Inhibition of the Reactivity of Coombs Sera with Igg-Sensitized Human Erythrocytes by Streptococcal Protein-G (Spg)

Streptococcal protein-G (SpG), type III bacterial Fc receptor, is a small globular protein produced by several Streptococcal species and it is composed of two or three nearly identical domains, each of 55 amino acids. Streptococcal protein G has been shown to have high binding affinity to sera from various mammalian species including rabbit, human, pig, goat, sheep, cow and many other animal species. Of concern are patients with invasive infections by Streptococcus spp, where large amount of secreted SpG could interfere with the outcome of the gel technique by getting false negative tests. It has been shown and reported that the bacterial protein SpA was already found to inhibit the Coomb's test. We hypothesize that SpG as well as many other immunoglobulin-binding bacterial proteins with binding affinity to human IgG could cause false negative results in patients with bacteraemia. With the intention of proving this hypothesis we conducted two sets of experiments, which proved that SpG has the potential of inhibiting the gel test for the detection of sensitized erythrocytes in vitro. We concluded that is important to exercise caution, when evaluating a result of gel technique in patients with septicemia caused by IgG-producer Streptococci. The experiments used in this research were novel modifications of existent techniques and they proved reliable in demonstrating our hypothesis.