Comparison of responsiveness to the monocytosis-producing activity of Listeria monocytogenes in mice genetically susceptible or resistant to listeriosis.

Injection of a monocytosis producing activity (MPA) from Listeria monocytogenes caused a dose-dependent elevation in numbers of macrophage colony forming units (CFU-m) in bone marrow and in peripheral blood. The increase could be seen as early as 8 h after injection of MPA and persisted until 72 h after injection. Serum from MPA-treated animals, capable of inducing monocytosis, also caused an elevation in numbers of CFU-m. The effect of MPA on CFU-m numbers in peripheral blood was measured in inbred strains of mice with differing susceptibility to listeriosis. Resistant C57B1/6 and B10. A mice responded best to MPA. C3H/HeJ and CBA mice, sensitive to Listeria, gave a slow, intermediate response. A/J mice, lacking the 5th component of complement, failed to respond to MPA. Since both B10.D2 old (C5-deficient) and B10.D2 new (C5-sufficient) mice, responded comparably to MPA, the unresponsiveness in A/J mice cannot be attributed solely to lack of C-5. Our results are consistent with the idea that A/J mice lack or are unresponsive to the MPA-induced serum factor which promotes monocytosis.