Phagocytes from flora-defined and germfree athymic nude mice do not demonstrate enhanced antibacterial activity

In this study we directly compared the in vitro antibacterial activities of resident and inflammatory phagocytes obtained from athymic (nu/nu) and euthymic (nu/+) mice. Resident peritoneal macrophages obtained from flora-defined nu/nu and nu/+ mice both demonstrated little ability to restrict the growth of Listeria monocytogenes in vitro. Inflammatory peritoneal neutrophils and macrophages obtained from flora-defined nu/nu and nu/+ mice did not differ in their ability to kill L. monocytogenes in vitro. Likewise, inflammatory peritoneal neutrophils obtained from germfree nu/nu and nu/+ mice killed equivalent numbers of listeria. In marked contrast, however, inflammatory macrophages obtained from germfree nu/nu and nu/+ mice demonstrated very limited antilisteria activity in vitro. The reduced antilisteria activity of macrophages from germfree nu/nu and nu/+ mice was associated neither with reduced phagocytosis of L. monocytogenes nor with an inability to generate an oxidative response. Unlike those of previous reports, these data suggest that resident and inflammatory phagocytes obtained from athymic mice that were maintained under conditions such that they were not subjected to underlying infections are not constitutively activated for enhanced antibacterial activity. Furthermore, these data suggest that stimulation of macrophages by products of the bacterial flora is required for the expression of macrophage antibacterial activity.