Blood will tell: profiling Sézary syndrome

In this issue of Blood, Najidh and colleagues add important insights to the current understanding of immunophenotypic and transcriptional profiles in the circulating malignant cells of patients with Sézary syndrome.¹ 

Sézary syndrome is a rare, aggressive leukemic variant of primary cutaneous T-cell lymphomas characterized by erythroderma (>80% body surface involvement), generalized lymphadenopathy, and circulating atypical lymphocytes called Sézary cells. Even though the circulating tumor clones are easily accessible for immunophenotyping in the blood, unequivocal identification and quantification of Sézary cells can be a challenge. Marker panels may fail to identify these cells, and there is considerable patient-to-patient variability. In the article by Najidh et al, blood cells from patients with Sézary syndrome were prospectively analyzed by both flow cytometric and RNA sequencing methods, which represent a highly sensitive and standardized approach based on the previously validated multiparameter EuroFlow marker panels. The authors demonstrate that conventional flow cytometry, generally used in clinical practice, either over- or underestimates the overall tumor burden when compared with EuroFlow. This finding adds a new aspect to the longstanding debate regarding whether percentages or absolute counts of circulating tumor cells are the best measure to use in diagnosis and monitoring.² 

Currently, loss of CD26 or CD7 is used for the routine detection of Sézary cells. This study revealed an enormous range across patient samples: 1.13% to 100% and 0.05% to 100% antigen loss, respectively. Importantly, loss of CD26 or CD7 was not restricted to malignant cells; it was also found in reactive (ie, nonmalignant) CD4⁺ T cells, thus limiting the specificity of conventional flow cytometry for assessing blood tumor burden. EuroFlow gating, aimed at a more comprehensive phenotyping, detected Sézary cells in all samples analyzed in the Najidh et al study. However, aberrant cell marker expression went beyond loss of CD26 and/or CD7 in 92% of the patients. Despite the advantage that this more precise methodology offers, identification of Sézary cells may remain an obstacle due to the complexity of multiparameter flow cytometry, because this tool is not widely available in routine clinical settings. This challenge could be overcome in the future with novel markers such as KIR3DL2, which has proven high diagnostic sensitivity and specificity.³ Notably, initial reports of the clinical use of an anti-KIR3DL2 antibody are promising.⁴ 

Although the wide variety of immunophenotypic aberrancies were not linked to previous treatment in this study, heterogeneous phenotypic shifts occurred in patients over time. The authors confirmed that Sézary cells also showed a wide range of T-helper cell phenotypes, encompassing almost all maturation stages investigated, with central memory cells being the most frequent, supporting the current dogma that Sézary cells derive from central memory T cells.⁵ Interestingly, recent data suggest a skin-resident T-cell origin for Sézary syndrome similar to that in mycosis fungoides, with an ultraviolet (UV) signature of potential driver gene mutations in both entities.⁶ Of note, data from T-cell biology suggest that CD4⁺ T-cell clones can exist simultaneously as skin resident and circulating memory cells, even under physiological conditions,⁷ further supporting the remarkable plasticity of lymphoma cells.⁸ Najidh et al confirmed the presence of the skin-homing receptor CCR4 at high frequencies in Sézary cells, which is supported by the efficacy data for mogamulizumab, an approved anti-CCR4 therapeutic antibody.

In daily practice, opportunistic infections caused by profound overall immune dysfunction in patients with primary cutaneous lymphoma can be a challenge. Although infections contribute to overall mortality per se in Sézary syndrome, microbial dysbiosis may directly drive disease progression, as shown by immunostimulatory toxins derived from colonization by Staphylococcus aureus.⁵ Najidh et al postulated that there is an overall imbalanced immune cell homeostasis in patients with Sézary syndrome. They showed that there are altered levels of innate and adaptive circulating leukocytes with a significant reduction of absolute cell counts of CD8⁺ cytotoxic T cells, B cells, natural killer cells, and others when compared with healthy controls. By contrast, neutrophils, dendritic cells, and monocytes were significantly increased. Transcriptional profiles of monocytes from patients with Sézary syndrome have previously been shown to differ remarkably from those in healthy controls. In vitro, monocyte profiles could be reshaped after interferon-γ (IFN-γ) stimulation, which might explain the clinical efficacy of IFN-γ in cutaneous lymphomas.⁹ 

Interestingly, differentiation of monocytes into dendritic cells is thought to be relevant for the immunomodulatory effects of extracorporeal photopheresis, an established immunotherapy for Sézary syndrome.¹⁰ 

The gene expression data from Nadijh et al demonstrated that phenotypically distinct Sézary cell populations within individual patients were still clonally related, further demonstrating the phenotypic plasticity of Sézary cells. On a transcriptomic level, the authors found strong downregulation of THEMIS and LAIR1, markers involved in T-cell receptor function. Importantly, these genes were recently linked to the pathogenesis of adult T-cell leukemia, which is clinically closely related to Sézary syndrome, suggesting some common pathogenetic mechanism.

In sum, the presented strategy of integrating EuroFlow-based multiparameter flow cytometry, fluorescence-activated cell sorting (FACS) and RNA sequencing provided a comprehensive exploration of the heterogeneity of Sézary cells and offers a precise method to quantify the overall tumor burden. These explorative results may initiate a revision of standardized international criteria for diagnosing blood involvement and responses for informed treatment decisions in Sézary syndrome.