3.2 Serum sCD25 levels and clinical extent of LCH disease
The overall median serum level of sCD25 was 3908 pg/ml (ranging from 231 to 44 000 pg/ml). Levels of sCD25 did not differ significantly between the test and validation cohorts, and medians were 3067 and 4097 pg/ml, respectively (P = 0.058). sCD25 were higher in the dabrafenib group than in the test or validation cohorts (median: 15586 pg/ml for dabrafenib group; both P < 0.001; Fig.1A). Patients younger than aged two years had higher levels of sCD25 than those older than two years (Fig.1B). There was a significant difference in sCD25 levels among patients with three disease extents categorization, with the highest in MS RO+ and the lowest in SS LCH (medians: 13 530 pg/ml for MS RO+, 4912 pg/ml for MS, 2979 pg/ml for SS, P < 0.001; Fig.1C). Increased sCD25 were associated with the involvements of risk organs, skin, lung, lymph node, or pituitary (all P < 0.05; Supplementary Fig.S2). Intriguingly, sCD25 levels were significantly higher in patients without bone involvement than in those with bone involvement (P = 0.0019). Notably, MS RO+ patients with macrophage activation syndrome-Hemophagocytic lymphohistiocytosis (MAS-HLH) had higher sCD25 than RO+ patients without this syndrome (P < 0.001). sCD25 levels did not differ significantly among patients carrying BRAF -V600E, MAP2K1 or BRAF-other mutations (Fig.1E). Patients with positive cfBRAF -V600E mutations had significantly higher levels of sCD25 (P < 0.001, Fig.1F).
Furthermore, we found there was only mild correlation between serum levels of sCD25 and several other cytokines, positively correlated with IL-10 (Spearman’s ρ = 0.22, P < 0.001) and IFN-γ (ρ = 0.20, P < 0.001), but negatively related to TNF-α (ρ = -0.15, P = 0.003). The sCD25 levels exhibited no correlation with IL-6, IL-4, and IL-2 levels (all P values < 0.05; Fig.1G).
Moreover, we determined sCD25 levels after one week of induction treatment in six available serum samples from LCH patients and revealed a rapid decrease of sCD25 after induction treatment (P = 0.031; Fig.2A). Similarly, sCD25 levels dropped in five available patients after one week of dabrafenib administration compared to the baseline levels (P = 0.008; Fig.2B).
3.3 Prognostic significanceof sCD25 levels at diagnosis
To assess the predictive abilities of LCH prognosis, we performed ROC curve analysis for sCD25 levels in 146 patients treated with the first-line treatment of the test cohort. The results revealed that sCD25 could efficiently predict LCH progression and relapse after given standard first-line treatment, with the area under the curve (AUC) 0.606 (95% CI: 0.512 - 0.701, P = 0.028; Fig.3A). An optimal cutoff value for sCD25 was determined at 2921 pg/ml, with a sensitivity, specificity, PPV, and NPV of 66.2%, 58.0%, 55.8%, and 68.1%, respectively. According to the optimal cutoff sCD25 value, we divided the test cohort into the high-sCD25 group (≥ 2921 pg/ml; n = 77) and the low-sCD25 group (< 2921 pg/ml; n = 69). We found that patients in the high-sCD25 group had a significantly worse PFS than those in the low-sCD25 group (5-year PFS were 40.4% ± 5.8% and 68.1% ± 5.6%, respectively, P < 0.001; Fig.3B). The median time to progression or relapse was 38.2 months for patients with high-sCD25 levels compared with 61.8 months for patients with low-sCD25 levels (P < 0.001).
We then used univariate analysis to study the effects of baseline characteristics on the PFS of LCH (Supplementary Table S1). We revealed that the prognostic indicators were significantly associated with clinical-biological features, including patients diagnosed before the age of 2 years (P = 0.014), multisystem disease (P< 0.001), RO involved (P < 0.001), the organ involvements of skin (P = 0.003), lung (P = 0.004) and ear (P = 0.013). In patients assessable for BRAFstatus, the presence of the BRAF -V600E mutation in tissue lesion or plasma was associated with a worse prognosis and increased risk of recurrence/progression (P values were 0.029 and < 0.001, respectively).
Clinical characteristics associated with an increased prognostic risk on univariate analysis (P < 0.05) and sCD25 levels grouping were subjected to multivariate Cox regression analysis to identify independent prognostic factors. BRAF -V600E mutation status was not included in the multivariate analysis, given that data were missing for about one-third of the patients. According to the multivariate analysis, multisystem disease, RO involvements, and high levels of sCD25 were confirmed to be independent prognostic factors for PFS of LCH patients (Fig.3C). Patients with high-sCD25 had an increased risk of progression or relapse compared to the patients with low-sCD25 (Hazard ratio: 2.577, 95% CI: 1.367-4.858, P = 0.003). RO involvement was found to be the strongest independent poor prognostic factor (Hazard ratio: 4.905, 95% CI: 2.028-11.863, P < 0.001).
Furthermore, we validated the prognosis impact of sCD25 levels in the independent validation cohort. When the 213 treated LCH patients in the validation cohort were assigned into the two subgroups according to the cut-off levels of sCD25 (2921 pg/ml), analysis of survivals also showed a poorer PFS in the high-sCD25 group compared to the low-sCD25 group (3-year PFS were 60.4% ± 4.3% and 75.8% ± 5.3%, and the median PFS time were 39.9 months and 48.2 months, respectively, P = 0.023; Fig.3D).