PASSIVE PROTECTION OF MICE AGAINST LETHAL FRANCISELLA-TULARENSIS (LIVE TULAREMIA VACCINE STRAIN) INFECTION BY THE SERA OF HUMAN RECIPIENTS OF THE LIVE TULAREMIA VACCINE

The relative role that humoral immunity plays in protection against in fection with the intracellular bacterium, Francisella tularensis, rema ins controversial. Cellular immunity is thought to play the major and perhaps only role. The authors, in this article, investigate the immun ologic and protective properties of immune serum collected from human recipients of the live tularemia vaccine (LVS). Sera of recipients of the vaccine demonstrated reactivity with the vaccine strain by enzyme- linked immunosorbent assay and Western blot analysis. This reactivity appeared to be directed primarily against the lipopolysaccharide of LV S and demonstrated complete cross-reactivity with fully virulent F. tu larensis (Schu4). Pooled immune sera protected mice fully against a 10 ,000 LD50 challenge with the LVS strain relative to nonimmune sera. Th e protection was abrogated by dilution or preadsorption with the LVS s train but not by preadsorption with Escherichia coli, which suggests s pecificity of protection. The authors conclude that antibodies to the LVS strain of F. tularensis are generated by live vaccination in human s and play a significant role in protection of mice against lethal cha llenge with the same organism. These antibodies crossreact completely with fully virulent F. tularensis, but whether they play a role in pro tection against fully virulent human tularemia strains requires furthe r experimentation.