IL4I1 and glioblastoma: 26 Another study also found that IPYA acts as a ligand for AhR to enhance glioblastoma migration and inhibit the proliferation of CD8+ T cells.27 It is noteworthy that in both studies where IPYA was reported to be pro-oncogenic, IPYA was produced by host cells metabolizing tryptophan via interleukin 4-induced 1 (IL4I1), and was not of microbial source.