Peripheral expansion of cytotoxic T lymphocytes (CTL) derived from the graft in the initial stages of allogeneic haematopoietic stem cell transplantation (alloHSCT) immune recovery is a well-known physiological event. The description of symptomatic large granular lymphocyte leukaemia in this setting may generate uncertainty, mostly in those cases in which the CTL expansion (CTLe) persists beyond the early transplantation period. We aimed to assess the nature of CTLe during the post-alloHSCT period in 154 adult patients with a long-term surveillance. We studied the longitudinal kinetics of those expansions, their relationship to clinical events, and their phenotypic and molecular features, including recently reported CTL leukaemia-STAT3 mutations. Persistent relative CTLe cases are frequent (49%), related with thymoglobulin prophylaxis (P ≤ 0·001), acute graft-versus-host disease (GVHD, P = 0·02), and reduced intensity conditioning (P = 0·04). Absolute CTLe are scarce (9%) and related to chronic GVHD. T cell receptor rearrangement was reported as clonal and oligoclonal in the majority of patients with CTLe. The absence of STAT3 mutations and the CD8/CD4 declining longitudinal kinetics in the late period supports its benign nature, expressed clinically by the null detrimental impact of these expansions on post-transplant outcome and/or serious infectious events.

An increasing numbers of patients are being diagnosed with asymptomatic early-stage chronic lymphocytic leukemia (CLL), with no treatment indication at baseline. We applied a high-throughput deep-targeted analysis, especially designed for covering widely TP53 and ATM genes, in 180 patients with inactive disease at diagnosis, to test the independent prognostic value of CLL somatic recurrent mutations. We found that 40/180 patients harbored at least one acquired variant with ATM (n=17, 9.4%), NOTCH1 (n=14, 7.7%), TP53 (n=14, 7.7%) and SF3B1 (n=10, 5.5%) as most prevalent mutated genes. Harboring one ‘sub-Sanger' TP53 mutation granted an independent 3.5-fold increase of probability of needing treatment. Those patients with a double-hit ATM lesion (mutation+11q deletion) had the shorter median time to first treatment (17 months). We found that a genomic variable: TP53 mutations, most of them under the sensitivity of conventional techniques; a cell phenotypic factor: CD38-positive expression; and a classical marker as β2-microglobulin, remained as the unique independent predictors of outcome. The high-throughput determination of TP53 status, particularly in this set of patients frequently lacking high-risk chromosomal aberrations, emerges as a key step, not only for prediction modeling, but also for exploring mutation-specific therapeutic approaches and minimal residual disease monitoring.

Thymic-independent peripheral expansion of CD8+ cells derived from the graft in the initial stage of post-HSCT immune recovery is a well-known physiological event. Nevertheless, the description of symptomatic LGL leukemias and aggressive malignant cases in this setting may generate uncertainty, mostly in those cases in which the cytotoxic T lymphocyte expansion CTLe persists beyond the early transplantation period. We aimed to assess the nature of CTLe in adults during the post-alloHSCT period in a series of 154 patients with a long term surveillance. We studied the longitudinal kinetics of those expansions, their relation to clinical events, and their phenotypic and molecular features, including recently reported CTL leukemia-STAT3 mutations. In our study, trying to adhere to the WHO annotation of T-LGL, we considered two definitions for a CTL expansion: an absolute increase (≥ 2000 x109/L), and a relative expansion (a CD8/CD4 ratio ≥ 1.5), persisting more than six months in both cases. Persistent relative CTLe cases are frequent (49%) and related with timoglobulin prophylaxis (p≤0.001), acute graft versus host disease (GVHD, p=0.02), reduced intensity conditioning (p=0.04) and fungal and viral infections in the early post-HSCT. No differences in the number of serious infectious events from day 180 was found. Absolute CTLe are scarce (9%), related with chronic GVHD and absence of relapses. TCR rearrangement was reported as clonal and oligoclonal in the majority of patients with CTLe. We studied in a cross sectional manner with an extended immunophenotypic panel 17 patients: 5 patients with an absolute CTLe and 12 cases with a relative CTLe. A similar cytotoxic T αβ-effector phenotype was observed in all cases, with slight differences in the expression of CD25, CD16 and 1a. One patient with a relative CTLe expressed CD56 intensely: his ratio normalized at day 730 and no immune-related events were recorded. DNA stored during the post-alloHSCT setting was available from 68/75 relative CTLe patients (14/14 absolute CTLe cases). All of them went through molecular TCR rearrangement and STAT3 exon 21 mutations determination. In the relative CTLe cohort, TCR rearrangement was described as clonal, oligoclonal or polyclonal in 77%, 16% and 7%, respectively. Regarding absolute CTLe patients, TCR rearrangement was described as clonal in all the patients (n=14) of this subset. To increase the sensibility of the Sanger PCR, it was performed on DNA from CD3+ sorted cells in 54 out of 68 cases. No STAT3 mutation could be found in the CD3+ sorted fraction of relative or absolute defined CTLe. Not using an absolute threshold would establish a diagnosis of a persistent CTL expansion in 49% of our cohort of allo-transplanted patients. Additional diagnostic tools, as an effector phenotype, the presence of a NK marker or a monoclonal TCR rearrangement would not reduce significantly that percentage: CD57 was invariably expressed in CTLe cases, and 80% of our patients with expansions showed a TCR monoclonal pattern. STAT3 mutations resulting in persistent proliferation of CTL clones are a frequent event in large granular lymphocytic leukemia, and those clones have also been described in autoimmunity-driven disorders as acquired aplastic anemia and hypocellular myelodysplastic syndromes. We establish in this study the absence of exon 21 STAT3 mutations in the persistent CTL expansions found in a large series of patients with a long-term post-alloHSCT surveillance. The absence of STAT3 mutations and the CD8/CD4 declining longitudinal kinetics in the late period, supports its benign nature, expressed clinically by the null detrimental impact of these expansions on post-transplant outcome and/or serious infectious events. Figure 1. Relative and absolute CTLe kinetics. A) Linear representation over time of the CD8/CD4 ratio in the 75 patients with relative CTLe. B) Trend linear plot of 25 patients with a relative CTLe and a follow up of, at least, 1440 days from transplantation. Each line depicts a patientxs longitudinally measured CD8/CD4 ratio. Patients are grouped by similar pattern of ratio behaviour through follow up. The line "slope" depicts the magnitude of the change between time points. C) Linear representation over time of the CD3/CD8 count in PB in the 14 patients with an absolute CTLe. D) Trend linear plot illustrating the CD8/CD4 ratio behaviour of the 14 patients with an absolute CTLe. Figure 1. Relative and absolute CTLe kinetics. A) Linear representation over time of the CD8/CD4 ratio in the 75 patients with relative CTLe. B) Trend linear plot of 25 patients with a relative CTLe and a follow up of, at least, 1440 days from transplantation. Each line depicts a patientxs longitudinally measured CD8/CD4 ratio. Patients are grouped by similar pattern of ratio behaviour through follow up. The line "slope" depicts the magnitude of the change between time points. C) Linear representation over time of the CD3/CD8 count in PB in the 14 patients with an absolute CTLe. D) Trend linear plot illustrating the CD8/CD4 ratio behaviour of the 14 patients with an absolute CTLe. Disclosures No relevant conflicts of interest to declare.

Background: We and others have reported on the impact of recurrent somatic mutations not only in the multistep pathogenetic process, but also in the clinical heterogeneity of chronic lymphocytic leukemia (CLL) patients. Immunophenotyping, as part of the diagnostic workout, is used for assessing clonality, as a differential diagnosis tool, and to examine the expression of molecules associated with a worse prognosis. Recently, NOTCH1 mutations have been linked to low CD20 levels in CLL and with a relative resistance to anti-CD20 immunotherapy in vitro. But to date, there is limited information on the correlation between cell surface marker expression and the presence of somatic mutations in CLL. The aim of this study was to evaluate potential associations between an extended phenotypic panel and the mutational status of 13 recurrently mutated genes in CLL detected by deep sequencing. Patients and Methods: To this end, we performed targeted NGS sequencing of blood samples, collected at diagnosis, from 131 CLL patients. Every patient underwent, at baseline, a flow cytometry characterization with a panel including (sIg)λ, (sIg)κ, CD19, CD5, CD11b, CD81, CD10, CD79b, CD29, CD38, FMC7, CD22, CD45, CD103, CD11c, CD25, ZAP70, CD11a, and CD24. We designed a TruSeq Custom Amplicon panel (Illumina, Inc. San Diego, CA, USA) containing 13 genes and covering 28.099 bases. The average amplicon size was 238 base pairs and ~ 99.1% of the regions were covered on both strands. Paired-end sequencing (2x250 bp) was performed with MiSeq v2.2 chemistry, and a mean depth of 998 reads/base within the regions of interest was obtained. Raw data were analyzed with IlluminaonJboard Real Time Analysis (RTA v.2.4.60.8) software and MiSeq Reporter. Results: With a median age of 68 y.o. (range, 33-95) and a slight male predominance, the median follow up time of our cohort was 43 months (24-104). We found that 47/131 (35%) patients harbored at least one mutation, with NOTCH1 (n = 13, 10%), ATM (n = 10, %), TP53 (n = 8, %), and SF3B1 (n = 8, 5.5%), as the most frequently mutated genes. Those patients with a NOTCH1 mutation showed a lower CD25 expression (25 mean fluorescence intensity units (MFIu)) than those without a mutation (45 MFIu), p=0.001. In addition, a higher expression of CD5 (265 vs. 219 MFIu, p= 0.02), of the monoclonal light chain (90.5 vs. 58.6 MFIu, p=0.03), and a higher percentage of CD38+ cells in the CD19+CD5+ compartment (37% vs. 19%, p=0.006) were significantly associated with the presence of, at least, one somatic mutation. We could not validate the recently reported association between the presence of NOTCH1 mutations and a low expression of CD20. In our cohort, the MFI expression in NOTCH1 mutated and non-mutated patients was 176 and 135 units, respectively (p=0.2) In the multivariate Cox analysis, the presence of a somatic variant in TP53 and a higher percentage of positive CD38 cells in the tumour population showed both a worse overall survival and shorter time to first treatment. The independence of these two variables was also supported by not finding a significative difference percentage of CD38 positive cells between TP53 mutated and non mutated cases (p=0.5). Conclusions: The associations described herein suggest potential pathogenic pathways in CLL, in particular the CD25-NOTCH1 axis, with a significative inferior expression of CD25 when activating NOTCH1 mutations are present. The relationship found between these two variables, with an inversed direction to that found in physiological conditions, has also been shown in the setting of NOTCH1-mutated acute lymphoblastic leukemia, emerging as a potential targetable pathway in this subset of CLL patients. Disclosures Maciejewski: Apellis Pharmaceuticals Inc: Membership on an entity's Board of Directors or advisory committees; Alexion Pharmaceuticals Inc: Consultancy, Honoraria, Speakers Bureau; Celgene: Consultancy, Honoraria, Speakers Bureau.