Heterogeneity of circulating CD57+ cytotoxic cell subsets across disease stage and aggressiveness in cutaneous T cell lymphoma

Abstract Background: CD57 is a terminal differentiation marker expressed on subsets of T- and NK cells with high cytotoxic potential, and its expansion has been reported in viral infection, autoimmune disease, and aging. We previously reported that clonal CD8⁺CD57⁺CD16⁻ large granular lymphocytes (LGLs) are seen in cutaneous T cell lymphoma (CTCL) and associated with favorable outcomes in Sézary syndrome (SS). However, the phenotypic variation of circulating CD57⁺ subsets across disease states, and their relationship to clinical aggressiveness, remain undefined. Cytotoxic CD4⁺CD57⁺ T cells represent a specialized subset of T cells traditionally associated with terminal differentiation or replicative senescence. First identified in humans in the setting of chronic HIV infection, these cells have since been recognized for their ability to exert direct cytotoxicity. Notably, they can target and eliminate MHC class II-expressing tumor cells and have been detected within the tumor microenvironment of various solid malignancies. Here, we assessed the distribution of circulating CD57⁺ subsets in mycosis fungoides (MF) and SS patients, with a particular focus on SS. Methods Peripheral blood CD57⁺ T- and NK cell subsets were analyzed from 48 patients with MF (n=8), SS at diagnosis (n=29), and treated SS (n=11). SS at diagnosis was defined as the first available peripheral blood flow cytometry sample at Sézary-stage disease, including MF transformed to SS. Patients with erythroderma and progressive counts near the ≥1,000 Sézary cells/μL threshold were also included to capture evolving SS. Subsets measured by multicolor flow cytometry included CD57⁺CD4⁺ T cells, CD57⁺CD8⁺ T cells, CD57⁺ NK cells (CD57⁺CD56dim⁺CD16⁺), and CD57⁺γδ⁺ T cells. SS patients were further stratified into four clinical subgroups: (1) SS at diagnosis, less aggressive (n=14), (2) SS at diagnosis, aggressive (n=15), (3) treated SS, less aggressive (n=9), and (4) treated SS, aggressive (n=2). Aggressive course was defined as death from disease within two years or progression requiring systemic HDAC inhibitors, chemotherapy, or alemtuzumab after failure of mogamulizumab and extracorporeal photopheresis. Kruskal-Wallis and Mann-Whitney U tests were used for groupwise and pairwise comparisons. Tests were two-sided with p<0.05, and median values, sample sizes, and p-values were reported for each marker comparison. Results Median CD57⁺CD4⁺ T cell levels differed significantly across disease states (p=0.0069), with the highest levels in SS at diagnosis (70.96 cells/μL), compared to MF (39.79 cells/μL) and treated SS (29.55 cells/μL). Pairwise comparisons showed significantly higher levels in SS at diagnosis compared to treated SS (p=0.0058) and a borderline difference compared to MF (p=0.0469). CD57⁺ NK cells, CD57⁺CD8⁺ T cells, and CD57⁺γδ⁺ T cells did not differ significantly across disease states. In SS patients stratified by aggressiveness and treatment status, CD57⁺CD4⁺ T cells approached significance across subgroups (p=0.0549). Median levels were 81.24 cells/μL (aggressive SS at diagnosis), 55.90 cells/μL (less aggressive SS at diagnosis), 29.55 cells/μL (less aggressive treated SS), and 136.03 cells/μL (aggressive treated SS; n=2). CD57⁺ NK cells differed significantly across subgroups (p=0.015), with higher levels in aggressive SS at diagnosis (58.78 cells/μL) and less aggressive treated SS (55.36 cells/μL), compared to aggressive treated SS (41.94 cells/μL) and less aggressive SS at diagnosis (18.22 cells/μL). CD57⁺CD8⁺ T cells and CD57⁺γδ⁺ T cells did not differ significantly across subgroups, though intergroup variability was observed. Conclusion Circulating CD57⁺ subsets demonstrate significant variation across CTCL disease states and SS subgroups. CD57⁺CD4⁺ T cells were most elevated at SS diagnosis, particularly in aggressive cases, suggesting a potential role in disease onset or progression. CD57⁺ NK cells were increased in both aggressive SS at diagnosis and less aggressive treated SS, indicating a possible immune response from treatment or disease burden rather than aggressiveness alone. These findings underscore the heterogeneity of CD57⁺ immune subsets in SS, implying differential roles in pathogenesis versus immune surveillance. It remains unclear if these proliferations are related to immune response to the disease or an exhausted immune state. Future studies with larger sample sizes, longitudinal characterization, and functional assays are underway.