Higb-frequency interferon-gamma-secreting splenocytes specific for murine cytomegalovirus immediate-early-1 (IE-1) peptide (YPHFMPTNL176)-Y-168 are insufficient to provide complete protection from viral challenge

Infection of Balb/c mice with murine cytomegalovirus (MCMV) has been used extensively as a model system with which to study host mechanisms of immunity to cytornegaloviruses. In this model, the cytotoxic T-lymphocyte (CTL) response crucial for clearing infected cells is dominated by CTLs specific for the MCMV nonapeptide (YPHFMPTNL176)-Y-168 encoded by the immediate-early 1 (IE-1) gene. The intradermal injection of plasmid pcDNA89 encoding IE-1 has been shown to offer some protection from viral challenge. In the present studies, the protective efficacy of immunisation with pcDNA89 given by intradermal injection was compared with particle-mediated DNA delivery (PMDD) and contrasted with that induced by injection with the K181 MCMV strain and with temperature-sensitive mutants (tsm) derived from the K181 strain. Modest protection was afforded by pcDNA89 immunisation given by PMDD, but none was observed after intradermal injection. PMDD immunisation induced a frequency of (YPHFMPTNL176)-Y-168-specific interferon-gamma (IFN-gamma)secreting splenocytes, which was equivalent to that after K181 infection and significantly higher than tsm immunisation. Whereas tsm-immunised mice were completely protected from MCMV challenge, PMDD-immunised mice were only weakly protected. Tsm immunisation protected mice completely against challenge with natural isolates having sequence variation in the IE-1 nonapeptide, while PMDD-immunised mice were weakly protected from isolates encoding (YPHFMPTNL176)-Y-168 and were not protected against isolates encoding (YPHFMPPSL176)-Y-168 or (YLDFMPPNL176)-Y-168. Thus, while IE-1-specific IFN-gamma-secreting splenocytes do contribute to immunity from MCMV challenge, their presence in isolation is insufficient to provide complete protection and they may not be involved in the protection observed against MCMV isolates having IE-1 sequence variation.