T cells raised against allogeneic HLA-A2/CD20 kill primary follicular lymphoma and acute lymphoblastic leukemia cells

Advances in immune therapies in hematological malignancies

Immunotherapy in cancer takes advantage of the exquisite specificity, potency, and flexibility of the immune system to eliminate alien tumor cells. It involves strategies to activate the entire immune defense, by unlocking mechanisms developed by tumor cells to escape from surrounding immune cells, as well as engineered antibody and cellular therapies. What is important to note is that these are therapeutics with curative potential. The earliest example of immune therapy is allogeneic stem cell transplantation, introduced in 1957, which is still an important modality in hematology, most notably in myeloid malignancies. In this review, we discuss developmental trends of immunotherapy in hematological malignancies, focusing on some of the strategies that we believe will have the most impact on future clinical practice in this field. In particular, we delineate novel developments for therapies that have already been introduced into the clinic, such as immune checkpoint inhibition and chimeric antigen receptor T-cell therapies. Finally, we discuss the therapeutic potential of emerging strategies based on T-cell receptors and adoptive transfer of allogeneic natural killer cells.

Presence of T cells directed against CD20-derived peptides in healthy individuals and lymphoma patients

Preclinical and clinical studies have suggested that cancer treatment with antitumor antibodies induces a specific adaptive T cell response. A central role in this process has been attributed to CD4⁺ T cells, but the relevant T cell epitopes, mostly derived from non-mutated self-antigens, are largely unknown. In this study, we have characterized human CD20-derived epitopes restricted by HLA-DR1, HLA-DR3, HLA-DR4, and HLA-DR7, and investigated whether T cell responses directed against CD20-derived peptides can be elicited in human HLA-DR-transgenic mice and human samples. Based on in vitro binding assays to recombinant human MHC II molecules and on in vivo immunization assays in H-2 KO/HLA-A2⁺-DR1⁺ transgenic mice, we have identified 21 MHC II-restricted long peptides derived from intracellular, membrane, or extracellular domains of the human non-mutated CD20 protein that trigger in vitro IFN-γ production by PBMCs and splenocytes from healthy individuals and by PBMCs from follicular lymphoma patients. These CD20-derived MHC II-restricted peptides could serve as a therapeutic tool for improving and/or monitoring anti-CD20 T cell activity in patients treated with rituximab or other anti-CD20 antibodies.

Targeting B-cell neoplasia with T-cell receptors recognizing a CD20-derived peptide on patient-specific HLA

T cells engineered to express chimeric antigen receptors (CARs) targeted to CD19 are effective in treatment of B-lymphoid malignancies. However, CARs recognize all CD19 positive (pos) cells, and durable responses are linked to profound depletion of normal B cells. Here, we designed a strategy to specifically target patient B cells by utilizing the fact that T-cell receptors (TCRs), in contrast to CARs, are restricted by HLA. Two TCRs recognizing a peptide from CD20 (SLFLGILSV) in the context of foreign HLA-A*02:01 (CD20p/HLA-A2) were expressed as 2A-bicistronic constructs. T cells re-directed with the A23 and A94 TCR constructs efficiently recognized malignant HLA-A2(pos) B cells endogenously expressing CD20, including patient-derived follicular lymphoma and chronic lymphocytic leukemia (CLL) cells. In contrast, a wide range of HLA-A2(pos)CD20(neg) cells representing different tissue origins, and HLA-A2(neg)CD20(pos) cells, were not recognized. Cytotoxic T cells re-directed with CD20p/HLA-A2-specific TCRs or CD19 CARs responded with similar potencies to cells endogenously expressing comparable levels of CD20 and CD19. The CD20p/HLA-A2-specific TCRs recognized CD20p bound to HLA-A2 with high functional avidity. The results show that T cells expressing CD20p/HLA-A2-specific TCRs efficiently and specifically target B cells. When used in context of an HLA-haploidentical allogeneic stem cell transplantation where the donor is HLA-A2(neg) and the patient HLA-A2(pos), these T cells would selectively kill patient-derived B cells and allow reconstitution of the B-cell compartment with HLA-A2(neg) donor cells. These results should pave the way for clinical testing of T cells genetically engineered to target malignant B cells without permanent depletion of normal B cells.

T-cell receptor gene therapy--ready to go viral?

T lymphocytes can be redirected to recognize a tumor target and harnessed to combat cancer by genetic introduction of T-cell receptors of a defined specificity. This approach has recently mediated encouraging clinical responses in patients with cancers previously regarded as incurable. However, despite the great promise, T-cell receptor gene therapy still faces a multitude of obstacles. Identification of epitopes that enable effective targeting of all the cells in a heterogeneous tumor while sparing normal tissues remains perhaps the most demanding challenge. Experience from clinical trials has revealed the dangers associated with T-cell receptor gene therapy and highlighted the need for reliable preclinical methods to identify potentially hazardous recognition of both intended and unintended epitopes in healthy tissues. Procedures for manufacturing large and highly potent T-cell populations can be optimized to enhance their antitumor efficacy. Here, we review the current knowledge gained from preclinical models and clinical trials using adoptive transfer of T-cell receptor-engineered T lymphocytes, discuss the major challenges involved and highlight potential strategies to increase the safety and efficacy to make T-cell receptor gene therapy a standard-of-care for large patient groups.

Invariant chain as a vehicle to load antigenic peptides on human MHC class I for cytotoxic T-cell activation

Protective T-cell responses depend on efficient presentation of antigen (Ag) in the context of major histocompatibility complex class I (MHCI) and class II (MHCII) molecules. Invariant chain (Ii) serves as a chaperone for MHCII molecules and mediates trafficking to the endosomal pathway. The genetic exchange of the class II-associated Ii peptide (CLIP) with antigenic peptides has proven efficient for loading of MHCII and activation of specific CD4(+) T cells. Here, we investigated if Ii could similarly activate human CD8(+) T cells when used as a vehicle for cytotoxic T-cell (CTL) epitopes. The results show that wild type Ii, and Ii in which CLIP was replaced by known CTL epitopes from the cancer targets MART-1 or CD20, coprecipitated with HLA-A*02:01 and mediated colocalization in the endosomal pathway. Furthermore, HLA-A*02:01-positive cells expressing CLIP-replaced Ii efficiently activated Ag-specific CD8(+) T cells in a TAP- and proteasome-independent manner. Finally, dendritic cells transfected with mRNA encoding IiMART-1 or IiCD20 primed naïve CD8(+) T cells. The results show that Ii carrying antigenic peptides in the CLIP region can promote efficient presentation of the epitopes to CTLs independently of the classical MHCI peptide loading machinery, facilitating novel vaccination strategies against cancer.

Alloreactive cytotoxic T cells provide means to decipher the immunopeptidome and reveal a plethora of tumor-associated self-epitopes

HLA molecules presenting peptides derived from tumor-associated self-antigens (self-TAA) are attractive targets for T-cell-based immunotherapy of cancer. However, detection of such epitopes is hampered by self-tolerance and limitations in the sensitivity of mass spectrometry. Here, we used T cells from HLA-A2-negative donors as tools to detect HLA-A2-bound peptides from two leukemia-associated differentiation antigens; CD20 and the previously undescribed cancer target myeloperoxidase. A high-throughput platform for epitope discovery was designed using dendritic cells cotransfected with full-length transcripts of self-TAA and HLA-A2 to allow presentation of all naturally processed peptides from a predefined self-protein on foreign HLA. Antigen-reactive T cells were directly detected using panels of color-coded peptide-HLA multimers containing epitopes predicted by a computer algorithm. Strikingly, cytotoxic T cells were generated against 37 out of 50 peptides predicted to bind HLA-A2. Among these, 36 epitopes were previously undescribed. The allorestricted T cells were exquisitely peptide- and HLA-specific and responded strongly to HLA-A2-positive leukemic cells with endogenous expression of CD20 or myeloperoxidase. These results indicate that the repertoire of self-peptides presented on HLA class I has been underestimated and that a wealth of self-TAA can be targeted by T cells when using nontolerized T-cell repertoires.

On the future of HLA

As the immunobiological function of the HLA (human leucocyte antigen) class I and II molecules was revealed, we have seen an explosive development of the HLA field. Today, the HLA complex occupies a central position in basic and clinical immunology. In this Opinion article, I will briefly discuss some challenges which in my opinion are more important than others in the near future of HLA, with a focus on products of the classical HLA class I and II genes. Matching for HLA antigens will continue to be of importance in organ and hematopoietic stem cell transplantations. In the latter field, induction of graft-versus-leukemia effects will receive greater attention, where HLA will play a central role. It is anticipated that we will see an extensive development in our knowledge of the etiology and pathogenesis of autoimmune diseases, where some HLA class I and II genes by far are the strongest predisposing genes. To predict and prevent autoimmune diseases will be a major challenge for the HLA field in the future. HLA will also be of increasing importance in pharmacogenomics, vaccinations and anthropology. Together, this will leave the HLA field with many new challenges and opportunities, which in the future will require more focus on functional aspects of the immunogenetics of HLA.