Platelet-derived growth factor mediates survival of leukemic large granular lymphocytes via an autocrine regulatory pathway

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Abstract
Large granular lymphocyte (LGL) leukemia results from chronic expansion of cytotoxic T cells or natural killer (NK) cells. Apoptotic resistance resulting from constitutive activation of survival signaling pathways is a fundamental pathogenic mechanism. Recent network modeling analyses identified platelet-derived growth factor (PDGF) as a key master switch in controlling these survival pathways in T-cell LGL leukemia. Here we show that an autocrine PDGF regulatory loop mediates survival of leukemic LGLs of both T- and NK-cell origin. We found high levels of circulating PDGF-BB in platelet-poor plasma samples from LGL leukemia patients. Production of PDGF-BB by leukemic LGLs was demonstrated by immunocytochemical staining. Leukemic cells expressed much higher levels of PDGFR-β transcripts than purified normal CD8+ T cells or NK cells. We observed that phosphatidylinositol-3-kinase (PI3 kinase), Src family kinase (SFK), and downstream protein kinase B (PKB)/AKT pathways were constitutively activated in both T- and NK-LGL leukemia. Pharmacologic blockade of these pathways led to apoptosis of leukemic LGLs. Neutralizing antibody to PDGF-BB inhibited PKB/AKT phosphorylation induced by LGL leukemia sera. These results suggest that targeting of PDGF-BB, a pivotal regulator for the long-term survival of leukemic LGLs, may be an important therapeutic strategy.