Suppressive and additive effects in protection mediated by combinations of monoclonal antibodies specific for merozoite surface protein 1 of Plasmodium yoelii

Abstract

Background: The merozoite surface protein (MSP)-1 is a target antigen of protective immunity and a malaria vaccine candidate. The nature of this protective immune response warrants further investigation: although specific antibody is thought to play a major role, the mechanisms of protection are still unclear. Monoclonal antibodies (mAbs) specific for the C-terminus of MSP-1 from Plasmodium yoelii have been shown previously to provide protection against challenge infection when administered by passive immunization to mice. Three protective mAbs were re-examined and, in particular, the effect of combinations of antibodies on the protection provided was studied. It was found that a combination of two antibodies can either provide additive protective effects or result in a suppression of protection. In this report the importance of antibody subclass and epitope specificity in the outcome of these passive immunization experiments are discussed.

Methods: The minimum protective dose (MPD) for each mAb was determined, and then combinations of antibody at their MPD were investigated for their ability to control parasitaemia and promote survival in groups of mice. Mice were inoculated over three days with the MPD and challenged with a blood stage infection of the virulent P. yoelii 17 XL. The resultant parasitaemia was assessed daily on Giemsa-stained blood films. Following the infection the presence of MSP-1 specific antibodies in the sera was monitored, and the proliferative responses of cells in the spleen of protected mice were measured.

Results: Combining antibodies resulted in either an additive effect on protection, with reduced peak parasitaemia and better survival, or resulted in a suppression of protection over that achieved by a single antibody alone. An additive effect was observed when B6 and F5 that have the same isotype and similar fine specificity, were combined. However, a combination of mAb D3, an IgG2a, with either B6 or F5 (both IgG3) suppressed protection, an effect that may have been due to the combination of different isotypes or to the different fine specificity of the antibodies.

Conclusions: These results suggest that a combination of protective antibodies with either the same or different isotypes can produce either an additive or a suppressive effect in passive immunization. This phenomenon may be important in better understanding immunity in this experimental mouse model of malaria.

Figure 1

Passive immunization with the minimal protective dose (MPD) of mAb D3. The bars represent the percentage survival and the lines the percentage parasitaemia for the control group of animals and the group receiving mAb D3.

Figure 2

Passive transfer experiment with the mAbs D3 and B6 in combination. The bars represent the percentage of survival and the lines the percentage of parasitaemia.

Figure 3

Passive transfer experiment with the mAbs D3 and F5 in combination. The bars represent the percentage of survival and the lines the percentage of parasitaemia.

Figure 4

Passive transfer experiment with the mAbs B6 and F5 together in combination. The bars represent the percentage of survival and the lines the percentage of parasitaemia.

Figure 5

Western blot analysis of the sera from the surviving animals from the passive transfer experiments (PTE). (1). Normal mouse serum, (2). Serum from PTE of D3+B6, (3). Serum from PTE of D3+F5, (4). Serum from PTE of B6+F5, (5). Serum from PTE of D3, (6). Serum from PTE of B6, (7). Serum from PTE of F5. The arrow indicates the location of the recombinant 46 kDa GST-MSP-1₁₉ protein.