CD38 Expression Marks CLL Cells with Invasive Properties

Abstract 2422

CLL is characterized by a dynamic balance between proliferating cells in the lymphoid organs and circulating cells resisting programmed cell death. Regulating this equilibrium entails complex interactions between tumor and host, modulated by a set of surface molecules expressed by the CLL cell according to environmental conditions.

CD38 is a negative prognostic marker, involved in the homing process. We previously reported that CD38⁺ cells are significantly more sensitive to CXCL12, a critical chemokine for the recirculation of neoplastic cells, both in vitro and in vivo. Activation of CD38 by means of agonistic mAbs promotes chemotaxis, while block of the molecule impairs it. CD38 is also co-expressed with CD49d, the alpha4 integrin subunit and a further independent negative prognostic marker for CLL. The two molecules appear to be intertwined in a dynamic loop which involves CD31 (the CD38 ligand predominantly expressed by endothelial cells) and VCAM-1 (the CD49d ligand).

Attention has now been focused on MMP-9, the main matrix metalloproteinase expressed by CLL cells, due to the relevance of extra-vasation in the homing process. The final aim is to clarify whether CD38 may represent a key player, taking part to all the main steps of CLL cells recirculation.

Results obtained analyzing a large cohort of CLL patients indicate that i) CD38⁺ cases are characterized by a higher expression and activity of MMP-9, as measured by gelatin zymography. Moreover, ii) the analysis of CLL patients with a bimodal expression of CD38 indicate that the CD38⁺ fraction of the clone is the one expressing higher levels of MMP-9 compared to the negative one. A formal proof of the connection between CD38 and this gelatinase has been obtained using a lentiviral technique that allows genetic manipulation of CLL cells. iii) De novo expression of CD38 is followed by secretion of high amounts of the active form of MMP-9, suggesting that de novo CD38⁺ cells digest extracellular matrix more readily. Furthermore, iv) the engagement of CD38 by means of an agonistic antibody is followed by an increased MMP-9 activation, while blocking anti-CD38 mAbs are highly effective in the modulation of MMP-9 secretion by CLL cells. Finally, v) CD38 appears to co-localize with MMP-9 in the same membrane areas, as inferred by confocal microscopy analysis.

Considered together, these information pinpoint CD38 as a connecting element between chemokines, adhesion molecules and matrix metalloproteinases. The finding of a physical proximity of all these molecules suggests that they form a large supramolecular complex, with the characteristics of the invadosome, a podosome of neoplastic cells that controls diffusion and metastasis. If confirmed, these results would link the ability to migrate and invade tissues with an inferior clinical outcome also in leukemia and not only in solid tumors.