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The number of malaria sporozoites delivered to a host by mosquitoes is thought to have a significant influence on the subsequent course of the infection in the mammalian host. We did studies with Anopheles stephensi mosquitoes with salivary gland infections of Plasmodium berghei sporozoites expressing a red fluorescent protein. After individual mosquitoes fed on an ear pinna or the ventral abdomen of a mouse, fluorescence microscopy was used to count numbers of sporozoites. Mosquitoes allowed to feed on the ear for periods of 3 versus 15 min deposited means of 281 versus 452 sporozoites, respectively, into the skin; this may have epidemiological implications because mosquitoes can feed for longer periods of time on sleeping hosts. Mosquitoes feeding on the ventral abdomen injected sporozoites not only into the skin but also into the underlying peritoneal musculature. Although mosquitoes injected fewer sporozoites into the abdominal tissues, more of these were reingested into the mosquito midgut, probably a consequence of easier access to blood intake from the abdominal area. The most consistent parameter of sporozoite transmission dynamics under all conditions of mosquito probing and feeding was the relatively slow release rate of sporozoites (approximately 1 to 2.5 per second) from the mosquito proboscis. The numbers of sporozoites introduced into the host by mosquitoes and the transmission efficiencies of sporozoite delivery are multifactorial phenomena that vary with length of probing time, skin site being fed upon, and numbers of sporozoites within the salivary glands.

A monoclonal antibody directed against the circumsporozoite protein on the surface of Plasmodium berghei sporozoites inhibited sporozoite motility in vitro. These immobilized sporozoites could adhere to but not invade target cultured cells. Other sporozoite-immobilizing agents also inhibited sporozoite invasion into cultured cells and did not prevent sporozoite adherence. These results indicate that sporozoite invasiveness is associated with sporozoite motility. Thus, the immobilizing effect of this antibody could explain its functional activity against sporozoite invasion in vivo.

We investigated the effect of both immune and normal sera on the binding of free Plasmodium berghei by resident and activated macrophages. Resident macrophages bound plasmodia to a greater extent than did activated macrophages, regardless of treatment. Resident macrophages bound free plasmodia, predominantly trophozoites, in the presence of normal serum by a mechanism inhibited by N-acetylglucosamine and N-acetylmannosamine. Macrophages activated through treatment with Propionibacterium acnes ("Corynebacterium parvum"), on the other hand, did not bind free plasmodia in the presence of normal serum through systems inhibited by N-acetylmannosamine or N-acetylglucosamine. The binding of free plasmodia by activated macrophages was greatest in the presence of immune serum and could be inhibited by immune complexes but not by N-acetylmannosamine or N-acetylglucosamine. These results suggest that a receptor for a carbohydrate component of a normal serum opsonin mediates initial adherence of plasmodial antigen onto resident macrophages, triggering both the immunological cascade and macrophage activation. After activation, the macrophages no longer have the carbohydrate-specific receptor but do have functional Fc receptors which mediate the adherence of immune-serum-opsonized plasmodia.

Morphological and cytochemical surface characteristics of isolated malaria parasites (Plasmodium berghei) and host erythrocytes were compared by electron microscopy by using thin section and carbon replica techniques. Erythrocytes were uniform in shape and had fine, granular surfaces. In contrast, free parasites exhibited a variety of sizes, shapes, and surface textures. Fine surface stippling was a common topographical feature of isolated parasites. Small, infective forms often had patterned surfaces resulting from the protuberance of an underlying thick intermediate layer. Results of cytochemical analysis using a sialophilic colloidal iron stain indicated that the malaria parasite's surface lacked exposed sialic acid groups which would normally give rise to a net negative surface charge common to erythrocytes. Biochemical assay demonstrated that malaria parasites contained about one-half the amount of sialic acid per unit weight as did control red cell extracts. Similarly, external acidic mucopolysaccharide coats of free parasites, as revealed by ruthenium red staining were extremely thin as compared with the thick glycocalyx layer of red cells. Lipid plaques at the surface of parasites and red cells were localized by lipophilic iron colloid staining. Although the gross patchwork distribution of plaques was somewhat similar for the two cell types, the parasites were stained more intensely and had a closer-knit patchwork pattern than those exhibited by the erythrocytes. Such findings indicate that there are slight differences in the arrangement of phospholipids at the surfaces of limiting membranes of host cells and parasites. The significance of the above cytochemical surface properties of the malaria parasite (which are seemingly akin to those of intracellular organelles is discussed in relation to certain host-parasite interactions, such as parasite adhesion to target cells and enhanced clearance of extracellular parasites.