Comparative Virulence and Immunogenicity of the Towne Strain and a Nonattenuated Strain of Cytomegalovirus

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Abstract
The Towne strain cytomegalovirus and a low-passage strain, Toledo-1, were compared for virulence and immunogenicity in healthy adult male subjects to determine the suitability of the Towne strain for vaccination. Five seropositive subjects who received the Toledo-1 strain developed infections ranging in severity from laboratory abnormalities to mild mononucleosis syndromes (mean incubation, 4.7 wk). None of the 4 seronegative subjects receiving the Towne strain developed systemic infection, but all developed delayed local reactions at the injection sites. All subjects developed cytotoxic lymphocyte responses specific to cytomegalovirus, usually HLA-restricted, but these were of greater magnitude and duration in the Toledo-1 recipients. The latter also developed natural killer cell and interferon responses, atypical lymphocytosis, inversion of helper/suppressor cell ratios, and depressed responses to T-cell mitogens, none of which occurred in Towne strain recipients. The results further substantiate the avirulence and immunogenicity of the Towne strain cytomegalovirus.