Cell-mediated and humoral immunity induced by a live Francisella tularensis vaccine

Live attenuated Francisella tularensis vaccine induced long-lasting humoral and cell-mediated immune responses in all 13 subjects studied. Lymphocyte blast transformation reactivity to F. tularensis appeared 2 weeks after vaccination in most subjects and remained unchanged for up to 1.5 years. Similarly, in most recipients, antibodies against F. tularensis were detectable by both the enzyme-linked immunosorbent assay (ELISA) and the agglutination method from 2 weeks after vaccination, although diagnostically significant agglutination titers (greater than or equal to 80) were not detectable until 4 weeks after vaccination. Maximal agglutination titers of 80 to 2,560 appeared at 4 to 8 weeks, and in spite of decreasing tendency, titers as high as 320 were still present 1.5 years after vaccination. ELISA showed the simultaneous, but not parallel, appearance of different immunoglobulin classes, immunoglobulin M (IgM) reaching individual maximal values 1.8 months after vaccination on average, at the same time as agglutinating antibodies, 1 week earlier than IgA, and about 1 month earlier than IgG. All of these immunoglobulin classes persisted in significant amounts up to 1.5 years, with IgG generally dominant. Long-lasting IgA and IgM responses after vaccination, as also after infection, suggested that the serodiagnosis of tularemia generally requires two consecutive serum samples with a significant increase in the titer.