Novel glycolipid TLR2 ligands of the type Pam2Cys-α-Gal: Synthesis and biological properties

A more complete understanding of the mechanism of action of TLR agonists has fueled the investigation of new synthetic immunoadjuvants. In this context, we designed and synthesized glycolipids of the type Pam2Cys-α-Galactose as novel immunoadjuvants. Their synthesis required modifying a hydrophobic tBoc-[2,3-bispalmitoyloxy-(2R)-propyl]-R-cysteinyl moiety, i.e. the minimal structure required for TLR2 agonist activity, by addition of a hydrophilic head, either an α-Galactosylpyranose or an α-Galactosylfuranose to gain respectively Pam2CGalp and Pam2CGalf. While preparing a carbohydrate building block, an unexpected stereoselectivity was observed during a halide ion-catalytic process on a protected galactofuranose: the alpha anomer was obtained with surprisingly high selectivity (α/β ratio > 9) and with good isolated yield (51%). The TLR2 binding properties of Pam2CGalp and Pam2CGalf were then fully evaluated. Their efficiency in triggering the proliferation of BALB/c mouse splenocytes was also compared to that of Pam2CAG and Pam3CAG, two well-established ligands of TLRs. Moreover, the maturation state of murine dendritic cells previously incubated with either Pam2CGalp or Pam2CGalf was monitored by flow cytometry and compared to that induced by lipopolysaccharide. Pam2CGalp and Pam2CGalf were found to be equivalent TLR2 agonists, and induced splenocyte proliferation and DC maturation. With very similar activity, Pam2CGalp and Pam2CGalf were also 10-fold to 100-fold better than Pam2CAG and Pam3CAG at inducing B cell proliferation. This represents the first time a glucidic head has been added to the tBoc-[2,3-bispalmitoyloxy-(2R)-propyl]-R-cysteinyl moiety whilst maintaining the immunomodulating activity. This should greatly enrich the data available on Pam2C structure/activity relationships.

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► Glycolipid TLR2 ligands were synthesized and characterized for the first time.
► The available data on Pam2C structure/activity relationships are thus enriched.
► An unexpected stereoselectivity was observed when preparing a galactofuranose ring.
► Toll-Like Receptor 2 (TLR2) agonist properties were fully demonstrated.
► Comprehensive binding and functional tests highlighted their interest as immunoadjuvants.