SLAMF1/CD150 IS A SIGNALING RECEPTOR EXPRESSED BY A SUBSET OF CHRONIC LYMPHOCYTIC LEUKEMIA PATIENTS CHARACTERIZED BY A FAVORABLE PROGNOSIS

Introduction. Human SLAMF1 (CD150) is the prototype member of a family of receptors expressed on the surface of hematopoietic cells. SLAMF-1 cognate binding initiates distinct signal transduction networks in T cells, natural killer cells and antigen presenting cells. Gene expression profiling has identified SLAMF1 as part of the genetic signature characterizing chronic lymphocytic leukemia (CLL) patients with favorable prognosis. The aim of this work is to validate the prognostic impact of SLAMF-1 in a cohort of CLL patients and to investigate its role in the activation of signaling pathways influencing CLL homeostasis. Methods. SLAMF-1 expression was tested in a cohort of 270 clinically and molecularly characterized CLL patients by flow cytometry. To start signaling, SLAMF-1 was ligated with an agonistic antibody and the ensuing cytoplasmic events determined by biochemical analyses. The autophagic pathway was studied using confocal and transmission electron microscopy. Apoptosis was determined by AV/PI assays. Results. The analysis of SLAMF1 surface expression on the CD19+ fraction of 270 CLL patients revealed highly variable levels (1- 95%). Statistical analyses of the data indicate that patients characterized by a good prognosis (in terms of disease stage at diagnosis or treatment requirements) express higher levels of SLAMF1 compared to the counterpart. Moreover, patients with > 6% SLAMF1+/CD19+ CLL cells had a significantly longer treatment free survival (median 6.4 in SLAMF1+ vs 1.2 years in SLAMF1- patients, P=.002). Consistently, SLAMF1 expression was also inversely correlated with CD38 and CD49d, two molecular markers of unfavorable prognosis and positively associated with the presence of somatic mutations in the IgHV genes. Engagement of SLAMF1 by a specific mAb started a signaling cascade mediated by phosphorylation of the EAT-2 adaptor and downstream activation of Vav-1, p38 and JNK. Co-crosslinking of SLAMF1 with sIgM prolonged phosphorylation of p38 and JNK and increased the percentage of CLL cells undergoing apoptosis, as compared to either signal alone. Furthermore, activation of the SLAMF1 pathway for a period of 6 hours led to the increased appearance of autophagic vescicles, as confirmed by confocal and transmission electron microscopy. The relative modulation of apoptosis and autophagy was mediated by the sequential phosphorylation of JNK and Bcl-2: the end result is the activation of Bcl-2 and the release of beclin-1, an essential member of the autophagic complex. Conclusions. In conclusion, SLAMF-1 represents a novel marker for the subset of CLL patients characterized by an indolent clinical course. Functional data suggest that it may function together with the BCR in regulating CLL apoptosis and that it can keep the autophagic process active, maintaining homeostasis of CLL cells. These findings suggest a hypothetical link between the relative modulation of autophagy and apoptosis and a more favorable clinical outcome in CLL.