Rapamycin-resistant mTORC1 restrains dendritic cell B7-H1 expression that requires IL-1β to enhance regulatory T cell induction (P1349)

Introduction: The mammalian Target of Rapamycin (mTOR) is a central regulator of dendritic cell (DC) function that performs the catalytic activity of mTOR complex (mTORC)1 and 2. mTORC2 functions independently from mTORC1 and is resistant to inhibition by rapamycin (RAPA); however, mTORC1 has both RAPA-sensitive and -resistant outputs. Our goal was to ascertain the role of RAPA-resistant mTOR in DC. Methods: WT C57BL/6 or B7-H1-/- bone marrow-derived DC were generated with the addition of RAPA or ATP-competitive mTOR inhibitor, which blocks all mTOR signaling. DC lacking rictor, an mTORC2-specific subunit, were generated from conditional rictor KO mice. DC induction of regulatory T cells (Treg) was determined in MLR, using BALB/c CD4+CD25- T cell responders. Results and Conclusion: RAPA and mTORC2 deletion reduced DC B7-H1 expression, but ATP-competitive mTOR inhibitors enhanced B7-H1 expression. Augmented B7-H1 expression was blocked by STAT3 inhibition and correlated with reduced expression of the STAT3 negative regulator, SOCS3. DC exposed to ATP-competitive mTOR inhibitors increased Treg induction, which was dependent on DC B7-H1. IL-1β neutralization additionally reduced Treg induction by B7-H1-/- ATP-competitive mTOR inhibitor-exposed DC, suggesting that IL-1β and B7-H1 act additively to promote Treg induction by these DC. These findings establish a RAPA-resistant mTORC1 pathway that acts through SOCS3 and STAT3 to regulate DC B7-H1 expression and Treg induction.

Flt3 ligand expands and activates myeloid-derived suppressor cells in a STAT3-independent manner (P1074)

Introduction: Fms-like tyrosine kinase 3 ligand (Flt3L) promotes dendritic cell (DC) expansion; however, its influence on myeloid-derived suppressor cells (MDSC), an immature population of myeloid cells that suppresses T cell immunity, has not been studied. Methods: BALB/c mice were treated with Flt3L (10 μg/d i.p., 10 d) in the presence or absence of STAT3 inhibitor S31-201 (5 mg/kg/d). Splenic DC and MDSC were isolated by CD11c+ and Gr1+ magnetic bead selection, respectively. Stimulation and suppression of CD3+ T cell responses were assayed by MLR. Syngeneic MDSC were adoptively transferred (5x106) to BALB/c recipients 1 d before C57BL/6 heart transplantation and survival monitored. Results and Conclusions: Flt3L increased MDSC (CD11b+Gr1+) frequency and absolute numbers. MDSC from Flt3L-treated mice potently suppressed CD3+ T cell proliferation greater than control MDSC from naïve splenocytes. Conversely, Flt3L-expanded DC stimulated increased proliferation of allogeneic T cells compared to those from naïve control mice. While STAT3 is considered crucial for MDSC expansion and activation, STAT3 inhibition reduced Flt3L-mediated DC, but not MDSC, expansion. STAT3 inhibition augmented MDSC expansion by Flt3L, without affecting their suppressive capacity. Flt3L-expanded MDSC, but not control MDSC, demonstrated in vivo suppressive activity by prolonging fully MHC-mismatched heart transplant survival. Together, these data identify a novel immunomodulatory function of Flt3L.