CD20 Antibodies of Human IgA Isotype Mediate CDC, and ADCC By Myeloid Effector Cells

Introduction: Three unconjugated CD20 antibodies have been approved for the treatment of lymphoma patients. All three are of human IgG1 isotype, but nevertheless they differ in their modes of action: type I antibodies (e.g. rituximab and ofatumumab) trigger effective complement-dependent cytotoxicity (CDC), whereas type II antibodies (e.g. obinutuzumab) are potent inducers of direct cell death, while both type I and II antibodies can induce antibody-dependent cellular cytotoxicity (ADCC). Studies in syngeneic mouse models suggest that myeloid cells are the predominant effector cell type for CD20 antibodies (Uchida et al. J Exp Med 199:1659, 2004). However, human myeloid cells, particularly PMN, are activated more effectively by human IgA than by IgG1 antibodies (Dechant et al. Blood 100:4574, 2002) - especially when the latter are engineered for enhanced FcγRIII affinity (Peipp et al. Blood 112:2390, 2008). Antibodies of IgA isotype constitute an integral part of the mucosal immune system, and differ from IgG antibodies in their pharmacokinetic properties and immune effector mechanisms (Boross et al. EMBO Mol Med 5:1213, 2013, Lohse et al. Cancer Res 76:403, 2016). Here, we compared the efficiency of IgG1 and IgA2 isotype variants of the type I CD20 antibody 1F5 in killing lymphoma cells in vitro and in vivo.

Methods: Recombinant antibodies against human CD20 were produced by co-transfecting BHK cells with vectors encoding the 1F5 variable, Igα2 or Igγ1 heavy, and κ light chain constant regions, respectively. The resulting isotype variants were compared for their biochemical characteristics as well as Fab- and Fc- mediated effector functions using human lymphoma cell lines as targets. NOD.Cg-Prkdc^scidIl2rgtm1Wjl/SzJ (NSG) mice xenotransplanted with human RAJI lymphoma cells were employed to investigate the therapeutic efficacy of human CD20 antibodies. Additionally, in vivo depletion of human CD20 transgenic B cells was evaluated in a syngeneic B cell depletion model using wildtype, human FcαRI transgenic and C3 knock-out mice.

Results and Discussion:In vivo studies with xenotransplanted RAJI cells in NSG mice, which lack functional T and NK cells, demonstrated significantly prolonged survival of treated as compared to non-treated mice, indicating that myeloid effector cells may contribute to the therapeutic efficacy of CD20 antibodies against human lymphoma cells. In vitro, neither IgG1 nor IgA2 variants of 1F5 showed efficient Fab-mediated effects such as direct cell death induction and homotypic aggregation compared to type II antibodies. However, human IgA2 but not IgG1 antibody variants against CD20 effectively triggered ADCC by human PMN, the most numerous myeloid effector cell population. Although IgA does not bind C1q, CD20 IgA antibodies also triggered CDC against several lymphoma cell lines. CDC was predominantly mediated by the alternative pathway, as evidenced by the kinetics of lysis, the requirement for higher serum concentrations and inhibition by the C3 inhibitor compstatin. Further in vivo experiments demonstrated that 1F5-IgA2 effectively depleted B cells in a syngeneic human CD20 transgenic B cell depletion model. However, studies in human FcαRI transgenic or C3 knock-out mice indicated that B cell depletion was not mediated by FcαRI or complement - suggesting that other currently undefined mechanisms contribute to the in vivo efficacy of IgA antibodies against CD20.

Conclusions: Together, the presented results suggest that CD20 antibodies of human IgA isotype constitute promising immunotherapeutic reagents with unique effector functions. Additional studies are required to further elucidate their effector mechanisms in vitro and in vivo.