Interleukin-35 Signalling

Interleukin-35 (IL35) can signal via IL35 receptors triggering the JAK/STAT pathway downstream. Signal transducer and activator of transcription 4-alpha/beta (STAT4) binds to the JAK associated-IL35 receptor complex and are activated via phosphorylations. Subsequently, phosphorylated p-STAT4 dissociates from the receptor complex (Collison et al. 2012). This is a black box event because there is no literature evidence about the exact mechanism of the release of STAT4 from the receptor complex. Signal transducer and activator of transcription 1-alpha/beta(STAT1) and Signal transducer and activator of transcription 4 (STAT4) after phosphorylation dissociates from the complex of ligand receptor and will dimerize (Delgoffe & Vignali 2013). This is a Black Box event because the mechanism of the release of STAT1 and STAT4 from the receptor complex is unclear. However, it is reported that simultaneous activation of IL12RB2 or IL6ST homodimer receptors do not promote the formation of pSTAT1:pSTAT4 and only the activation of the IL12RB2:IL6ST heteromeric receptor favours pSTAT1:pSTAT4 formation. For this reason, STAT1 and STAT4 are simultaneously considered in the binding, activation and release events.

EBI3 monomer can be found at the endoplasmic reticulum and is not secreted since it remains associated to the protein calnexin (CANX). So is suggested EBI3 needs the association with another subunit to be released to the extracellular region (Devergne et al. 1996).
Interleukin-35 affects and is specifically produced by regulatory T cells and regulatory B cells (Niedbala et al. 2007, Wang et al. 2014, Egwuagu et al. 2015, Fonseca-Camarillo et al. 2015. This interleukin is secreted and inhibits T cell proliferation (Collison et al. 2007).
This reaction is a black box event because we know the Interleukin-35 heterodimeric protein is secreted but there is no reported evidence from which compartment and which mechanism is involved in the traslocation.
For this reason, this event is assigned a black box status. First, IL6ST binds to site 2 in the Interleukin 6(IL 6), Ciliary neurotrophic factor (CNTF) and LIF (Leukemia inhibitory factor) complexes and thus could do so in the IL 35 IL35R complex, leaving IL12RB2 to bind site 3.
Second, the Interleukin 35-Interleukin 35 receptor complex could form symmetric homohexameric assemblies (2:2:2) analogous to the Interleukin 6 receptor complex, thus allowing IL 12RB2 and IL6ST to each bind to site 3 on IL12A.
Third, IL6ST and IL12RB2 could both be capable of binding to site 2 and site 3 and therefore exist in an interchanging equilibrium of heterotetrameric complexes (at a ratio of 1 Signal transducer and activator of transcription 1-alpha/beta(STAT1) and Signal transducer and activator of transcription 4 (STAT4) after phosphorylation dissociates from the complex of ligand receptor and will dimerize (Delgoffe & Vignali 2013). This is a Black Box event because the mechanism of the release of STAT1 and STAT4 from the receptor complex is unclear. However, it is reported that simultaneous activation of IL12RB2 or IL6ST homodimer receptors do not promote the formation of pSTAT1:pSTAT4 and only the activation of the IL12RB2:IL6ST heteromeric receptor favours pSTAT1:pSTAT4 formation. For this reason, STAT1 and STAT4 are simultaneously considered in the binding, activation and release events. This reaction is presented as a black box because the exact mechanism of traslocation to the nucleus is unclear.