Mimicking the tumour microenvironment: three different co‐culture systems induce a similar phenotype but distinct proliferative signals in primary chronic lymphocytic leukaemia cells

Interactions in the tumour microenvironment can promote chronic lymphocytic leukaemia (CLL) cell survival, proliferation and drug resistance. A detailed comparison of three co-culture systems designed to mimic the CLL lymph node and vascular microenvironments were performed; two were mouse fibroblast cell lines transfected with human CD40LG or CD31 and the third was a human microvascular endothelial cell line, HMEC-1. All three co-culture systems markedly enhanced CLL cell survival and induced a consistent change in CLL cell phenotype, characterized by increased expression of CD38, CD69, CD44 and ITGA4 (CD49d); this phenotype was absent following co-culture on untransfected mouse fibroblasts. In contrast to HMEC-1 cells, the CD40LG and CD31-expressing fibroblasts also induced ZAP70 expression and marked CLL cell proliferation as evidenced by carboxyfluorescein succinimidyl ester labelling and increased Ki-67 expression. Taken together, our data show that co-culture on different stroma induced a remarkably similar activation phenotype in CLL cells but only the CD40LG and CD31-expressing fibroblasts increased ZAP70 expression and CLL cell proliferation, indicating that ZAP70 may play a critical role in this process. This comparative study reveals a number of striking similarities between the co-culture systems tested but also highlights important differences that should be considered when selecting which system to use for in-vitro investigations.