Regulation of Antigen-Specific Immunoglobulin G Subclasses in Response to Conserved and PolymorphicPlasmodium falciparumAntigens in an In Vitro Model

Abstract

Cytophilic antibodies (Abs) play a critical role in protection against Plasmodium falciparum blood stages, yet little is known about the parameters regulating production of these Abs. We used an in vitro culture system to study the subclass distribution of antigen (Ag)-specific immunoglobulin G (IgG) produced by peripheral blood mononuclear cells (PBMCs) from individuals exposed to P. falciparum or unexposed individuals. PBMCs, cultivated with or without cytokines and exogenous CD40/CD40L signals, were stimulated with a crude parasite extract, recombinant vaccine candidates derived from conserved Ags (19-kDa C terminus of merozoite surface protein 1 [MSP1 19 ], R23, and PfEB200), or recombinant Ags derived from the polymorphic Ags MSP1 block 2 and MSP2. No P. falciparum-specific Ab production was detected in PBMCs from unexposed individuals. PBMCs from donors exposed frequently to P. falciparum infections produced multiple IgG subclasses when they were stimulated with the parasite extract but usually only one IgG subclass when they were stimulated with a recombinant Ag. Optimal Ab production required addition of interleukin-2 (IL-2) and IL-10 for all antigenic preparations. The IgG subclass distribution was both donor and Ag dependent and was only minimally influenced by the exogenous cytokine environment. In vitro IgG production and subclass distribution correlated with plasma Abs to some Ags (MSP1 19 , R23, and MSP2) but not others (PfEB200 and the three MSP1 block 2-derived Ags). Data presented here suggest that intrinsic properties of the protein Ag itself play a major role in determining the subclass of the Ab response, which has important implications for rational design of vaccine delivery.1 Passive transfer of hyperimmune immunoglobulin G (IgG) to malaria patients has shown that antibodies (Abs) play a key role in protection against Plasmodium falciparum blood stages (12,25,30). In vitro studies have indicated that efficient parasite destruction is achieved through interaction of Abs with monocytes. Binding of Abs to an infected red blood cell results in opsonization of the infected cell (24), and Ab-dependent cellular inhibition of parasite growth is triggered by the binding of an IgG-merozoite complex to monocytes (5, 6, 40). Both mechanisms have been reported to involve IgG1 and IgG3 but not IgG2, which is normally noncytophilic (4, 8, 23). The efficiency of Ab-dependent cellular inhibition of parasite growth depends on the relative proportion of parasite-specific IgG1 and IgG3 compared to IgG2 (4). The data indicate that the subclass distribution of IgG Abs reacting with merozoite surface antigens (Ags) is an important parameter for protection against P. falciparum blood stages. In order to develop efficient blood stage vaccines, we need to better understand anti-P. falciparum IgG subclass production and/or switching.Identification of the parasite and host factors affecting Ab production by B cells might indicate ways in which vaccineinduced immune responses can be directed to ensure terminal differe...

Immunoglobulin G Isotype Responses to Variant Surface Antigens ofPlasmodium falciparumin Healthy Gabonese Adults and Children during and after Successive Malaria Attacks

“…The preferential induction of IgG3 responses to various P. falciparum asexual-stage antigens, including merozoite surface protein 1 (MSP-1), -2, and -3 and par- asite glycosylphosphatidylinositols, is a well-described phenomenon (8,12,17,45). This preference seems to be associated with the degree of polymorphism associated with the target antigen or epitope thereof, an observation with which in vitro antibody induction assays are largely concordant (19). The prominence of IgG2 that we observed in the adults' anti-VSA antibody profile deserves further comment, since the role of IgG2 antibodies with specificity for plasmodial antigens remains a subject of some controversy.…”

Target Antigen, Age, and Duration of Antigen Exposure Independently Regulate Immunoglobulin G Subclass Switching in Malaria

“…Polarized IgG1 responses are also reported for Pf155/RESA (17), crude schizont lysate (33), RAP-1 (44), and MSP-6 (50), while IgG3 responses are seen for a polymorphic C-terminal region (block 2) of MSP-1 (11), MSP-3 (11, 31), MSP-4 (51), and MSP-7 (50). A crucial role for antigens per se in driving these patterns of subclass switching has been confirmed experimentally both in vitro with human B cells (21) and in a mouse model where different recombinant P. falciparum antigens-expressed with the same fusion protein tag and administered with the same adjuvant-induced very different IgG subclasses (47). This raises the important question of whether there are intrinsic features of individual malarial antigens which influence their interactions with antigenpresenting cells, T cells, or B cells, leading to the preferential 19 -or AMA-1-specific IgG1 inhibited the simultaneous generation of IgG3 responses to MSP-2, then significantly less than 25% of sera would give ODs above the median for both.…”

Immunoglobulin G Subclass-Specific Responses against Plasmodium falciparum Merozoite Antigens Are Associated with Control of Parasitemia and Protection from Symptomatic Illness

“…It is unclear whether individuals have a bias toward producing a specific subclass regardless of the antigen or if instead the IgG subclass response is generated independently for each antigen and how this relates to protective immunity. While factors determining subclass responses to antigens are not clearly defined, antigen properties, host age, cumulative exposure, and genetic determinants have been linked with the nature of subclass responses (2,4,17,33,34,41,42,47,48). Some studies have suggested that increasing age (and therefore malaria exposure) leads to an increasing polarization of IgG subclass responses to merozoite antigens (41,48).…”