Type I CD20 Antibodies Recruit the B Cell Receptor for Complement-Dependent Lysis of Malignant B Cells

Abstract

Human IgG1 type I CD20 Abs, such as rituximab and ofatumumab (OFA), efficiently induce complement-dependent cytotoxicity (CDC) of CD20 B cells by binding of C1 to hexamerized Fc domains. Unexpectedly, we found that type I CD20 Ab F(ab') fragments, as well as C1q-binding-deficient IgG mutants, retained an ability to induce CDC, albeit with lower efficiency than for whole or unmodified IgG. Experiments using human serum depleted of specific complement components demonstrated that the observed lytic activity, which we termed "accessory CDC," remained to be dependent on C1 and the classical pathway. We hypothesized that CD20 Ab-induced clustering of the IgM or IgG BCR was involved in accessory CDC. Indeed, accessory CDC was consistently observed in B cell lines expressing an IgM BCR and in some cell lines expressing an IgG BCR, but it was absent in BCR B cell lines. A direct relationship between BCR expression and accessory CDC was established by transfecting the BCR into CD20 cells: OFA-F(ab') fragments were able to induce CDC in the CD20BCR cell population, but not in the CD20BCR population. Importantly, OFA-F(ab') fragments were able to induce CDC ex vivo in malignant B cells isolated from patients with mantle cell lymphoma and Waldenström macroglobulinemia. In summary, accessory CDC represents a novel effector mechanism that is dependent on type I CD20 Ab-induced BCR clustering. Accessory CDC may contribute to the excellent capacity of type I CD20 Abs to induce CDC, and thereby to the antitumor activity of such Abs in the clinic.

Response to Comment on “Type I CD20 Antibodies Recruit the B Cell Receptor for Complement-Dependent Lysis of Malignant B Cells”

“…control, to investigate the contribution of the BCR in anti-CD20-mediated complementdependent cytotoxicity (CDC) that we proposed (1). We agree with the notion that loss of accessory CDC after BCR knockout would provide additional evidence for the recruitment and activation of complement by CD20-mediated BCR clustering.…”

Comment on “Type I CD20 Antibodies Recruit the B Cell Receptor for Complement-Dependent Lysis of Malignant B Cells”

“…W e read with great interest the article by Engelberts and colleagues (1), which concludes that presence of the BCR on a CD20-expressing cell line can render these cells susceptible for lysis via a type I Ab with a missing or defective C1q binding site. This phenomenon was termed accessory complement-dependent cytotoxicity (CDC).…”

Effector mechanisms of IgA antibodies against CD20 include recruitment of myeloid cells for antibody‐dependent cell‐mediated cytotoxicity and complement‐dependent cytotoxicity

“…Thus, IgA2 and IgG1 differed in the types of effector cells they recruited (MNC versus PMN), and in the conditions under which they activated the complement pathway. Recent studies demonstrated that different CD20 antibody constructs, which cannot bind C1q themselves, recruited the surface immunoglobulin of B cells for C1q binding (Engelberts et al , ) – a mechanism that could also explain CDC by IgA antibodies.…”