Host-pathogen interaction in bovine respiratory disease: (1) DNA immunization against bovine herpesvirus-1. (2) Role of CD18 in Mannheimia haemolytica leukotoxin-induced cytolysis of bovine leukocytes

Bovine herpesvirus 1 (BHV-1) and Mannheimia haemolytica are important pathogens causing bovine respiratory disease. BHV-1 undergoes latency and induce immunosuppression, necessitating development of alternative immunization strategies that do not involve live virus. DNA immunization represents such an alternative. This study was undertaken to determine its potential to induce BHV-1-specific cytotoxic T-lymphocytes (CTLs), which are critical for defense against BHV-1. Mice were injected by intramuscular (IM) or intradermal (ID) route with Sindbis virus-based plasmid encoding BHV-1 glycoprotein D (gD). Splenocytes from immunized mice, upon in vitro restimulation, specifically lysed gD-transduced syngeneic fibroblasts in MHC class I-restricted manner. IM route induced better CTL response than ID. These results indicate the potential of DNA immunization to induce CTLs against BHV-1.; Leukotoxin (Lkt) secreted by M. haemolytica is the major virulence factor that specifically targets ruminant leukocytes. Previous studies have identified β₂ integrins as the receptor for the Lkt. β ₂ integrins have a common β subunit (CD18) that associates with three distinct α chains (CD11a, CD11b, CD11c). Our previous study demonstrated that Lkt binds to all three β₂ integrins, suggesting that the common β subunit, CD18, may be the subunit involved in binding the Lkt. Irrefutable identification of bovine CD18 (bCD18) as the Lkt receptor could be made by rendering Lkt-non-susceptible cells susceptible to Lkt by recombinant expression of bCD18 on these cells. Therefore, in this study, the Lkt-non-susceptible mouse cell line, P815, was transfected with bCD18 cDNA and transfectants were tested for expression of bCD18. Transfectants expressing bCD18 on their surface were effectively lysed by the Lkt in concentration-dependent manner. Furthermore, co-immunoprecipitation with MAb to mouse CD11 a and bCD18 suggests that bCD18 is expressed as heterodimer with mouse CD11a on the surface of transfectants. These results indicate that bCD18 acts as the receptor for Lkt, which paves the way for developing means to interrupt Lkt-binding, and hence the disease.