Human CD38 Is a Potential Therapeutic Target for Sekected Chronic Lymphocytic Leukemia cases.

Chronic lymphocytic leukemia (CLL), the most common leukemia in Europe and North America, is characterized by a highly heterogeneous clinical behavior. Absence of mutations in the immunoglobulin variable (IgV) genes, expression of the surface receptor CD38 and the cytoplasmatic kinase ZAP-70 represent independent molecular markers of unfavorable prognosis.

Recent data indicates that the proliferative core of the disease resides within peripheral lymphoid organs. There, the neoplastic cells have access to the antigen as well as to microenvironmental signals necessary to the maintenance and expansion of the clone. Our group has investigated the role of CD38 in CLL not only as a disease marker but also as a pathogenetic agent. We showed that CD38 ligation by agonistic monoclonal antibodies (mAbs) or the CD31 ligand is followed by proliferation and blast transformation of a subset of CLL cells. CD38/CD31 interactions occur predominantly in the lymph nodes and spleen where CD31 is abundantly expressed by stromal and nurse like cells (NLC). The signaling pathway initiated upon CD38 engagement relies upon tyrosine phosphorylation of ZAP-70 as a necessary step. CLL cells expressing CD38 and ZAP-70 also show the highest migration propensity towards CXCL12, a critical chemokine produced by NLC to attract CLL cells to lymph nodes. Collectively, these results point to a crucial role for the CD38/ZAP-70 pathway in directing CLL cells from the blood to peripheral lymphoid organs and suggest that targeting CD38 might impair CLL cells homing to sites favorable for neoplastic growth.

In order to obtain a genome-wide signature of the transcriptional events taking place upon CD38/CD31 contacts CLL cells from 10 patients were co-cultured with CD31 transfected (L-CD31⁺) cells and L-mock controls for 5 days. Microarray analyses revealed 1,645 sequences selectively modulated as a consequence of CD38/CD31 interactions with 84 differentially regulated pathways: 12 of these pathways regulated cell adhesion/movement phenomena and 14 lymphocyte signalling.

Attention was next focused on the impact of blocking anti-CD38 mAbs on CLL homing in vivo. To investigate the homing of CLL cells from blood to spleen and BM, PBMC were injected in the tail vein of female NOD/SCID mice and left to home for 24 hours. Next, mice were sacrificed and human cells tracked in spleen and BM by staining with specific reagents. Homing was significantly reduced when human cells were pre-incubated with anti-CD38 mAbs immediately prior to in vivo delivery. The in vivo data provided a rationale for a more in depth in vitro investigation of the effects of anti-CD38 mAbs on the CXCL12/CXCR4 signaling pathway. Results indicate that CXCL12-mediated chemotaxis can be blocked by anti-CD38 mAbs. Pre-treatment of CLL cells with anti-CD38 mAbs impairs CXCR4 signaling, as indicated by lack of Ca2+ mobilization, ERK1/2 activation and receptor internalization. These effects are specific for CD38⁺ samples, as no interference in migration was recorded when anti-CD38 mAbs were incubated with CD38⁻ cells.

The molecular explanation may be found at least in part in a physical proximity between CD38 and CXCR4 on the cell membrane, as shown by confocal microscopy and coimmunoprecipitation studies. In line with this working hypothesis, CXCL12 binding to CXCR4 is effectively inhibited following pre-incubation with anti-CD38 mAbs.

In conclusion, these results offer biological evidence of a functional cross-talk between CD38 and CXCR4 signaling pathways. They also provide a preliminary rationale for the use of anti-CD38 ligands in therapy of a subset of CLL patients, representing a novel approach interfering with deleterious growth circuits and increasing the susceptibility of leukemic cells to conventional chemotherapy.