Development and suppression of a population of late-adhering macrophages in mouse malaria

Changes in phagocytic and adherent cell numbers were compared during the course of infections of mice with Plasmodium yoelii (Py) and P. berghei (Pb) and in vaccinated mice challenged with homologous parasites. Nucleated cells in the spleen increased in number in Py-infected mice and were maximal at the time of recovery. The number of phagocytic cells increased in parallel, as did the number of blood leucocytes. Rates of increase were accelerated in vaccinated mice. Changes in Pb-infected mice resembled controls and blood leucocytes showed no consistent increase. In infected mice, the number of spleen and bone marrow cells which adhered to plastic rose above normal. At some stages of infection, cells which did not adhere in 24 h did so in 72 h. Such late-adhering cells, which resembled macrophages in morphology, were most numerous at the time of recovery. They appeared to be derived from monocyte precursors which matured in culture. Sometimes cells adherent at 24 h suppressed the development of the late-adhering population. Silica invactivated these suppressive macrophages but did not affect the precursors which developed into late-adhering cells. It is concluded that malarial infection stimulates the production of precursors of the macrophage-monocyte series and that their development is regulated by the presence of mature macrophages.