Experimental maedi infection in sheep. 2. Antibody response.

All 4 sheep inoculated via the respiratory tract with 7×10 TCID50 af maedi M88 strain developed complement fixing (CF) antibodies within 3 months after inoculation, and a gradual rise in CF titers was found during the first year. The antibody titers have been maintained, though with some fluctation, through the following year, and the titers vary from 64 to 256. Virus neutralizing activity against maedi M88 strain was detected in the sera of all intrapulmonarily inoculated sheep within 8 months after inoculation. Titers have been maintained or have slightly increased. The level of titers, ranging from 8 to 256, was clearly different between individual sheep. One of the 4 sheep inoculated intracerebrally with 5×10 TCID50 of maedi M88 strain developed CF antibodies 1 month after inoculation, but no neutralizing antibodies until death 11 months after inoculation. The rest of the intracerebrally inoculated sheep displayed no evidence of CF or neutralizing antibodies within 18 months after inoculation in spite of numerous virus isolations from peripheral blood leukocytes. The absence of antibodies might perhaps be attributed to phenomena such as differences in tropism, provirus state, immunological tolerance and size of inoculum. One sheep hyperimmunized with repeated s.c. and i.v. injections of maedi M88 strain developed high CF antibody titers but lower neutralizing antibody titers. The 2 uninoculated control sheep developed no CF or neutralizing antibodies within 18 months after inoculation.