Advanced Pathogenetic Concepts in T-Cell Prolymphocytic Leukemia and Their Translational Impact

T-cell prolymphocytic leukemia (T-PLL) is the most common mature T-cell leukemia. It is a typically aggressively growing and chemotherapy-resistant malignancy with a poor prognosis. T-PLL cells resemble activated, post-thymic T-lymphocytes with memory-type effector functions. Constitutive transcriptional activation of genes of the T-cell leukemia 1 (TCL1) family based on genomic inversions/translocations is recognized as a key event in T-PLL’s pathogenesis. TCL1’s multiple effector pathways include the enhancement of T-cell receptor (TCR) signals. New molecular dependencies around responses to DNA damage, including repair and apoptosis regulation, as well as alterations of cytokine and non-TCR activation signaling were identified as perturbed hallmark pathways within the past years. We currently witness these vulnerabilities to be interrogated in first pre-clinical concepts and initial clinical testing in relapsed/refractory T-PLL patients. We summarize here the current knowledge on the molecular understanding of T-PLL’s pathobiology and critically assess the true translational progress around this to help appraisal by caregivers and patients. Overall, the contemporary concepts on T-PLL’s pathobiology are condensed in a comprehensive mechanistic disease model and promising interventional strategies derived from it are highlighted.

Extricating human tumor-unique immune alterations from non-malignant tissue inflammation

Immunotherapies to treat cancer have achieved remarkable successes, but major challenges persist. An inherent weakness of current treatment approaches is that therapeutically targeted pathways are not only found in tumors, but also in tissue microenvironments, particularly inflamed tissues. This confounding overlap complicates treatment as well as predictions of treatment outcome. In an effort to identify potential tumor-unique immunotherapeutic targets that are distinct from general tissue inflammation, we used complementary single-cell analysis approaches to interrogate immune cell alterations and interactions in human squamous cell carcinomas and site-matched non-malignant, inflamed tissues. We found that a distinct population of intratumoral regulatory T cells (Tregs) received T cell receptor (TCR) signals from antigen-presenting cells and this Treg population was uniquely identified by co-expression of ICOS and IL-1 receptor type 1 (IL-1R1). Intratumoral IL-1R+ Tregs appeared activated and a TCR signal was sufficient to convert IL-1R1- Tregs to IL-1R1+ Tregs ex vivo. Overall, our work identifies an intratumoral Treg population that recognizes antigen in the tumor microenvironment and two biomarkers that allow for specific depletion of these Tregs. Finally, our approach also provides a blueprint for extricating tumor-unique therapeutic targets distinct from general inflammatory patterns in other tumors.

T cell self-reactivity during thymic development dictates the timing of positive selection

Functional tuning of T cells based on their degree of self-reactivity is established during positive selection in the thymus, although how positive selection differs for thymocytes with relatively low versus high self-reactivity is unclear. In addition, preselection thymocytes are highly sensitive to low-affinity ligands, but the mechanism underlying their enhanced TCR sensitivity is not fully understood. Here we show that murine thymocytes with low self-reactivity experience briefer TCR signals and complete positive selection more slowly than those with high self-reactivity. Additionally, we provide evidence that cells with low self-reactivity retain a preselection gene expression signature as they mature, including genes previously implicated in modulating TCR sensitivity and a novel group of ion channel genes. Our results imply that thymocytes with low self-reactivity down-regulate TCR sensitivity more slowly during positive selection, and associate membrane ion channel expression with thymocyte self-reactivity and progress through positive selection.

T cell self-reactivity during thymic development dictates the timing of positive selection

Functional tuning of T cells based on their degree of self-reactivity is established during positive selection in the thymus, although how positive selection differs for thymocytes with relatively low versus high self-reactivity is unclear. In addition, preselection thymocytes are highly sensitive to low-affinity ligands, but the mechanism underlying their enhanced TCR sensitivity is not fully understood. Here we show that murine thymocytes with low self-reactivity experience briefer TCR signals and complete positive selection more slowly than those with high self-reactivity. Additionally, we provide evidence that cells with low self-reactivity retain a preselection gene expression signature as they mature, including genes previously implicated in modulating TCR sensitivity and a novel group of ion channel genes. Our results imply that thymocytes with low self-reactivity down-regulate TCR sensitivity more slowly during positive selection, and suggest that modulation of membrane ion channel function may play a role in regulating TCR tuning throughout development.Impact StatementDeveloping T cells whose TCRs have relatively low reactivity experience very brief TCR signaling events, delayed positive selection, and do not fully down regulate their TCR sensitivity as they mature.

The order and logic of CD4 versus CD8 lineage choice and differentiation in mouse thymus

CD4 and CD8 mark helper and cytotoxic T cell lineages, respectively, and serve as coreceptors for MHC-restricted TCR recognition. How coreceptor expression is matched with TCR specificity is central to understanding CD4/CD8 lineage choice, but visualising coreceptor gene activity in individual selection intermediates has been technically challenging. It therefore remains unclear whether the sequence of coreceptor gene expression in selection intermediates follows a stereotypic pattern, or is responsive to signaling. Here we use single cell RNA sequencing (scRNA-seq) to classify mouse thymocyte selection intermediates by coreceptor gene expression. In the unperturbed thymus, Cd4+Cd8a- selection intermediates appear before Cd4-Cd8a+ selection intermediates, but the timing of these subsets is flexible according to the strength of TCR signals. Our data show that selection intermediates discriminate MHC class prior to the loss of coreceptor expression and suggest a model where signal strength informs the timing of coreceptor gene activity and ultimately CD4/CD8 lineage choice.

IL-7 and IL-15 combined with strong TCR stimulation decrease Treg suppressive activity in healthy donors and patients with rheumatoid arthritis

Homeostatic proliferation (HP) is a physiological process to reconstitute the T-cell pool after lymphopenia with IL-7 and IL-15 being the key cytokines regulating the process. However, there is no evidence whether these cytokines influence the function of regulatory T cells (Tregs). Since lymphopenia often accompanies autoimmune diseases, we decided to study the proliferation rate and function of Tregs stimulated by IL-7 and IL-15 in patients with rheumatoid arthritis (RA) compared to healthy donors (HD). The study used peripheral blood from 14 RA patients and 18 HD. Proliferation of purified CD3CD4CD25CD127 cells was assessed by flow cytometry using CFSE. Tregs were stimulated by anti-CD3, IL-7, IL-15, IL-7, or IL-15 combined with anti-CD3, and by IL-2+anti-CD3, and their functional activity was evaluated in each case by CD4 and CD8 cells proliferation inhibition. The suppressive activity of peripheral Tregs did not differ between RA and HD; however, it significantly decreased when IL-7 or IL-15 were applied together with strong TCR stimulation with anti-CD3 antibodies. Herewith Treg proliferation caused by IL-7 and IL-15 was lower in RA than in HD. The revealed decrease in Treg suppressive activity can lead to the proliferation of potentially self-reactive T-cell clones, which can receive relatively strong TCR signals. This may be another explanation of why lymphopenia is associated with the development of autoimmune diseases. The revealed decrease of Treg proliferation under IL-7 and IL-15 may lead to a delay in Treg pool reconstitution in patients with rheumatoid arthritis.