Research Fellow Monash Univ. Fac. of Med., Nursing and Hlth. Sci. Clayton, Victoria, Australia
Disclosure(s):
Praveena Thirunavukkarasu, PhD: No financial relationships to disclose
Introduction/Rationale: The human gut microbiota comprises more than 50% of Bacteroides species that produce small diffusible molecules like sphingolipids that play a key role in modulating the host’s immune responses. In particular, Bacteroides fragilis produces glycosphingolipids termed ‘BfaGCs’ that can activate type I Natural Killer T (NKT) cells. BfaGCs exhibit distinct structural characteristics, including truncated sphinganine chains, varied branching patterns, and specific functional groups, distinguishing them from the canonical type I NKT cell marker, α-galactosylceramide (KRN7000).
Methods: Using a combinatorial cellular immunology, mass spectrometry and X-ray crystallography approach, we provide the first insights into the molecular mechanism of recognition of four novel BfaGCs presented by the antigen-presenting molecule CD1d, in complex with the type I NKT TCR.
Results: The co-culture assay performed with bone marrow-derived dendritic cells and NKT cells in the presence of specific BfaGCs indicated that branching in their sphinganine chain is a critical determinant of NKT cell activation. The three-dimensional complex structures revealed that the TCR adopted a parallel docking topology atop the F’-pocket of CD1d in recognising the presented BfaGCs, reminiscent of other published type I NKT TCR lipid complexes. Nevertheless, the terminal sphinganine branching of BfaGCs facilitates unique interactions within the CD1d F’-pocket, thereby underpinning their distinctive agonistic properties. Also, the NKT TCR demonstrates high binding affinities for both stimulatory and non-stimulatory forms of CD1d-presented BfaGCs, affirming BfaGCs as bona fide CD1d ligands that modulate host immune defences via NKT cells.
Conclusion: Thus, BfaGCs were demonstrated to function as immunomodulatory mediators influencing the host’s defence in the context of NKT cells. Collectively, our findings enhance understanding of the symbiotic interplay between lipid-producing gut microbes and the host.
