High-Affinity CD20-Specific T-Cell Receptors Suitable for Adoptive Immunotherapy in the Treatment of CD20^low B-Cell Malignancies

Therapeutic monoclonal antibodies (mAb) such as Rituximab and Ofatumumab have demonstrated the clinical efficacy of targeting the B-cell restricted antigen CD20 for the treatment of B-cell lymphomas and leukemia. Although CD20 is also expressed on healthy B-cell cells which are depleted in the course of therapy, long-term B-cell aplasia is well manageable. However, non-responsive or refractory disease to CD20-targeted mAb treatment has been reported with various mechanisms of resistance: downregulation of CD20 expression, internalization of CD20:mAb complex, inhibition of complement-dependent cytotoxicity and absence of an effector cell repertoire in patients treated with chemotherapy prior to mAb infusion. Therefore, additional therapeutic strategies are required.

T-cell receptor (TCR) gene transfer is an attractive strategy to equip T-cells with TCRs of defined antigen-specificity. Due to their high sensitivity for cognate antigen presented in HLA, TCRs can induce T-cell activation even when antigen expression is very low. However, the broad application of TCR-based adoptive immunotherapy directed against self-antigens such as CD20 is hampered by lack of an effective immune response against self-antigens. T-cells carrying high-affinity TCRs reactive to such self-antigens are deleted by negative selection during thymic development to prevent auto-reactivity. An attractive strategy to target self-antigens is to exploiting the immunogenicity of such antigens presented in the context of allogeneic HLA (alloHLA).

Here, we used the CD20-derived peptide SLFLGILSV (CD20_SLF) binding in HLA-A2 to isolate CD20-reactive T-cells carrying high-affinity TCRs. From peripheral blood mononuclear cells of HLA-A*0201 (HLA-A2)-negative healthy individuals CD8⁺ T-cells binding to peptide-HLA tetramers composed of CD20_SLF bound to HLA-A2 were isolated and clonally expanded. Two high-avidity T-cell clones were identified specific for HLA-A2-bound CD20_SLF. CD20-dependent recognition was demonstrated for both clones by transducing the CD20 gene in HLA-A2-positive cell lines which otherwise lack CD20 expression. Both CD20-specific T-cell clones efficiently recognized CD20-expressing HLA-A2-positive primary B-cell malignancies including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). In addition, the CD20-specific T-cell clones were able to more efficiently recognize ALL cell-lines than CD20-specific mAbs. We demonstrated that on target cells with only very low CD20 surface expression, the CD20-specific T-cell clones could still efficiently recognize endogenously processed CD20-derived peptide in the context of HLA-A2. Furthermore, no recognition of HLA-A2-positive but CD20-negative cell subsets including CD34⁺hematopoietic progenitor cells, T-cells, immature and mature dendritic cells could be demonstrated. Additionally, recognition of HLA-A2-positive non-hematopoietic cells such as fibroblasts even under simulated inflamed conditions was absent.

Transduction of the identified TCRs resulted in efficient expression of the introduced CD20-specific TCRs and conferred CD20-specificity onto recipient cells.

In summary, we exploited the immunogenicity of alloHLA to raise high-avidity T-cells against self-antigens such as CD20. The identified CD20-specific T-cell clones efficiently recognized CD20-expressing primary ALL, CLL and MCL. These T-cells clones more efficiently recognized B-cell malignancies than CD20-targeted mAbs while no recognition of CD20-negative hematopoietic and non-hematopoietic cells was observed. Transduction of these CD20-specific TCRs conferred CD20-specificity onto recipient cells. These CD20-specific TCRs can be useful to treat patients with CD20^low B-cell malignancies by administering TCR-engineered T cells with potent effector function.