Abstract
A hallmark of malaria and several other intravascular infections is the development of splenomegaly associated with splenic macrophage hyperplasia. These macrophages do not arise as a consequence of in situ proliferation of preexisting splenic macrophages, but rather from blood monocytes. To investigate the mechanism of accumulation of blood monocytes in the spleen during the development of splenomegaly, we studied mice and monkeys infected with malaria and mice treated with methyl cellulose. Employing a modified Boyden chamber chemotaxis assay, extracts of washed spleen cells of malaria-infected but not control mice or monkeys were found to contain a factor chemotactic for homologous and human peripheral mononuclear cells. In mice, this factor was present within 4 hours of infection, persisted for the duration of infection, and anteceded a detectable increase in splenic macrophage number by 48 hr. Extracts of lysed parasitized erythrocytes contained no chemotactic activity. Supernatants from in vitro cultures of spleen cells from malaria-infected but not control mice also contained mononuclear cell chemotactic activity. The spleen-derived chemotactic factor was present in nonadherent cells, was water soluble, heat labile, trypsin sensitive and had an estimated m.w. of 80,000 daltons (Sephadex G-150). It contained no serine esterase activity. These observations suggest that infection with malaria and administration of other agents inducing splenomegaly result in the in vivo elaboration of an endogenous spleen-derived mononuclear cell chemotactic factor. This factor, presumably derived from lymphocytes, may be responsible for or contribute to the trapping of blood monocytes in the development of macrophage hyperplasia in the spleen.
