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Abstract

Herpes simplex type 1 (HSV-1) is a neurotropic virus which establishes
lifelong latency in human trigeminal ganglia (TG). Currently, two
nonexclusive control mechanisms of HSV-1 latency are discussed:
antiviral CD8(+) T cells and viral microRNAs (miRNAs) encoded by the
latency associated transcript (LAT). We investigate here to what extent
these mechanisms may contribute to the maintenance of HSV-1 latency. We
show that only a small proportion of LAT(+) neurons is surrounded by T
cells in human TG. This indicates that viral latency in human TG might
be controlled by other mechanisms such as viral miRNAs. Therefore, we
assessed TG sections for the presence of HSV-1 miRNA, DNA, and mRNA by
combining LAT in situ hybridization, T-cell immunohistochemistry, and
single cell analysis of laser-microdissected sensory neurons.
Quantitative reverse transcription-PCR (RT-PCR) revealed that LAT(+)
neurons with or without surrounding T cells were always positive for
HSV-1 miRNAs and DNA. Furthermore, ICP0 mRNA could rarely be detected
only in LAT(+) neurons, as analyzed by single-cell RT-PCR. In contrast,
in LAT(-) neurons that were surrounded by T cells, neither miRNAs nor
the DNA of HSV-1, HSV-2, or varicella-zoster virus could be detected.
These data indicate that the majority of LAT(+) neurons is not directly
controlled by T cells. However, miRNA expression in every latently
infected neuron would provide an additional checkpoint before viral
replication is initiated.