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Abstract: Mycoplasma gallisepticum infection in chickens leads to tracheitis, airsacculitis, poor feed conversion and reduced egg production, resulting in considerable economic hardship on the poultry industry. The chemokines and cytokines responsible for recruitment, activation and proliferation of leukocytes in affected tissues have not been described. In the current study, chemokine and cytokine gene expression profiles were investigated in tracheas of chickens inoculated with M. gallisepticum strains R low (pathogenic) and GT5 (attenuated) at days 1, 4 and 8 post-inoculation. Expression of lymphotactin mRNA was higher in R low-inoculated chickens than GT5- or PBS-inoculated chickens, while CXCL13/BCA1 mRNA expression level was higher in both GT5- or R low-inoculated chickens than in PBS-inoculated controls on day 1 post-inoculation. However, both R low and GT5 strains induced a down-regulation in mRNA expression of CCL20, IL-1β, IL-8 and IL-12p40 genes, with CCL20 and IL-12 mRNA levels remaining lower on days 4 and 8 post-inoculation. On day 4, R low-inoculated chickens exhibited significantly higher tracheal lesion scores and higher levels of lymphotactin, CXCL13, CXCL14, RANTES, MIP-1β, IL-1β and IFN-γ mRNA compared to PBS-inoculated controls. The mRNA levels of these genes were also higher in R low-inoculated chickens that had moderate to severe tracheal lesion scores on day 8 post-inoculation. These results reflect the importance of lymphocyte and monocyte chemotactic factors in the development of tracheal lesions in chickens inoculated with M. gallisepticum strain R low. Our data also suggest that M. gallisepticum may modulate the host response causing dramatic decreases in CCL20, IL-8 and IL-12 mRNA levels in GT5- or R low-inoculated chickens as early as one day post-inoculation. [Copyright &y& Elsevier]