TGFB1 and glioblastoma: However, in GBM, they often exist as a heterogeneous population comprising the IL-4- and IL-13-activated M2a subset, the M2b subset (activated by IL-1R ligands or immune complexes plus lipopolysaccharides, LPS), and the M2c subset (activated by IL-10 and transforming growth factor beta, TGF-β) (26, 27).