Therefore, it appears that two conditions were necessary (not sufficient) to elicit the pro-activation phenomenon: a positive basal activation/proliferation response of the CLL B cell to the ‘microenvironment’ stimuli (either CD40L + IL-4 or CpG + IL-15) and the presence of accessory ‘microenvironment’ bystander cells in the culture (either murine fibroblasts or autologous NLCs). Here, IL4 is linked to B-cell chronic lymphocytic leukemia.