The level of HIF1α transcription is controlled by nuclear factor-κΒ, but its activity is mainly controlled post-translation by an oxygen-mediated ubiquitination and degradation selleckchem controlled by the Von Hippel–Lindau tumor suppressor complex and by positive regulation via a TORC1-mediated phosphorylation. The differentiation of naive T cells under hypoxic conditions has also been suggested to enhance
FOXP3 expression and the development of regulatory activity, but it is not clear whether this is a direct effect of HIF1α on FOXP3 expression, or whether it is acting indirectly, as HIF1α activation can also inactivate mTOR. Hypoxia is associated with raised levels of AMP within the cell, which activates AMP-activated protein kinase and consequently inhibits mTOR via tuberous sclerosis complex 1/2. Other sources of AMP that may activate this pathway are downstream of G protein signalling where the generated cAMP from ATP is subsequently broken down to AMP by cAMP phosphodiesterases. In addition, extracellular adenosine can generate Selumetinib in vivo cAMP via activation surface receptors
(e.g. the A2AR on T cells[40, 41]) or can be directly taken up by specific transporters where, once inside the cell, it will be rapidly converted to AMP by adenosine kinase, one of the most abundant enzymes present in mammalian cells. Adenosine is particularly relevant to immune regulation, as TGF-β is able to induce in a range of haematopoietic cells the co-expression of two ectoenzymes, CD39 and CD73, that are constitutively expressed
on Treg cells. These enzymes act to convert extracellular sources of ATP, which is associated with Amisulpride inflammation and cell necrosis, into the anti-inflammatory product adenosine (Fig. 2). Although there is some evidence that this pathway may be relevant to tumours escaping immune surveillance,[45, 46] it remains, however, to be resolved just how important adenosine is as a component of the anti-inflammatory microenvironment within tolerated tissues. It has only recently become clear that tolerance can be maintained by Treg cells acting within a highly localized microenvironment to induce a state of acquired immune privilege.[47, 48] This can best be demonstrated in experiments where donor alloantigen-specific tolerance has been induced to a skin graft (e.g. by a short period of co-receptor blockade with anti-CD4 and anti-CD8 monoclonal antibodies), and then that tolerated graft is removed and re-transplanted onto a secondary recipient with no T cells of its own (e.g. a recombinase activating gene 1 knockout mouse). As expected, this skin graft is accepted by the secondary recipient because it has no T cells to cause rejection. If, however, we treat the recipient at the time of grafting with monoclonal antibodies that deplete or inactivate FOXP3+ Treg cells (e.g. anti-CD25, or anti-hCD2, if the original recipient carries the hCD2.