In vitro activation of CD38 caused by stimulating mAbs induces a portion of the CLL clone (10%-30%) to proliferate, giving rise to an immunoblast-like component that does not secrete Igs; these effects are dependent on significant amounts of soluble IL-2, in keeping with the observation that T lymphocytes and accessory cells are needed in vivo. This was a first indication that CD38 transduces signals, leading or contributing to CLL cell growth, even in the absence of stimuli coming from the BCR.
First indications that CD38 acts as a molecular signpost that routes leukemic cells to specialized niches arose from in vivo findings that the number of CD38 molecules expressed in BM and LN is higher than in circulating lymphocytes. In addition, nurse-like cells, differentiated in vitro from circulating CD14+ cells and also presumably present in solid lymphoid tissues in vivo, express CD31, the CD38 ligand. Interactions between CD38 and CD31 in this system promote proliferation and survival through a molecular circuit that can be interrupted by blocking anti-CD31 or anti-CD38 mAbs. CD38/CD31 interaction results in a genetic signature that includes more than 1500 modulated sequences. Pathway analysis drastically reduces this complexity, with proliferation and migration emerging as the main elements characterizing this receptor/ligand system.
CD38 signaling may be influenced by the presence of ZAP-70. Patients with CD38+ZAP-70+ clones are more responsive to activation of intracellular proteins than CD38+ZAP-70− persons. This may explain why the simultaneous expression of the two molecules offers a more efficient identification of the high-risk patient subset. CD38+ZAP-70+ CLL cells also migrate better in response to the CXCL12 chemokine and exhibit a genetic signature primarily consisting of genes involved in cell locomotion. A functional cooperation between CD38 and CXCR4 was demonstrated by showing that binding of stimulating anti-CD38 mAbs and de novo expression of CD38 by lentiviral infection increases chemotaxis in response to CXCL12.
These effects on proliferation and chemotaxis are altered by blocking reagents, which interfere with CXCL12-mediated migration in vitro and block CLL homing in an NOD/SCID mouse model. A possible explanation is that CD38 and CXCR4 are associated on the same membrane patches; and hence, stimulating and blocking anti-CD38 mAbs have the potential to interfere positively or negatively with CXCL12 binding to the CXCR4 receptor, changing responses. Recent data indicate that this signaling platform of molecules is more complex and includes adhesion molecules, such as CD49d and matrix metalloproteases, such as MMP-9.