Functional Glycomics homepage

Article navigation

Featured Articles

Carbohydrate vaccines: Desperately seeking non-self antigens

Functional Glycomics (14 October 2010) | doi:10.1038/fg.2010.32

Structure-activity relationships of antigenic carbohydrates reveal that 'non-self' sugars can break immune tolerance while promoting 'self' recognition.

HIV remains a serious and prevalent illness despite improved treatments that block viral reproduction at a number of steps. The development of an HIV vaccine would significantly advance our capacity to control this virus, but relevant protein epitopes in gp120 and gp41, two proteins in the HIV-1 envelope spike, undergo frequent mutations and are hidden by host-derived N-glycans. Although the availability of these glycan side chains, and in particular a mannose tetrasaccharide 'D1-arm', suggests that these carbohydrates might provide alternative antigens, their native source limits their immunogenicity. Ian Wilson, Ben Davis and colleagues now report in the Proceedings of the National Academy of Sciences USA the surprising discovery that replacement of the terminal sugar in the D1-arm with non-natural residues yields high-affinity, immunogenic ligands with cross-reactivity to host structures.

Though HIV is thought to be refractory to detection by the human immune system, a very small number of antibodies have been isolated from HIV patients that do provide protection from the virus. One such antibody, 2G12, recognizes D1-arm carbohydrates via a unique domain-swapped structure that provides additional contact points for the mannose tetramer. Binding assays of a panel of monosaccharides with 2G12 have revealed a further surprise: the protein bound fructose with higher affinity than it bound its native ligand mannose (see Further reading).

Wilson, Davis and colleagues speculated that fructose might be acting as a mannose mimic, and that this sugar, or similar mannose mimics, might provide a means to create successful non-self antigens. To test this hypothesis, the authors first obtained a crystal structure of fructose bound to 2G12 that explained the high affinity of the molecule, as they identified several intermolecular interactions with fructose that would not be possible with mannose. On the basis of this crystal structure and additional molecular modeling to identify possible substitution points, the authors synthesized a panel of monosaccharides using a synthetic strategy optimized for introduction of diverse functional groups. Tests of these compounds revealed one compound, C6-methylated mannose, that was as active as fructose; when this glycan was incorporated into a D1-arm tetrasaccharide mimic using a convergent synthetic scheme, it bound more tightly to the antibody by over two orders of magnitude as compared to mannose, and showed a four-fold improvement over the natural D1-arm. Crystal structures of 2G12 with the mono- and tetrasaccharides confirmed that the methylated sugar takes part in several interactions observed for fructose, but makes additional contacts to strengthen binding.

With a high-affinity ligand in hand, the authors tested whether this molecule could serve as an antigen and whether any resultant antibodies would have cross-reactivity with the natural mannosylated epitope. They attached ∼300 copies of an azide-modified tetrasaccharide with mannose or C6-methyl mannose groups to the surface of the virus-like particle Qβ using click chemistry, and then immunized rabbits with these particles. The 'non-self' antigen yielded higher titers of antibodies than the 'self' epitope. Additionally, antibodies isolated from the non-self immunization did cross-react with proteins such as cowpea mosaic virus and bovine serum albumin displaying the natural tetrasaccharide and more complex sugars containing this motif, and this recognition was more effective than for antibodies generated against the native sequence. Strangely, though, the antibodies from the non-self immunization were not effective at recognizing HIV itself.

Although it remains to be seen how these antibodies distinguish the D1-arm in different protein contexts, these results successfully demonstrate the use of non-self carbohydrates as a new approach for engaging the immune system against natural carbohydrate epitopes.

Catherine Goodman

Original research paper

  1. Doores, K. J. et al. A “nonself” sugar mimic of the HIV glycan shield shows enhanced antigenicity. Proc. Natl. Acad. Sci. USA 107, 17107–17112 (2010). doi: 10.1073/pnas.1002717107  | Article | 

Further reading

  1. Calarese, D. A. et al. Antibody domain exchange is an immunological solution to carbohydrate cluster recognition. Science 300, 2065–2071 (2003). doi:10.1126/science.1083182 | Article |