Objective: To study the pathological damage of thymus and thymus cell apoptosis of male rats infected with Toxoplasma gondii.
Methods: Fifty Wistar male rats (7-8-week-old) were randomly divided into infection group (40) and control group (10). Rats in infection group were infected with 5 x 10(4) tachyzoites by intraperitoneal injection, while those in control group received same volume of PBS. On the 3rd, 6th, 9th and 12th day post infection, ten rats from infection group and two from control group were sacrificed, the thymus glands were removed. The thymus tissue sections were stained with hematoxylin and eosin (HE) for observation on histopathological changes. Single thymus cell suspensions were prepared. Cell cycle analysis was performed by flow cytometry, and proliferation index was calculated. Thymus frozen sections were stained with Hoechst 33258, and morphologic changes in apoptotic nuclei were observed under fluorescence microscope. Expression of Bcl-2 and Bax proteins were determined by using immunohistochemistry.
Results: Microscopic examination showed that pathological changes occurred in thymus grand on the 3rd day after infection. The space between connective tissue capsules was widened, cells in cortex and medulla cells were sparse, and more phagocytes and extravasated blood were found in thymus. On the 6th day post infection the thymus damage was aggravated, and no significant improvement was seen on day 12. On the 3rd, 6th, 9th and 12th day after infection, thymocyte proliferation index was (11.15 +/- 0.99)%, (6.17 +/- 1.02)%, (5.45 +/- 0.96)% and (6.63 +/- 1.52)%, respectively, and each of them was significantly lower than that of the control [(13.81 +/- 1.18)%] (P < 0.01). On the 3rd day after infection, the number of apoptotic cells increased, significantly increased on day 6, and there was no much difference in the number of apoptotic cells between day 6 and day 12. The immunohistochemistry results showed that on the 3rd, 6th, 9th and 12th day post-infection, the gray scale value of Bax positive cells was 88.21 +/- 4.74, 64.69 +/- 6.82, 83.62 +/- 5.79, and 101.09 +/- 6.72, respectively, and each of them was significantly lower than that of the control (128.69 +/- 8.95) (P < 0.01), while there was no significant change in the Bcl-2 protein level (P > 0.05).
Conclusion: T. gondii causes severe pathological damage in host thymus tissue with a decrease in the proliferation index, an increase in the number of apoptotic cells, and high expression of Bax protein.