Immunoglobulin A protease is a virulence factor for gram-negative pneumonia

Secretory immunoglobulin A (SIgA) is the principal immunologic defense of respiratory and other mucosal surfaces in the body. SIgA is relatively stable in mucosal secretions. However, cleavage of SIgA by bacterial proteases might render it immunologically inactive and thus contribute to the development of pneumonia as well as other infections. Bacterial species and infection sites might be important in the expression of bacterial protease activity and serve as the impetus to this study. Bacterial isolates from respiratory and nonrespiratory sites were incubated with SIgA in vitro. SIgA degradation was determined by size exclusion ultrafiltration and gel electrophoresis. IgA protease activity was evident in gram-negative but not gram-positive respiratory isolates. Gram-negative isolates from nonrespiratory sources did not exhibit IgA protease activity. Expression of IgA protease activity might be important in the development and subsequent outcome of gram-negative pneumonia in the patient in the surgical intensive care unit.