Intermedilysin plays a key role in the invasion of human cells by Streptococcus intermedius

Streptococcus intermedius, which causes endogenous infections leading to abscesses, produces intermedilysin (ILY), a human-specific cytolysin. Because of the significant correlation between higher ILY production levels by S. intermedius and deep-seated abscesses, we constructed an ily knockout mutant and two complementation strains, (i) with the wild-type ily gene and (ii) with an ILY SS-mutant gene (the latter was not cytolytic without reductant due to an intramolecular SS bridge between domains 2 and 3), from ILY high level expressing strain UNS38, in order to investigate the role of ILY in deep-seated infections. Infection with the wild strain and the complementation strain secreting cytolytic ILY reduced the viability of human liver cell line HepG2 at infection but not of rat liver cell line BRL3A, and increased intracellularly concomitantly with increasing HepG2 cell damage. Isogenic ily knockout mutant strain and the complementation strain secreting ILY SS-mutant were not cytotoxic and showed no proliferation in either cell line. Masking of cytotoxic strains with anti-ILY antibody drastically decreased adherence and invasion of HepG2. Moreover, coating ily knockout mutant strain with ILY partially restored adherence to HepG2 but without subsequent bacterial growth. These results indicate that: 1) surface-bound ILY on S. intermedius is an important factor for invasion of human cells by this bacterium; 2) secretion of ILY within host cells is essential for subsequent host cell death; and 3) cell membrane poration by ILY is required in infection steps. These strongly suggest that ILY is an important factor in the pathogenesis of abscesses in vivo by this streptococcus.