CD84 (SLAMF5) As a Potential Target for Immunomodulation in Cutaneous T-Cell Lymphoma (CTCL)

Background: Cutaneous T-cell lymphoma (CTCL) develops from clonally expanded, skin-homing CD4 + T cells within a background of chronic inflammation. Mycosis fungoides (MF) and Sézary syndrome (SS) are the most common subtypes of CTCLs. Patients with advanced-stage MF/SS experience progressively worsening disease and decreased overall survival (Scarisbrick et al., 2015). Immune cells and immune checkpoints (ICs) are essential factors for tumor growth and the host immune response in CTCL. Tumor-associated macrophages (TAMs [M2-like]) play a key role in CTCL growth, and neoplastic T cells escape immune surveillance via the PD-L1/PD1 axis (Wu et al., 2014). However, the approach of ICs inhibition with monoclonal antibodies (mAbs) in CTCL has been challenged due to modest efficacy and remaining immunosuppression. Hence, novel therapeutic strategies are needed to improve anti-tumor immunity in CTCL. We previously characterized CD84 (SLAMF5) as one of the key players in the tumor microenvironment (TME) by regulating multiple ICs, including PD-L1 and PD1, and modulating myeloid cell fractions and T cells in our preclinical MM models. Our CD84 targeting strategy using anti-CD84 mAb significantly reduced PD1 on effector CD8 + T cells and PD-L1 on the tumor and myeloid cells, decreased tumor burden, and increased the survival of the mice with myeloma (Lewinsky and Gunes et al., 2021). The role of CD84 in ICs regulation and anti-tumor immunity in CTCL remains to be explored. Methods and Results: In this study, we evaluated the role of CD84 in CTCL and, for the first time, reported CD84 as a potential target to reduce immunosuppression through the PD-L1/PD1 axis in CTCL. We found upregulated CD84 CD274 (PD-L1) , and PDCD1 (PD1) gene expressions in CTCL patients (n=5) compared to healthy individuals (n=4). We also noted higher CD84 geneexpression in SS patients (n=8) than in MF patients (n=6). We next evaluated CD84 expression on the formalin-fixed paraffin-embedded (FFPE) tumor sections derived from SS patients' skin lesions. We noted that CD84 is highly expressed in the TME and co-expressed in Ki67 + Sézary cells. We further evaluated the CD84 expression in immune cell clusters in SS patients and healthy individuals using publicly available single-cell RNA sequencing (scRNA-seq) datasets. We noted a profound CD84 expression in CD4 + T cells and CD163 + M2-like macrophages in SS patients. We also noted a strong correlation between CD84 and CD274 (p=0.0086) and CD84 and PDCD1 (p=0.0002) in SS patients. We next assessed the expressions of CD84, PD-L1 on CD163 + M2-like macrophages, and PD1 on T cell subsets in a coculturing setting with human SS cell line, Hut78, to determine the impact of Sézary cells on these cell compartments in the TME. We observed a significant increase in the percentages of CD163 + M2-like macrophages and CD4 + T cells and elevated CD84, PD-L1, and PD1 expressions on these cells following the coculturing of peripheral blood mononuclear cells (PBMCs) derived from healthy donors (HDs) with Hut78 cells (n=3, *P < 0.05, **P < 0.01). While the percentages of CD8 + T cells were slightly reduced, CD84 and PD1 expressions were elevated in these cells following the coculturing with Hut78 cells (n=3, *P < 0.05, ***P < 0.001). Dysregulation of STAT proteins, particularly STAT3, has been shown in MF/SS (Olszewska et al., 2020). To explore whether Sézary cells regulate STAT3 in CD163 + M2-like macrophages, we assessed pSTAT3 levels in CD163 + M2-like macrophages following the coculturing of HDs PBMCs with Hut78 cells. We found that Hut78 cellssignificantly increased intracellular pSTAT3 levels inCD163 + M2-like macrophages (n=3, *P < 0.05). Furthermore, we noted the inhibition of STAT signalingusing a pan-STAT inhibitor decreased PD-L1 and PD1 expression onCD163 + M2-like macrophages and T-cell subsets, respectively. Notably, we found the stimulation of CD84 signaling with an agonist anti-CD84 mAb enhanced pSTAT3 levels in CD163 + M2-like macrophages, suggesting the STAT pathway might regulate PD-L1 expression in immunosuppressive CD163 + M2-like macrophages through CD84 signaling in SS. Conclusion: Our preliminary data from this exploratory study indicate that CD84 may be a potential target to reduce immunosuppression in CTCL. Further studies are needed to explore the functions of CD84 as an immune receptor in CTCL TME and its therapeutic potential in CTCL.