Moreover, both in monocytes and in DCs, calcium influx induces secretion of pro-IL-1 and pro-IL-18 (Gardella et al., 2000a, Peptide 17 2001; Andrei et al., 2004). by substances that promote autophagy and, conversely, the enhancement observed with compounds that block autophagy (Crisan et al., 2011; Harris et al., 2011; Shi et al., 2012) and by the results obtained in mice deficient of autophagy genes (Saitoh et al., 2008; Nakahira et al., 2011). A possible explanation for these contrasting findings is that formation of pro-IL-1 containing autophagosomes is a pre-requisite for IL-1 secretion, and it is induced by TLR activation. Autophagosomes may then undergo exocytosis, with secretion of IL-1, or fuse to lysosomes, with degradation of pro-IL-1, depending on the Peptide 17 type and strength of the stimuli that trigger IL-1 producing cells. Thus, strong stimuli (e.g., high doses of TLR agonists) or exposure to inducer of autophagy such Rabbit Polyclonal to ATG16L2 as rapamycin, would privilege maturation of autophagosome to autolysosome, and degradation of its content by the hydrolases provided by lysosomes. Weaker stimuli, such as continuous triggering by lower doses of TLR agonists would instead favor amphisome fusion with the plasma membrane, resulting in secretion (Figure ?(Figure22). The Travel of Cytokines from Inside to Outside during Evolution. Both Passive Release and Active Secretion Account for IL-1 and IL-33 Externalization The founding member of IL-1 family is probably IL-1 because of its close homology to acidic FGF, in turn one of the most ancient cytokines. It is conceivable that, at the beginning, IL-1F cytokines were, like FGF, intracellular growth factors, and repair molecules interacting with DNA as transcription factors (Dinarello, 2010) and were passively externalized by dying cells. When later in evolution Immunoglobulins appeared, they were used by extracellular cytokines/growth factors as cell surface receptors, and cytokine-mediated signaling evolved. Some IL-1F members including IL-1 and IL-33 retain intracellular (nuclear) function (Carriere et al., 2007; Cayrol and Girard, 2009). Interestingly, unlike IL-1 and IL-18, IL-1, and IL-33 can activate their receptors on target cells as full-length molecules: thus when released from injured cells they can exert their biological activity in the absence of a proteolytic processing (Chen et al., 2007; Eigenbrod et al., 2008; Cayrol and Girard, 2009). Based on the above observations, IL-1 family members can be divided into a group of cytokines that retain some intracellular function and are passively externalized upon cell lysis (the prototype being IL-1), and a second group including cytokines that are stored into the cell cytosol before secretion, but do not play intracellular function and undergo regulated processing and secretion (the prototype being IL-1). Still, the situation is more complex. In fact, a number of reports indicate the possibility that also IL-1 and IL-33 are actively released by cells that maintain their integrity. IL-1 was reported to be secreted in response to heat shock (Mandinova et al., 2003) and through an unknown mechanism requiring caspase-1 (Gross et al., 2012). In the case of IL-33, intracellular calcium increase, regulated autocrinally by ATP and purinergic receptor stimulation, induces translocation from nucleus to Peptide 17 cytoplasm and release of full-length IL-33 (Kouzaki et al., 2011). Extracellular ATP is a well-known inducer of inflammasome activation and IL-1/IL-18 processing. The mechanism through which ATP induces IL-33 secretion seems to be different, since the unprocessed, full-length molecular form of Peptide 17 IL-33 is secreted. However, we have previously observed that in human monocytes ATP drives exocytosis of pro-IL-1 containing vesicles also if caspase-1 is inhibited, resulting in secretion of the precursor form of the cytokine (Andrei et al., 2004). Moreover, both in monocytes and in DCs, calcium influx induces secretion of pro-IL-1 and pro-IL-18 (Gardella et al., 2000a, 2001; Andrei et Peptide 17 al., 2004). Thus it is conceivable that the pathway described for IL-33 makes use of mechanisms (purinergic receptor stimulation and calcium influx) which are old and conserved during evolution. The ATP-mediated signaling may then have further specialized adding to the older function of inducing exocytosis the newer function of controlling inflammasome.