Extracellular Nicotinamide Phosphoribosyltransferase (NAMPT) Shapes the CLL Microenvironment Promoting Macrophage M2 Polarization Via a Non-Enzymatic Mechanism

Cancer-associated microenvironment provides to malignant cells the right cocktail of signals to increase survival, reprogram their metabolism and escape the control of the immune system. Nicotinamide phosphoribosyltransferase (NAMPT) is the only human enzyme that metabolizes nicotinamide, the main NAD precursor, in a rate-limiting step in NAD biosynthesis. Beside this canonical activity, NAMPT can be secreted in the extracellular milieu (eNAMPT), where it exerts cytokine/adipokine-like actions in different tumor models, as well as in acute and chronic inflammatory-metabolic diseases. Here we investigated the functional role of eNAMPT in the microenvironment of chronic lymphocytic leukemia (CLL), an indolent lymphoproliferative disorder, strongly dependent on a growth supportive environment.

Results indicate that: i) B-CLL lymphocytes express higher levels of NAMPT mRNA and intracellular protein than normal circulating B lymphocytes obtained from age- and sex-matched donors; ii) plasma levels of eNAMPT are also significantly higher in CLL patients (n=95) compared to controls (n=20) and iii) eNAMPT is produced by CLL lymphocytes upon B-cell receptor, toll-like receptor and NF-KB signaling pathway activation.

We then asked whether this cytokine plays an active role in the leukemic microenvironment. Data shows that eNAMPT acts on resting monocytes, polarizing them towards tumor-supporting M2 macrophages, known as nurse-like cells (NLC) in CLL. These cells express high levels of the scavenger receptor CD163, mannose receptor CD206 and indoleamine 2,3-dioxygenase (IDO), and secrete immunosuppressive (IL-10, CCL18) and tumor-promoting (IL-6, IL-8) cytokines. NAMPT-primed NLCs activate ERK1/2, STAT3 and NF-kB signaling, promote leukemic cell survival and reduce T cell responses. These effects are independent of the enzymatic activity of NAMPT, as inferred from the use of an enzymatically inactive mutant.

From the translational point of view, drugs interfering with CLL signaling, such as Ibrutinib, can efficiently suppress NAMPT transcription, reducing NAMPT protein levels. Furthermore, NLCs differentiated in the presence of the immunomodulatory drug lenalidomide express lower levels of NAMPT, again suggesting that drugs that restore immune functions interfere with the production of eNAMPT.

To conclude, we propose that a vicious circle based on CLL cell activation through antigen and accessory signals increase eNAMPT and CCL3 production. CCL3 serves as an attractant for circulating monocytes, which - in the presence of high levels of eNAMPT - differentiate into NLC, with an enhanced M2 phenotype and increased functional characteristics, contributing to CLL survival, activation and proliferation and inhibition of T cell responses.