New monoclonal antibodies (mAb) against different antigens expressed by B-CLL, developed within a European program, were
evaluated. Binding and internalisation assays were performed with 125Iradiolabelled antibodies on various cell lines including the original and the CD5-transfected leukaemia cell line JOK1 and JOK1-5.3, respectively. mAb mediated cell proliferation inhibition was tested with a radiolabelled-thymidine analogue and FACS analysis. Antitumour efficacy was tested in an intra-peritoneal (i.p.) JOK1-5.3 B-CLL model in SCID mice using early and delayed (1 and 7 days, respectively) antibody treatment initiation. Mice were sacrificed at appearance of significant disease. Radiolabelled mAbs against CD5, CD32, CD71 and anti-HLA-DR showed specific direct bindings of ~45-70% on target cells and induced variable internalisation effects. Anti-HLA-DR strongly inhibited cellular proliferation of all cell lines tested and directly induced apoptosis. Anti-CD71 inhibited the proliferation of most cell lines with an accumulation of cells in early S-phase, but had no effect on JOK1-5.3 cell growth. No proliferation inhibition was observed with anti-CD5 or anti-CD32 mAbs. JOK1-5.3 tumour mice treated with anti-CD71 or anti-HLA-DR showed a prolonged survival of about 20 days compared with untreated or isotype matched irrelevant mAb treated controls. Similar results were observed after treatment with anti-CD5 as compared to untreated controls or sham-treated JOK1
tumour grafted mice (CD5-). In conclusion, all mAbs inhibited JOK1-5.3 leukemia development while showing variable in vitro effects. The broader in vivo efficacy of certain mAbs might be explained by their IgG2 subclass, allowing recruitment of effector functions, but the in vivo efficacy of the anti- CD71 mAb, of mouse IgG1 subclass, and having no effect on JOK1 5.3 in vitro, remains currently unexplained.