Several membrane polypeptides of the live vaccine strain Francisella tularensis LVS stimulate T cells from naturally infected individuals.

The currently used live vaccine strain Francisella tularensis LVS was derived several decades ago from a wild strain of the species. In the present report, several membrane polypeptides of LVS are shown to be recognized by T cells from individuals immunized by natural infection with F. tularensis. Bacterial membranes of a capsule-deficient mutant of LVS were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Thereafter, gels were divided into seven fractions, each fraction containing a different number of peptide bands. From other gels, four bands were excised, each containing one major polypeptide. Eluates of each fraction and of each polypeptide band induced a proliferative response and an interleukin-2 response in lymphocytes from most of the individuals. When the lymphocytes were separated after induction, most of the proliferative response was found to occur in CD4+ T cells. Lymphocytes from nonimmune individuals responded poorly to all membrane polypeptides. To study the possible heterogeneity of antigen determinants among the polypeptides, T-cell clones were raised towards F. tularensis and tested for proliferative response to the four major membrane polypeptides. Five clones, all CD4+ CD8-, responded to one or more of the polypeptides, each clone with a unique pattern of response. In conclusion, F. tularensis possesses a high number of T-cell-reactive membrane polypeptides. There seems to be a heterogeneity of T-cell determinants among these polypeptides. Determinants involved in immunization by natural infection are well conserved in LVS.