High throughput screening for antibody induced complement-dependent cytotoxicity in early antibody discovery using homogeneous macroconfocal fluorescence imaging

Regulation of complement and modulation of its activity in monoclonal antibody therapy of cancer

The complement system is a powerful tool of the innate immune system to eradicate pathogens. Both in vitro and in vivo evidence indicates that therapeutic anti-tumor monoclonal antibodies (mAbs) can activate the complement system by the classical pathway. However, the contribution of complement to the efficacy of mAbs is still debated, mainly due to the lack of convincing data in patients. A beneficial role for complement during mAb therapy is supported by the fact that cancer cells often upregulate complement-regulatory proteins (CRPs). Polymorphisms in various CRPs were previously associated with complement-mediated disorders.

In this review the role of complement in anti-tumor mAb therapy will be discussed with special emphasis on strategies aiming at modifying complement activity. In the future, clinical efficacy of mAbs with enhanced effector functions together with comprehensive analysis of polymorphisms in CRPs in mAb-treated patients will further clarify the role of complement in mAb therapy.

The role of complement in mAb-based therapies of cancer

The ability of complement to promote lysis of antibody-opsonized cells is well-established. Virtually all of the molecular details of this reaction have been elucidated and numerous points of regulation have also been delineated. Use of this information, along with the techniques that were first applied in the fundamental studies of complement, has allowed for investigations of the role of complement in mAb-based immunotherapies of cancer. These studies, which have often combined in vitro investigations with parallel correlative clinical measurements, have revealed that several FDA-approved mAbs make use of complement as an effector function in promoting opsonization and killing of targeted malignant cells. We describe the key methods used in this work, and discuss how the results of these studies provide rational approaches for making more effective use of complement in mAb-based cancer immunotherapy.

Boosting ADCC and CDC activity by Fc engineering and evaluation of antibody effector functions

In recent years, therapy with monoclonal antibodies has become standard of care in various clinical applications. Despite obvious clinical activity, not all patients respond and benefit from this generally well tolerated treatment option. Therefore, rational optimization of antibody therapy represents a major area of interest in translational research. Animal models and clinical data suggested important roles of Fc-mediated effector mechanisms such as antibody dependent cell-mediated cytotoxicity (ADCC) or complement dependent cytotoxicity (CDC) in antibody therapy. These novel insights into the mechanisms of action mediated by monoclonal antibodies inspired the development of different engineering approaches to enhance/optimize antibodies' effector functions. Fc-engineering approaches by altering the Fc-bound glycosylation profile or by exchanging amino acids in the protein backbone have been intensively studied. Here, advanced and emerging technologies in Fc-engineering resulting in altered ADCC and CDC activity are summarized and experimental strategies to evaluate antibodies' effector functions are discussed.

The natural human IgM antibody PAT-SM6 induces apoptosis in primary human multiple myeloma cells by targeting heat shock protein GRP78

In contrast to other haematological malignancies, targeted immunotherapy has not entered standard treatment regimens for de novo or relapsed multiple myeloma (MM) yet. While a number of IgG-formatted monoclonal antibodies are currently being evaluated in clinical trials in MM, our study aimed to investigate whether the fully human IgM monoclonal antibody PAT-SM6 that targets a tumour-specific variant of the heat shock protein GRP78 might be an attractive candidate for future immunotherapeutic approaches. We here show that GRP78 is stably and consistently expressed on the surface on tumour cells from patients with de novo, but also relapsed MM and that binding of PAT-SM6 to MM cells can specifically exert cytotoxic effects on malignant plasma cells, whereas non-malignant cells are not targeted. We demonstrate that the induction of apoptosis and, to a lesser extent, complement dependent cytotoxicity is the main mode of action of PAT-SM6, whereas antibody dependent cellular cytotoxicity does not appear to contribute to the cytotoxic properties of this antibody. Given the favourable safety profile of PAT-SM6 in monkeys, but also in a recent phase I trial in patients with malignant melanoma, our results form the basis for a planned phase I study in patients with relapsed MM.

Penetration of antibody-opsonized cells by the membrane attack complex of complement promotes Ca(2+) influx and induces streamers

We have reported that during complement-mediated cytolysis of B cells promoted by the CD20 mAbs rituximab or ofatumumab (OFA), long, thin structures that we call streamers (≥ 3 cell diameters) are rapidly generated and grow out from the cell surface. Streamers appear before cells are killed and contain opsonizing mAbs and membrane lipids. By exploiting the differential Ca(2+) requirements of discrete steps in the complement cascade, we determined that mAb-opsonized cells first tagged with C3b using C5-depleted serum are killed on addition of serum and EDTA, but the cells do not produce streamers. Also, cells first opsonized with OFA are lysed in serum containing Mg-EGTA by the alternative complement pathway but streamers are not produced. These findings indicate that Ca(2+) influx is necessary for streamer formation. Other mAbs that promote complement-mediated cytolysis also induce streamers on target cells. Streamer-like structures called nanotubes have been reported in several cellular systems, and are thought to promote intercellular communication/signaling. We tested whether this signaling could influence the susceptibility of neighboring cells contacted by streamers to complement attack and found that complement-mediated cytolysis of OFA-opsonized cells increases the resistance of unopsonized indicator cell populations to subsequent lysis when these cells are exposed to OFA and complement.