A n electronmicroscopy study o f the spleen fro m mice infected withThe predominantly intraerythrocytic habitat of malarial parasites and the fact that gross changes in the splenic microarchitecture occur during m alaria8 , make the spleen an im portant organ in which to study host -parasite interactions. A t this site parasites are removed from the blood, phagocytosed, and both induce and are subjected to the effects of the immune response. Although macrophages play a central role in ali these events, our understanding of their functions is still very fragmentary and several important questions remain unanswered. This study was therefore undertaken to investigate the nature of phagocytic cells from different areas of the spleen during malaria infection, and the mechanisms by which they remove parasites from the blood. strain) -infected erythrocytes, and eight days later, anesthetized by ether 30 minutes after an injection of 150 units of heparin intravenously. A polythene cannula (0.61 mm externai diameter) was inserted caudally into the thoracic aorta and its tip secured at the levei of emergence of the coeliac artery. After clamping the distai aorta and sectioning the portal vein, animais were perfused with phosphate buffered saline at 37°C at a pressure of 163 cm of water. After bleaching of the liver, perfusion was switched to the fixation solution of 1.5% glutaraldehyde in 0.1M sodium cacodylate bufferpH 7.3. Spleens were remov ed after 15 min perfusion with the fixative, sliced into sections of 1 mm submitted to a further 2 hour fixation with 4% buffered glutaraldehyde and washed with cacodylate buffer containing 10% sucrose. Tissues were post fixed in veronal-acetate buffered 1 % osmium tetroxide, dehydrated in ethanol, embedded in Araldite and stained with uranyl acetate and lead citrate, and examined by electron microscopy.A reas comprising each major cell compartment of the spleen (red pulp, marginal zone, periarteriolar lymphocyte sheath and germinal centre) were selected in toluidine blue-stained sections, and relocated on the surface of the corresponding block. Phagocytic cells were characterized by the presence of ingested parasites or malarial pigment. The latter was recognized as osmiophilic trapezoidal crystals, sometimes partially dissolved by the lead citrate used for processing the tissue. 31
The effects ofone non-lethal species ofmalarialparasite, Plasmodium yoelii, and one lethal species, P. berghei, on the mononuclear phagocyte system (MPS)
BA LB /c mice).The mononuclear phagocyte system (MPS) represents a major site of interactions between malarial parasite and the host. During infection, macrophages perform a crucial role in the clearance of parasites 2, and also appear to play a part in the modulation of the immune response 21. It is therefore conceivable that the particular way in which this interaction occurs may influence the outcome of the infec tion. This possibility was investigated in two murine malarias, one lethal caused by Plasmo dium berghei, and one self-limiting caused by P. yoelii.
M A T E R IA LS AN D M ETHODS
Host-parasite systemsTwo to four month old female BALB/c mice bred in our Animal Unit were used throughout Recebido para publicação em 0 3 /0 1 /8 3 this study. They were infected with 105 parasitized erythrocytes i.v. of either P. yoelii strain 17X or P. berghei A N K A strain, which were maintained as frozen stabilates and passed once in isogenic mice before use. P yoelii 17X causes a self-curing infection after an acute attack lasting 2-3 weeks, while P. berghei gives rise to a lethal infection with two peaks of mortality, on days 6-8 and 21-23.
Histological techniquesLivers and spleens were fixed in Carno/s solution and stained either with haematoxylineosin, Giemsa solution, pyronin-methyl green,
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