Elevated production of Legionella-specific immunoglobulin A in A/J mice is accompanied by T-helper 1-type polarization.

Legionella pneumophila (Lpn) is a Gram-negative bacterium and an intracellular parasite that causes Legionnaires' disease. Secretion of immunoglobulin A (IgA) against Lpn on the mucosal surface of the upper respiratory tract may be important as a self-defense mechanism. A/J mice have been demonstrated to be permissive as to Lpn replication in macrophages due to a natural mutation in neuronal apoptosis inhibitory protein 5. We compared A/J and BALB/c mice as to IgA production after repeated intranasal immunization using a heat-killed Lpn in the presence of cholera toxin as a mucosal adjuvant. A/J mice secreted more Lpn-specific IgA in nasal washes than BALB/c mice. The Lpn-specific serum IgA level was also higher in A/J than BALB/c mice. Because both BALB/c and A/J mice are known to exhibit T-helper 2 (Th2)-biased immune responses, we examined whether the Lpn-specific IgA production is related to the stronger Th2 bias. There was no difference in IgG1 (Th2-controlled) while A/J mice produced more IgG2a (Th1-controlled), suggesting that the elevated IgA response was rather correlated with Th1-controlled isotype switching. Our results also suggest that A/J mice will be useful hosts for Lpn-specific IgA production such as for the preparation of IgA monoclonal antibodies.