The circumsporozoite protein is the main sporozoite coat protein. It has a central area consisting of repeat sequences that are highly immunogenic and present in the differing malaria species. Studies reveal that the central repeat region can elicit a B-cell immune response, producing antibodies that block sporozoites in culture. The CS protein may also be an important stimulator of CTLs. One experiment has shown that mouse models and a recent vaccine trial have indicated the importance of T-cell immunity to the CS protein. However, one of the major problems of the development of a CSP-based vaccine is that human T-cell epitopes, identified on the CSP, span regions of significant point mutational polymorphism (Zevering).

The immunogenic properties of three different multispecies multiple antigen constructs (MACs) containing CSP repeats of P. falciparum and P. vivax have been characterized. Results demonstrated that naturally immunogenic epitopes from different species of malaria parasites can be incorporated in a single vaccine construct to induce immune responses against multiple epitopes (Udhayakumar).

SSP2 is a protein found on the surface of sporozoites from the murine malaria parasite P. yoelii. The equivalent protein found in P. falciparum is known as thrombospondin-related anonymous protein (TRAP). It possesses a sequence homologous to the conserved region II of the CS protein. Antibody raised to recombinant P. falciparum TRAP was demonstrated to inhibit sporozoite invasion of hepatoma cells in vitro. The passive transfer of TRAP/SSP2 specific CD8+ CTL protected naïve recipients against sporozoite challenge. In naturally infected individuals living in Africa and also in sporozoite-immunized volunteers, CD8+ CTL specific for P. falciparum TRAP/SSP2 have been detected (Nardin and Nussenzweig).

The liver stage-specific antigen 1 is first expressed in infected liver cells. LSA-1 has a 17 amino acid repeat that is immunogenic. Studies involving naturally exposed West Africans revealed that they produced CTLs against a peptide from LSA-1 which correlated with protection from severe malaria. An enzyme- linked immunosorbent assay (ELISA) has been used to show increased serum antibody reactive to synthetic peptides representing the LSA- 1-based repeat sequence in mice immunized with irradiated sporozoites. In a study in Papa New Guinea, resistance to falciparum malaria was correlated with CD8+ T-cell IFN-g responses to a LSA-1 epitope that contained an HLA All restricted sequence. Since All is more than 40% frequency in this population, it has been proposed that P. falciparum has driven genetic selection of certain HLA haplotypes (Hollingdale).

MSP-1 is the major surface antigen of merozoites and is the best studied merozoite stage protein. During the invasion process, several portions of the 195-kDa merozoite surface protein 1 are shed, leaving a highly conserved 19-kDa C-terminal processing fragment (MSP-1(19)) that contains epitopes targeted by antibodies that inhibit erythrocytic invasion. This 19-kDa antigenic domain is a potential malaria vaccine candidate. Four known alleles (E-TSR, E-KNG, Q-KNG, and Q-TSR) of this domain have been identified. A 1998 study revealing the presence of three new alleles (E-KSG-L, E-KSR-L, and E-KNG-F) suggests that it may be possible to predict the complexity of the genetic makeup of natural parasite populations (Qari).

A recent 1998 study compared the immunogenicity of conserved and polymorphic regions using recombinant antigens representing the conserved N terminus (Block 1), the conserved C terminus, and the three main types of the major polymorphic region (Block 2). These antigens were used to determine the specificity and longitudinal patterns of IgG Ab repsonses to MSP-1 in individuals. Out of a study population of 52 donors, responses to the conserved C-terminal Ag occurred in the majority of acutely infected individuals and thus were a reliable indicator of recent clinical infection. Responses to the polymorphic Block 2 region were recognized by many, but not all, individuals after clinical malaria infections. Responses to all of these Ags declined within a few months of drug treatment and parasite clearance, indicating that naturally induced human Ab responses to MSP-1 are short-lived (Cavanagh).

Another experiment demonstrated that serum antibodies from malaria immune donors can inhibit merozoite dispersal by forming immune complexes through surface-accessible regions of membrane associated antigens. These merozoite forms are known as immune clusters of merozoites (ICM). Antibodies dissociated from ICM of P. falciparum were shown to identify a restricted subset of Ags, including MSP-1 (Lyon).

MSP-2 is a second merozoite surface antigen with a molecular size of 45-kDa. MSP-2 has a central repeat region that has significant variation among isolates while the N- and C- terminal regions are well conserved. The antibody response is directed almost completely towards variant regions of MSP-2. The conserved regions are rarely recognized. Data suggest that age/exposure-related acquisition of IgG3 antibodies to MSP-2 may contribute to the development of clinically protective immunity to malaria (Taylor).

EBA-175 is a transmembrane protein characterized by two conserved cysteine-rich domains and is expressed on the surface of merozoites. It thought to be involved in attaching merozoites to erythrocyte receptors via sialic acid during invasion. Immunizing rabbits with portions of EBA- 175 has been shown to elicit blocking activity of merozoite invasion of erythrocytes (Clough).

RESA is expressed on the surface of erythrocytes infected with ring stage parasites. It is also known as Pf155. Although antibodies to RESA have not been found to affect the intra-erythrocytic development of ring stages or trophozoites, they have decreased the number of ring stages after reinfection. Antibodies to RESA have been implicated in the inhibition of merozoite invasion. RESA conjugated with CSP and immunoadjuvants such as lauroyl tetrapeptide induced high titre and affinity antibodies when compared to RESA alone (Chaba).

Antibodies to the non-repeat cytoadherence-related motif of Pf155/RESA, as well as antibodies raised against a multiple antigen peptide (MAP) based on a corresponding non-repeat band 3 motif, inhibited cytoadherence but not parasite growth. In contrast, antibodies to sequences adjacent to the Pf155/RESA cytoadherence motif inhibited parasite growth in vitro but had no effect on cytoadherence (Siddique).

The proliferative and Th cell responses to the P. chabaudi adami DS homologue of the P. falciparum AMA-1 has been investigated. A limited number of major T cell sites were identified in both conserved and variable regions of the protein. Several cryptic epitopes that evoked T cell responses following immunization with peptides, but not after protein immunization, were also identified (Amante).

PfEMP1 is a heterogeneous family of proteins that are involved in adhesion of P. falciparum infected erythrocytes to the capillary endothelium of the host. Antibodies to variable regions of these proteins, measured by agglutination, correlates with clinical protection against falciparum malaria.. During falciparum malaria infection, adhesion of parasitized red blood cells (PRBCs) to endothelial cells is mediated by the interaction of PfEMP-1 on the PRBC surface with receptors on the surface of endothelial cells, including CD36 (Staalso).

A recombinant 179-residue subfragment of PfEMP-1 (rC1-2[1-179]), which encompasses the CD36-binding region, inhibits and reverses adhesion of PRBCs to CD36 under physiologically relevant flow conditions. This activity was not strain-specific and virtually inhibited adhesion of four different parasite lines. The effect of rC1-2[1-179] was, however, specific for CD36-mediated adhesion and had no effect on adhesion mediated by CSA (Cooke).

Sequestrin is a 270-kDa surface knob protein that is expressed on the surface of infected erythrocytes and it is a ligand for CD36, which is a recognition protein expressed by vascular endothelium. This knob has been identified and characterized to a certain extent. Immunotherapeutic agents can be developed to block as well as reverse the cytoadherence phenomenon. This surface protein is also a good vaccine candidate except that it shows a high rate of antigenic variation (Sharma).