Immune responsiveness of nematode-resistant or susceptible Romney line-bred sheep to continuous infection with Trichostrongylus axei

Two divergent lines of Romney sheep have been selected on the basis of differences in faecal egg counts (FEC) to natural poly-generic parasite challenge in New Zealand. However, it is not known if the expression of resistance or susceptibility extends to parasitic nematodes that were not a major part of the selection challenge. To examine this, the immune response to infection with Trichostrongylus axei was examined in these sheep lines. Changes in the proportions of CD5+, CD4+, CD8+, T19+ and CD45R+ lymphocytes and parasite specific antibody titres in peripheral blood were monitored each week in six resistant and six susceptible line lambs that were maintained indoors in pens during the course of 14 weekly infections with 10 000 T. axei larvae. No difference in FEC was observed. Similarly, no significant difference in T. axei specific antibody titre between sheep lines was seen although antibody titre increased steadily from Week 4. Significant increases in the proportions of CD5+, CD4+, CD8+ and T19+ cells occurred in both resistant and susceptible line lambs during Weeks 1–4 of infection. Following peak levels, proportions of CD5+, CD4+ and CD8+ cells fell with the rate of decline of CD5+ and CD4+ cells significantly greater in the resistant line lambs. Proportions of CD45R+ cells showed significant changes with time that were the inverse of those of CD5+, CD4+ and CD8+ cells. Susceptible line lambs showed higher proportions of CD5+ and lower proportions of CD45R+ cells than resistant lambs before infection with T. axei. Overall, during infection, these differences were maintained while CD4+ and T19+ levels were higher in the susceptible line lambs.