TCL1: A Shared Tumor-Associated Antigen for Immunotherapy Against B-Cell Lymphomas

Abstract 105

Rituximab-based chemotherapy regimens have improved complete response rates, progression-free survival, and overall survival of B-cell non-Hodgkin lymphoma patients. However, most patients relapse and die of their lymphoma. To further improve clinical outcome, novel strategies that eradicate minimal residual disease (MRD) after induction therapy are needed. Therapeutic vaccines may induce antitumor antibody and T-cell responses and may eradicate MRD by complementary mechanisms. A recent randomized phase III clinical trial showed that patient-specific idiotype vaccination improves disease-free survival when administered in the setting of MRD in follicular lymphoma (FL), providing proof of principle that therapeutic vaccines can improve clinical outcome (Schuster et al, J Clin Oncol, 2011). However, generation of a custom-made vaccine formulation for each patient can be expensive and time consuming. To overcome these difficulties, identification of novel shared lymphoma-associated antigens is necessary.

The T-cell leukemia/lymphoma 1 (TCL1) oncoprotein encoded by the TCL1 gene is a co-activator of Akt and promotes cell proliferation and survival. Ectopic expression of TCL1 in B cells in transgenic mice results in the development of B-cell malignancies and TCL1 was reported to be aberrantly expressed in multiple human B cell malignancies. Here, we determined whether TCL1 can serve as a novel shared tumor-associated antigen in B cell lymphomas. We analyzed the expression pattern of TCL1 in human B cell lymphomas and normal tissues by real-time PCR, flow cytometry, immunohistochemistry, and Western blotting. TCL1 mRNA transcripts were hyperexpressed in multiple types of primary B cell lymphomas [chronic lymphocytic leukemia (CLL), n=7; mantle cell lymphoma (MCL), n=6; FL, n=12, diffuse large B-cell lymphoma (DLBCL), n=5] as compared with B cells derived from normal donors. TCL1 mRNA was not detected in other normal tissues that included T cells, adipose tissue, esophagus, ovary, spleen, bladder, heart, placenta, brain, kidney, prostate, liver, muscle, thyroid, colon, lung, intestine, trachea, thymus, and cervix. Very low TCL1 mRNA was detected in testis. By flow cytometry, immunohistochemistry, and/or Western blotting we observed that the TCL1 protein is hyperexpressed in CLL, MCL, DLBCL, Burkitts lymphoma, and FL, but not splenic marginal zone lymphoma. To determine whether TCL1 is immunogenic, we synthesized overlapping 15-mer peptides spanning the entire length of the TCL1 protein and stimulated PBMC from HLA-A*0201⁺ (HLA-A2⁺) normal donors to generate peptide-specific T cells. We found that TCL1_65–79 peptide (TQIGPSLLPIMWQLY) consistently induced T cells that secreted significant amount of IFN-γ from 3 normal donors. By intracellular cytokine assay, we determined that the TCL1_65–79 peptide-specific IFN-g was produced by CD8⁺ T cells but not CD4⁺ T cells. Using a panel of peptide-pulsed EBV-transformed B-cell lines that were mismatched at the MHC Class I locus and MHC Class I blocking antibodies, we determined that HLA-A2 is the restriction element of the TCL1_65–79 peptide-specific T cells. Consistent with this, TCL1_65–79 peptide-specific CD8⁺ T cells lysed TCL1 expressing HLA-A2⁺ MCL cell lines but did not lyse, TCL1 expressing HLA-A2⁻ myeloma cell line. Using a panel of peptides truncated progressively by one amino acid at the N- and C-termini of the TCL1_65–79peptide, we determined that TCL1_71–78(LLPIMWQL) is the minimal epitope that bound to HLA-A2. Furthermore, cytotoxic T lymphocytes (CTL) specific to this peptide efficiently killed lymphoma cell lines and primary lymphoma cells from CLL, MCL, DLBCL, and FL in an HLA-A2-restricted manner. Using TCL1_71–78 peptide-specific HLA-A2 tetramers, we found that TCL1 epitope-specific CTLs were present in the peripheral blood and/or tumor-infiltrating lymphocytes of B-cell lymphoma patients (n=8). More importantly, these TCL1 epitope-specific CTLs could be expanded in vitro and lysed autologous and allogeneic tumor cells in an HLA-A2-restricted manner. In conclusion, our results suggest that TCL1 is naturally processed and presented on the surface of primary lymphoma cells for recognition by CTL and can serve as a novel, shared tumor-associated antigen for therapeutic vaccine development against common B-cell lymphomas including CLL, MCL, DLBCL, and FL.