Chronic lymphocytic leukaemia (CLL) is the most common adult leukaemia accounting for approximately 1,000 new cases per year in Australia and an accompanying mortality of approximately 300 patients per year (AIHW, 2009). As its name suggests, CLL is a chronic condition and approximately 30% of diagnosed cases will remain clinically indolent and will not require treatment other than monitoring of their disease. In recent years there has been an increase in the number of immunotherapies being used or trialed in CLL. These new therapeutic antibodies are likely to become increasingly important in standard of care for CLL patients.
We previously reported that CLL cells in short term primary cultures increase the expression of CD62L (Clin Cancer Res. 2013 Oct 15;19(20):5675-85). The induction of CD62L expression on CLL cells in patients is known to drive the homing and retention of CLL cells in the lymphoid tissues. We now show that a therapeutic antibody against CD62L is able to induce an antibody-dependent cell mediated cytoxicity (ADCC) in primary cultures of CLL cells. Significantly, we find that sensitivity to CD62L-Ab dependent killing varies between patients from highly sensitive to insensitive. Moreover, in a small cohort of patients with long term follow up of their disease we observed that patients with stable disease retained sensitivity to CD62L-Ab whilst patients whose disease progressed became insensitive to CD62L-Ab. Using strategies to enrich for NK cells, T cells and monocytes we were able to show that the CD62L-Ab dependent killing was attributable to an FcγR-dependent mechanism within the monocyte derived cell (MDC) fraction. We noted that the MDCs (often referred to as “nurse-like cells”) from sensitive and insensitive patients were both able to bind CD62L-Ab-bound CLL cells equally. Using pharmacological inhibitors against the activating pathway of FcγR signaling and the FcγRIIB inhibitory pathway we were able to show that the insensitivity of CLL cells could be attributable to the inhibitory actions of FcγRIIB in the MDCs of insensitive patients. These findings were replicated with the anti-CD20 antibody, obinutuzumab. These data establish, for the first time, that MDCs derived from CLL patients may switch from an ADCC sensitive phenotype to an ADCC-insensitive phenotype as disease progresses. Significantly, we show that this switch in phenotype may involve the emergence of a predominant FcγRIIB pathway in MDCs. Finally, using therapeutic antibodies, we show that resistance to MDC-mediated ADCC may be reversed by the inhibition of FcγRIIB with pharmacological modifiers.