Langerhans Cell Histiocytosis (LCH) is characterized by inflammatory lesions with pathologic CD207+ dendritic cells (DCs) and constitutively activated ERK. LCH lesions are associated with a local cytokine storm and exhibit a heterogeneous collection of inflammatory infiltrates (eosinophils, neutrophils and lymphocytes). The critical role of the immune system in regulating tumor growth and progression has been highlighted with the extraordinary impact of dis-inhibiting cytotoxic T cells in some adult cancers. However, the mechanisms behind tumor formation, both accumulation of pathologic CD207+ DCs as well as recruitment of inflammatory infiltrate in LCH, remain to be defined. Mutually exclusive somatic mutations in MAPK pathway genes have been identified in approximately 75% of LCH cases, including recurrent BRAF -V600Eand MAP2K1 mutations. However, the inflammatory infiltrate within LCH lesions is composed of cells without any BRAF mutation or ERK activation. We hypothesized that the pathologic inflammation in LCH is orchestrated by interactions between the pathologic MAPK-activated DCs and normal immune cells. Defining the nature and function of infiltrating T cells in LCH has the potential to not only provide insights into mechanisms of pathogenesis and potential therapeutic targets, but may also inform novel physiologic mechanisms of DC-mediated T cell function. Clonality, investigated by exploring T cell receptor beta (TCRB) rearrangement in LCH patients, identified clonal populations among LCH lesion-infiltrating CD3+ T cells. The complementarity-determining region 3 (CDR3) of the TCRBV07 gene demonstrated marked monoclonal expansion, suggesting a potential involvement of antigen selection in the pathogenesis of LCH. High dimensional mass cytometric analysis identified multiple tumor-infiltrating T lymphocyte subsets in LCH. Functional analysis indicated hypercytokinemia and defective alloreactive CD8 effector T cells. Spanning tree progression of density normalized events (SPADE) analysis of infiltrating CD3+ T cells revealed high expression of T cell immunoglobulin mucin (TIM-3), lymphocyte activated gene 3 (LAG3) and programmed cell death protein 1 (PD-1) compared to healthy controls, indicating that effector T cell exhaustion might be an important mechanism of immune dysregulation in LCH patients. In addition we found pathologic DCs in LCH lesions expressing ligands for the inhibitory receptors (PDL-1/L2, Galectin-9 and MHC Class II). We also found that patients harboring BRAF -V600E mutation express higher ligand expression compared to patients with wild type BRAF . Checkpoint inhibitor therapy by inhibiting PD-1 in an ex vivo setting to reinvigorate exhausted T cells only partially rescued CD8 effector cells alloreactivity in LCH patients. We are currently evaluating the efficacy of combination therapy in restoring effector T lymphocyte function. The results from this study thus support a model in which LCH lesion DCs with activated MAPK signaling select and recruit activated T cells with potential for clonal expansion to LCH lesions. While the CD207+ DCs in LCH lesions is histologically uniform across the disease spectrum, the composition of the immune microenvironment is variable. The variability of the inhibitory receptor ligand expression in BRAF -V600E vs BRAF wild-type lesions suggests that specific mechanisms of MAPK activation may have some distinct impact on T cell recruitment and activation. Finding more effective ways to reinvigorate T cells and have them regain functions could prove to be incredibly useful for efficacious therapeutic intervention in patients with LCH.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.