Serological responses in cats vaccinated with FeLV ISCOM and an inactivated FeLV vaccine

Various approaches have been considered for generation of effective and safe vaccines against retroviruses, including HIV, with limited success. In the present vaccination study, encompassing 137 household cats, we have composed an experimental ISCOM subunit vaccine containing gp70 of feline leukaemia virus (FeLV)--the external glycosylated envelope protein, and the transmembrane protein p15E, with a commercial available inactivated FeLV vaccine (Leukocell). The two vaccines were estimated to contain approximately the same amount of gp70 antigen and the cats were immunized three times according to the recommendations of the commercial vaccine. A control preparation not containing gp70 or p15E was also included. During the observation period of 200 days all cats remained healthy and no virus was isolated during the isolation attempts. The serological responses were measured in ELISA, membrane immunofluorescence (MIF) and virus neutralization (VN) tests. In contrast to the cats in the other groups almost all ISCOM-vaccinated cats responded by seroconversion or increased titres in the three tests. The development of specific antibodies to gp70 and p15E were confirmed in Western blot. These results clearly illustrate the potential of the ISCOM structure for the development of safe and effective vaccines against retroviruses.