New method that uses binding of immunoglobulin A to group A streptococcal immunoglobulin A Fc receptors for demonstration of microbial immunoglobulin A protease activity

A new method is described for the detection of bacterial immunoglobulin A (IgA) protease which splits IgA into Fab and Fc fragments. The method takes advantage of a recent finding that receptors for IgA fragments occur commonly among type 4 group A streptococci. The bacterial preparation to be tested for protease activity was first incubated with radiolabeled purified IgA1 myeloma protein, and the proportion of radioactivity bound to a standard suspension of the streptococci was then measured. Since isolated Fab fragments do not bind to streptococcal IgA receptors, a decrease in the amount of radioactivity bound to the streptococci, as compared with the amount before digestion, indicates the presence of protease in the test preparation. Using this method, protease activity was detected in Neisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae, and Streptococcus sanguis, but not in Escherichia coli or Branhamella catarrhalis.