A nonself sugar mimic of the HIV glycan shield shows enhanced antigenicity

Abstract

Antibody 2G12 uniquely neutralizes a broad range of HIV-1 isolates by binding the high-mannose glycans on the HIV-1 surface glycoprotein, gp120. Antigens that resemble these natural epitopes of 2G12 would be highly desirable components for an HIV-1 vaccine. However, host-produced (self)-carbohydrate motifs have been unsuccessful so far at eliciting 2G12-like antibodies that cross-react with gp120. Based on the surprising observation that 2G12 binds nonproteinaceous monosaccharide D-fructose with higher affinity than D-mannose, we show here that a designed set of nonself, synthetic monosaccharides are potent antigens. When introduced to the terminus of the D1 arm of protein glycans recognized by 2G12, their antigenicity is significantly enhanced. Logical variation of these unnatural sugars pinpointed key modifications, and the molecular basis of this increased antigenicity was elucidated using high-resolution crystallographic analyses. Virus-like particle protein conjugates containing such nonself glycans are bound more tightly by 2G12. As immunogens they elicit higher titers of antibodies than those immunogenic conjugates containing the self D1 glycan motif. These antibodies generated from nonself immunogens also crossreact with this self motif, which is found in the glycan shield, when it is presented in a range of different conjugates and glycans. However, these antibodies did not bind this glycan motif when present on gp120.

High-Mannose Glycan-Dependent Epitopes Are Frequently Targeted in Broad Neutralizing Antibody Responses during Human Immunodeficiency Virus Type 1 Infection

“…Glycan-specific antibody responses can be an attractive line of vaccine development, since some glycan-specific MAbs demonstrate great neutralizing breadth and potency (51,54). It has also been shown that glycan-binding antibodies can be induced by synthetic glycans (2,3,17,19,26,57) or HM-glycan on non-HIV-1 proteins (1, 25, 34, 35). Unfortunately, a glycan-binding antibody capable of neutralizing diverse strains of primary HIV-1 has yet to be achieved.…”

“…HM-glycan and C-glycan are different in size and electric charge and thus perform different biological functions. Natural mammalian glycoproteins contain predominantly C-glycan (53), so a high content of HMglycan on the virion-associated Env of HIV-1 logically makes it somewhat foreign to the human immune system, as well as potentially immunogenic (15,17,37). In contrast, recombinant HIV-1 Envs carry mainly C-glycan and a relatively low level of HMglycan even when expressed in mammalian cells such as 293T cells and CHO cells (21-23, 30, 37, 43).…”

2G12-Expressing B Cell Lines May Aid in HIV Carbohydrate Vaccine Design Strategies

“…We previously investigated the potential of a nonself variant of the D1 arm displaying a methyl group at the C-6 position of the terminal mannose residue to elicit HIV-reactive antibodies (19). Although this glycan had enhanced 2G12 antigenicity and there was a slightly higher immune response than with the self D1 arm, no HIV-neutralizing antibodies were elicited.…”

“…First, we were interested to determine whether multivalent displays of Man 4 and Man 9 antigens on icosahedral Q␤ bacteriophage particles could activate any of our 2G12 B cell lines (18,19,26). We have previously shown that 2G12 WT can bind these immunogens with nanomolar affinity (20 to 50 nM).…”

“…There have been many attempts to elicit HIV broadly neutralizing carbohydrate-specific antibodies using both chemically and biochemically prepared multivalent and clustered displays of the 2G12 glycan antigens Man 4 (D1 arm) and Man 9 . These have included whole yeast cells (13)(14)(15), bacteria (16), oligodendrons (17), and Q␤ particles (18,19). Although many of these immunogens have generated mannose-specific antibodies, thus far, none have generated a broadly neutralizing response against HIV.…”