Epitope specificity and capacity to inhibit parasite growth in vitro of human antibodies to repeat sequences of the Plasmodium falciparum antigen Ag332

Abstract

It has earlier been shown that the Plasmodium falciparum-reactive human monoclonal antibody 33G2 inhibits parasite growth in vitro as well as cytoadherence of infected red blood cells to melanoma cells in vitro. MoAb 33G2 recognizes an epitope of the P. falciparum antigen Ag332 and cross-reactive determinants in Pf155/RESA and Pf11.1 located in repetitive regions containing sequences of regularly spaced pairs of glutamic acid. To study whether antibodies of this specificity frequently occur in human immune sera and if they could be of importance for protective immunity, antibodies were affinity purified on MoAb 33G2 reactive Ag332 peptides. The epitope specificity of the affinity purified antibodies, determined by the Pepscan method, resembled that of MoAb 33G2, but showed differences in fine specificity. The antibodies cross-reacted to some extent with Pf11.1 and Pf155/RESA repeat peptides as detected by peptide ELISA and Pepscan. In indirect immunofluorescence all purified antibodies displayed a dotted pattern of staining of late stage infected red blood cells of two lines of the P. falciparum strain FCR3, including a Pf155/RESA deficient line. The in vitro growth of these two lines was efficiently inhibited by the affinity purified antibodies, indicating that their inhibitory effect was mainly due to reactivity with antigens other than Pf155/RESA. This, and the fact that Pf11.1 has been shown not to be expressed by the asexual stages suggests that Ag332 may be an important target for potentially protective antibodies in vivo and that Ag332 based immunogens are of interest for development of malaria subunit vaccines.

Human immune responses to the highly repetitive Plasmodium falciparum antigen Pf332.

“…9 Moreover, analysis of the epitope specificity of human Pf332-reactive antibodies suggested that a multitude of Pf332 repeats constitute B cell epitopes. 10 The importance of Pf332-reactive antibodies for protective immunity against P. falciparum asexual blood stages is indicated by several findings. Antibodies to Pf332 repeats, raised in animals or affinity purified from human serum, inhibit in vitro parasite growth.…”

Differential effect and interaction of monocytes, hyperimmune sera, and immunoglobulin G on the growth of asexual stage Plasmodium falciparum parasites.

“…4,5 This approach has led to the identification of several vaccine candidate antigens from the blood stage of P. falciparum. [6][7][8] Involvement of antibodies in immune protection against P. falciparum blood stages has been established by passive antibody transfer studies in humans. In these studies, several investigators have found that passive transfer of IgGs from immune adults to nonimmune infected children causes a marked decrease in P. falciparum parasitemias 1,[9][10][11][12] Investigations from the laboratory of Druilhe identified the cooperation of antibodies with monocytes in protection against malaria.…”

Immune responses in congenic mice to multiple antigen peptides based on defined epitopes from the malaria antigen Pf332

“…The Pf332-derived epitope, VTEEI, is recognized by the parasite neutralizing human mAb 33G2. 18,20,25 Moreover, human antibodies, affinity purified from malaria immune sera on synthetic peptides comprising this Pf332 epitope, also have the capacity to inhibit parasite growth in vitro, 16,17 as have rabbit antibodies induced by MAP1 and MAP2. 26 When H-2 congenic mice were injected with MAP1, high antibody levels were induced in H-2 d mice, whereas the responses in H-2 q and H-2 k (and other I-A k -expressing mice) were lower.…”

“…12,14,15 Pf332-reactive antibodies have been reported to inhibit parasite growth in vitro. 16,17 Furthermore, the growth inhibitory Pf332-reactive human monoclonal antibody (mAb) 33G2 also inhibits cytoadherence of PRBC to endothelial cells in vitro. 18,19 The motif VTEEI and structurally related sequences occurring multitudinously in Pf332 constitute epitopes for mAb 33G2, as well as for growth inhibitory Pf332-reactive antibodies isolated from human malaria immune sera.…”

“…18,19 The motif VTEEI and structurally related sequences occurring multitudinously in Pf332 constitute epitopes for mAb 33G2, as well as for growth inhibitory Pf332-reactive antibodies isolated from human malaria immune sera. 17,20 In the present study, MAP containing either trimers of the VTEEI peptide or a repeat sequence comprising the sequence SVTEEIAEEDK were injected into H-2 congenic mice. Our main objective was to compare the ability of the assembled polyvalent subunit immunogens to elicit B-and T-cell responses in mice expressing various H-2 haplotypes.…”