IgA Proteases of Haemophilus Influenzae Dividing in Human Milk are Inhibited by IgA Antibody in the Milk

Abstract

IgA proteases^1–4 are extracellular enzymes produced by bacteria specifically pathogenic for human beings. These enzymes cleave both serum and secretory human IgA immunoglobulins (S-IgA), and thus may be virulence factors, or may contribute to colonization of mucosal surfaces by these bacteria. All clinical isolates of the important childhood pathogens Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitides are IgA protease-positive, and the non-pathogenic members of these genera are enzyme-negative. IgA proteases are produced by all strains, those harmlessly colonizing the upper respiratory mucosa of children, and those isolated from infected sites e.g., the cerebrospinal fluid in meningitis. The role of these enzymes in disease has been difficult to verify because of their high specificity for human IgA as substrate, so animal models are inappropriate, and human tissues in vitro ⁵ cannot precisely reproduce in vivo conditions to examine this question.