CELL-MEDIATED-IMMUNITY IN MERINO LAMBS WITH GENETIC-RESISTANCE TO HAEMONCHUS-CONTORTUS

Lymphocyte proliferation and skin hypersensitivity responses to parasi te antigens were measured as indicators of cell-mediated immunity in g enetically resistant and random-bred lambs following infection with Ha emonchus contortus. Responses of PBMC to the mitogens PHA, PWM and LPS were also assessed to determine if infection was associated with supp ression of general immune function. All lambs demonstrated significant antigen-specific lymphocyte proliferation prior to experimental infec tion. The responses did not differ between resistant and randombred la mbs 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 bla stogenic responses to both larval and adult antigens than PBMC from ra ndom-bred lambs. Addition of cyclosporin A to the antigen-stimulated c ultures resulted in significant inhibition of blastogenesis, suggestin g 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 tr ansient suppression of lymphocyte reactivity to PHA and PWM was noted in ail infected lambs. Responses to LPS remained unaffected during inf ection. There was no difference in the immediate hypersensitivity resp onses to intradermal inoculation of parasite antigens between the geno types. However, the delayed-type hypersensitivity responses to parasit e 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 immu ne responses when compared with randombred lambs and that this may con tribute to differences in the susceptibility of two genotypes to haemo nchosis.