Metabolic reprogramming during T cell activation is required to support the increased biosynthetic demands of CD8+ Teff cell proliferation, effector function and epigenetic remodeling needed to enforce differentiation. Metabolites also directly modulate the activity of signaling pathways (for example, amino acids regulate mTOR signaling pathway), tying T cell signaling to metabolite acquisition. While changes in polar metabolites during CD8+ T cell activation have been studied, how lipidome changes support CD8+ effector T cell differentiation is not well understood.
It has been a longstanding question whether discrete PIP2 pools are essential for specific cellular effects. We hypothesized that T cells at different differentiation stages are equipped with a PIP2 pool containing with saturation levels of acyl chains (with 0–2 double bonds as saturated vs. with 3–4 double bonds as polyunsaturated). Therefore, specific T cell programs may utilize these different PIPn to drive signaling events. Saturated PIPn mediate Teff cell signaling, while polyunsaturated PIPn exclusively mediate signaling at these differentiation stages.
This study demonstrates how to utilize lipidomics technology to investigate the important roles of lipidome during T cell differentiation. In addition, we also applied 13C-glucose stable isotope tracing approach to help us better understand which metabolic events drive synthesis of saturated PIPn in Teff cells.
Our Approach
To investigate how lipid metabolism changes support CD8+ effector cell differentiation and how polar metabolites impact lipid signaling pathways, we firstly performed lipidomics analysis with stimulated and unstimulated CD8+ T cells and identified 11 PI species that increased in Teff cells. Our findings indicate that PIPn species with saturated acyl chains accumulate in mouse CD8+ Teff cells in vitro and in vivo. Next, with the genetically edited CDIPT- T cell model, we confirmed that CD8+ Teff cells rely on CDIPT-dependent de novo PI synthesis to synthesize saturated PI and saturated PIPn synthesis via CDIPT is required for CD8+ Teff cell fitness and function during cancer and infection. Furthermore, we applied 13C-glucose stable isotope tracing, and we identified that glycolytic metabolism drives synthesis of saturated PIPn in Teff cells.
Our Conclusion
In summary, diverse pools of PIPn with differing acyl chain saturation are critical regulators of distinct CD8+ T cell differentiation programs. Polyunsaturated PIP2 sufficiently supports T cell signaling early after TCR stimulation. As CD8+ T cells commit to their full Teff cell function marked by intense proliferation and an increased demand for second messengers in the light of waning PLCγ1 activity, rapid synthesis and conversion of saturated PIPn from glucose becomes essential for sustained Teff cell signaling. Thus, fatty acid chain saturation in PIPn is a new mechanism that gov erns T cell signaling at different stages of CD8+ T cell differentiation. Given the central role of Teff cells in antitumor immunity and in autoimmune diseases, our findings may provide an avenue for modulating PIPn saturation as a means to balance Teff cell activity.