Abstract: 【Objective】The objective of study is to detect susceptibility differences to M. hyopneumoniae among Chinese native pigs, western pigs and their crossbreed pigs, screen differentially expressed genes and explore the molecular genetic basis of M. hyopneumoniae infection in swine. It wouldThe results of the study will provide a basis and possibility for researchers to investigate the molecular mechanisms of M. hyopneumoniae infection in swine. 【Method】 In this study, Meishan pigs (sensitive to M. hyopneumonia, n=20 ), Landrace (excellent tolerance to M. hyopneumonia, n=20) and Suzhong pigs (cultivated breed derived from crossbred progeny of Meishan and Landrace pigs, n=20) were selected for artificial infection with M. hyopneumoniae. And the pigs were allocated randomly into the treatment group (15 pigs of each breed) or control group (5 pigs of each breed). The pigs of treatment group were administered with M. hyopneumoniae strain JS. Meanwhile, the control pigs were injected with physiological saline solution. The treatment and control groups were isolated and fed an antibiotic-free diet throughout the experimental period. And then, clinical symptoms were observed daily, and average daily weight gain, antibodies level, X-ray transmission and M. hyopneumoniae pathogen were detected at 18 and 28 days post M. hyopneumoniae challenge, respectively. Twenty-eight days post challenge, the pigs were sacrificed and inspected for hepatization of the lung lobes as an indicator of Mycoplasma pneumonia. Then, 2 infected individuals and 2 non-infected individuals of each breed pigs by M. hyopneumoniae were selected to perform Agilent Pig 4x44K Gene Expression Microarray analysis, and the double comparison method of inter- and intra-breed was used to investigate differential expression genes. Furthermore, Gene Ontology (http://www. geneont ology. org) and KEGG pathway (http://www. genome.jp/kegg/) were applied to analyze the roles of these differentially expressed genes involved in signaling pathway and regulatory network to discuss the molecular genetic basis in the process of M. hyopneumoniae infection in swine.【Result】 The results showed that, from day 1 to day 18 of experiment period, the average daily weight gain of the treated pigs was significantly lower than that of the control pigs (0.01≤P＜0.05), especially for Meishan pigs. From day 19 to day 28, negative growth (an average daily weight gain of less than 0.00 kg) and a drop in average daily weight gain were recorded for pigs in the treatment group. And the average daily weight gain of treated pigs was significantly lower than that of the control pigs (P＜0.01). Eighteen days post M. hyopneumoniae challenge, the antibody level of Landrace pigs and Suzhong pigs had little change and were low with the negative s/p ratio. But the s/p ratios of Meishan pigs were rapidly up to positive level, and were significantly higher than that of Landrace and Suzhong pigs (0.01≤P＜0.05), respectively. Twenty-eight days after treatment, the antibody level of Meishan pigs was up to 0.97±0.26, which was significantly higher than those of Landrace (0.15±0.10) and Suzhong pigs (0.46±0.20) (P＜0.01, respectively). At 18 days after treatment, clinical symptoms of mycoplasma pneumonia appeared in 11 Meishan pigs in the treatment group. Only two Landrace pigs showed signs of a slight cough. Five Suzhong pigs had early symptoms of mycoplasma pneumonia, such as coughing and lethargy. Twenty-three days into the experiment, one Meishan pig from the treatment group died after infection of M. hyopneumoniae. At the end of study, day 28, all Meishan pigs in the treatment group showed typical clinical symptoms and signs of mycoplasma pneumonia. Seven Landrace pigs from the treatment group had intermittent cough, lethargy and decreased appetite. Five Suzhong pigs had typical clinical symptoms of mycoplasma pneumonia, and further more six pigs had a slight cough. The results of M. hyopneumoniae pathogen detection were generally in accordance with description above. Moreover, on day 18 after treatment administration, the results of X-ray transmission detection revealed that 15 Meishan pigs in the treatment group had cloudy flocculent shadow, and amongst them, six animals had typical clinical symptoms and signs of mycoplasma pneumonia. While, only one of the Landrace pigs had a cloudy flocculent shadow and showed increased lung markings in the lung field, and four pigs had a small cloudy flocculent shadow. The number and degree of M. hyopneumoniae infection of Suzhong pigs appeared to be between that of Meishan and Landrace pigs. On day 28, all Meishan pigs in the treatment group had typical clinical symptoms and signs of mycoplasma pneumonia. Seven Landrace pigs and thirteen Suzhong pigs had clinical pathological evidence of mycoplasma pneumonia. At the end of experiment period, pathological examination of infected pigs indicated that the lung lesion scores of Meishan pigs (mean 23.20±2.54) were significantly higher than that of Landrace pigs  (mean 10.27±8.18) (P＜0.01) and Suzhong pigs (mean 18.71±4.78) (0.01≤P＜0.05). Profile microarray results showed that 119 differential genes were identified in Mycoplasma pneumonia pigs, including 49 up-regulated and 70 down-regulated genes, compared with the control group. These genes were involved in 18 signal pathways, such as B cell receptor signaling pathway, complement and coagulation cascades, steroid hormone biosynthesis and so on.【Conclusion】The obvious sensitivity differences do exist in the occurrence of swine mycoplasma pneumonia among various pig breeds. The primary immunodeficiency, Toll-like receptor signaling, and steroid metabolism pathways play important roles in the regulation of inflammatory response to M. hyopneumoniae infection in swine.

Key words: pig, mycoplasma pneumonia, susceptibility difference, molecular genetic basis