Synthetic Neoglycoconjugates of Hepta- and Nonamannoside Ligands for Eliciting Oligomannose-Specific HIV-1-Neutralizing Antibodies

Abstract

Oligomannose-type glycans on the spike protein of HIV-1 constitute relevant epitopes to elicit broadly neutralizing antibodies (bnAbs). Herein we describe an improved synthesis of α- and β-linked hepta- and nonamannosyl ligands that were subsequently converted into BSA and CRM₁₉₇ neoglycoconjugates. We assembled the ligands from anomeric 3-azidopropyl spacer glycosides from select 3-O-protected thiocresyl mannoside donors. Chain extensions were achieved using [4+3] or [4+5] block synthesis of thiocresyl and trichloroacetimidate glycosyl donors. Subsequent global deprotection generated the 3-aminopropyl oligosaccharide ligands. ELISA binding data obtained with the β-anomeric hepta- and nonamannosyl conjugates with a selection of HIV-1 bnAbs showed comparable binding of both mannosyl ligands by Fab fragments yet lesser binding of the nonasaccharide conjugate by the corresponding IgG antibodies. These results support previous observations that a complete Man₉ structure might not be the preferred antigenic binding motif for some oligomannose-specific antibodies, and have implications for glycoside designs to elicit oligomannose-targeted HIV-1-neutralizing antibodies.

Keywords: HIV/AIDS; glycoconjugates; glycosylation; oligomannosides; synthesis.

Fig. 1.

Binding of Fab fragments of bnAb PGT125, PGT126, PGT128, and PGT130 to the β-heptamannosyl conjugates 30 (MAC093 and NIT211–6) as well as the nonamannosyl conjugate 32 (MAC095). The two β-heptamannosyl and nonamannosyl conjugates were coated as solid-phase antigens onto ELISA plates at 76, 81, and 81 nM concentration.

Fig. 2.

Binding assay of PGT IgG bnAbs to the β-heptamannosyl conjugates 30 (MAC093 and NIT211–6) and the nonamannosyl conjugate 32 (MAC095). The coating concentration of the conjugates is the same as in Fig. 1.

Scheme 1.

Previously used route to introduce the β-anomeric spacer group.^[13]

Scheme 2.

Synthesis of the anomeric spacer glycoside derivatives 8a–8f.

Scheme 3.

Synthesis of the tetrasaccharide acceptor 12.

Scheme 4.

Synthesis of tri- and pentasaccharide glycosyl donors 15 and 17.

Scheme 5.

Synthesis of α-anomeric hepta- and nonasaccharide spacer derivatives 19 and 21.

Scheme 6.

Synthesis of the tetrasaccharide acceptor 24

Scheme 7.

Synthesis of the β-anomeric hepta- and nonamannoside derivatives 26 and 28.

Scheme 8.

Synthesis of the hepta- and nonamannosyl BSA and CRM₁₉₇ conjugates 29–32.