Early pathogenesis of Listeria monocytogenes infection in the mouse spleen

Histological observations suggested that in the spleen, blood-borne Listeria monocytogenes bacteria were preferentially ingested by two morphologically distinct mononuclear phagocyte populations present in the marginal zone of the white pulp. The morphologies of these phagocytes corresponded to those of marginal zone macrophages or marginal zone dendritic cells. Moreover, during the first day of infection, the same phagocytes containing listeria apparently translocated from the marginal zone into the white pulp where they established secondary infectious foci. This event was associated with a large influx of neutrophil polymorphonuclear leucocytes (PMNLs) into infected white pulp, and with the disappearance of lymphocytes from this compartment. White pulp lymphocytopenia also occurred in the spleens of listeria-infected mice selectively depleted of neutrophil PMNLs, indicating that these phagocytes were not responsible for displacing or destroying lymphocytes. The implications of these findings for explaining the virulence and immunogenicity of L. monocytogenes are discussed.