We have shown previously that BALB/c mice can be protected against a fatal infection with Leishmania major by adoptive transfer of a T cell line recognizing a protective soluble fraction (fraction 9) of promastigotes. We now describe the isolation and characterization of a T cell clone (9.1-2) that also transfers protective immunity against Leishmania. After Ag or mitogen stimulation, this clone secrets IL-2 and IFN-gamma, but not IL-4 or IL-5. The clone preferentially recognizes L. major fraction 9, and in addition, soluble Ag from Leishmania donovani, Leishmania amazonensis, and Leishmania braziliensis, but not from the related flagellates, Trypanosoma cruzi or Crithidia fasciculata. Besides being contained in fraction 9, the stimulatory Ag is also released from the parasite, because concentrated promastigote culture supernatants induced IFN-gamma production by 9.1-2. By means of T cell immunoblotting, we determined that 9.1-2 recognized a protein with a relative m.w. between 8,000 and 12,000, and within this region is a predominant protein contained in fraction 9 of approximately 10,000 m.w. These data identify a new candidate Ag for immunization against protozoa belonging to the genus Leishmania.

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