Activatory and inhibitory Fc{gamma} receptors augment rituximab-mediated internalisation of CD20 independent of signalling via the cytoplasmic domain [Membrane Biology]

January 7th, 2015 by Vaughan, A. T., Chan, C. H. T., Klein, C., Glennie, M. J., Beers, S. A., Cragg, M. S.

Type I anti-CD20 mAb such as rituximab and ofatumumab engage with the inhibitory FcγR, FcγRIIb on the surface of B cells, resulting in immunoreceptor tyrosine-based inhibitory motif (ITIM) phosphorylation. Internalisation of the CD20:mAb:FcγRIIb complex follows, the rate of which correlates with FcγRIIb expression. In contrast, although type II anti-CD20 mAb such as tositumomab and obinutuzumab also interact with and activate FcγRIIb, this interaction fails to augment the rate of CD20:mAb internalisation, raising the question of whether ITIM phosphorylation plays any role in this process. We have assessed the molecular requirements for the internalisation process and demonstrate that in contrast to internalisation of IgG immune complexes, FcγRIIb-augmented internalisation of rituximab-ligated CD20 occurs independently of the FcγRIIb ITIM, indicating that signalling downstream of FcγRIIb is not required. In transfected cells, activatory FcγRI, FcγRIIa and FcγRIIIa augmented internalisation of rituximab-ligated CD20 in a similar manner. However, FcγRIIa mediated a slower rate of internalisation than cells expressing equivalent levels of the highly homologous FcγRIIb. The difference was maintained in cells expressing FcγRIIa and FcγRIIb lacking cytoplasmic domains and in which the transmembrane domains had been exchanged. This difference may be due to increased degradation of FcγRIIa, which traffics to lysosomes independently of rituximab. We conclude that the cytoplasmic domain of FcγR is not required for promoting internalisation of rituximab-ligated CD20. Instead, we propose that FcγR provide a structural role in augmenting endocytosis that differs from that employed during the endocytosis of immune complexes.
  • Posted in Journal of Biological Chemistry, Publications
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