The production of cytokines by lymphoid cells, isolated from non-infected and Haemonchus contortus-infected lambs, was investigated. Particular attention was paid to differences in T helper 1- (Th1) and Th2-type immune profiles between genetically resistant and random-bred animal groups. Non-infected resistant and random-bred lambs produced equivalent levels of interferon-γ (IFN-γ) and interleukin-5 (IL-5), from isolated abomasal lymph node cells (ALN), mesenteric lymph node cells (MLN) and spleen cells (SC), in response to in vitro stimulation with T-cell mitogen (concanavalin A) or larval parasite antigen. ALN and MLN cells derived from infected resistant and random-bred lambs produced relatively lower levels of IFN-γ, following in vitro stimulation with parasite antigen, when compared with their uninfected counterparts. In contrast, infected lambs of both groups showed enhanced mitogen- and antigen-stimulated production of IL-5, in comparison with uninfected controls, at days 5 and 28 postinfection (p.i.). Mitogen- and antigen-stimulated IL-5 responses were higher among resistant lambs compared with random-bred lambs, with the highest overall production of IL-5 by parasite antigen-stimulated ALN and MLN cells. Among day 28 p.i. lambs, levels of cell culture-derived parasite-specific immunoglobulin G1 (IgG1) and IgE antibodies were higher in resistant lambs than in random-bred lambs, following in vitro stimulation of SC or ALN cells with parasite antigen. Finally, after 28 days p.i., histological examination of abomasal tissue revealed higher densities of mast cells and eosinophils in the mucosa of resistant lambs than in random-bred lambs. Taken together, these data support the notion of a strong Th2-type immune response to Haemonchus infection in genetically resistant sheep, and support the claim for a Th1/Th2 dichotomy in ruminants
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