Isotype switching: Mouse IgG3 constant region drives increased affinity for polysaccharide antigens

For many microbes, their polysaccharides (PS) are a critical part of their interaction with the mammalian immune system. Given their lower immunogenicity compared to protein peptides, PS can provide protection against both opsonization and phagocytosis, and in many cases actively protects bacteria against elements of both the innate and adaptive immune systems. Many bacterial species have traditionally been defined by their capsular polysaccharides (CPS) and polysaccharide O-antigens. These two PS are often highly variable between bacterial strains (e.g.). The biosynthesis of both CPS and O-antigens generally results in long chains (often 10s to 100s of saccharide units) that consist of repeats of a short oligosaccharide. Although sugar moieties are poor substrates for eliciting an efficient adaptive immune response in vivo, PS based vaccines have delivered highly effective protection to humans against a range of microbial infections. PS generally have to be administered conjugated to a carrier molecule (usually a protein) in order to engage T cells. Indeed, many of the effective licensed vaccines couple PS to bacterial toxins that generally provide a strong adjuvant effect. In the absence of a carrier, PS stimulate B cells independently by cross-linking antigen receptors. This produces an initial IgM mediated antibody response, which generally switches to an IgG response upon repeated boosting with antigen. The IgG response affords the greater part of the long term immunological memory to the antigen. The mouse model is used for the large majority of initial vaccination studies. It has a very long track record of success, and the large volume of data available on the murine immune response makes it ideal for comparative studies. However, mice have known immunological differences compared to humans, and such discrepancies have the potential to confound some studies. Mice have different IgG subclasses that respond differently to certain cytokine treatments and different IgG receptors compared to human. Furthermore, mouse B cells tend to only switch antibody classes from IgM to IgG3 when stimulated with T-cell independent antigens. Humans, in contrast, tend to switch to the IgG2 subclass. Although the variable (antigen binding or Fab) regions of the antibodies remain identical, the different constant regions (or Fc) that the subclasses provide have a significant effect not only on antibody avidity but also on affinity. The former largely depends on the different capabilities of Fc regions to multimerise in vivo and in vitro; while the latter depends on the effect Fc regions have on Fab’s structural and conformational properties. Understanding the nature of this change in avidity and/or affinity is important for understanding the likely impacts that antibody class switching will have on transplanting vaccines between species. Furthermore, it is essential for the use of "humanized" antibodies for passive vaccination, in which there is increasing interest with the rise of multiple antimicrobial resistant bacteria (e.g.,). In this issue of Virulence, Dillon et al. demonstrate that class switching has a profound impact on IgG3 antibodies against the CPS of the globally distributed emerging human pathogen Burkholderia pseudomallei. Interestingly, the CPS under investigation is an unusual polysaccharide, consisting of a linear homopolymer of a 2-O-acetyl-6-deoxy-heptopyranose. As the simplest form of polymer, the potential number of antigen sites is very dense. Indeed, a hexamer conjugated to protein is sufficient to confer protection in a mouse model. Dillon et al. demonstrate that an IgG3 anti-CPS antibody (3C5; composed of Vh6 and IgKV19/28 chains) raised against heat killed B. pseudomallei has a strong affinity for