Feline Leukemia Virus Envelope Glycoprotein Vaccine: Preparation and Evaluation of Immunizing Potency in Guinea Pig and Cat<xref ref-type="fn" rid="FN2">2</xref>

Envelope glycoprotein of feline leukemia virus (FeLV), strain FeLV-1, was extracted with 2% Triton X-100 and 0.43 M NaCI In 10 mM Tris (pH 7.5) and purified by ion-exchange chromatography on phosphocellulose. Purified FeLV envelope gp75 had a molecular weight of 75,000 daltons and induced antibodies of high titers in the goat. These antibodies specifically neutralized FeLV, precipitated radiolabeled FeLV envelope gp75, and lysed FL74 cells. The immunogenicity of purified FeLV envelope gp75 with and without adjuvant was tested and compared in guinea pigs and cats. Administration of 20 μg of FeLV envelope gp75 to guinea pigs in either alum or adjuvant 65 formulation induced neutralizing antibody [geometric mean titer (GMT), 8-25], radioimmune precipitating antibody (GMT, 1,382-6,450), and cytotoxic antibody (titer, ≥2) in all animals. Aqueous FeLV envelope gp75 vaccine induced neutralizing antibody (GMT, <4) in only 50% of the guinea pigs. In contrast to results with guinea pigs, immunization of speclflc-pathogen-free cats, 10-15 weeks old, with twice the guinea pig dose level of FeLV envelope gp75 (40 μg) induced significantly less immune response to FeLV when its envelope gp75 was administered in alum or Freund's adjuvant and little or no response when FeLV envelope gp75 was given as aqueous or adjuvant 65 formulation. Cats immunized with 40 μg FeLV envelope gp75 in alum or Freund's adjuvant developed radiolmmune antibody with GMT's of 127-207 [100% seroconverslon (SC)], but little or no neutralizing antibody (22-38% SC; GMT, <2) or cytotoxic antibody (0-44% SC) developed.