Latent infections with periodic reactivation are a common outcome after acute infection with many viruses. The latency-associated transcript ( LAT ) gene is required for wild-type reactivation of herpes simplex virus (HSV). However, the underlying mechanisms remain unclear. In rabbit trigeminal ganglia, extensive apoptosis occurred with LAT − virus but not with LAT + viruses. In addition, a plasmid expressing LAT blocked apoptosis in cultured cells. Thus, LAT promotes neuronal survival after HSV-1 infection by reducing apoptosis.

The latency-associated transcript (LAT) gene the only herpes simplex virus type 1 (HSV-1) gene abundantly transcribed during neuronal latency, is essential for efficient in vivo reactivation. Whether LAT increases reactivation by a direct effect on the reactivation process or whether it does so by increasing the establishment of latency, thereby making more latently infected neurons available for reactivation, is unclear. In mice, LATnegative mutants appear to establish latency in fewer neurons than does wild-type HSV-1. However, this has not been confirmed in the rabbit, and the role of LAT in the establishment of latency remains controversial. To pursue this question, we inserted the gene for the enhanced green fluorescent protein (EGFP) under control of the LAT promoter in a LAT-negative virus (⌬LAT-EGFP) and in a LAT-positive virus (LAT-EGFP). Sixty days after ocular infection, trigeminal ganglia (TG) were removed from the latently infected rabbits, sectioned, and examined by fluorescence microscopy. EGFP was detected in significantly more LAT-EGFP-infected neurons than ⌬LAT-EGFP-infected neurons (4.9% versus 2%, P < 0.0001). The percentages of EGFP-positive neurons per TG ranged from 0 to 4.6 for ⌬LAT-EGFP and from 2.5 to 11.1 for LAT-EGFP (P ‫؍‬ 0.003). Thus, LAT appeared to increase neuronal latency in rabbit TG by an average of two-to threefold. These results suggest that LAT enhances the establishment of latency in rabbits and that this may be one of the mechanisms by which LAT enhances spontaneous reactivation. These results do not rule out additional LAT functions that may be involved in maintenance of latency and/or reactivation from latency.Following primary ocular infection, herpes simplex virus type 1 (HSV-1) establishes a lifelong latent infection in sensory neurons of the trigeminal ganglia (TG). At various times throughout the life of the infected individual, the virus can reactivate, return to the eye, be shed in tears, and produce recurrent corneal disease and scarring leading to impaired vision. Recurrent ocular HSV-1 results in over 400,000 doctor visits per year in the United States and is a leading infectious cause of corneal blindness (15).During latency, abundant viral transcription is consistently detected only in the region of the latency-associated transcript (LAT) gene (25,30). LAT is located in the long repeats of the HSV-1 genome and is therefore present in two copies per genome. The primary LAT transcript is 8.3 kb (33, 34). It gives rise to a family of LAT RNAs (LATs) including a very stable 2-kb LAT that appears to be an intron spliced from the primary transcript (4). LAT null mutants (i.e., LAT transcriptionnegative mutants) have been shown to reactivate poorly by explant or induced reactivation in the mouse (9, 10, 28, 29), by induced reactivation in the rabbit (8, 32), and by spontaneous reactivation in the rabbit (16, 19).The molecular mechanisms by which LAT enhances reactivation are not understood. It is also not known (i) whether LAT functions to enhance the establishmen...

Herpes simplex virus type 1 (HSV-1) is a large (152-kb) double-stranded DNA virus with neurotropic properties. HSV-1 establishes lifelong latent infections in host sensory neurons. This virus is wide spread in the general population. When the eye is infected, the virus travels up nerves and establishes latent infection in neurons of the trigeminal ganglia (TG). During neuronal latency, HSV-1 has no apparent impact on the infected individual. The latent virus can reactivate at various times throughout the life of the individual. This occurs through a mechanism(s) that is currently not completely understood. HSV-1 reactivation in the TG results in virus returning to the eye via the same route previously used to get from the eye to the TG. At the eye, reactivated HSV-1 can produce recurrent disease. Recurrent HSV-1 infection in the eye can cause corneal scarring leading to loss of vision. As a result, HSV-1 is one of the most common infectious causes of corneal blindness in the developed world.During neuronal latency, LAT (latency-associated transcript) is the only abundantly transcribed viral gene (30, 37). The primary LAT transcript is ca. 8.3 kb long (7, 45) and overlaps two viral genes, ICP0 and ICP34.5, in an antisense direction (30,37). A very stable intron, the 2-kb LAT is spliced from the primary transcript (9) and is the major LAT RNA detected during latency (7,33,36,(42)(43)(44).LAT enhances the induced and spontaneous reactivation phenotypes in the rabbit ocular model (12, 21) and the induced reactivation phenotype in mice (1a, 6, 19, 26, 31, 35). The reduced reactivation phenotypes of LAT Ϫ mutants does not necessarily imply that LAT is directly involved in the molecular mechanism of HSV-1 reactivation from latency. LAT might enhance reactivation by increasing the initial amount of latency established and/or by maintaining a high level of latently infected neurons. The larger pool of latently infected neurons and/or the larger pool of neurons containing high copy numbers of the HSV-1 genome would be expected to increase reactivation. Several reports have, in fact, shown that in experimentally infected animals more neurons become latently infected with LAT ϩ viruses compared to LAT Ϫ viruses (29,31,40). In addition, we have recently shown that LAT has anti-* Corresponding author. Mailing address:

Macrophage-specific expression of the apoA-IMilano gene is more effective than wild-type apoA-I in reducing atherosclerosis and plaque inflammation despite comparable circulating levels of the transgene and lipid profile.

Herpes simplex virus type 1 latency-associated transcript (LAT)-null mutants have decreased reactivation but normal virulence in rabbits and mice. We report here on dLAT1.5, a mutant with LAT nucleotides 76 to 1667 deleted. Following ocular infection of rabbits, dLAT1.5 reactivated at a lower rate than its wild-type parent McKrae (6.1 versus 11.8%; P ‫؍‬ 0.0025 [chi-square test]). Reactivation was restored in the marker-rescued virus dLAT1.5R (12.6%; P ‫؍‬ 0.53 versus wild type), confirming the importance of the deleted region in spontaneous reactivation. Compared with wild-type or marker-rescued virus, dLAT1.5 had similar or slightly reduced virulence in rabbits (based on survival following ocular infection). In contrast, in mice, dLAT1.5 had increased virulence (P < 0.0001). Thus, deletion of LAT nucleotides 76 to 1667 increased viral virulence in mice but not in rabbits. In contrast, we also report here that LAT2.9A, a LAT mutant that we previously reported to have increased virulence in rabbits (G. Herpes simplex virus type 1 (HSV-1) is a double-stranded neurotropic DNA virus. It is ubiquitous in the general population and establishes lifelong latent infections in host sensory neurons. Following ocular infection, the virus travels up nerves and establishes latent infection in neurons of the trigeminal ganglia (TG). Via a mechanism that remains to be fully elucidated, the latent virus can reactivate at various times throughout the life of the individual and produce recurrent disease. Recurrent HSV-1 infection in the eye can result in corneal scarring, leading to loss of vision. As a result, HSV-1 is one of the most common infectious causes of corneal blindness in the developed world.CDuring neuronal latency, the latency-associated transcript (LAT) is the only viral gene that is abundantly transcribed (4,11,27,30,32,33). The primary LAT transcript is 8.3 kb long (35, 40) and overlaps two viral genes, ICP0 and ICP34.5, in an antisense direction (27,33). A very stable intron, the 2-kb LAT, is spliced from the primary transcript (9) and is the major LAT RNA detected during latency (6,30,31,36,37,39).LAT is essential for wild-type levels of spontaneous and induced reactivation in the rabbit ocular model (12,20). This function maps to within the first 1.5 kb of the primary LAT (22). This 1.5-kb region does not overlap ICP0 or ICP34.5, encompasses only the first 837 nucleotides of the stable 2-kb LAT, and does not retain the 2-kb LAT stability (22). Thus, LAT appears to enhance spontaneous reactivation in the rabbit by a mechanism that does not involve antisense regulation of ICP0, does not require production of the 2-kb LAT, and does not require the presence of a highly stable LAT RNA. We have recently shown that LAT can promote cell survival in vitro and block apoptosis in rabbit TG during acute infection (13,19). This may increase the number of neurons that become latently infected (25,28,34), thereby increasing spontaneous reactivation by increasing the pool of latently infected neurons available for subsequent ...