Yersinia pestis, the causative agent of plague, must survive in blood in order to cause disease and to be transmitted from host to host by fleas. Members of the Ail/Lom family of outer membrane proteins provide protection from complement-dependent killing for a number of pathogenic bacteria. The Y. pestis KIM genome is predicted to encode four Ail/Lom family proteins. Y. pestis mutants specifically deficient in expression of each of these proteins were constructed using lambda Red-mediated recombination. The Ail outer membrane protein was essential for Y. pestis to resist complement-mediated killing at 26 and 37°C. Ail was expressed at high levels at both 26 and 37°C, but not at 6°C. Expression of Ail in Escherichia coli provided protection from the bactericidal activity of complement. High-level expression of the three other Y. pestis Ail/Lom family proteins (the y1682, y2034, and y2446 proteins) provided no protection against complement-mediated bacterial killing. A Y. pestis ail deletion mutant was rapidly killed by sera obtained from all mammals tested except mouse serum. The role of Ail in infection of mice, Caenorhabditis elegans, and fleas was investigated.

Background: LcrG, a negative regulator of the Yersinia type III secretion apparatus has been shown to be primarily a cytoplasmic protein, but is secreted at least in Y. pestis. LcrG secretion has not been functionally analyzed and the relevance of LcrG secretion on LcrG function is unknown.

There is increasing evidence that mast cells play a role in host defenses, both via an IgE-FcεR mediated, allergy-like, response, as well as an IgG-FcγR mediated response. We provide evidence here that the inhibition of mast cell degranulation greatly enhances susceptibility of HLA-DQ8αβ transgenic (tg) mice to the plague. DQ8 tg mice are significantly more resistant to Yersinia pestis infection compared to out bred Swiss Webster mice, LD50 of >104 and 20 CFUs respectively. The magnitude of the resistance observed in DQ8 tg mice allowed us to examine the contribution of various immunological components during Y. pestis infection. To assess the role of mast cells, DQ8 tg mice were treated with cromolyn, a specific inhibitor of mast cell degranulation, prior to infection with Y. pestis. The enhanced resistance observed in DQ8 tg mice was completely eliminated with cromolyn treatment, suggesting an important role for mast cells in the host response of DQ8 tg mice to Y. pestis. We have previously reported that Y. pestis infected DQ8 tg mice develop a strong total IgG response, while infected Swiss Webster mice do not have any elevation in total IgG levels. The presence of elevated IgG levels may trigger the mast cell response in the DQ8 tg mice. These findings provide further evidence for a role of mast cells in host defenses and provide a model in which the molecular mechanisms of this interaction may be better understood.