In vivo immunomodulatory effects of ixodid ticks on ovine circulating T‐ and B‐lymphocytes

SUMMARY Selected aspects of the ovine immune system were examined during the course of repeated infestations with the ixodid ticks, Haemaphysalis bispinosa and Hyalomma anatolicum anatolicum that naturally infest sheep, either individually or together. By the use of flow cytometry it was shown that total T‐lymphocyte numbers were significantly reduced from the sixth through the ninth days of all infestations. Gamma/delta (γδ+) and CD8+ T‐lymphocytes were significantly depleted during tick feeding in all infested groups. CD4+ T‐lymphocyte levels were significantly increased during secondary H. bispinosa and mixed species infestations. Hyalomma anatolicum anatolicum caused a significant increase in circulating B‐lymphocytes over several days in both initial and secondary infestations. All infested sheep had increased CD4/CD8 and decreased T/B lymphocyte ratios during exposure to both ticks. Bromodeoxyuridine (BrdU) ELISA was used to measure in vitro proliferation of peripheral blood mononuclear cells stimulated with the T‐lymphocyte mitogen Concanavalin A (Con A) after their collection from infested sheep. Significant suppression of in vitro proliferation occurred during first and secondary infestations with H. bispinosa, H. a. anatolicum and with both tick species together, beginning on the sixth day of infestation in all cases. These important tick species of sheep significantly modulate the numbers of immune effector cells and proliferation of T‐lymphocytes derived from infested animals.