Lymphocyte proliferation and skin
hypersensitivity responses to parasite
antigens were measured as indicators of cell-mediated immunity in genetically resistant and random-bred lambs following
infection with Haemonchus contortus. Responses of PBMC to the
mitogens PHA, PWM and LPS were also assessed to determine if
infection was associated with suppression of general immune function. All lambs demonstrated significant
antigen-specific lymphocyte proliferation prior to experimental
infection. The responses did not differ between resistant and random-bred lambs and the correlation between pre-
infection stimulation indices and resistance to challenge
infection was also not significant. Following
infection, PBMC from resistant lambs consistently exhibited higher blastogenic responses to both larval and adult
antigens than PBMC from random-bred lambs. Addition of
cyclosporin A to the
antigen-stimulated cultures resulted in significant inhibition of blastogenesis, suggesting that the cells responding to parasite
antigens were predominantly of the T-helper cell phenotype. Proliferation responses of resistant and random-bred sheep were similar for all
mitogens tested. However, a transient suppression of lymphocyte reactivity to PHA and PWM was noted in all infected lambs. Responses to LPS remained unaffected during
infection. There was no difference in the
immediate hypersensitivity responses to intradermal inoculation of parasite
antigens between the genotypes. However, the delayed-type
hypersensitivity responses to parasite
antigens were significantly greater in resistant lambs than those in random-bred lambs. Together, these data suggest that resistant lambs have an enhanced ability to mount parasite-specific cell-mediated immune responses when compared with random-bred lambs and that this may contribute to differences in the susceptibility of two genotypes to haemonchosis.