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In a recent study (1), we demonstrated a role for the macrophage (MO) type 3 complement receptor (CR3) in serum-independent binding of Leishmania donovani promastigotes to murine resident peritoneal Mo (RPM). Earlier studies had also established a role for CR3 in serum-independent binding of the yeast wall product zymosan to murine and human MO (2) and to human neutrophils (3). In all three studies, binding of the parasite/particle was inhibited by mAbs (M1/70, Mot, OKM1, MN-41, and anti-Leu-15) directed against the a-chain of CR3 . In the neutrophil system it was argued that binding of zymosan to CR3 involved direct lectin-like recognition processes not mediated by complement, as binding was inhibited by an anti-CR3 mAb (OKM1) not specifically directed against the iC3b binding site, but not inhibited by Fab anti-C3 (3). In the MO system it was postulated that complement proteins secreted by MO mediate local iC3b-opsonization of zymosan and promastigotes, both of which are good activators of the alternative complement pathway (1, 2). This was supported by three independent lines of evidence . First, binding of zymosan to MO was also reduced after treatment of M¢ with cyclohexamide, an inhibitor of protein synthesis (2). Second, the potent inhibition of promastigote binding to RPM using MI/70 was completely mimicked with Fab anti-C3 or the nucleophile sodium salicyl hydroxamate (1). The latter is a potent inhibitor of the covalent binding of activated C3 to the activator surface (4), thus providing strong evidence that cleaved C3, in the degraded form iC3b, on the surface of promastigotes mediates binding to CR3. Finally, under assay conditions identical to those used in binding assays, it was shown (2) by SDS polyacrylamide gel analysis that MO-derived C3 (iC3b) can be deposited on zymosan in the absence of exogenous complement . In this study, the evidence for local iC3b-oponization is extended by direct visualization of Mo-derived C3 on the surface of L. donovani promastigotes using an anti-C3 antibody and a protein A-gold conjugate in electron microscope sections .

The sheath and cuticle of microfilariae of Brugia pahangi were examined by electron microscopy and the presence of various proteins, carbohydrate and enzymes sought. The epicuticle of microfilariae consists of a pentalaminate structure (24.0 +/- 1.4 nm), a cortex (13.7 +/- 3.6 nm) and a basal zone (27.8 +/- 4.8 nm) which is often banded in appearance. The pentalaminate layers are not continuous at the base of the interannular grooves. The sheath and the epicuticle of B. pahangi stained positively with concanavalin A and saccharated iron oxide. The sheath of approximately 50% of microfilariae showed activity for acid phosphatase, 5' nucleotidase and peroxidase, but not for ATPases, alkaline phosphatase or esterase. No enzymes were detected in the epicuticle although the cortex and basal layers of the cuticle did show enzymic activity. Structures beneath the cuticle in the main body of the worms contained considerable enzymic activity. Microfilariae directly isolated from the blood of infected cats were found by immunochemical means to carry serum proteins on their sheaths but not on their cuticles. These studies extend the definition of the outer structures of microfilariae and confirm that they significantly differ in morphology and enzyme content from typical mammalian cell membranes.