration that resulting in ‘short-circuiting’ of the complement path- lead to significant inflammatory responses. In addition, way.17 delivery of viral vectors to the CNS is typically In addition, wild-type HSV evades the host immune performed by stereotactic surgery resulting in a tempor- system by interfering with MHC class I cytotoxic T- ary breach of the blood–brain barrier (BBB) and a lymphocyte (CTL) recognition of the infected cell. One potential influx of immune effectors from the periphery. manner by which HSV achieves this is by extinguishing Two disparate HSV-1-based delivery platforms capable global protein synthesis. The tegument of the HSV virion of gene transfer have been developed: recombinant and harbors several viral proteins that act to initiate viral amplicon vectors.26–32 Each vector demonstrates efficient gene expression and/or to establish a favorable environ- gene delivery to a variety of tissues and cell types, ment for viral propagation within infected cells. One of including cells resident in the CNS. However, because of these proteins, designated virion host shutoff (vhs), acts the inherent qualities specific to each of the platforms, as an mRNase to disrupt polysomes and degrade host recombinant and amplicon vectors are engaged differ- cell mRNA. During the process of global protein ently by the host immune system. The following will synthesis