Listeriolysin as a virulence factor in Listeria monocytogenes infection of neonatal mice and murine decidual tissue

Listeriolysin is a 60-kDa protein which allows the growth of Listeria monocytogenes in macrophages and other cells and has been shown to be a virulence factor in Listeria infections of adult mice. However, the neonate and fetoplacental unit are major populations susceptible to listeriosis. Recent data indicate that macrophage and T-cell functions are markedly inhibited in these young mice, and the virulence of listeriolysin-negative (HLY^-) and listeriolysin-positive (HLY⁺) Listeria cells in the setting of such inhibited macrophage and T-cell functions has not previously been examined. We now compare CNL 85/162, a transposon-induced, HLY^- Listeria strain, and CNL 85/163, a spontaneous HLY⁺ revertant. We found that all 18 neonates injected with CNL 85/163 (HLY⁺) died within 12 days after an injection of 10⁴ Listeria cells per mouse. In contrast, all 16 neonates injected with 1,000 times more CNL 85/162 (HLY^-) cells survived more than 14 days. Three days after injection, growth of CNL 85/163 (HLY⁺) in the internal organs was more than 5 log greater than that of CNL 85/162 (HLY^-). We also found that CNL 85/162 (HLY^-) did not proliferate well in decidual tissue, which is a major component of the placental region. Our studies indicate that HLY^- bacteria are not virulent in the neonate and the fetoplacental unit despite the inhibited immune functions at these sites.