Sin1/mTORC2 regulate B cell growth and metabolism by activating mTORC1 and Myc.

Proper control of B cell growth and metabolism is crucial for B-cell-mediated immunity, but the underlying molecular mechanisms remain incompletely understood. In this study, Sin1, a key component of mTOR complex 2 (mTORC2), specifically regulates B cell growth and metabolism. Genetic ablation of Sin1 in B cells reduces the cell size at either the transitional stage or upon antigen stimulation and severely impairs metabolism. Sin1 deficiency also severely impairs B-cell proliferation, antibody responses, and anti-viral immunity. At the molecular level, Sin1 controls the expression and stability of the c-Myc protein and maintains the activity of mTORC1 through the Akt-dependent inactivation of GSK3 and TSC1/2, respectively. Therefore, our study reveals a novel and specific role for Sin1 in coordinating the activation of mTORC2 and mTORC1 to control B cell growth and metabolism.

Active mTORC2 Signaling in Naive T Cells Suppresses Bone Marrow Homing by Inhibiting CXCR4 Expression.

Recirculation of naive T cells between secondary lymphoid organs to receive survival cues and scan for signs of infection or other pathologic conditions is important for immune homeostasis and effective immune responses. Although the mechanisms that specifically guide the entry of naive T cells into secondary lymphoid organs are well studied, the mechanisms that keep them from fluxing into inappropriate or undesirable compartments, such as healthy tissues or bone marrow, are less well understood. In this study, we report an unexpected finding that under steady state, bone marrow homing of naive T cells is actively suppressed by mTORC2 signaling. We found that in mice, T cell-specific deletion of an essential mTORC2 component Sin1 results in increased accumulation of naive T cells in the bone marrow. Mechanistically, we show that loss of mTORC2 signaling in naive T cells results in enhanced FOXO1 activity, which leads to increased CXCR4 expression and chemotactic response to CXCL12, a key chemokine that promotes bone marrow homing and retention of T cells. Together, the results of our study reveal a novel role of mTORC2 in T cell homeostasis via active suppression of naive T cell bone marrow homing by the mTORC2-FOXO1-CXCR4 axis.