Epstein-Barr virus (EBV) B cell lymphomas in post-transplant lymphoproliferative disorder (PTLD) is a major problem in organ transplantation. Reduction of immunosuppression sometimes result in tumor regression but also puts the graft at risk for rejection. Therefore, treatments that combine anti-tumor and anti-rejection properties would be of great importance to PTLD patients.

Previous studies have shown that multiple cellular signal transduction pathways, including the PI3K/Akt/mTOR pathway, are dysregulated in EBV+ B cell lymphomas, and that inhibiting mTOR with rapamycin results in partial inhibition of proliferation. We hypothesized that targeting molecules upstream of mTOR would inhibit EBV+ B cell lymphoma proliferation. Moreover, blockade of this pathway may prolong graft survival, since this pathway is also known to be activated downstream of the T cell receptor.

Phosphorylation of the PI3K/Akt/mTOR pathway was determined by protein array on cellular lysates from EBV+ B cell lymphoma lines from patients (n=5) with PTLD. The effect of small molecular inhibitors of the pathway on cellular proliferation was also determined. Finally, the effect of these inhibitors on graft survival was assessed using a fully MHC mismatched heterotopic mouse heart transplant model (Balb/c to C57Bl/6).

EBV+ B lymphoma cell lines showed hyperactivation of multiple nodes along the PI3K/Akt/mTOR pathway, including mTOR, 4E-BP1, S6K, and GSK3α/β, compared to normal human B cells and EBV- Burkitt's lymphoma cells. Several upstream signals such as PDK3 and Akt were also constitutively phosphorylated, supporting their role as potential therapeutic targets. Hierarchical clustering distinguished the EBV+ B cell lines from EBV- B cells. Inhibiting upstream PI3K with the small molecule inhibitor CAL101 resulted in a dose-dependent decrease in proliferation (x=51.6±6.6%, range 39.8-62.6%) of the EBV+ B cell lymphoma lines (n=3). Likewise, inhibition of Akt with MK2206 also decreased proliferation (x=52.1±3.5%, range 46.5-58.5%) in a dose-dependent manner.

Finally, in vivo treatment with either CAL-101 or MK2206 prolonged allograft survival from 8.75 days (control) to 21.2 (SEM ±4.48, n=6) and 20.5 (±4.31, n=6) days, respectively. Thus, inhibition of signaling nodes upstream of mTOR, such as PI3K and Akt, demonstrates anti-tumor properties, and the attractiveness of these regimens clinically is complemented by their immunosuppressive effects.

CITATION INFORMATION: Sang A, Ivison G, Esquivel C, Krams S, Martinez O. Targeting the Constitutively Active PI3K/Akt/mTOR Signaling Pathway in Epstein-Barr Virus B Cell Lymphomas. Am J Transplant. 2016;16 (suppl 3).

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