Improving treatment for chronic lymphocytic leukemia by rational use of monoclonal antibodies

Monoclonal antibodies (mAbs) are central treatment components for patients with chronic lymphocytic leukemia (CLL). Different mechanisms contribute to tumor cell depletion after mAb binding to surface antigens. In complement-dependent cytotoxicity (CDC), mAbs promote activation and membrane binding of a cascade of proteins that generate the membrane attack complexes, producing pores, subsequent osmotic swelling and cell lysis [1,2]. In antibody-dependent cellular cytotoxicity (ADCC), the Fc portion of the mAb is recognized by natural killer cells, macrophages and granulocytes. Tumor cells are killed by phagocytosis or the release of cytotoxins [3], while direct induction of apoptosis can be induced by triggering signal transduction pathways after Ab binding [4]. In addition, a “vaccination” effect has been suggested to contribute to the effect of mAbs [5].

CD20 is a cell surface glycoprotein which is highly expressed on B-cells throughout their development from pre-B to the immunoblast stage. CD20 has been shown to be an ideal target molecule. Stem cells and pro-B-cells are lacking CD20 surface protein, which allows reestablishment of B-cell populations after treatment [6]. Probably more importantly, other cell lineages do not express CD20, and are therefore not affected by CD20 targeting. Rituximab (RTX), which targets CD20, has revolutionized treatment of B-cell lymphoma and CLL [7]. Recently, the Food and Drug Administration (FDA) approved a second-generation anti-CD20 antibody for fludarabine and alemtuzumab refractory patients – ofatumumab (OFA). OFA binds to a different CD20 epitope closer to the cell surface, and has shown better efficiency in CDC in vitro [8]. Another clinically exploited surface molecule is CD52, targeted by alemtuzumab (ALM). CD52 is a membrane glycoprotein, and is highly expressed on B and T lymphocytes at most levels of differentiation (except stem cells), as well as on natural killer cells, monocytes and dendritic cells. ALM therefore targets not only malignant B-cells but also other leukocytes, increasing the risk of opportunistic infections [9].

Based on the widespread use of CD20 antibodies across lymphoma subtypes, surprisingly little is known about its predominant mechanism of action and mechanisms of resistance to CD20 antibodies. The work of Baig et al. [1] in this issue of Leukemia and Lymphoma tries to systematically dissect mechanisms of primary resistance to CDC induced by mAbs as well as comparing combination strategies for antibody-based treatment approaches. Baig et al. [1] concentrate on the activation of CDC mechanisms in CLL by studying different mAbs, and combinations thereof, on primary CLL cells. In their study, the authors showed that ALM more effectively induced CDC than did OFA, while RTX induced CDC only to a small extent in vitro. The researchers tried to advance combination approaches by assessing the added effect of ALM with RTX or OFA. Not surprisingly, because of the CDC assay used, the most effective combination in vitro was ALM and OFA. Whether this finding can indeed be exploited clinically is currently unclear, but clinical trials are under way.

Impaired CDC has been suggested as an important mechanism of resistance of mAbs in vivo [10]. It has been suggested that the addition of complement proteins from frozen plasma can enhance the effect of RTX in patients with CLL [11]. Baig et al. [1] have contributed to the existing literature by identifying a subpopulation of CLL cells resistant to induction of CDC by any of the mAbs tested. They show that in their in vitro system the basis of resistance is neither depletion of complement nor the differential expression of membrane complement regulatory proteins (CD46, CD55, CD59). Initial stages of complement activation monitored by C3b and C5b-9 binding levels were not impaired in the resistant cell population, but for unknown reasons this population, in contrast to sensitive cells, did not generate the membrane attack complexes. Cells resistant to CDC may turn out to be a good cellular model for studying the molecular basis of this mechanism in vitro; however, the clinical parallel is still unclear. Indeed, both OFA and ALM very effectively deplete CLL cells from the peripheral blood, and it remains to be shown whether the remaining cell populations still survive after in vivo exposure because of the mechanism identified and described by the authors.

In summary, mechanisms of resistance to mAbs used in the therapy of CLL are far from being well understood. The current work constitutes a step forward in understanding the mechanisms of resistance to CDC. In addition to the findings by Baig et al. [1], there is evidence that treatment with mAbs causes down-regulation [12] and/or shaving of surface targets [13]. Potential avenues to overcome resistance to mAbs may well include modulation of the expression of target surface proteins (e.g. histone deacetylase [HDAC] inhibitors have been shown to up-regulate CD20 expression [14]) or the future development and construction of improved antibodies [15]. Rational combination of monoclonal antibodies based on the improved understanding of synergism, or combination with agents modulating surface targets, could well increase the efficiency of mAbs in vivo.

Potential conflict of interest

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