Novel holistic immunotherapy to specifically target the malignant clone in Sézary syndrome, a rare cancer of T lymphocytes

A rare cancer of T lymphocytes

Sézary syndrome (SS) is an aggressive leukemic form of cutaneous T-cell lymphoma characterized by a triad of exfoliative erythroderma, lymphadenopathy and circulating atypical malignant T lymphocytes (Sézary cells). It is a very rare disorder (Orphanet 3162) normally diagnosed  in  adults  over  60  with  an  annual  incidence  of  1/106-107,  about  3%  of  skin lymphomas. SS has been linked to a wide range of mutations and chromosomal anomalies, but its etiology remains unclear. Although SS is sometimes referred to as a variant of another cutaneous non leukemic T-cell lymphoma called mycosis fungoides, these two cancers are considered separate conditions.

Lacking an efficient treatment

Patients with SS have a poor prognosis (median survival approximately 5 years) and current treatment options, including extracorporeal photopheresis, chemotherapy (bexarotene, doxorubicin, gemcitabine) and/or biotherapy (interferon-alpha, alemtuzumab, mogamulizumab) show high rates of relapse. In cases of relapse, treatment may include total skin electron beam therapy or allogeneic hematopoietic stem cell transplantation, which have high rates of long- term adverse effects. Thus, there is an unmet need for an efficient treatment.

Targeting T cell cancers is difficult

B cell lymphomas can be successfully targeted by immunotherapy using CD20- or CD19- specific monoclonal antibodies (mAb) or CAR T cells. Although such strategies also eliminate normal B cells, these can be replaced by exogenous immunoglobulins. In contrast, normal T lymphocytes cannot be replaced, thus immunotherapy against T cell cancers should be tumor- specific. Therefore, there is a need to identify and target T cell tumor-specific antigens.

Sézary T cells have unique targetable epitopes

Each SS patient has a single malignant T cell clone, thus expressing a clonal TCR on its surface. The clonal TCR has two chains, alpha and beta, and each contains a variable domain, which in turn has three regions termed CDR (complementarity-determining regions) because they bind unique peptides on HLA molecules. Of the three regions, CDR1 and CDR2 primarily recognize HLA, are largely germline-encoded, and thus shared by some normal patient T cells, whereas CDR3 recognize peptides and include both palindromic and non-germline-encoded amino acids, which are unique to each malignant T cell clone. Therefore, TCRa and TCRb CDR3 and the full peptide-binding variable TCR regions as single-chain (sc) TCR or scTv are unique targetable epitopes for patient-specific immunotherapy using mouse B cells as responders to generate malignant T cell clone-specific mAb.

In addition, each SS patient has a number of mutated proteins that are unique and can be defined by comparing exomes from SS T cells and normal patient cells. Some of these mutated proteins, together with TCR CDR3 sequences, can be filtered immunoinformatically or directly identified by peptidome analysis to define unique neopeptides or CDR3 peptides which can be restricted by each patient’s HLA class I or II molecules. Therefore HLA-restricted neopeptides and CDR3 peptides are unique targetable epitopes for patient-specific immunotherapy using normal autologous or HLA-identical human T cells as responders to generate malignant T cell clone-specific T cells.

Our hypothesis is that it is possible to generate new immunotherapeutic reagents (mAb) and cells (CAR T cells, pHLA-specific T cells) directed against surface SS TCR or HLA-restricted epitopes.

Our aim is to develop and test a novel holistic preclinical immunotherapy to specifically target the malignant T cell clone

Here we propose a combined two-pronged immunotherapy strategy against SS T cells by targeting unique B cell (a) and T cell (b) epitopes thereof using novel approaches.

1. a) Targeting SS clonal TCR epitopes using mAb and CAR T We will produce SS TCR- specific mAb by immunizing mice with clonal TCR peptides or sc proteins. The mAb will be humanized and used directly or as scFv for the generation of SS TCR-specific CAR T cells, as reported for B cell lymphomas. The specificity and efficacy of both moAb and CAR T cells will be tested in preclinical models in vitro (patient-derived malignant SS T cells or SS TCR- transfected Jurkat T cells) and in vivo (mouse models using patient samples or transfected Jurkat T cells).

1. b) Targeting SS T cell HLA-restricted neopeptides and CDR3 peptides using specific T cells. We will produce SS pHLA-specific T cells lines and clones by incubating in vitro normal lymphocytes from patients or, if available, HLA-identical siblings with autologous or HLA- identical mature dendritic cells stimulated with nanostructured mesoporous silicon micro/nanoparticles containing immunoinformatically-predicted or peptidome-defined SS T cell clone HLA-restricted neopeptides and CDR3 peptides. The specificity and efficacy of SS pHLA- specific T cells lines and clones will be tested in vitro (patient-derived malignant SS T cells) and in vivo (humanized mouse models using patient samples).