Activation of Fc Gamma Receptor-Dependent Responses to Therapeutic Antibodies By Nurse like Cells Requires PI3Kdelta

There is a continued reliance on antibody therapies for treating chronic lymphocytic leukaemia (CLL). Whilst the inclusion of antibody therapies in many standard treatment regimes results in good outcomes, acquired resistance remains a significant clinical challenge for many CLL patients resulting in insensitivity to the antibody treatment. Thus, understanding the mechanisms driving treatment resistance is likely to lead to therapies to reverse resistance and improve patient outcomes. Earlier studies from our laboratory have shown that resistance to therapeutic antibodies, in CLL, is due to a reduced ability of monocyte derived macrophages (MDMs) to participate in FcγR-dependent antitumour responses (e.g. ADCC and antibody-dependent phagocytosis (ADP). In this regard, we recently showed that SYK and BTK activation are downstream of the FcγRs (Oncogene, 36(17):2366-2376, 2017). Moreover, we showed that signalling through the FcγR pathway was reduced in antibody-resistant MDMs and could be reversed using inhibitors of SHIP1. These studies indicated that knowledge of FcγR signalling could exploited to reverse resistance. Unfortunately, knowledge of the exact signalling events controlling FcγR activity in MDMs from CLL patients is unclear.

In this study we investigated the involvement of PI3K isoforms in FcγR-dependent ADCC and ADP in MDMs from CLL patients. PI3K isoforms have been shown to be important pathway regulators for immune-receptor function in various immune cells such as T cells, B cells and NK cells as well as in cancerous cells. In the first instance, I examined the expression of PI3K isoforms in MDMs from CLL patients. This showed that PI3Kα, β, and δ are expressed in MDMs whereas PI3Kγ is below the limit of detection. Next, I examined the involvement of the different PI3K isoforms to contribute to FcγR-dependent ADCC by MDMs. For this we used a suite of isoform-selective inhibitors to target each PI3K isoform and examined their effect on ADCC responses by MDMs. The PI3Kδ-selective inhibitor, idelalisib and the pan PI3K inhibitor BKM120 (Buparlisib) were able to inhibit ADCC responses to the CD20-targeting therapeutic antibody, obinutuzumab. Similarly, both buparlisib and idelalisib were able to inhibit AKT phosphorylation at concentrations that also inhibited ADCC. In contrast. None of the other isoform-selective inhibitors were able to suppress ADCC responses to obinutuzumab. These studies have been repeated using isoform-specific siRNAs.

This is the first report to show that PI3Kδ is involved in FcγR signalling in MDMs from CLL patients or in MDMs from any tumour type. Based on these findings we conclude that PI3Kδ is a critical effector molecule for antitumour responses to therapeutic antibodies in CLL.