If one prepares a list of the diverse microorganisms that require an intact T-cell-dependent immune response for control or eradication, and compares this with a list of pathogens that regularly infect patients with acquired immunodeficiency syndrome (AIDS), the two are virtually superimposable. This observation is not particularly surprising (and, indeed, seems readily predictable) given the uniform and profound impairment in the AIDS patient's T-cell-mediated immune responses. There is, however, one well-known intracellular opportunistic pathogen, Listeria monocytogenes, which rarely infects AIDS patients1; this finding is curious and remains unexplained.
Because it was used to originally delineate the basic mechanisms of cell-mediated immunity, L monocytogenes maintains a special niche in the annals of laboratory science. The work of Mackaness and other investigators has demonstrated that the sensitized T lymphocyte directs the eradication of established L monocytogenes infection by elaborating soluble mediators (lymphokines), which, in turn, activate macrophages to display microbicidal effects.