Circumsporozoite protein is required for development of malaria sporozoites in mosquitoes.

Malaria parasites undergo a sporogonic cycle in the mosquito vector. Sporozoites, the form of the parasite injected into the host during a bloodmeal, develop inside oocysts in the insect midgut, then migrate to and eventually invade the salivary glands. The circumsporozoite protein (CS), one of the major proteins synthesized by salivary gland sporozoites, is a surface-associated molecule which is important in sporozoite infectivity to the host. Here, by gene targeting, we created Plasmodium berghei lines in which the single-copy CS gene was disrupted. The CS(-) and wild-type parasites produced similar numbers of oocysts of comparable size in the mosquito midgut. In the CS(-) oocysts, however, sporozoite formation was profoundly inhibited. CS therefore appears to have a pleiotropic role and to be vital for malaria parasites in both the vector and the host: in mosquitoes, CS is essential for sporozoite development within oocysts, and in the vertebrate host it promotes sporozoite attachment to hepatocytes.

The transcript encoding the circumsporozoite antigen of Plasmodium berghei utilizes heterogeneous polyadenylation sites.

We have employed polymerase chain reaction-based techniques to examine the transcript encoding the circumsporozoite (CS) antigen, the immunodominant coat protein of the infectious stage of the murine parasite Plasmodium berghei. Earlier studies suggested that the 3' terminus of the CS message might be determined by transcription termination rather than by cleavage and polyadenylation, as in most eukaryotes. Here we report that a subset of CS messages are polyadenylated. Moreover, the poly(A) tails are added at multiple sites clustered within a short region 300 bp downstream from the stop codon. Whether 3' end heterogeneity is peculiar to the CS message or a common feature of plasmodial transcripts remains to be determined.

Cellular and humoral immune responses to a recombinant P. falciparum CS protein in sporozoite-immunized rodents and human volunteers.

The immune responses of sporozoite-immunized rodents and of human volunteers exposed to multiple bites of irradiated Plasmodium falciparum infected mosquitos have been investigated using a yeast-derived recombinant P. falciparum circumsporozoite (rPfCS) protein. The murine immune response to immunization with rPfCS was not genetically restricted. Nine different murine haplotypes, when immunized with rPfCS, developed high levels of antisporozoite antibodies detectable by IFA and RIA. In addition, injection of rPfCS induced a secondary antibody response in P. falciparum sporozoite-primed mice. Murine T-cell epitopes were mapped in the C terminus of the rPfCS protein using overlapping synthetic peptides. The human T-cell response was investigated using T-cell clones derived from peripheral blood lymphocytes (PBL) of a P. falciparum sporozoite-immunized volunteer. A total of 40 CD4+ T-cell clones were obtained. Stimulation indices ranged from 2.5 to 103.4 following challenge with rPfCS in the presence, but not in the absence, of antigen-presenting cells. The clones were specific for rPfCs and did not proliferate or secrete lymphokines when challenged with yeast-derived recombinant P. vivax or P. berghei CS protein or with a yeast-extract control. The clones also recognized the native CS protein in extracts of P. falciparum, but not P. berghei or P. cynomolgi, sporozoites.

Conserved repetitive epitope recognized by CD4+ clones from a malaria-immunized volunteer.

T cell clones obtained from a human volunteer immunized with Plasmodium falciparum sporozoites specifically recognized the native circumsporozoite (CS) antigen expressed on P. falciparum sporozoites, as well as bacteria- and yeast-derived recombinant falciparum CS proteins. The response of these CD4+ CD8- cells was species-specific, since the clones did not proliferate or secrete gamma interferon when challenged with sporozoites or recombinant CS proteins of other human, simian, or rodent malarias. The epitope recognized by the sporozoite-specific human T cell clones mapped to the 5' repeat region of the CS protein and was contained in the NANPNVDPNANP sequence.

Interferon-gamma inhibits the intrahepatocytic development of malaria parasites in vitro.

In this study, we examined the activity of recombinant interferon (IFN)-gamma against Plasmodium berghei exoerythrocytic forms (EEF) grown in vitro within the highly differentiated human hepatoma cell line HEPG2. We assayed the effect of IFN-gamma on parasite growth by DNA hybridization using a P. berghei specific DNA probe. The specific activity of IFN-gamma against EEF is very high, and depends upon the time of lymphokine addition. When IFN-gamma is added to HEPG2 cells containing intracellular EEF, 6 hr after sporozoite invasion, parasite DNA replication is inhibited by approximately 75% at 10(3) U/ml and 50% at 1 U/ml. This treatment can either abolish or greatly reduce the infectivity of EEF for mice. When added earlier, 3 hr after completion of sporozoite invasion, IFN-gamma inhibits parasite replication to an even greater degree. The highest levels of inhibition were obtained when IFN-gamma was added 6 hr prior to sporozoite invasion (100% inhibition at 10(2) U/ml, approximately 55% inhibition at 0.1 U/ml, and 17% inhibition at 0.001 U/ml). We found that HEPG2 cells express approximately 44,000 surface receptors for IFN-gamma. These data are consistent with the view that IFN-gamma exerts its antimalarial activity by binding to surface receptors on hepatocytes and inducing intracellular changes unfavorable for parasite development. Tryptophan starvation does not appear to be involved in this process. These findings also support the idea that IFN-gamma, released from immune T cells upon encountering sporozoite antigen, may be an important effector mechanism in sterile immunity to sporozoite challenge.

Use of a DNA probe to measure the neutralization of Plasmodium berghei sporozoites by a monoclonal antibody.

A specific DNA probe has been used to quantify the neutralizing effects of monoclonal antibodies (3D11) against the circumsporozoite protein of Plasmodium berghei sporozoites. The amount of parasite DNA was measured in the livers of Norway Brown rats at the peak of proliferation of the exoerythrocytic forms (EEF). In vitro treatment of 1.5 X 10(5) sporozoites with 0.36 microgram/0.5 ml of whole 3D11 IgG neutralized about 90% of the sporozoite infectivity. When the dose was 3.6 micrograms no signal was detected, indicating that less than ten sporozoites developed into EEF in the liver. In contrast, 3.6 micrograms of Fab obtained from 3D11 neutralized sporozoite infectivity by only 60%. Although the neutralizing effect of 3D11 was very marked, the infected rats developed parasitemias after a prolonged delay in patency, suggesting that a small proportion of sporozoites was resistant to the effects of 3D11. The sporozoites were subjected to four cycles of 3D11-mediated selection, each one involving treatment of sporozoites with the antibodies, injection of the mixture into rats, infection of hamsters with blood stage parasites obtained from the rats, feeding of Anopheles stephensi on these hamsters, and obtaining sporozoites from the salivary glands of the infected mosquitoes. After four cycles of selection, the susceptibility of the resulting sporozoites to different concentrations of 3D11 was compared with that of nonselected sporozoites. No differences were detected, indicating that the capacity of a few sporozoites to escape the neutralizing effect of 3D11 antibodies is not inherited.

Carrier-linked primaquine in the chemotherapy of malaria.

The antimalarial effect of intravenously administered primaquine (PQ) can be improved and its toxicity diminished by linking it to a macromolecular carrier protein. A thiol-containing primaquine derivative 8-[[4-(2-amino-3-mercaptopropionamido)-1-methylbutyl]amino]-6- methoxyquinoline was synthesized. This compound could readily be linked via a disulfide bond to a carrier protein containing (pyridyldithio)propionate groups. The derivative was coupled to serum albumin as well as to serum albumin that contained covalently linked lactose residues. The protein-drug conjugates were tested for their antimalarial activity in mice inoculated with Plasmodium berghei. The causal prophylactic activity of the conjugate with the lactosaminated serum albumin was 2 times higher than that of the free drug; the mean causal prophylactic doses (CPD50) were 6 and 13 mg of primaquine base/kg, respectively. Moreover, its acute lethal toxicity had decreased at least 6.5-fold (mean lethal dose (LD50) greater than 85 mg of primaquine base/kg). The therapeutic index of this conjugate was at least 12 times higher than that of the free drug. This allowed the administration of a dose that cured 100% of the animals (17.5 mg of primaquine base/kg), in a single injection. With unmodified serum albumin the conjugate showed an increased therapeutic efficacy (the CPD50 was approximately 10 mg of primaquine base/kg) and a strongly reduced lethal toxicity.