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Lipidomic scanning of self-lipids identifies headless antigens for natural killer T cells
Tan-Yun Cheng1, T Praveena2, Srinath Govindarajan3,4
1Division of Rheumatology, Inflammation, and Immunity, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02210.
Insights
Researchers identified novel "headless" lipid antigens, ceramides, that interact with natural killer T (NKT) cells. These findings reveal a new pathway for NKT cell responses to cellular stress.
Area of Science:
- Immunology
- Lipidomics
- Structural Biology
Background:
- Natural killer T (NKT) cells are crucial immune cells that recognize lipid antigens presented by CD1d molecules.
- The full spectrum of self-lipids recognized by NKT cells remains incompletely understood.
- Endoplasmic reticulum (ER) stress and cellular damage can alter lipid metabolism and immune responses.
Purpose of the Study:
- To comprehensively identify the range of cellular self-antigens that bind to NKT cell receptors (TCRs) via CD1d.
- To investigate the structural and functional characteristics of these CD1d-lipid-TCR interactions.
- To explore the role of newly identified lipid antigens in NKT cell responses, particularly under cellular stress conditions.
Main Methods:
- Development of a sensitive lipidomics system to analyze lipids within CD1d-NKT TCR complexes.
- Separation of protein complexes based on CD1d-TCR interaction strength.
- Lipid elution, mass spectrometry, and crystal structure determination of TCR-ceramide-CD1d complexes.
Main Results:
- Identification of fifteen self-lipid molecules, primarily phospholipids and novel neutral lipids (ceramides, deoxyceramides, diacylglycerols).
- Structural analysis revealed that 'headless' lipids like ceramides allow TCRs to predominantly contact CD1d, supporting CD1d autoreactivity.
- Ceramides demonstrated physiological binding affinity, mediating weak NKT cell responses and tetramer binding in human and mouse NKT cells.
Conclusions:
- Ceramides and related headless lipids represent a new class of NKT cell antigens.
- The ceramide-NKT cell axis provides a molecular link to sterile cellular stress, apoptosis, and transformation.
- These findings, alongside sphingomyelin's role, offer insights into NKT cell regulation during cellular stress.
Abstract:
To broadly measure the spectrum of cellular self-antigens for natural killer T cells (NKT), we developed a sensitive lipidomics system to analyze lipids trapped between CD1d and NKT T cell receptors (TCRs). We captured diverse antigen complexes formed in cells from natural endogenous lipids, with or without inducing endoplasmic reticulum (ER) stress. After separating protein complexes with no, low, or high CD1d-TCR interaction, we eluted lipids to establish the spectrum of self-lipids that facilitate this interaction. Although this unbiased approach identified fifteen molecules, they clustered into only two related groups: previously known phospholipid antigens and unexpected neutral lipid antigens. Mass spectrometry studies identified the neutral lipids as ceramides, deoxyceramides, and diacylglycerols, which can be considered headless lipids because they lack polar headgroups that usually form the TCR epitope. The crystal structure of the TCR-ceramide-CD1d complex showed how the missing headgroup allowed the TCR to predominantly contact CD1d, supporting a model of CD1d autoreactivity. Ceramide and related headless antigens mediated physiological TCR binding affinity, weak NKT cell responses, and tetramer binding to polyclonal human and mouse NKT cells. Ceramide and sphingomyelin are oppositely regulated components of the "sphingomyelin cycle" that are altered during apoptosis, transformation, and ER stress. Thus, the unique molecular link of ceramide to NKT cell response, along with the recent identification of sphingomyelin blockers of NKT cell activation, provide two mutually reinforcing links for NKT cell response to sterile cellular stress conditions.
