Novel Immunotherapies for EBV-Associated Cancers

Abstract

Epstein-Barr virus (EBV) is a gamma herpes virus persistently infecting over 90% of the adult population worldwide. It has been aetiologically linked to a number of human malignancies, including more than 90% of post transplant lymphoproliferative disease (PTLD), 50% of Hodgkin’s lymphoma (HL), virtually all undifferentiated nasopharyngeal carcinoma (NPC), and approximately 10% of gastric carcinoma (GC). As EBV infection in healthy individuals is mainly controlled by virus specific cytotoxic T lymphocytes (CTLs), we hypothesise that engineering T cells with chimeric T cell receptors (cTCRs) specific for EBV latent membrane proteins (LMPs) will confer on these cells the ability to target and kill the malignant cells of cancers associated with Epstein-Barr virus. Thus, the aim of this project was generate these engineered T cells and to set up a severe combined immunodeficient (SCID) mouse model in which to test their effectiveness. Three EBV-infected cell lines derived from HL, NPC and GC gave rise to tumours in 11 of 12 (92%), 12 of 12 (100%) and 10 of 10 (100%) SCID mice respectively, when 1x107 cells were injected subcutaneously. Immunohistochemical analysis showed that the HL SCID tumours were CD4-, CD15-, CD20+, CD30+, consistent with a HL Reed-Sternberg cell phenotype, and NPC and GC SCID tumours expressed the epithelial cell marker cytokeratin. Furthermore, all tumours expressed EBVencoded RNAs (EBERs) and LMP1. This was identical to parent cell line expression patterns, and hence growth in vivo did not affect cell phenotype. T cells were successfully transduced with a retroviral vector encoding a CD19-specific cTCR (CD19- cTCR) with a mean transduction rate of 13%±6%. Transduced cells were cytotoxic for HL-derived L591 cells in vitro, with specific lysis of 24%±11% at an effector to target ratio of 20:1. This was significantly higher than specific lysis seen in mock transduced cells (p>0.05). At a tumour inoculation dose of 5x106, in vivo sc transfer of 5x107 CD19-cTCR transduced cells was able to prevent HL tumour development in 6 of 6 (100%) test mice, whereas 17 of 22 (77%) control mice and 2 of 3 (66%) mice treated with unmodified EBV-specific CTLs developed tumours. Moreover, iv transfer of 5x107 CD19-cTCR transduced cells mediated complete regression of HL SCID tumours in 3 out of 6 (50%) mice. Phage display selection experiments to isolate a single chain antibody fragment (scFv) specific for viral LMPs for incorporation in a cTCR were performed. Linear, biotinylated and cyclised biotinylated peptides derived from the external reverse turn loops of LMP2 were used as target antigens. Despite extensive testing, no reactive clones specific for the peptides were identified. The ability of CD19-cTCR transduced cells to specifically lyse HL cells in vitro, and clear tumour burden in vivo, supports a future role for engineered T cells in the treatment of HL. Despite the lack of success in isolating a scFv for LMP2, the use of viral antigen specific, cTCR redirected T cells remains in principle a valuable therapeutic alternative for EBV-associated malignancies. The SCID models for HL, NPC and GC will provide a useful preclinical tool for investigation of their efficacy in vivo.