HLA-DR meets ERK

In this issue of Blood, Stein and colleagues present a novel mechanism by which MHC class II antibodies induce apoptosis in lymphoid malignancies. These are important findings that provide fresh insight into a common antibody target.

While HLA-DR and other MHC class II molecules are typically known for their role in antigen presentation and elicitation of immune response, their increased expression in lymphoid and other hematologic malignancies has made them a prime target for antibody therapy in these diseases. Much has been published on the ability of HLA-DR directed antibodies to mediate death, but the pathway by which this occurs has remained elusive.

In this issue of Blood, Stein et al describe a new humanized monoclonal antibody to HLA-DR, IMMU-114.¹  This antibody has not yet entered the clinic but is in line for development with a promise of being different from other agents such as rituximab, ofatumumab, and alemutuzumab that are currently approved for marketing by the FDA for treatment of lymphoid malignancies. One drawback for current antibody therapies is the dependence on antibody-dependent cellular cytotoxicity or complement-dependent cytotoxicity, which requires competent immune effector cells and functional complement, respectively. Additionally, complement activation may also lead to significant side effects. IMMU-114, however, has been designed specifically to disable the ability to mediate ADCC and CDC, while maintaining direct cytotoxicity toward target cells. In fact, the effect of IMMU-114 direct cytotoxicity is equivalent in several lymphoid cell lines in vitro even in the absence of a secondary cross-linking antibody. This is not the case with monoclonal antibodies to other surface molecules such as CD74 and CD20 that require cross-linking for cytotoxicity, which suggests a novel mechanism of action for IMMU-114.

How then does a monoclonal antibody specifically kill tumor cells independently of antigen expression and ADCC? To address this question, the authors explore downstream signaling events following treatment IMMU-114, specifically looking at JNK-MAPK signaling activation. The MAPK pathway is constitutively active in several hematologic malignancies and is generally thought to prevent apoptosis.²  However, in CLL, cytotoxicity with the anti-CD20 antibody rituximab has been demonstrated to be dependent on the p38-MAPK pathway.³  In this particular study, while both ERK and JNK were phosphorylated following CD20 cross-linking, these pathways were not required for cell death.

IMMU-114 also induces phosphorylation of both ERK and JNK, which again occurs independently of the presence of secondary cross-linking antibody. Furthermore, the activation occurs quickly and correlates with increased ROS production and mitochondrial membrane depolarization. However, in contrast to previous studies with rituximab, inhibition of ERK, JNK, or both pathways combined using either pharmacologic inhibitors or siRNA completely abrogates the cytotoxic effect of IMMU-114. These studies establish a novel death pathway activated by IMMU-114 and provide justification for its differentiation from other therapeutic antibodies utilized for the treatment of lymphoid malignancies.

Overall, this article from David Goldenberg's laboratory presents exciting data, indicating that the induction of ERK and JNK signaling pathways play an important role in the cytotoxicity of the HLA-DR monoclonal antibody IMMU-114, providing a novel mechanism for this class of antibodies. More importantly, this manuscript takes the importance of signal transduction mediated by therapeutic antibodies to another level of significance by examing how these agents ultimately produce clinical response in patients with CLL and related B-cell malignancies. Given the importance of signal transduction through the ERK pathway by IMMU-114 to mediate cytotoxic effect in B-cell tumors, Stein et al have established this as a potential pharmacodynamic marker to utilize in clinical trials to predict who will respond to treatment. Additionally, this paper provides the basis to justify studies of what is different about the ERK signaling pathway in those who fail to respond to IMMU-114. An important question not addressed by the study is the influence of the microenvironment present in patients that also can contribute to activation or inhibition of specific signal transduction pathways. As IMMU-114 moves forward through preclinical studies necessary for introduction into clinical trials, it will be important to examine whether this same signaling pathway is relevant to primary tumor cells when placed under the protective auspices of stromal or cytokine microenvironment.