Chemical Biology of αGalCer: A Chemist's Toolbox for the Stimulation of Invariant Natural Killer T (iNKT) Cells

Abstract

Often referred to as the “Swiss Army knife” of the immune system, invariant natural killer T (iNKT) cells are a subpopulation of T lymphocytes stimulated by the synthetic glycolipid α‐galactosylceramide (αGalCer) when in complex with the CD1d receptor of antigen presenting cells. Through their ability to produce TH1 and TH2 cytokines and co‐stimulate several other lymphocytes, iNKT cells have emerged as central players in directing the immune response in a range of physiological processes, such as infections, autoimmune diseases, and cancer. Over the years, synthetic chemistry has advanced the field of iNKT cell stimulation with the development of more efficient approaches to prepare αGalCer, and, additionally, with the chemical synthesis of αGalCer analogs in the search of better TH1/TH2 cytokine skewing compounds for therapeutic applications. Here, we review the strategies for the synthesis of αGalCer and its analogs, including synthetic probes, together with the most important advances in the understanding of the mechanism of action of these compounds, as a guide to the available tools for interrogating the iNKT cell−αGalCer−CD1d complex and inspiration for future research.

Diversity-Oriented Synthesis of a Molecular Library of Immunomodulatory α-Galactosylceramides with Fluorous-Tag-Assisted Purification and Evaluation of Their Bioactivities in Regard to IL-2 Secretion

“…Many structural analogs of α-GalCer based on the modification of KRN7000 were found to be potent immunomodulatory agents that act by CD1d-dependent iNKT cell stimulation [ 12 ]. SAR studies of α-GalCer analogs include two major classes of modification; variation in the hydrophilic polar carbohydrate head group that directly interacts with the T-cell antigen receptor (TCR) and variation in the acyl chain and the sphingoid base structures of the ceramide (Cer) moiety responsible for CD1d binding [ 13 ].…”

“…Overall, synthetic analogs that enhance CD1d protein–α-GalCer complex stability by noncovalent interaction, such as fatty acid tails containing aromatic rings, tend to stimulate a T H 1-like immune response, while those that lower the stability of the CD1d-α-GalCer complex, such as truncation of the Psp chain of Cer of α-GalCer, induce a T H 2 bias due to its short retention time on the surface of CD1d. Therefore, it may be possible to provide versatility to the generation of new α-GalCer analogs for SAR study by constructing an α-GalCer scaffold that allows post-glycosylation modifications both at the fatty acyl tail and Psp backbone derivatization of the natural C18 chain by olefination with virtually any olefination reagent [ 12 ].…”

“…Due to its wide range of biological activities, numerous strategies for the synthesis of α-GalCer and its derivatives have been developed. Most common methods involve the stereoselective union of a variety of glycosyl donors with a Psp acceptor [ 12 , 27 ]. Although iodides, phosphates, and other anomeric leaving groups were employed for glycosylation with Psp, glycosyl trichloroacetimidates and aryl thioglycosides were widely used donors because of their popularity in preparative carbohydrate chemistry and the mild conditions required for their activation [ 28 , 29 , 30 , 31 , 32 , 33 ].…”

A New iNKT-Cell Agonist-Adjuvanted SARS-CoV-2 Subunit Vaccine Elicits Robust Neutralizing Antibody Responses

“…The relatively short-lived and partially antagonistic nature of the mix of Th1 and Th2 cytokines produced by activated iNKT cells, followed by long-term anergy of iNKT cells, may restrict the efficacy of αGC . Due to the lack of optimal effects of α-GalCer in clinical studies, further research has been directed toward finding αGC analogues with more attractive activity profiles to be used in potential therapeutic applications. , For example, α-C-GC and 7DW8-5 induce Th1-biased immune responses, whereas OCH and C20:2 have Th2-type cytokine profiles. These typical Th1- or Th2-biasing analogues have been used as adjuvants to combat infectious diseases .…”

“…10 to the lack of optimal effects of α-GalCer in clinical studies, further research has been directed toward finding αGC analogues with more attractive activity profiles to be used in potential therapeutic applications. 11,12 For example, α-C-GC 13 and 7DW8-5 14 induce Th1-biased immune responses, whereas OCH 15 and C20:2 16 have Th2-type cytokine profiles. These typical Th1-or Th2-biasing analogues have been used as adjuvants to combat infectious diseases.…”