Epitope-Specific Regulation of Immunoglobulin Class Switching in Mice Immunized with Malarial Merozoite Surface Proteins

Abstract

Antibodies that bind to Fc receptors and activate complement are implicated in the efficient control of pathogens, but the processes that regulate their induction are still not well understood. To investigate antigen-dependent factors that regulate class switching, we have developed an in vivo model of class switching to immunoglobulin G2b (IgG2b) using the malaria antigen Plasmodium falciparum merozoite surface protein 2 (MSP2). C57BL/6 mice were immunized with recombinant proteins representing discrete domains of MSP2, and a T-cell epitope (C8) was identified within the conserved C terminus of the protein that preferentially induces IgG2b antibodies. The ability of C8 to induce IgG2b is ablated in both homozygous gamma interferonnegative and interleukin 10-negative mice. The IgG2b-inducing properties of C8 override the IgG1-inducing properties of both the fusion protein partner, glutathione S-transferase, and the adjuvant. Furthermore, when attached to other proteins that normally induce IgG1 responses, C8 induces a switch to IgG2b secretion. This is the first description of a defined T-cell epitope that drives specific IgG2b subclass switching, and our data offer proof of the concept that chimeric vaccines incorporating specific T-cell "switch epitopes" might be used to enhance qualitative aspects of the antibody response.

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

“…The lack of a predisposition to produce a particular subclass response regardless of the antigen further supports the hypothesis that the factors influencing the subclass response are determined mainly by the antigen rather than the host. Prior studies of mice identified a T-cell epitope in MSP2 that appears to promote cytophilic IgG subclass responses (47). Although polymorphisms in AMA-1 were not associated with differences in subclass responses, it is conceivable that the component of the antibody response directed against the conserved epitopes may have affected our ability to detect differences in subclass responses to allele-specific epitopes.…”

“…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).…”

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.…”

“…In particular, alterations in the concentrations of IL-10 and/or gamma interferon produced by APL-restimulated memory T cells may influence the IgG1/IgG3 balance, as shown for human antibody responses to schistosomiasis (37) and, in a model system, murine IgG subclass responses to P. falciparum MSP-2 (47). Shifts in helper T-cell phenotype with increasing age and increasing malaria exposure as a result of encountering APLs during successive infections with genetically distinct malaria parasites might also explain the age and exposure dependence of the IgG3 response reported here.…”

Acute Plasmodium chabaudi Infection Dampens Humoral Responses to a Secondary T-Dependent Antigen but Enhances Responses to a Secondary T-Independent Antigen

“…The IgG subclass produced in response to infection can dramatically affect the ability of humoral immunity to confer protection against disease (17,18). The Abs made in response to soluble Plasmodium vaccine candidate proteins are heavily skewed toward the IgG3 isotype in humans (19,20), which is equivalent to IgG2b in mice (21). In most experimental mouse models of Plasmodium infection, there is a heavy skewing toward an IgG2a or IgG2c isotype during an in vivo infection (22).…”