Immune responses in Indonesian thin tail and Merino sheep during a primary infection with Fasciola gigantica: Lack of a specific IgG₂ antibody response is associated with increased resistance to infection in Indonesian sheep

After a primary infection with Fasciola gigantica, the immune responses in a resistant (Indonesian thin tail) and a susceptible (Merino) breed of sheep were analysed. The number of adult flukes recovered from the livers of the Indonesian thin tail sheep were significantly lower than those found in the Merino animals. On days 8, 14 and 25 p.i., Indonesian thin tail sheep exhibited a significantly higher eosinophilia than Merino sheep, whereas neutrophilia was significantly elevated in the Indonesian thin tail sheep on days 36 and 48 p.i. Serum from both sheep breeds demonstrated IgM, IgG₁ and IgE responses to F. gigantica. In contrast, the Indonesian thin tail sheep produced significantly lower levels of IgG₂ antibodies relative to the high level detected in Merino sheep. The IgE response was biphasic in both sheep breeds with the first response detected by day 14 and the second response developing from days 30 to 60 p.i. Western blotting showed that a similar profile of adult fluke antigens was recognised by IgG₁ and IgE antibodies in both the Indonesian thin tail and Merino sheep. The IgE response was directed to a major antigen at about 92 kDa. We postulate that IgG₂ could act as a blocking antibody for protective effector responses against F. gigantica in sheep and that the Indonesian thin tail sheep, by downregulating IgG₂ responses, have an enhanced capacity for killing F. gigantica in vivo. Copyright (C) 1999 Australian Society for Parasitology Inc.