Mokhtari Amirmajdi et al. : Apoptosis of Human Lymphocytes after Exposure …. Discussion E. granulosus has considerable defense mechanisms against the immune response of the host, which warrants cyst survival in the host body (9). Hydatid cyst can live as long as 53 years in the human body (22). This shows that this parasite has strategies for evading protective immune responses. Sequestration and antigenic disguise (23), interference with complement activity (24), alteration of lymphocyte proliferative re- sponses (25), and inhibition of the effector cell chemotaxis (9) are some of the proposed strategies that permit the parasite to survive for a longer period (12). Mitogenic re- sponses induced by protoscoleces of Echino- coccus species were previously showed by investigations evaluating growth kinetics of leukocyte cell lines cultured with hydatid fluid (10, 15). These responses decrease the efficacy of immune system, as mitotic cycles induced by hydatid products are not com- pleted (15). Along with alteration in lympho- cyte proliferative responses, depletion of T lymphocytes is another strategy, by which the parasite impairs the immune response of the host (26). Although not thoroughly identified, apop- tosis might be another mechanism. Cyto- toxic features of E. granulosus was pre- viously verified (18). This study shows that apoptosis was significantly higher in lymphocytes that were treated by fertile hydatid fluid regarding to control cells. Expression of the Bax gene as a pro-apop- totic molecule was increased and the expres- sion level of Bcl-2 mRNA as an anti-apop- totic molecule was reduced in the fertile HF- treated lymphocytes relative to the infertile HF-treated lymphocytes and control. Activ- ity of caspase-3 was also significantly higher in the first group. All these supports this no- tion that protoscolex of E. granulosus (which is present in fertile hydatid cysts) in- 14 duces apoptosis in the lymphocytes of hosts. In consistent with our results, Macintyre et al. showed that hydatid fluid could induce T- cell mitosis with enhanced membrane expression of CD25 and CD38 on human peripheral blood lymphoblasts, and dimin- ished that of CD28 and reduce co-stimula- tion with subsequent T-cell anergy or apop- tosis (18). In another study, Li et al. ex- plained apoptosis as an important mecha- nism of CD4+ T cell death in experimental alveolar echinococcosis. The apoptosis level of CD4+ and CD8+ T cells was significantly higher after the infection (27). Results also showed that apoptosis was more in the fertile HF-treated lymphocytes compared with the infertile HF-treated lymphocytes. This could be resulted from the presence of protoscole- ces in the cysts, which may release inducer of apoptosis. However, in a study by Janssen et al., it was demonstrated that protoscoleces had no cytotoxic impact on the macrophages. Albeit in that study, it was suggested that some toxic substances are probably secreted by protoscoleces, which induce reduction in the viability of the macrophages in vitro (28). It was previously verified that hydatid fluid toxins have lytic effects on mouse peritoneal macrophages (29). Thus, whether by se- creted toxins from the protoscoleces or di- rectly by the antigens of protoscoleces, apoptosis pathways are activated after expo- sure to hydatid fluid containing protoscole- ces. This shows that fertility of hydatid cyst might be an important factor in the survival of cyst in the host’s body. This apoptotic feature offers a significant advantage to this pathogenic parasite, as in the fertile stage, the organism defend by induction of apop- tosis in the leukocytes invading to the newly growing cyst. Induction of apoptosis and a possible anergy in the T-cells might result in a significant decrease in the effectiveness of