Abstract：Seventy-two lactating SD rats were randomly divided into control and treatment groups(n =36). The 100 μL of sterile 0.01 mol/L，pH 7.2 PBS(phosphate-buffered saline)(control group) and CpG 200 μg/rat(treatment group) were injected intramuscularly into tibialis anterior of the left leg after parturition 0 h respectively. Then 100 μL /rat of bacterial suspension containing 2×1012 cfu(colony forming unit)/mL(both sides) of Escherichia coli were inoculated into the fourth (abdominal) mammary gland via the teat duct 72 h after parturition respectively. Before defined at o h and after 8,16,24,48 and 72 h (n =6) of inoculation, all the rats euthanatized and the mammary glands and serum were harvested. The histopathologic evaluations showed that polymorphonuclear leukocytes (PMN) accumulation in alveoli peaked 16 h postinfection. The PMN in treatment group influxed more rapidly and no PMN was found in alveoli 72 h after infection. Bacteria counts of E. coli in mammary gland peaked at 16 h postinfection and CpG-DNA induced significant decrease of viable bacteria at 16, 24 and 48 h postinfection. CpG-DNA induced significant increase of IL-2 in mammary gland before infection. Significant increases in IL-6 and TNF-αin mammary gland were observed at different time points in both group. The maximal increases in IL-6 were at 16 h postinfection, the time that was coincident with peak infiltration of PMN. CpG-DNA could induced significant increase of IL-6 in mammary gland at 16 h postinfection. No significant changes of N-acetyl-β-D-glucosaminidase (NAGase) in mammary gland from treatment group were observed. The result showed that CpG-DNA induced more prompt migration of PMN from blood to mammary gland at the initial stage of E. coli infection, stimulated the secretion of IL-2 and IL-6, decreased E.coli counts in mammary gland and attenuated the destroy of inflammation-mediator to cell. The study indicates that CPG DNA protects against mastitis at some extent induced by E. coli infection in rat.
