Actinobacillus pleuropneumoniae iron transport and urease activity:: Effects on bacterial virulence and host immune response

Actinobacillus pleuropneumoniae, a porcine respiratory tract pathogen, has been shown to express transferrinbinding proteins and urease during infection. Both activities have been associated with virulence; however, their functional role for infection has not yet been elucidated. We used two isogenic A. pleuropneumoniae single mutants (Delta exbB and Delta ureC) and a newly constructed A. pleuropneumoniae double (Delta ureC Delta exbB) mutant in aerosol infection experiments. Neither the A. pleuropneumoniae Delta exbB mutant nor the double Delta ureC Delta exbB mutant was able to colonize sufficiently long to initiate a detectable humoral immune response. These results imply that the ability to utilize transferrin-bound iron is required for multiplication and persistence of A. pleuropneumoniae in the porcine respiratory tract. The A. pleuropneumoniae Delta ureC mutant and the parent strain both caused infections that were indistinguishable from one another in the acute phase of disease; however, 3 weeks postinfection the A. pleuropneumoniae Delta ureC mutant, in contrast to the parent strain, could not be isolated from healthy lung tissue. In addition, the local immune response-as assessed by fluorescence-activated cell sorter and enzyme-linked immunosorbent spot analyses-revealed a significantly higher number of A. pleuropneumoniae-specific B cells in the bronchoalveolar lavage fluid (BALF) of pigs infected with the A. pleuropneumoniae Delta ureC mutant than in the BALF of those infected with the parent strain. These results imply that A. pleuropneumoniae urease activity may cause sufficient impairment of the local immune response to slightly improve the persistence of the urease-positive A. pleuropneumoniae parent strain.