CD40LG and B-cell chronic lymphocytic leukemia: Hence, to mimic the lymph node microenvironment and induce CLL cell proliferation in vitro, we deployed co-cultures with CD40 ligand (CD40L)-expressing stroma, as previously described by us and others.4, 18 Under these conditions, <1% of cells entered S phase as early as 24 h, as evidenced by EdU incorporation (Figure 1a).