Findings and conclusions. 1. In vitro study: we observed an overall increased mRNA expression of cytokines TNFα, IL-1β, IL-8, IL-10 and IL-12 when bovine alveolar macrophages were challenged with LKT-deficient strain compared to the wt-strain of M. haemolytica. LKT-induced cell cytotoxicity was observed in BAM which suggested that the difference in cytokine mRNA expression was due to LKT-induced cell cytotoxicity and cytokine expression is also mediated by LPS and other virulence factors.

2. In vivo study: we challenged calves with the wild type and LKT-deficient M. haemolytica. Following intratracheal inoculation, inflammatory cells were separated from the bronchioalveolar lavage collected on day -4, 1, 3, and 6. The effects on leukocyte function were measured by quantifying cytokines IL-1β, IL-8, IL-10, IL-12 and TNFα response using ELISA and real-time RT-PCR. Calves challenged with wt-strain had a higher mean clinical score and one death compared to calves challenged with the LKT-deficient strain. The composition of bronchioalveolar lavage fluid was variable between the groups. Though a statistically significant difference in the expression level of cytokines was not observed between the groups challenged with LKT-deficient strain and wt-strain of M. haemolytica, increased expression of IL-8 and IL-10 was observed in wild type and LKT-deficient M. haemolytica challenged calves, respectively. The expression of these cytokines may be correlated with the clinical severity in calves.

3. The results presented in these in vitro and in vivo studies demonstrate that virulence factors other than LKT, in particular LPS, play an important role in BPP and through the use of multiple virulence factors M. haemolytica has a selective effect on the induction of various cytokine mRNA synthesis by inflammatory cells.