The protective immunity induced by 3 experimental FeLV
vaccines were evaluated: Prototype inactivated FeLV
vaccine developed from a molecularly cloned FeLV isolate (FeLV-
FAIDS-61E-A); a mixture of immunodominant synthetic
peptides corresponding to regions of the FeLV-Gardner-Arnstein-B (FeLV-GA-B) envelope
proteins; and an adjuvant-disrupted but non-activated virus prepared from a non-cloned FeLV field isolate comprised of subgroup A and B viruses (FeLV-05821-AB). Included as controls were parallel groups of cats inoculated with adjuvants alone or with an established commercial FeLV
vaccine. After each inoculation and after virulent virus challenge exposure, sera from all cats were assayed for ELISA-reactive antibody against purified FeLV, FeLV neutralizing (VN) antibody, and FeLV antigenemia/
viremia--viral
p27 antigen in serum and within circulating leukocytes. Immunity was challenged by oral/nasal exposure of vaccinated and control cats with FeLV-FAIDS-61E-A or FeLV-05821-AB, an infective, noncloned, tissue-origin, FeLV field isolate containing subgroup-A and -B viruses.
Vaccine-induced immunity was assessed by comparing the postchallenge-exposure incidence of persistent
viremia and the pre- and postchallenge exposure titers of VN and ELISA antibody in cats of the control and
vaccine groups. The percentage of cats, that resisted development of persistent
viremia after FeLV challenge exposure and the preventable fraction (PF) for the
vaccine groups (which adjusts for the severity of the challenge and the degree of innate resistance in the controls) were as follows: adjuvant controls, 26%; FeLV-FAIDS-61E-A inactivated virus
vaccine, 95% (PF = 93.2%); FeLV-GA-B
peptide vaccine, 5% (-28.4%); FeLV-05821-AB noninactivated
vaccine, 67% (55.4%); and commercial FeLV
vaccine, 35% (12.2%). The prechallenge exposure mean VN antibody titer for each group was: less than 1:8 in the adjuvant controls; 1:43 in the FeLV-FAIDS-61E-A-vaccinated cats; less than 1:8 in the
peptide-vaccinated cats; 1:38 in the noninactivated virus-vaccinated cats group; and 1:12 in the cats vaccinated with the commercial
vaccine. Thus, induction of VN antibody in the vaccinated cats, although modest, appeared to be correlated with induction of protective immunity as defined by resistance to FeLV challenge exposure. Results of these studies indicate that inoculation of cats with an experimental inactivated virus
vaccine prepared from a molecularly cloned FeLV isolate was most effective in stimulating protective immunity against heterologous and homologous FeLV challenge exposure.