The fate of the circumsporozoite antigens during the exoerythrocytic stage of Plasmodium berghei

There has been considerable interest in the circumsporozoite proteins due to their potential use in anti-malarial vaccines. Previous authors have shown that these proteins persist from the invading sporozoite throughout the growing exoerythrocytic or liver stage. We show that the different distributions of these proteins seen during the development of the exoerythrocytic parasite of Plasmodium berghei closely follow morphological changes, which can be recognized under the light microscope. At the end of the exoerythrocytic cycle, the majority of the remaining circumsporozoite proteins were associated with the spongy stroma in which the emerging exoerythrocytic merozoites lay. Cell-mediated immunity originally directed against sporozoites might recognize the stroma as a second target resulting in the indirect destruction of the exoerythrocytic merozoites.

Ookinete antigens of Plasmodium berghei. Appearance on the zygote surface of an Mr 21 kD determinant identified by transmission-blocking monoclonal antibodies

Zygotes and ookinetes of the rodent malaria Plasmodium berghei can be enriched 50-fold, from whole blood cultures by ammonium chloride lysis. Three monoclonal antibodies (MoAbs) raised against such enriched preparations specifically bind to a determinant of Mr 21 kD as assessed by 125I-labelled goat anti-mouse IgG probed immunoblots of Western transfers of SDS-PAGE gels. Indirect immunofluorescence indicates that the 21 kD determinant bound by specific MoAbs, whilst not detectable on gametocytes or gametes, appears on the parasite surface within 2 h of exflagellation/fertilization and increases thereafter. The three MoAbs specifically binding the 21 kD determinant block oocyst development in mosquitoes by at least 90%, as assessed either by in-vitro membrane feeds or by live feeds on passively immunized mice. These MoAbs reduce ookinete formation in vitro by between 52 and 100%. Possible mechanisms of action of these MoAbs are discussed.

Immunoelectron microscopic localization of circumsporozoite antigen in the differentiating exoerythrocytic trophozoite of Plasmodium berghei

The distribution of the circumsporozoite (CS) antigens in the 24 hour exoerythrocytic trophozoite of P. berghei was studied using Lowicryl immunogold electron microscopy. These antigens were present on the plasmalemma of the parasite, in disrupted areas of the host cell cytoplasm adjacent to the trophozoite and around inclusions of the host cell cytoplasm. There was evidence of a redistribution of the CS antigens away from the pellicular region of the sporozoite.

The complete development in vitro of the vertebrate phase of the mammalian malarial parasite Plasmodium berghei

All three 'vertebrate' stages of the rodent malarial parasite Plasmodium berghei berghei were grown in vitro in the absence of the vertebrate host. The parasite was introduced into culture from infected mosquitoes and 2 in vitro culture methods were used sequentially to complete the 'vertebrate' phases of development in hepatoma and erythrocyte host cells. The resultant blood infection produced mature schizonts and male and female gametocytes. The protocol, which is now being extended to the human pathogen P. falciparum, may assist future studies on this important group of parasites.

The in vitro cultivation of P. falciparum ookinetes, and their enrichment on Nycodenz density gradients

Employing a simple method of growing ookinetes of Plasmodium falciparum in culture, 40% of mature gametocytes convert to macrogametes, 4% reach the retort-form ookinete stage and 0.45% become mature ookinetes. A single-step gradient centrifugation method on 12.5% Nycodenz enriches both gametocytes and ookinetes.

Ookinete antigens of Plasmodium berghei: a light and electron-microscope immunogold study of expression of the 21 kDa determinant recognized by a transmission-blocking antibody

The expression of a 21 kDa transmission-blocking determinant on the malarial parasite Plasmodium berghei was studied by using the immunogold method at the light, scanning-electron and transmission-electron microscope levels. The determinant was shown to be expressed exclusively on the macrogamete and its immediate progeny the zygote, ookinete and oocyst. It is first detected on the plasmalemma two hours after the escape of the parasite from the red blood cell, reaches a maximal density on the young ookinete some ten hours later, and is still found on the oocyst after six days. The antigen is distributed evenly over the entire surface of the zygote and ookinete, but is readily shed from the parasite surface. The general applicability of the silver-enhanced immunogold method in parasitological research is emphasized.

Fine structure of the malaria parasite Plasmodium falciparum in human hepatocytes in vitro

Recent advances in the ability to culture the hepatic forms of mammalian malaria parasites, particularly of the important human pathogen Plasmodium falciparum have provided novel opportunities to study the ultrastructural organisation of the parasite in its natural host cell the human hepatocyte. In this electron-microscopic and immunofluorescence study we have found the morphology of both parasite and host cell to be well preserved. The exoerythrocytic forms, which may be found at densities of up to 100/cm2, grow at rates comparable to that in vivo in the chimpanzee. In the multiplying 5- and 7-day schizogonic forms of the ultrastructural organisation of the parasite bears striking resemblances to other mammalian parasites, e.g., the secretory activity and distribution of the peripheral vacuole system, but also homology with avian parasites, e.g., in nuclear and nucleolar structure and mitochondrial form. The latter homologies support earlier suggestions of the close phylogenetic relationship of P. falciparum with the avian parasites. Evidence is also presented showing the persistence of the cytoskeleton of the invasive sporozoite within the cytoplasm of the ensuing rapidly growing vegetative parasites.

Identification of the meiotic division of malarial parasites

Zygotes of Plasmodium berghei were cultured 15-25 h in vitro to yield mature infective ookinetes. Samples taken in the first 5 h of culture were examined by electron microscopy. Meiotic figures were detected in the nuclei of the zygotes. Threadlike leptotene chromatids (chromosomes) condensed from attachment plaques on the nuclear envelope; chromatid pairing followed (zygotene), with synaptonemal complexes subsequently appearing (pachytene). These complexes persisted into metaphase but dissociated when the chromatids rapidly decondensed during anaphase. At telophase of the first meiotic division the kinetochores were retracted toward two small spindle complexes, which were found at widely separated poles in the nuclear envelope. The observations are consistent with a haploid genome of 8-10 chromosomes.

The development of Plasmodium ookinetes in vitro: an ultrastructural study including a description of meiotic division

Ookinetes have been cultured in vitro using modifications to the method of Weiss & Vanderberg (1977). Significant improvements in technique were produced by culture in medium at pH 8.4 and at a blood dilution at or over 1/10. Ookinetes produced were infective to mosquitoes by membrane feeding techniques. Ultrastructural analyses were made of nuclear, cytoskeletal, crystalloid and microneme development. The first intranuclear division in the zygote has been recognized as meiosis. Chromosome condensation during prophase follows the classical stages of leptotene, zygotene and pachytene. Diplotene and diakinesis are not present - the synaptonemal complexes persist into metaphase I. Chromosomes separate at anaphase and rapidly de-condense prior to telophase. We have not recognized a second meiotic division in the ookinete. The implication of these findings to the molecular and Mendelian organization of the parasite genome are discussed.

A cell biologist's view of host cell recognition and invasion by malarial parasites

The migration of various Apicomplexan parasites through host cells and tissues is examined. Two approaches are taken. First a comparative examination of the invasive stages from an ultrastructural and biochemical viewpoint. Second a critical review of the mechanism of recognition and invasion of host cells. The ultrastructural organization of the invasive stages is highly conserved and invariably includes three classes of organelle whose probable functions can be deduced. These organelle classes are: The rhoptry/microneme/osmiophilic body/microsphere complex. These electron-dense, membrane-limited organelles are situated close to the parasite plasmalemma, and are often concentrated or specifically localized within the apical complex. Their contents are probably secreted by the parasite and induce various modifications in the host cell plasmalemma. However, it is concluded that the exact role of these organelles has not yet been determined nor their mechanism of action adequately evaluated. The microtubule cytoskeleton and associated structures (e.g., apical rings and conoid, etc.). These are structural organelles, and the microtubules probably direct the locomotion of the parasites. Locomotion is possibly achieved by a directed capping reaction. The plasmalemma and inner membrane vacuoles. The plasmalemma has a glycocalyx of varying complexity which includes molecules capable of host cell recognition and binding. It is speculated that host cell invasion is achieved by the directed capping of these molecules. Whilst appreciating the very specific nature of host cell recognition and invasion in some parasite stages (e.g., RBC invasion by the malarial merozoite), a marked contrast is observed in the ability of other stages of the malarial life-cycle (sporozoite and ookinete) to invade, migrate through, and escape from varied types of host cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Observations on early and late post-sporozoite tissue stages in primate malaria. III. Further attempts to find early forms and to correlate hypnozoites with growing exo-erythrocytic schizonts and parasitaemic relapses in Plasmodium cynomolgi bastianellii infections

Rhesus monkeys were heavily infected with sporozoites of Plasmodium cynomolgi bastianellii in an attempt to demonstrate the site of invasion of sporozoites into tissue cells and their growth there. Further attempts were made to correlate the appearance and loss of hypnozoites with parasitaemic relapses. Hypnozoites were demonstrated and once again shown to decrease in numbers over 229 days during which time the infection showed parasitaemic relapses. Liver biopsies taken at two-day intervals for 12 days showed that hypnozoites decreased in numbers over-all and growing schizonts were demonstrated in the liver. At this time a parasite the size of a hypnozoite was seen with two nuclei and another was seen with an elongate, possibly dividing nucleus in one monkey. an attempt to find the location of the early intracellular exoerythrocytic forms in the liver at various times less than 40 hours after infection using smears and immunological staining with newly prepared anti-sera failed. Large numbers of sporozoites of P. knowlesi were also injected into a rhesus monkey the liver of which on the fifth day after infection showed no hypnozoites among 157 sections of growing schizonts and no parasites at all on the 42nd day after infection. In P. cynomolgi bastianellii infections parasites, mostly hypnozoites, were found in the liver up to 229 days after infection.

Gametocytogenesis of Plasmodium falciparum in vitro: a simple technique for the routine culture of pure capacitated gametocytes en masse

A technique is described for the growth of pure gametocyte cultures of Plasmodium falciparum. Using the classification of Hawking, Wilson & Gammage (1971) these cultures contain gametocytes of stages III, IV and V alone. Routine sexual cultures, varying from 0.35 ml static cultures to 500 ml shaking cultures, are exposed to an inhibitor of DNA synthesis (mitomycin C at 10 micrograms/ml) on the 11th day of culture, and the culture is harvested on the 14th day when capacitated stage V gametocytes are present. All other stages are killed by the drug and are morphologically degenerate within 2 days of the addition of the inhibitor.

Studies on the kinetics of uptake and distribution of free and liposome-entrapped primaquine, and of sporozoites by isolated perfused rat liver

The kinetics of uptake and the distribution of free primaquine differed markedly from that of liposome-entrapped primaquine. The uptake of the liposome-entrapped drug (LPQ) was gradual, reaching a plateau of 60% of the initial load after 20 minutes of perfusion. However, clearance of the free drug was almost immediate, reaching its maximum uptake of 44% within five minutes. Some interactions also seen between liposomes and malarial sporozoites. In mixed perfusions the removal of LPQ was enhanced whilst the uptake of sporozoites remained normal. Liposome uptake was significantly lower in livers obtained from silica-treated animals, in which Küpffer cell numbers are depleted. Further, analysis of the radioactive content of hepatocyte and Küpffer cell fractions following perfusion with 3H and 14C labelled liposomes suggested that the vesicles were concentrated in the latter cell type.

Ultrastructural observations on the infection of rat liver by Plasmodium berghei sporozoites in vivo

The invasion of liver parenchymal cells by sporozoites of Plasmodium berghei Vincke & Lips, 1948, was studied in vivo using transmission electron microscopy. Livers of Brown Norway rats were examined 30 and 60 min after intraportal injection of 15 million sporozoites each. Sporozoites found after incorporation into vacuoles in hepatocytes were often located near a bile canaliculus at the lateral cell surface, surrounded by hepatocyte lysosomal structures; however, degradation of sporozoites caused by lysosomal digestion inside hepatocytes was never observed. Due to the crescent shape of sporozoites, serial sections were necessary to demonstrate the actual process of invasion of the hepatocyte. The hepatocyte's plasmalemma appeared to invaginate due to the sporozoite's action, thereby creating a parasitophorous vacuole. It was suggested that the sporozoite actively penetrated the hepatocyte; however, no visible depletion of rhoptries and micronemes was observed.

The cell biology of sexual development in plasmodium

This review examines the sexual development of Plasmodium spp. particularly relating the ultrastructural organization of their cells to the limited, though rapidly expanding, body of metabolic and biochemical studies. Thus it is hoped the article may provide a useful background of information for those undertaking studies on the sexual parasites with the objective of developing methods for the immunological and chemotherapeutic control of malaria transmission. These objectives, however, should not dominate our clear recognition that the three phases of sexual development, gametocytogenesis, gametogenesis and fertilization contain within them examples of control and assembly of organelles without peer amongst eukaryotic cells.

Malaria parasites--discovery of the early liver form

Infections of mammalian malaria parasites start when sporozoites from an infected anopheline mosquito are injected into the bloodstream of the host. The sporozoites enter the hepatocytes and become transformed into exoerythrocytic schizonts. Since the discovery of the primate parasite Plasmodium cynomolgi in monkey hepatocytes and the rodent parasite Plasmodium berghei in hamster hepatocytes, the ultrastructure of these stages has been extensively studied both in primate and rodent plasmodia. These observations relate only to the development of the exoerythrocytic schizont 25 h after sporozoite injection until the final maturation (of P. berghei) 50 h post-inoculation. Recently, we have studied the route of entry of sporozoites across the cellular lining of liver sinusoids and invasion of the liver parenchymal cells by using transmission electron microscopy. The results of these studies in combination with other physiological experiments strongly suggested that the sporozoite was initially harboured by the Kupffer cell, from which the parasite escaped into the neighbouring hepatocyte. The migration of sporozoites from liver sinusoids to hepatocytes can be achieved within a few minutes. We present here the first ultrastructural observations on the natural transformation of intrahepatocytic sporozoites into exoerythrocytic forms in vivo, using the rodent malaria parasite P. berghei in a laboratory host, the Brown Norway rat. These observations complete the search for the final link in the life cycle of malaria parasites.