Rituximab primarily targets an intra-clonal BCR signaling proficient CLL subpopulation characterized by high CD20 levels

The use of the anti-CD20 antibody rituximab has improved the outcome of patients with chronic lymphocytic leukemia (CLL). Rituximab was shown to act via various mechanisms, such as antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), direct apoptosis, or sensitization to chemotherapy (reviewed in ref. [1]). However, the biological function of CD20 and the reasons for the impressive activity of rituximab and other anti-CD20 antibodies remain elusive. It has been suggested, but remains controversial, whether CD20 functions as a calcium channel, couples with CD40 and MHCII, or B-cell receptor (BCR), and whether it has a role in T cell-dependent and -independent immunity [1,2,3,4]. This is also of great clinical interest for the design of combinatorial treatment of rituximab with BCR inhibitors, DNA damaging or immunomodulatory drugs, or CAR T cells. We have previously shown that microenvironmental interactions upregulate the CD20 levels on CLL cells through the CXCR4/SDF-1 axis [5]. Here we describe that higher CD20 expression has a direct role in the BCR signaling in CLL cells, and a BCR-proficient intra-clonal CLL cell subpopulation is more efficiently eliminated by rituximab in vivo due to higher CD20 levels.