Ted Hall

Efficacy of Recombinant Circumsporozoite Protein Vaccine Regimens against Experimental Plasmodium falciparum Malaria

After initial successful evaluation of the circumsporozoite-based vaccine RTS,S/SBAS2, developed by SmithKline Beecham Biologicals with the Walter Reed Army Institute of Research, protective efficacy of several regimens against Plasmodium falciparum challenge was determined. A controlled phase 1/2a study evaluated 1 or 2 standard doses of RTS,S/SBAS2 in 2 groups whose members received open-label therapy and 3 immunizations in blinded groups who received standard, one-half, or one-fifth doses. RTS,S/SBAS2 was safe and immunogenic in all groups. Of the 41 vaccinees and 23 control subjects who underwent sporozoite challenge, malaria developed in 7 of 10 who received 1 dose, in 7 of 14 who received 2 doses, in 3 of 6 who received 3 standard doses, in 3 of 7 who received 3 one-half doses, in 3 of 4 who received 3 one-fifth doses, and in 22 of 23 control subjects. Overall protective efficacy of RTS,S/SBAS2 was 41% (95% confidence interval, 22%-56%; P=.0006). This and previous studies have shown that 2 or 3 doses of RTS,S/SBAS2 protect against challenge with P. falciparum sporozoites.

Long-Term Efficacy and Immune Responses following Immunization with the RTS,S Malaria Vaccine

The malaria sporozoite vaccine candidate RTS,S, formulated with an oil-in-water emulsion plus the immunostimulants monophosphoryl lipid A and the saponin derivative QS21 (vaccine 3), recently showed superior efficacy over two other experimental formulations. Immunized volunteers were followed to determine the duration of protective immune responses. Antibody levels decreased to between one-third and one-half of peak values 6 months after the last dose of vaccine. T cell proliferation and interferon-gamma production in vitro were observed in response to RTS,S or hepatitis B surface antigen. Seven previously protected volunteers received sporozoite challenge, and 2 remained protected (1/1 for vaccine 1, 0/1 for vaccine 2, and 1/5 for vaccine 3). The prepatent period was 10.8 days for the control group and 13.2 days for the vaccinees (P < .01). Immune responses did not correlate with protection. Further optimization in vaccine composition and/or immunization schedule will be required to induce longer-lasting protective immunity.

Specificities of antibodies that inhibit merozoite dispersal from malaria-infected erythrocytes

When malaria schizont-infected erythrocytes are cultured with immune serum, antibodies prevent dispersal of merozoites, resulting in the formation of immune clusters of merozoites (ICM) and inhibition of parasite growth. Antigens recognized by these antibodies were identified by probing two dimensional immunoblots of Plasmodium falciparum antigens with antibodies dissociated from immune complexes present at the surface of merozoites in ICM. Total immune serum recognized 88 of the 135 protein spots detected by colloidal gold staining, but antibodies dissociated from immune complexes recognized only 15 protein spots attributable to no more than eight distinct antigens. Antigens recognized by antibodies that inhibit merozoite dispersal include the precursor to the major merozoite surface antigens (gp195), a 126-kDa serine-repeat antigen (SERA), the 130-kDa protein that appears to bind to glycophorin (GBP130), and the approx. 45-kDa merozoite surface antigen. One other antigen (230/215-kDa doublet) was identified by using antibodies affinity purified from recombinant expression proteins. The identities of the other three antigens (150 kDa, 127 kDa and less than 30 kDa) were not determined. This approach provides a strategy for identifying epitopes accessible at the merozoite surface which may be important components of a multivalent vaccine against blood stages of P. falciparum.

Point mutations are associated with a gene duplication leading to the bloodstream reexpression of a trypanosome metacyclic VSG

African trypanosomes evade the immune response of their hosts by sequentially expressing different variant surface glycoproteins (VSGs). We isolated a bloodstream trypanosome clone of Trypanosoma brucei rhodesiense that expresses a VSG normally present during the metacyclic stage of the parasite in the insect vector. Associated with the bloodstream reexpression of this metacyclic VSG is a gene conversion in which the duplicated, expressed gene of 1650 nt contains 11 scattered point mutations when compared with its donor gene. Analysis of an uncloned population of bloodstream trypanosomes revealed another VSG reexpressor of the same donor gene in which the coding region had undergone 24 point mutations. The mutations are unique to the duplicated gene and appear to be nontemplated. The generation of these mutations provides a way for the trypanosome to increase further its antigenic diversity.

Expression site associated genes of Trypanosoma brucei rhodesiense

Upstream of at least some telomere-linked genes for the variant surface glycoproteins (VSGs) of African trypanosomes are expression site associated genes (ESAGs) whose transcription is co-ordinated with the transcription of the adjacent VSG gene [Cully et al. (1985) Cell 42, 173-182]. The function of the corresponding ESAG proteins is not known. Here we show the sequences of two members of the ESAG-I family that are upstream of the VSG genes expressed in metacyclic variant antigen types 4 and 7 of Trypanosoma brucei rhodesiense. The corresponding metacyclic ESAG-I proteins of about 330 amino acids display extensive positional identity both with each other and with two other ESAG-I proteins of Trypanosoma brucei brucei. Only about 7% of the positions are occupied by a different amino acid in each of the four putative ESAG proteins while 40% of the positions are identical. Thus, the ESAG-I proteins are much more highly conserved than are the VSGs studied to date.