Langerhans cell histiocytosis (LCH) is a rare disease involving inflammatory lesions that can occur in essentially any organ of the body. LCH lesions are defined by the presence of CD1a+/CD207+ myeloid lineage cells (LCH cells), which often have mutations within the RAS/RAF/MEK/ERK pathway and constitutive activation of ERK. A range of other immune cells are also present within lesions including an enrichment of Foxp3 regulatory T cells (Tregs).
The immune suppressive cytokine transforming growth factor beta (TGF-β), which is commonly produced by Foxp3 Tregs has also been detected within lesions and blood from patients with active LCH disease. Groups have suggested that LCH cells are a source of TGF-β and that TGF-β is one of the drivers of the LCH cell phenotype. Given the enrichment of Foxp3 Tregs and the presence of TGF-β within LCH lesions it is conceivable that Foxp3 Tregs too are a source of TGF-β production.
Historically the identification of Foxp3 Tregs has not allowed for functional studies to be conducted because staining for the Foxp3 transcription factor requires cell permeabilization. A newer surrogate gating strategy to detect CD3+CD4+CD25+CD127low lymphocytes allows for downstream functional assays on the Foxp3 Treg population. Our study aimed to better define the role of Tregs in LCH pathogenesis using this surrogate staining method.
A surrogate Treg identification method was employed to gate on CD3+CD4+CD25+CD127low lymphocytes in the blood from healthy donors and patients with LCH and in lesions from patients with LCH. Using this gating strategy we identified similarly to previous studies that the proportion of lesional Tregs (median = 12.85 %, IQR = 7.85-26.72 %, n = 6) was significantly (p < 0.0001) increased in the total T cell population when compared to those in the blood from healthy donors (median = 1.02 %, IQR = 0.84-1.20 %, n = 19). The proportion of Tregs in blood from patients with active LCH (median = 1.73 %, IQR = 1.58-3.80 %, n = 8) was also significantly (p = 0.232) increased when compared to those in the blood from healthy donors. We confirmed Foxp3 expression in Tregs in lesions from three independent LCH donors by staining for the transcription factor, and we confirmed that other lesional CD4+ T cells were mostly negative for Foxp3.
Due to our interest in unconventional T cells we analysed specimens for CD56 expression in conjunction with investigating Tregs and we unexpectedly found CD56 expression on a considerable proportion of the Treg population in LCH lesions (median = 36.48 %, IQR = 24.94-55.14 %, n = 6). The proportion of Tregs that displayed CD56 expression on their cell surface was significantly higher in the lesions from patients with LCH compared to the blood from healthy donors (median = 2.29 %, IQR = 0.61-4.10 %, n = 15, p = 0.0092) and patients with active LCH (median = 0.57 %, IQR = 0.11-0.85 %, n = 8, p < 0.0001). Additionally, we found that the proportion of CD56+ Tregs in total T cells from LCH lesions was directly correlated to the proportion of total Tregs in overall lesional T cells (r = 1, p = 0.0028). Importantly, we confirmed Foxp3 expression by both the CD56+ and CD56- Treg populations in lesions from three independent LCH donors.
We purified and then stimulated Tregs from healthy donors, and CD56+ and CD56- Treg populations from lesions from patients with LCH. Using supernatants from this assay we found that Tregs from lesions from patients with LCH produced active TGF-β when challenged with PMA and ionomycin for 16 hours (CD56+ Tregs n = 3, CD56- Tregs n = 3).
We have confirmed that both CD56+ and CD56- Treg populations from the lesions from patients with LCH are able to produce TGF-β. Given their cytokine potential, the functional status of Tregs in LCH lesions is therefore most likely to be suppressive in nature. This means that Tregs are possibly responsible for a component of the well documented expression of TGF-β within lesions.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.