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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.

Previously, we have identified a protein in Trypanosoma equiperdum that possesses homology with the regulatory (R) subunits of the mammalian cAMP-dependent protein kinase (PKA). The recombinant T. equiperdum PKA R-like protein was expressed in bacteria and purified to homogeneity. Mice polyclonal antibodies were raised against the recombinant R-like protein to serologically evaluate its humoral immune response. High titers of specific sera antibodies were obtained against the parasite R-like protein by indirect enzyme-linked immunosorbent assay (ELISA), and immunoblots revealed that this protein was specifically recognized by the hyperimmune mice sera. Cellular proliferation assays using splenic B cells from the immunized mice showed higher values when the recombinant T. equiperdum R-like protein was employed than when concanavalin A was utilized as an unspecific mitogen. Two healthy horses that were experimentally infected using either T. equiperdum or Trypanosoma evansi showed a curve response characterized by the appearance of anti-T. equiperdum PKA R-like protein antibody production in sera using indirect ELISA. The recombinant parasite PKA R-like protein was also recognized by sera from naturally trypanosome-infected horses using western blotting. These findings demonstrated that the T. equiperdum PKA R-like protein is an antigen that exhibits cross-reaction with T. equiperdum and T. evansi. [ABSTRACT FROM AUTHOR]

Polyclonal immunoglobulin Y (IgY) antibodies were produced in chicken eggs against the purified RII-subunit of the cAMP-dependent protein kinase (PKA) from pig heart, which corresponds to theSus scrofaRIIα isoform. In order to evaluate whetherTrypanosoma equiperdumpossessed PKA R-like proteins, parasites from the Venezuelan TeAp-N/D1 strain were examined using the generated anti-RIIIgY antibodies. Western blot experiments revealed a 57-kDa polypeptide band that was distinctively recognized by these antibodies. Likewise, polyclonal antibodies raised in mice ascites against the recombinantT. equiperdumPKA R-like protein recognized the PKA RII-subunit purified from porcine heart and the recombinant human PKA RIβ-subunit by immunoblotting. However, a partially purified fraction of the parasite PKA R-like protein was not capable of binding cAMP, implying that this protein is not a direct downstream cAMP effector inT. equiperdum. Although the function of theS. scrofaPKA RIIα and theT. equiperdumPKA R-like protein appear to be different, their cross-reactivity together with results obtained by bioinformatics techniques corroborated the high level of homology exhibited by both proteins. Moreover, its presence in other trypanosomatids suggests an important cellular role of PKA R-like proteins in parasite physiology. [ABSTRACT FROM AUTHOR]