Herpes simplex virus: neuroinvasiveness, neurovirulence and latency

A herpes simplex virus DNA polymerase mutation that specifically attenuates neurovirulence in mice

Herpes simplex virus can infect the mammalian brain causing lethal encephalitis (neurovirulence). Previously, herpes simplex virus mutants that are attenuated for neurovirulence have exhibited defects in replication in brain and/or blocks to replication in neuronal cells. We investigated the attenuation of neurovirulence of mutant PAAr5, which exhibits resistance to antiviral drugs due to altered viral DNA polymerase. Following intracerebral inoculation of 7-week-old CD1 mice, PAAr5 was 30-fold attenuated for neurovirulence compared to its wild-type parent. A drug-sensitive virus derived by marker rescue with DNA polymerase gene sequences exhibited neurovirulence that was essentially indistinguishable from that of wild-type virus, demonstrating that attenuation was due to a polymerase mutation. PAAr5 replicated in brain similarly to wild-type virus unlike another polymerase mutant, 615.8, that exhibited a similar degree of attenuation. The attenuation of PAAr5 was not associated with altered particle to PFU ratios nor with any obvious reductions in viral antigen expression in neurons, spread, histopathology, or TUNEL staining suggestive of apoptotic cells. Thus PAAr5 differs from other mutants that are attenuated for neurovirulence. Understanding how a polymerase mutation specifically attenuates neurovirulence may shed light on how herpes simplex virus can cause lethal encephalitis.

An intertypic herpes simplex virus helicase-primase complex associated with a defect in neurovirulence has reduced primase activity

R13-1 is an intertypic recombinant virus in which the left-hand 18% of the herpes simplex virus type 1 (HSV-1) genome is replaced by homologous sequences from HSV-2. R13-1 is nonneurovirulent and defective in DNA replication in neurons. The defect was localized to the UL5 open reading frame by using marker rescue analysis (D. C. Bloom and J. G. Stevens, J. Virol. 68:3761-3772, 1994). To provide conclusive evidence that UL5 is the only HSV-2 gene involved in the restricted replication phenotype of R13-1, we have characterized the phenotype of a recombinant virus (IB1) in which only the UL5 gene of HSV-1 was replaced by HSV-2 UL5. Data from 50% lethal dose determinations and the in vivo yields of virus suggested that IB1 has the same phenotypic characteristics as R13-1. UL5 is the helicase component of a complex with helicase and primase activities. All three subunits of this complex (UL5, UL8, and UL52) are required for viral DNA replication in all cell types. The intertypic complex HSV-2 UL5-HSV-1 UL8-HSV-1 UL52 was purified and biochemically characterized. The primase activity of the intertypic complex was 10-fold lower than that of HSV-1 UL5-HSV-1 UL8-HSV-1 UL52. The ATPase activity was comparable to that of the HSV-1 enzyme complex, and although the helicase activity was threefold lower, this did not interfere with the synthesis of leading strands by the HSV polymerase. One explanation for these findings is that the interactions between the subunits of the helicase-primase intertypic complex that are important for the full function of each subunit are inappropriate or weak.

A deletion at the UL/IR junction reduces pseudorabies virus neurovirulence

A recombinant pseudorabies virus (PRV), designated LLT beta delta 2, which contains a 3-kbp deletion spanning the junction of the unique long and internal repeat sequences was constructed. Compared with the parental strain and a virus rescued for the deleted sequences, LLT beta delta 2 exhibited similar replication characteristics in tissue culture. When inoculated intranasally in swine, LLT beta delta 2 was significantly reduced in virulence and did not produce neurological signs characteristic of PRV infection. LLT beta delta 2 replicated efficiently at the site of inoculation and in peripheral nervous tissues, but replication was restricted in the central nervous system. These results indicate the presence of a PRV neurovirulence determinant in the vicinity of the junction.

Glycoprotein B is a specific determinant of herpes simplex virus type 1 neuroinvasiveness

Herpes simplex virus type 1 strains ANG and KOS lack neuroinvasiveness when inoculated on the footpads of mice, and because the strains are able to complement each other, the genes associated with this phenotype differ. In this study, we used marker rescue techniques to show that at least two genes cloned from ANG are required to restore neuroinvasiveness to KOS. One of the two fragments required is the 6.3-kb BamHI-A/EcoRI-D fragment (0.15 to 0.19 map units). The second has been identified as the sequence encoding glycoprotein B (gB) (UL27). Analysis of ANG and KOS DNA sequences in the relevant region of the gB gene revealed two nucleotide differences which result in amino acid differences in the gB protein. One appears to be unique to the strain of KOS used in our laboratory. The second, at codon 523 of the mature gB protein, encodes a valine in KOS and an alanine in ANG. Recombinant KOS viruses which contained ANG sequences in this region were constructed, and two independently selected recombinants demonstrated increased neuroinvasiveness in mice. From these results, we conclude that gB significantly influences neuroinvasiveness. Mechanisms by which this might occur are discussed.

Viral pathogenesis and central nervous system infection

Both host defense and viral genetic factors influence the development of viral infection and disease. Due to the presence of the blood-brain barrier, infection of the central nervous system creates additional complexities in interactions between a virus and its host. Stages in viral pathogenesis defined as (1) virus entry, (2) spread, (3) tropism, (4) virulence and injury to the host, and (5) the outcome of infection are discussed for viral infections in general and those aspects unique to infections of the central nervous system. Information about neuronal physiology and function has also been revealed through studying virus infection. An increased understanding of viral pathogenetic mechanisms and host response to infection raises interesting possibilities for vaccine development and for basic studies in neurology and neurobiology.

The immune system response to viral infection of the CNS

In immunological terms the CNS is at a severe disadvantage in its ability to respond to infection by virus. First, both glial and neuronal cells normally do not express molecules of the major histocompatibility complex. Second, the most efficient cells for stimulating an immune response (leucocyte dendritic cells) are not present in the healthy CNS; and third, there is no specialized lymphatic drainage from the CNS to lymph nodes to enable the immune system to be quickly informed of the presence of an infection. Nevertheless, the immune system apparently copes with the vast majority of viral infections in the CNS. This is clearly evidenced by the reactivation of latent CNS viral infections in some immunosuppressed patients and the dramatic increase in the seventy of CNS disease in young or otherwise immunologically incompetent experimental animals infected with neurotropic viruses. The routes by which the CNS and the immune system may communicate and the varied ways in which an immune response may affect the outcome of a viral infection of the CNS are discussed.