Background/Purpose

Self-lipids play an increasingly appreciated role in immunity and inflammation. Lipid antigens are presented by CD1d and CD1a-d molecules in mouse and human, respectively, to T cells. Glycolipids (GL) such as βGluCer, and phospholipids (PL) such as phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanoloamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS), have been eluted and identified by mass spectrometry as natural human CD1d ligands, and PC and PE have been eluted from murine CD1d. Crystallography shows that complexes of CD1d bound to PL can exist. Diverse subsets of T cells that recognize self lipids have been reported, including invariant natural killer T cells (iNKT). Extensive work shows protective or pathogenic roles of GL αGalCer-reactive iNKT cells in a wide range of diseases. However, the functions of T cells that recognize abundant self PL are not known. Here, we identified and characterized self PL-reactive T cells (PLT), investigated their in vivo functions, and elucidated the cellular and molecular participants in PL mediated immune homeostasis.

Methods

CD1d tetramers were loaded with 8 PL or GL antigens, and cells analyzed by flow cytometry. Chemical binding of PL to CD1d was assessed by isoelectric focusing/gel shift analysis. CD1d-PA complex was crystallized by sitting drop vapour diffusion. Autoimmune hepatitis that is mediated by iNKT cells was induced by injecting concanavalin A (ConA), and assessed by serum ALT, morphology, and histology. 

Results

CD1d tetramers loaded with PL, namely PA, PC, PE, PI, PS and BMP (bismonoacylglycerophosphate), identify 0.4–4% T cells in the lymphoid organs of wild-type and iNKT–deficient Jα18^–/– mice but not in CD1d^–/– mice. PLT cells don’t recognize GL-loaded tetramers and don’t respond to αGalCer, suggesting that PLT cells are distinct from iNKT cells. PLT cells expand, express CD69, and produce cytokines upon in vivo priming. Chemical binding and crystal structure show that PA binds CD1d in the absence of lipid transfer proteins, and is centrally located in the CD1d-binding groove opening for TCR recognition. Although PA bound slightly weaker to CD1d than a self GL, it competed with αGalCer to load onto CD1d. All PL tested profoundly inhibited the proliferation and cytokine production by iNKT cells. Such PL-induced inhibition of iNKT cells was abrogated upon depletion of granulocytes by gemcitabine that preferentially depleted the MDSC subset called monocyte-MDSC (mMDSC). Furthermore, PL induced IL-10 producing mMDSC that inhibited iNKT cell proliferation in an IL-10-dependent manner. Finally, treatment with a PL ameliorated ConA–hepatitis, reduced pro-inflammatory cytokines, granulocyte accumulation and IFNγ⁺ mMDSC, but promoted IL-10⁺ mMDSC that upon adoptive transfer reduced the incidence/severity of ConA–hepatitis.

Conclusion

We identified a new role for self PL that activate a distinct subset of CD1d-restricted T cells that inhibit iNKT cells by competitive inhibition and via induction of IFNγ^–IL10⁺ mMDSC that ameliorate autoimmune hepatitis. These results have important implications for conditions with altered lipid metabolism and inflammation such as atherosclerosis and autoimmune disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/self-phospholipids-regulate-inflammation-via-activation-of-cd1d-restricted-t-cells-and-induction-of-anti-inflammatory-myeloid-derived-suppressor-cells-mdsc/