Phagocytosis of A. clavatus conidia by alveolar macrophages 'in vivo' and by peritoneal macrophages 'in vitro'.

Two types of inflammatory cell are notable as phagocytes, the neutrophil polymorphonuclear leucocyte and, in particular, the macrophage. In the murine inflammatory response invoked by conidia of Aspergillus clavatus, both cell types are seen to be actively phagocytic, although examples by macrophages are much the more common in an 'in vivo' setting. The process of phagocytosis was studied 'in vitro' using thioglycollate-stimulated peritoneal macrophages. The earliest examples of cultured macrophages in the presence of conidia as seen by light microscopy were examined using the(PAS) Periodic acid/Schiff staining technique which renders fungal cells red. Both scanning(SEM) and transmission(TEM) electron microscopy was then used to provide high power micrographs. Peritoneal macrophages are large and aggressively phagocytic cells covered by intricate folds of the outer cytoplasmic membrane. They are excellent for the identification of cell organelles within the cytosol. The earliest stages of phagocytosis as seen by SEM which involve chemotaxis and the development of cell receptor sites show that the cytoplasmic membrane enfolds the target conidia. Engulfed conidia can be seen within the phagocyte. When seen in TEM sections, membrane folds coalesce and fuse to enfold the conidia, each within a discrete phagosome. The conidial cell wall is mutilayered. A phagosome then accepts a wide range of active degradative enzymes, initially from primary lysosomes. In later stages of phagocytosis the conidia appear grossly distorted.