CD38 Regulates Homing and Engraftment of CLL Cells,

Abstract 3873

Chronic lymphocytic leukemia (CLL) is considered the result of a dynamic balance between proliferating cells in lymphoid organs and circulating cells resisting apoptosis. Re-circulation of leukemic cells from blood to growth-permissive niches represents an essential step in the maintenance and progression of the disease. This equilibrium is finely tuned by a set of surface molecules expressed by CLL cells and modulated in response to environmental conditions.

We previously reported that CD38, an enzyme and a receptor, functionally cooperates with the CXCL12/CXCR4 axis, enhancing the ability of CLL cells to home to bone marrow and lymph nodes. In addition, the use of anti-CD38 mAbs can enhance or impair the chemotactic behavior of the neoplastic cells. New evidence also indicates that CD38 synergizes with the CD49d integrin, increasing adhesion of CLL cells to VCAM-1 or the CS-1 fibronectin fragment, two known ligands of CD49d. To complete the picture, CD38 expression denotes a CLL subset with increased activity of the matrix metalloproteinases MMP-9. Ligation of CD38 with specific antibodies increases MMP-9 secretion and the invasive properties of CLL cells, using in vitro assays. The effects on chemotaxis, adhesion and invasion are obtained through modulation of a ERK1/2-dependent pathway.

To further confirm the involvement of CD38 in CLL homing to specific niches, in vivo experiments have been set using NOD/SCID/γ chain^−/− (NSG) mice. The CLL-like cell line Mec-1, constitutively CD38⁻/CD49d⁺, was adopted as a model and compared to transfectants stably expressing wild-type (wt) CD38, as well a mutant lacking enzyme activities. Results after i.v. injections of tumor cells indicate that de novo expression of CD38 by Mec-1 cells increases growth kinetics in vivo with a higher proliferation rate and metastatic potential, as compared to the Mec-1 mock-trasfected cells. Both these features are lost when the animals are injected with the enzyme-deficient variant of CD38, suggesting that the enzymatic activity is critical for in vivo growth and re-circulation of Mec-1 cells. Microarray data confirm that the genetic signature of the CD38-enzyme mutant overlaps with the wild-type cell line, clearly distinct from cells transfected with CD38. The latter cell line shows up-modulation of several genes involved in chemotaxis and adhesion.

All together, these results support the notion that CD38 is part of a complex network of molecules and signals, that regulate homing of CLL cells to growth-permissive niches, suggesting a relationship between the expression of CD38, the ability to migrate and invade and the poor clinical outcome of the CD38⁺ subset of patients.