The relationship between Tci -L4 - ES mediated lymphocyte suppression and Tci - L4 - ES antigen-specific lymphocyte proliferation. Five helminth-naïve lambs were trickle infected with 2000 T. circumcincta L3 three times a week for six weeks. PBMC were harvested at 0 and 6 weeks from the start of infection and cultured for 72 h with PBS alone, 5 μg/mL Con A, 5 μg/mL Con A + 30 μg/mL Tci-L4-ES (Con A + ES) or 30 μg/mL heat-inactivated Tci-L4-ES (HiES). Proliferation was assessed at 72 h by measurement of [3H] thymidine incorporation. Levels of IL-10 present in the cell culture supernatants at 54 h were determined by ELISA. (A) Percentage suppression of Con A-induced proliferation of PBMC by ES. (B) Proliferation of PBMC in response to HiES expressed as a stimulation index (SI). (C) Concentration of IL-10 in culture supernatants of PBMC incubated with Con A + ES. (D) Concentration of IL-10 in culture supernatants of PBMC incubated with HiES. Pre = PBMC harvested before infection; Post = PBMC harvested after infection. Open squares represent data from the same animal. *P < 0.05 (students t-test).