Abstract:

Objective To investigate the effect of soluble protein from Eimeria stiedai(EsSP) on tumor growth, survival and immune status in tumor-bearing mice. Methods The minimum number of tumor cells that caused tumorigenesis was determined to establish a mouse model of subcutaneous colorectal cancer (CT26). EsSP was prepared from 108 E.stiedai sporulated oocysts by ultrasonic intermittent emulsification. One hundred and five BALB/c mice were randomly assigned into 7 groups, each receiving a subcutaneous injection of 5×105 CT26 cells at the right flank. Six experiment groups (A-F) were also injected intraperitoneally with different doses of EsSP (100.00, 50.00, 10.00, 1.00, 0.10, and 0.01 μg) once a day for 5 days, respectively. The control group were injected with the same volume of PBS. The tumor diameter was measured at 7, 11, 13, 15, 17, 19, 21, 23 days after inoculation, and the relative tumor volume and relative tumor proliferation rate (T/C) were calculated. Five mice in each group were sacrificed on day 25 after inoculation. The tumor was weighed and tumor inhibition rate was calculated. Orbital blood was collected to isolate peripheral lymphocytes. The effect of EsSP on the proliferation of lymphocytes in tumor-bearing mice was assessed by MTS colorimetric assay, and the stimulus index (SI) was calculated. CD4+/CD8+ T lymphocyte ratio in peripheral blood was determined by flow cytometry. The time and number of death were recorded for 80 days. Data were analyzed by one-way ANOVA. Results The minimum amount of tumor cells for tumorigenesis was 5 × 105 cells. At 23 days after inoculation, the tumor volumes in groups A, B and C were (435.2±41.1) mm3, (366.3±29.2) mm3, and (460.2±28.5) mm3, respectively, which were significantly smaller than (761.2±33.22) mm3, (810.4±38.36) mm3 and (865.2±35.29) mm3 in groups E and F and control group, respectively （P<0.05). The T/C values in groups A, B and C were (39.0±6.7)%, (33.3±8.9)%, and (35.0±8.1)%, all < 40%. The tumor weight in group B was (1.109±0.432) g, which was significantly lower than (1.946±0.289) g in the control (P<0.05), presenting with the highest tumor inhibitory rate of(43.0±14.6)%. MTS colorimetric assay showed that the SI values in groups B and C were (1.75±0.15) and (1.70±0.32), both significantly higher than the control [(1.38±0.18)] (P<0.05). Flow cytometry revealed that the ratios of CD4+/CD8+ T lymphocyte subsets in peripheral blood in groups A, B and C were (1.58±0.24), (1.74±0.22) and (1.61±0.16), respectively [P<0.01 or P<0.05, compared with (1.34±0.15) in the control]. The 80-day observation reported that 5 mice survived in groups B and C, while only 2 survived in the control group (P<0.05). Conclusions EsSP can inhibit tumor growth, improve the survival, change the tumor-induced immunosuppressive state, and enhance the anti-tumor immune response in tumor-bearing mice.
 

Key words: Eimeria stiedai, Colorectal cancer, Tumor growth, Immunity