Summary Dioxygenases of the TET family impact genome functions by converting 5-methylcytosine (5mC) in DNA to 5-hydroxymethylcytosine (5hmC). Here, we identified TET2 as a crucial regulator of mast cell differentiation and proliferation. In the absence of TET2, mast cells showed disrupted gene expression and altered genome-wide 5hmC deposition, especially at enhancers and in the proximity of downregulated genes. Impaired differentiation of Tet2-ablated cells could be relieved or further exacerbated by modulating the activity of other TET family members, and mechanistically it could be linked to the dysregulated expression of C/EBP family transcription factors. Conversely, the marked increase in proliferation induced by the loss of TET2 could be rescued exclusively by re-expression of wild-type or catalytically inactive TET2. Our data indicate that, in the absence of TET2, mast cell differentiation is under the control of compensatory mechanisms mediated by other TET family members, while proliferation is strictly dependent on TET2 expression.

DNA methylation and specifically the DNA methyltransferase enzyme DNMT3A are involved in the pathogenesis of a variety of hematological diseases and in regulating the function of immune cells. Although altered DNA methylation patterns and mutations in DNMT3A correlate with mast cell proliferative disorders in humans, the role of DNA methylation in mast cell biology is not understood. By using mast cells lacking Dnmt3a, we found that this enzyme is involved in restraining mast cell responses to acute and chronic stimuli, both in vitro and in vivo. The exacerbated mast cell responses observed in the absence of Dnmt3a were recapitulated or enhanced by treatment with the demethylating agent 5-aza-2′-deoxycytidine as well as by down-modulation of Dnmt1 expression, further supporting the role of DNA methylation in regulating mast cell activation. Mechanistically, these effects were in part mediated by the dysregulated expression of the scaffold protein IQGAP2, which is characterized by the ability to regulate a wide variety of biological processes. Altogether, our data demonstrate that DNMT3A and DNA methylation are key modulators of mast cell responsiveness to acute and chronic stimulation.DNA methylation | epigenetics | inflammation | mast cells

Understanding the mechanisms that modulate T helper lymphocyte functions is crucial to decipher normal and pathogenic immune responses in humans. To identify molecular determinants influencing the pathogenicity of T cells, we separated ex vivo -isolated primary human memory T lymphocytes based on their ability to produce high levels of inflammatory cytokines. We found that the inflammatory, cytokine-producing phenotype of memory T lymphocytes was defined by a specific core gene signature and was mechanistically regulated by the constitutive activation of the NF-κB pathway and by the expression of the transcriptional repressor BHLHE40. BHLHE40 attenuated the expression of anti-inflammatory factors, including miR-146a, a negative regulator of NF-κB activation, and ZC3H12D, an RNase of the Regnase-1 family able to degrade inflammatory transcripts. Our data reveal a molecular network regulating the pro-inflammatory phenotype of human memory T lymphocytes, with the potential to contribute to disease.

The transcription factor NF-B regulates the expression of a broad number of genes central to immune and inflammatory responses. We identified a new molecular network that comprises specifically the NF-B family member NF-B1 (p50) and miR146a, and we show that in mast cells it contributes to the regulation of cell homeostasis and survival, while in T lymphocytes it modulates T cell memory formation. Increased mast cell survival was due to unbalanced expression of pro-and antiapoptotic factors and particularly to the complete inability of p50-deleted mast cells to induce expression of miR-146a, which in the context of mast cell survival acted as a proapoptotic factor. Interestingly, in a different cellular context, namely, human and mouse primary T lymphocytes, miR-146a and NF-B p50 did not influence cell survival or cytokine production but rather T cell expansion and activation in response to T cell receptor (TCR) engagement. Our data identify a new molecular network important in modulating adaptive and innate immune responses and show how the same activation-induced microRNA (miRNA) can be similarly regulated in different cell types even in response to different stimuli but can still determine very different outcomes, likely depending on the specific transcriptome.