S1PR1 and B-cell chronic lymphocytic leukemia: Finally, coculture of CLL cells on endothelial cells (HUVEC), stromal cells (HS-5), or CD40L-expressing fibroblasts to mimic cell interactions in the lymph node microenvironment showed that none of these accessory cells had any effect on the expression of S1PR1 or its upregulation by idelalisib, nor did they render CLL cells responsive to S1PR1 modulation by fostamatinib or ibrutinib (Fig. 3C, Supplemental Fig. 2G–I).