Amerikanou, Rodothea

We previously showed that both reactive oxygen species-inducing chemotherapy and some genetic subtypes of B precursor acute lymphoblastic leukaemia (ALL) can induce bone marrow (BM) mesenchymal stromal cells (MSC) to become cancer associated fibroblasts (CAFs). Exposure of MSC to ALL-cell conditioned media (CM) is sufficient to generate CAFs - direct cell-cell contact is not required (Burt, Dey et al, 2024). We showed that CAFs can lead to chemoresistance via transfer of mitochondria to ALL cells through tunnelling nanotubes (Burt, Dey et al 2019). We noted that CAFs in the ALL BM had features of senescence, prompting the current study in which we aimed to investigate the link between senescence and CAFs in ALL. We used 3 genetic subtypes of ALL; BCR::ABL1 (SD1 cells, representing “adult” ALL) , ETV6::RUNX1 (REH cells, representing “childhood” ALL) and KMT2A::AFF1 (SEM cells, known from our previous work NOT to induce CAFs). We exposed HS27a stromal cells to the ALL CM at a number of timepoints over a period of three weeks and characterised senescence using qPCR, cytochemical B-galactosidase assays, flow cytometry and immunofluorescence (IF). Cytokine arrays were performed to quantify the senescence associated secretory phenotype (SASP) components and assess the complexity of SASP components in all three ALL subtypes. A more prolonged exposure (> 10 days) to CM was required for senescence to develop in contrast to CAF formation which occurs early (2-3 days) in HS27a MSC cells. Senescence marker expression peaked at around day 11 to day 14 of CM exposure before subsiding to lower levels around day 21. On day 14, 70% of HS27a cells treated with SD1 CM were positive for beta galactosidase compared to below 25% positive for REH and SEM cells. Cell cycle arrest was demonstrated by P16 and Ki67 IF. 95% of SD1 CM treated HS27a cells were positive for P16 compared to less than 10% in RPMI control. Likewise, only 29% of HS27a exposed to SD1 CM were Ki67 positive compared to 66% in the RPMI control. Expression levels of other senescence-associated markers such as SERPINE1, SERPINB2, SOD2, NFKB1 were quantified by qPCR and were all significantly (p<0.001) upregulated in SD1 CM treated HS27a (2 to 40 fold) compared to SEM CM (-2 to 5 fold) and REH CM (-0.5 to 1.0) treated HS27a. SASP associated cytokines IL6 (1000-5000pg/ml), IL8 (2500-5000pg/ml), CXCL10 (500-1500pg/ml), CCL5 (1500-2250pg/ml), IL10 (25-50pg/ml), were significantly (p<0.0001) upregulated in the SD1 CM treated HS27a compared to controls and other ALL cell line exposed HS27a. Finally, HMGB2 and LMNB1 genes, loss of which is associated with senescence were significantly (p<0.001) downregulated in the SD1 CM (-2.5 to 0.5 fold) treated HS27a compared to controls. In view of the prominence of senescence in SD1-exposed HS27a cells, we tested the effects of two different senolytic drugs venetoclax (BCL2i) and navitoclax (BCL2i/BCLXLi) in vitro. Briefly, HS27a cells were exposed to SD1 CM for 11 days, and after washing, increasing concentrations of the drugs were added for up to 72 hrs followed by washing, fixing and DAPI-staining and counting. HS27a exposed to SD1 CM was highly sensitive to low dose treatment of navitoclax (0.5uM) with 80% cell death 48 hrs post treatment. HS27a cells exposed to SD1 CM were less sensitive to venetoclax with only 50% cell death observed at higher doses (5uM) even after 72 hrs post treatment. Further senolytic drug screening experiments are ongoing. Our data suggest that BCR::ABL1 but not ETV6::RUNX1 or KMT2A::AFF1 ALL cells, can induce paracrine senescence in HS27a MSC cells. We suggest that paracrine senescence in the stromal microenvironment may be a component of treatment response in B-ALL. ^^ and ** contributed equally

Purpose: The discovery of MAPK/ERK pathway mutations has led to the approval of MEK-inhibitor (MEKi) cobimetinib in histiocytic neoplasms. However, the pivotal clinical trial that led to the approval of cobimetinib included only 4 patients with Langerhans cell histiocytosis (LCH). Therefore, there is a need to define outcomes with MEKi therapy in patients with LCH. We undertook a multi-center collaboration through the Adult LCH Working Group for the Histiocyte Society to address this gap in knowledge. Methods: Adults with LCH who received MEKi therapy were included from five institutions. Objective response rate (ORR) was evaluated using PET/CT criteria, and CT/MRI if PET scan was not available; response categories were categorized into complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD). Progression-free (PFS) and overall survival (OS) were calculated from the time of targeted therapy initiation. Risk organ disease was defined as liver, spleen, or bone marrow involvement. Results: 25 adults with LCH were included in this study; median age at MEKi initiation 43y (range, 18-85). Most common systems involved at diagnosis were bone (52%), pituitary (44%), and lung (36%). Risk organ involvement was observed in 7 (28%, 6 liver, 1 liver and spleen) and 2 (8%) patients had intra-axial CNS involvement. Median lines of prior therapy received were 2 (range: 1-5), and 4 (16%) patients received MEKi in first-line. MAPK/ERK pathway status was assessed via target capture next generation sequencing (NGS) in 22 (96%) cases. Amongst those who underwent NGS, alterations identified included: BRAF V600E (2/22, 9%), BRAF deletions (11/22 50%), MAP2K1 or MEK mutations (4/22, 18%), and MAPK1 or ERK2 (n=1, 5%). Overall, 5 patients also received BRAF inhibitors (4 patients as first therapy prior to MEKi and 1 concomitantly due to BRAF V600E positive melanoma). Specific MEKi agents utilized included cobimetinib (n=13) and trametinib (n=12). Among the response evaluable patients (n=23), ORR to MEKi treatment was 78% (9 CR, 9 PR, 4 SD, 1 PD). Among the 7 patients with liver involvement, responses were seen in 2 (29%) cases. ORR did not significantly differ according to mutation status among those with NGS data (78% MAPK mutated vs. 100% unmutated; p=1.0). In this subcohort, all of the 4 cases that did not respond to MEKi had liver involvement; 3 had BRAF deletions (2 SD and 1 PD), and one had MAPK1 mutation (SD). Median follow-up duration after MEKi initiation in the entire cohort was 18 months (95% CI 6 - 30). The median PFS and OS were not reached; 2-year PFS and OS were 78.1% and 79.9%, respectively (Figure 1A). The median PFS was much shorter in those with liver involvement (7 months [95% CI 0 - 14]) compared with those without liver disease (not reached [95% CI 19-not reached] P=0.047; Figure 1B). PFS did not differ significantly between MAPK pathway mutated and unmutated patients (p=0.34) Adverse events (AEs) were reported in 36% (n=9) and most commonly included rash (n=5) and fatigue (n=3). Four patients experienced grade 3-4 AEs including rash, fatigue, and thrombocytopenia. Common causes of MEKi discontinuation included AEs (n=3), progression (n=2), and drug holiday (n=2). At last follow-up, 5 patients died and 18 remained on MEKi. Death was attributable to LCH (n=3), concomitant malignancy (n=1), or unknown causes (n=1). Conclusions: In our multi-institutional study of adults with LCH, we found MEKi therapy to be highly efficacious. However, liver involvement correlated with a lack of response and worse PFS, while mutational status did not. Future studies are needed to evaluate optimum treatment for high-risk subgroups of LCH like liver disease.