157P - Rituximab Induced Internalisation of B-Cells CD20 Receptor is Independent of the Inhibitory FC Receptor (CD32B) Intracellular Cell Signalling

Abstract

ABSTRACT The anti-CD20 monoclonal antibody, rituximab, has improved treatment outcomes in B-cell malignancies. However, several B-cell lymphomas either do not respond to rituximab or develop resistance. Internalisation of rituximab may be partly responsible for this resistance. We recently showed that the level of the inhibitory Fc receptor (CD32B) at the cell surface controls the rate of rituximab internalisation. However, the precise mechanism involved has not been elucidated. Different isoforms of CD32B exist (B1 and B2), only one of which (B2) has previously been associated with an ability to internalise after engagement of immune complexes. The B1 form in contrast contains an additional intracellular region that makes it more resistant to internalisation. We therefore investigated the role of the two different CD32B isoforms (B1 and B2) and the associated intracellular tail on rituximab internalisation. CD32B1 and CD32B2 isoforms were stably transfected into CD32B-ve mouse IIA1.6 and human Ramos lymphoma cells, and flow cytometry was then used to determine the relative rates of CD32B and CD20 internalisation. Additionally, we generated mutant versions of the CD32B receptors, including those lacking the entire cytoplasmic domain to assess the importance of intracellular signalling. In contrast to expectations both B1 and B2 CD32B isoforms were downregulated upon engagement with CD32B mAb as a surrogate for immune complexes, although in agreement with earlier reports the CD32B2 isoform internalised more extensively. However, the rate of rituximab internalisation occurred equally with both isoforms and was dependent on relative expression of CD32B and the CD20:CD32B ratio at the cell surface rather than any specific activity imparted by the CD32 intracellular domain. These studies suggest that the intracellular part of CD32B is redundant for rituximab-induced CD20 internalisation and imply that internalisation is augmented by CD32B through its physical ability to bind the Fc region of rituximab at the cell surface. Disclosure All authors have declared no conflicts of interest.