Regulation of mast cell growth and proliferation

Akt-dependent cytokine production in mast cells

Cross-linking of FcepsilonRI induces the activation of three protein tyrosine kinases, Lyn, Syk, and Bruton's tyrosine kinase (Btk), leading to the secretion of a panel of proinflammatory mediators from mast cells. This study showed phosphorylation at Ser-473 and enzymatic activation of Akt/protein kinase B, the crucial survival kinase, upon FcepsilonRI stimulation in mouse mast cells. Phosphorylation of Akt is regulated positively by Btk and Syk and negatively by Lyn. Akt in turn can regulate positively the transcriptional activity of interleukin (IL)-2 and tumor necrosis factor (TNF)-alpha promoters. Transcription from the nuclear factor kappaB (NF-kappaB), nuclear factor of activated T cells (NF-AT), and activator protein 1 (AP-1) sites within these promoters is under the control of Akt activity. Accordingly, the signaling pathway involving IkappaB-alpha, a cytoplasmic protein that binds NF-kappaB and inhibits its nuclear translocation, appears to be regulated by Akt in mast cells. Catalytic activity of glycogen synthase kinase (GSK)-3beta, a serine/threonine kinase that phosphorylates NF-AT and promotes its nuclear export, seems to be inhibited by Akt. Importantly, Akt regulates the production and secretion of IL-2 and TNF-alpha in FcepsilonRI-stimulated mast cells. Altogether, these results revealed a novel function of Akt in transcriptional activation of cytokine genes via NF-kappaB, NF-AT, and AP-1 that contributes to the production of cytokines.

Regulation of mast cell number and function

Mast cells are complex, multifunctional cells that have unique phenotypes and growth requirements. Regulation in vitro of human mast cell growth and function differs from regulation of rodent mast cells. Human cell yields in vitro vary depending on tissue of origin, use of mononuclear or CD34+ progenitor cells, presence of cytokines, and serum-free versus serum-containing mediums. This article presents a summary of recent advances in the understanding of cytokine regulation of mast cell numbers and function in rodents and humans.

In vitro and in vivo immunostimulatory potential of bone marrow-derived mast cells on B- and T-lymphocyte activation

BACKGROUND

Mast cells, which play a unique role in inflammatory and allergic responses, have also been shown to actively participate to the build-up of protective host defense mechanisms. Recently, they have been shown to stimulate resting B cells and to form heterotypic aggregates with activated T cells, resulting in mast cell degranulation.

OBJECTIVES

Our aim is to investigate the cytokine requirements and the mechanisms by which murine mast cells activate resting B and T lymphocytes.

METHODS

Mouse bone marrow-derived mast cells (BMMCs) or peritoneal mast cells were cocultured with resting splenocytes. Activation of B and T lymphocytes was assessed by measuring cell proliferation, blast formation, and cytokine release.

RESULTS

We report that addition of IL-4-treated BMMCs to normal spleen cells resulted within 48 hours in a B- and T-cell activation with substantial amounts of the T(H1) cytokines IFN-gamma and IL-12 and no detectable IL-4. We also demonstrate that mature mast cells in the peritoneal cavity are able to induce spleen cell activation and cytokine release. Addition of antileukocyte function-associated antigen 1 and anti-intercellular adhesion molecule 1 to the cocultures completely abrogates mast cell-induced blast formation and cytokine release. Experiments performed in vivo indicate that spleen cells from mice injected with BMMCs sustain their capacity of proliferation and cytokine production in vitro without any further stimulation.

CONCLUSION

These observations suggest that mast cells may exert a helper effect on B and T lymphocytes, initiate T(H1)-type immune responses, and may participate, through this mechanism, in the downregulation of allergic responses.

Nuclear Translocation of Upstream Stimulating Factor 2 (USF2) in Activated Mast Cells: A Possible Role in Their Survival

Multiple transcription factors are activated in the cytoplasm and translocated to the nucleus where they exert positive or negative control over cellular genes. Such subcellular traffic of transcription factors usually requires the presence of a positively charged nuclear localization sequence (NLS). Upstream stimulating factor 2 (USF2) is one of the few transcription factors that contain two potential domains for nuclear localization. In addition to the conventional basic NLS, USF2 contains a highly conserved USF-specific region that is involved in its nuclear translocation. In the present work, the induction of translocation of USF2 into the mast cell nucleus was observed and found to be dependent on activation of the cells either by IL-3 or IgE-Ag. It was also observed that the prevention of the translocation of USF2 to the nucleus, using a peptide derived from the specific USF-NLS region, significantly inhibited their IL-3-mediated survival. Thus, our findings show a direct connection between mast cell surface receptor-mediated USF2 nuclear translocation and cell viability.