Herpes simplex virus thymidine kinase expression in trigeminal ganglion infection: Correlation of enzyme activity with ganglion virus titer and evidence of in vivo complementation

Experimental trigeminal ganglion and corneal infection in mice was studied with three thymidine kinase-positive (TK⁺) strains of herpes simplex virus type 1 (HSV-1) and eight TK^- HSV-1 mutants. Viruses were extensively tested in cell culture to determine whether any were temperature sensitive (ts) for virus replication or for TK activity. TK^- mutants were no more ts than were TK⁺ viruses. By arabinosylthymine testing and measurement of thymidine phosphorylation, it was apparent that some TK^- mutants were, in fact, intermediate for TK activity (TK^±). After corneal inoculation of individual viruses it was observed with one exception that TK^-, TK^±, and TK⁺ viruses replicated in ocular tissues (3 days postinoculation, 2.2 × 10³-1.1 × 10⁵ PFU/eye). However, TK^- mutants were rarely isolated from trigeminal ganglia (<2.5-<5.6 × 10^-1 PFU/mg), whereas TK⁺ and to a lesser degree TK^± viruses were frequently (1.2 × 10³-1.2 × 10⁴ PFU/mg and 3.2 × 10⁰-2.1 × 10³ PFU/mg). In vivo complementation studies were performed by corneal inoculation of TK^--TK^- and TK^--TK^- virus mixtures. TK⁺ HSV complemented TK^- virus since significant amounts of TK^- HSV were isolated from trigeminal ganglia (complementation indices were >60->1600). In addition, after inoculation of certain TK^--TK^- pairs, complementation in ganglia was observed (complementation indices were >2.4->120). These studies support the hypothesis that HSV-1 TK expression is necessary for sensory ganglion (neuron) infection in three ways: HSV-1 TK mutants that replicated in ocular tissues and were not ts mutants did not replicate in vivo in trigeminal ganglia; (ii) there was a correlation between level of viral TK activity and trigeminal ganglion virus titer; and (iii) when complemented by TK⁺ or TK^- HSV, TK^- virus replicated in trigeminal ganglia.