The B cell dependence of Haemonchus contortus antigen-induced lymphocyte proliferation

The proliferative responses of ovine lymphocytes to soluble antigens of third-stage larvae of Haemonchus contortus (HcAg) were examined in vitro. Lymphocytes in mononuclear cell cultures responded consistently to HcAg even if the lambs were naive to H. contortus. Lymphocytes from sheep that were immune to reinfection with H. contortus also consistently responded to HcAg with a response significantly greater than that recorded from naive lambs. When the responses of T cell or B cell enriched fractions were examined, neither enriched cell population responded to HcAg, even though the T cell populations were shown to contain monocytes by histochemistry. Studies using the accessory cell dependent T cell mitogen Con A demonstrated that such T cell enriched populations had normal Con A-induced responses compared to unfractionated mononuclear cells. In addition, neither the T cell nor B cell fractions responded to HcAg in the presence of autologous mononuclear cells which contained monocytes and dendritic cells. Responses to HcAg in the T cell enriched population were restored, however, by the presence of mononuclear cells that were cultured with HcAg for 48 hours and then irradiated before the addition of the T cells. These results were seen with cells from both H. contortus naive lambs and immune sheep. This provides evidence that B cells and not dendritic cells or monocytes are acting as antigen presenting cells in Haemonchus antigen-induced lymphocyte responses in both naive lambs and immune sheep.