Inferred from mouse:
Mouse Soluble isoform of Interleukin-27 receptor alpha subunit (sIl27ra or WSX-1) can be produced by two mechanisms. There is experimental evidence of expression of sIl27ra in primary cortical neurons (PCNs) (Hashimoto et al. 2010). Human sIL27RA is produced by activated CD4(+) and CD8(+) T cells, B cells, myeloid cells and various cell lines.

sIL27RA is released as two N-glycosylated variants of around 90 and 70 kDa. Production is inhibited by the metalloprotease inhibitors GM6001 (galardin, ilomastat) and Tumor necrosis factor-alpha processing inhibitor-0 (TAPI-0). sIL27RA is a natural antagonist of Interleukin-27 (Dietrich et al. 2014)
As the origin of sIL27RA is unclear this is a black box event.