Bockarie, J

Bockarie, J. has been exerted in the past two decades in identifying, characterizing, and testing various stage-specific malaria antigens as potential vaccine candidates. As a result, a number of candidate vaccines have undergone phase I and phase II clinical trials, with promising results. Some of the antigens (30) that have been well-characterized include circumsporozoite protein (CS) and thrombospondin-related anonymous protein, expressed in sporozoites; merozoite surface protein 1 (MSP-1), apical membrane antigen 1, and erythrocyte binding antigen 175, expressed on asexual blood stages; and P25, P48/45, and P230, expressed around the sexual stages of the parasites. Though much is now known about the mechanisms of immunity and immune responses to some of these candidate vaccines, the main challenge that has been encountered in evaluating the functional in vivo efficacy of vaccine-induced immune responses is the lack of suitable small-animal models. By and large, most of the available assays are only in vitro surrogates, such as ILSDA (8) and hepatic invasion assays (17, 21, 31) for assessing neutralizing antisporozoite immunity, GIA (22, 24, 25, 32, 33) for assessing functional immune responses to asexual parasite stages, and MFA (3, 14, 18) for measuring immune responses against surface antigens present on gametocytes, gametes, or ookinete stages of the parasite. Nonhuman primates such as and monkeys can be infected with adapted human malaria parasites, enabling the assessment of functional immune responses against malaria antigens in vivo; however, these animal models are not widely accessible, and the cost of maintaining primates is usually a limiting factor (38). The availability of suitable small-animal models for the in vivo assessment of vaccine-induced functional immune responses may play a significant role in the development and functional assessment of vaccines against human malaria. High-efficiency transfection protocols (11) have enabled the transfer of genes from human malaria parasites into rodent malaria parasites with relative ease, and here, we discuss and review the potential and feasibility of using such transgenic parasites (Table ?(Table1)1) in assessing antibody responses to various human malaria parasite stage-specific target antigens. TABLE 1. Transgenic parasites with potential use in assessing the functionality of human malaria immunity in vivo parasite(s)hybrid parasite expressing the repetitive region of CS (PfCS). In this transgenic parasite, the repetitive region of CS made up of B- and CD4+ T-cell epitopes essential for antisporozoite protective immunity was replaced with the corresponding domains from the CS OF-1 of 18S RNA by real-time PCR (17). The OF-1 transgenic sporozoite neutralization assay has been used to analyze serum samples from humans immunized with the CS-based peptide (T1B)4 MAP (17) and a different CS-formulated vaccine comprising B- and CD4+ and universal T-cell epitopes (10). Thus, CS(Pf) transgenic parasites can PTEN1 be applied in in vitro functional OF-1 neutralization assays that may be more predictive of protection than assays such as enzyme-linked immunosorbent assays or immunofluorescence assays. Through the use of transgenic rodent parasites, it has become possible to quantify infectivity, as well as delineate humoral and cellular immune responses that are crucial in anti-CS immunity (10, 17). The use of the CS(Pf) transgenic parasites has also exhibited that linear peptides such as those described above need to be further explored for their potential as vaccines (6). Further vaccine immunogenicity and CS(Pf) sporozoite challenge studies can also help in dissecting the potential roles of various arms of cellular immune responses. The advantages of using CS(Pf) have been further highlighted by the findings of studies demonstrating good correlation between the neutralization observed in ILSDA and sterile immunity in vivo as shown by the lack of liver exoerythrocytic forms (28). Such studies are important.