On the mechanism of T cell-independent anti-Listeria resistance in nude mice.

Unlike the acute infection seen in normal mice, listeriosis in athymic nude mice is a chronic and eventual lethal infection, characterized by the persistence of large numbers of Listeria in internal organs for several weeks. Since normal anti-Listeria responsiveness can be restored by thymus implantation several weeks before infection, there is little doubt that T cells are required under natural circumstances for rapid and complete elimination of the organism from the tissues. Even so, the fact that the absence of T cells does not prevent nude mice from stabilizing bacterial numbers in internal organs for several weeks, means that a T cell-independent accessory mechanism must operate in defense against listeriosis. Indeed, the results of this study indicate that at least two accessory mechanisms are involved. There is a) a radioresistant, preexisting mechanism of antibacterial resistance that enables conventionally-raised nude mice to destroy a much larger proportion than normal of the bacterial inoculum during the first 12 hr of infection, and b) a subsequently acquired, radiosensitive mechanism of resistance that functions to prevent progressive bacterial multiplication after 24 to 48 hr. The protection afforded by these T cell-independent mechanisms of resistance is limited, however, in that infected nude mice die sporadically from listeriosis over a period of 6 weeks or so. Therefore, although results obtained with nude mice are consistent with the hypothesis that protection against listeriosis depends ultimately on the generation of T cell-mediated immunity, the hypothesis nevertheless needs to be modified.