Myeloid Suppressor Lines Inhibit T Cell Responses by an NO-Dependent Mechanism

Abstract

CD11b+Gr-1+ myeloid suppressor cells (MSC) accumulate in lymphoid organs under conditions of intense immune stress where they inhibit T and B cell function. We recently described the generation of immortalized MSC lines that provide a homogeneous source of suppressor cells for dissecting the mechanism of suppression. In this study we show that the MSC lines potently block in vitro proliferation of T cells stimulated with either mitogen or antigenic peptide, with as few as 3% of MSC cells causing complete suppression. Inhibition of mitogenic and peptide-specific responses is not associated with a loss in IL-2 production or inability to up-modulate the early activation markers, CD69 and CD25, but results in direct impairment of the three IL-2R signaling pathways, as demonstrated by the lack of Janus kinase 3, STAT5, extracellular signal-regulated kinase, and Akt phosphorylation in response to IL-2. Suppression is mediated by and requires NO, which is secreted by MSC in response to signals from activated T cells, including IFN-γ and a contact-dependent stimulus. Experiments with inducible NO synthase knockout mice demonstrated that the inhibition of T cell proliferation by CD11b+Gr-1+ cells in the spleens of immunosuppressed mice is also dependent upon NO, indicating that the MSC lines accurately represent their normal counterparts. The distinctive capacity of MSC to generate suppressive signals when encountering activated T cells defines a specialized subset of myeloid cells that most likely serve a regulatory function during times of heightened immune activity.

Identification and Characterization of Novel Bone Marrow Myeloid DEC205+Gr-1+ Cell Subsets That Differentially Express Chemokine and TLRs

“…This population of regulatory cells or so-called inhibitory macrophages is responsible, at least in part, for the profound depression of T cell responses observed in mice immunized with vaccines or bearing tumor cells (31). Interestingly we found that the DEC205 ϩ Gr-1 ϩ cells express CD31, F4/80, and low levels of MHC class II molecules, a phenotype consistent with that of "inhibitory macrophages" described in mouse spleens and bone marrow (30).…”

Nitric Oxide-Independent CTL Suppression during Tumor Progression: Association with Arginase-Producing (M2) Myeloid Cells

“…One well-documented suppressive mechanism for CD11b ϩ /Gr-1 ϩ cells was described to be mediated by IFN-␥-dependent NO production (17, 30 -33, 63). However, it is noteworthy that this type of suppressive activity was often tested toward the primary T cell proliferation induced by mitogens (30,63), anti-CD3 Abs combined with anti-CD28 or IL-2 (17,30,32,33), or unrelated Ags (63), but not toward the relevant Ag-specific T cells that the CD11b ϩ /Gr-1 ϩ cells regulate in vivo as in our study. Hence, the physiological relevance of these in vitro testing systems needs to be carefully evaluated.…”

VISTA is highly expressed on MDSCs and mediates an inhibition of T cell response in patients with AML

“…MDSCs can exert the inhibition of T cell activation through multiple mechanisms including 1) depleting amino acids that are essential for T cell proliferation by overproduction of arginase 1 (ARG1); 30 2) causing the loss of T cell receptor by releasing oxidizing molecules (e.g. iNOS) and generating oxidative stress; 31,32 3) inducing the development of Treg; 33-36 and 4) impairing T cell migration by down regulating trafficking related surface molecules. 37-39 In our study, we discovered that VISTA is highly expressed on MDSCs and knockdown of VISTA significantly diminished the MDSC-mediated inhibition of T cell proliferation.…”