6) and when mouse CD11b+ spleen cells were used as effector cells (data not shown). When tested in different donors, the % shaving observed with mouse AT80 was typically between 20 and 47%, whereas other mouse antibodies induced shaving at 60–90%. We then tested related human or chimeric antibodies BHH2, CD20-2, CD20-6, CD20-G and chimeric AT80 (chAT80). However, here find more we observed 67–84% shaving, which was comparable to the level observed with RTX (Fig. 7). Recently, it was reported that monocytes have an inhibitory effect on ADCC because they
can remove antibody such as RTX from the surface of target B cells and in this way cause a reduced ability of NK cells to bind RTX via the FcγRIII.11,12 Hence, monocytes seem to compromise RTX treatment, in particular in haematological malignancies with a large B-cell load.13 Here, we confirm these observations and demonstrate that the shaving mechanism is independent of endocytosis but relies on protease activity after monocyte binding to the Fc part of RTX. Also, we have screened a series of alternative type I and II anti-CD20 antibodies to identify antibodies with a reduced effect on monocyte-mediated shaving. This work demonstrated that monocytes are able to remove B-cell-bound RTX at monocyte : B-cell ratios of 1 : 2 in vitro and that this is dependent on the Fc part of RTX. Recent work has shown that the high-affinity receptor for IgG, FcγRI,
is responsible for this and expression of this receptor on monocytes provides a competitive advantage to hinder NK-cell-mediated ADCC through FcγRIII with lower affinity.12 This group also demonstrated that addition of human IgG could restore NK-cell-mediated ADCC MK-8669 in vitro in these co-cultures. However, in our assay, the addition of human IgG or anti-CD64 only had a minor effect on monocyte-mediated shaving. This could reflect that the addition of IgG in their assay had a direct effect on the NK cells, which also have an ability to perform shaving of target cells. Hence, monocytes could either be dependent on cross-linking
of even low numbers of free FcγRI to induce shaving or be activated in alternative ways. Interestingly, we also observed that second monocyte-derived dendritic cells can mediate the shaving reaction, and this could represent an additional mechanism whereby dendritic cells in the tumour microenvironment act as a ‘black hole’, hindering effective anti-tumour immune responses. Hyperosmolar sucrosis is an inhibitor of endocytosis. In our assay, hyperosmolar sucrosis did not lead to inhibition of the shaving reaction and this indicates that this phenomenon is not the result of B-cell-mediated endocytosis of the CD20/RTX complex or of simple endocytosis by monocytes. This observation is in line with detailed analysis from Beum et al.11 who recently demonstrated that the shaving reaction is similar to a processing mechanism originally described by Griffin et al.,10 which is now named trogocytosis.