Syk-induced phosphatidylinositol-3-kinase activation in Epstein-Barr virus posttransplant lymphoproliferative...

Posttransplant lymphoproliferative disorder (PTLD)-associated Epstein-Barr virus (EBV)+ B cell lymphomas are serious complications of solid organ and bone marrow transplantation. The EBV protein LMP2a, a B cell receptor (BCR) mimic, provides survival signals to virally infected cells through Syk tyrosine kinase. Therefore, we explored whether Syk inhibition is a viable therapeutic strategy for EBV-associated PTLD. We have shown that R406, the active metabolite of the Syk inhibitor fostamatinib, induces apoptosis and cell cycle arrest while decreasing downstream phosphatidylinositol-3'-kinase (PI3K)/Akt signaling in EBV+ B cell lymphoma PTLD lines in vitro. However, Syk inhibition did not inhibit or delay the in vivo growth of solid tumors established from EBV-infected B cell lines. Instead, we observed tumor growth in adjacent inguinal lymph nodes exclusively in fostamatinib-treated animals. In contrast, direct inhibition of PI3K/Akt significantly reduced tumor burden in a xenogeneic mouse model of PTLD without evidence of tumor growth in adjacent inguinal lymph nodes. Taken together, our data indicate that Syk activates PI3K/Akt signaling which is required for survival of EBV+ B cell lymphomas. PI3K/Akt signaling may be a promising therapeutic target for PTLD, and other EBV-associated malignancies.