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Background Designing biologically informative models for assessing the safety of novel agents, especially for cancer immunotherapy, carries substantial challenges. The choice of an in vivo system for studies on IgE antibodies represents a major impediment to their clinical translation, especially with respect to class‐specific immunological functions and safety. Fcε receptor expression and structure are different in humans and mice, so that the murine system is not informative when studying human IgE biology. By contrast, FcεRI expression and cellular distribution in rats mirror that of humans. Methods We are developing MOv18 IgE, a human chimeric antibody recognizing the tumour‐associated antigen folate receptor alpha. We created an immunologically congruent surrogate rat model likely to recapitulate human IgE‐FcεR interactions and engineered a surrogate rat IgE equivalent to MOv18. Employing this model, we examined in vivo safety and efficacy of antitumour IgE antibodies. Results In immunocompetent rats, rodent IgE restricted growth of syngeneic tumours in the absence of clinical, histopathological or metabolic signs associated with obvious toxicity. No physiological or immunological evidence of a “cytokine storm” or allergic response was seen, even at 50 mg/kg weekly doses. IgE treatment was associated with elevated serum concentrations of TNFα, a mediator previously linked with IgE‐mediated antitumour and antiparasitic functions, alongside evidence of substantially elevated tumoural immune cell infiltration and immunological pathway activation in tumour‐bearing lungs. Conclusion Our findings indicate safety of MOv18 IgE, in conjunction with efficacy and immune activation, supporting the translation of this therapeutic approach to the clinical arena.

Mast cells and basophils, which are activated by IgE and allergens through the high affinity IgE receptor (Fc epsilon RI), play a prominent role in anaphylaxis in the mouse. Mice deficient in this receptor become resistant to passive anaphylaxis. As a first step in developing an in vivo model that more closely mimics the IgE-mediated responses in man, we used a combination of transgenic and embryonic stem cell technology to generate a mouse line in which the murine Fc epsilon RI alpha-chain has been replaced with its human homologue. We demonstrate here that these mice express a tetrameric high affinity IgE receptor, in which the human alpha-chain associates with the murine beta- and gamma-chains, and that upon triggering with relevant Ag, this receptor mediates the initiation of the expected intracellular events. In addition, we show that the human alpha-chain restores an anaphylactic response to the nonresponsive alpha-deficient parental mouse line. This "humanized" mouse represents a potentially important model system, not only for studying the role of IgE in human immune responses, but also for testing potential therapeutic reagents that can interfere with responses mediated through the human Fc epsilon RI receptor.

We attempted to elicit active anaphylaxis to ovalbumin, or passive IgE- or IgG(1)-dependent anaphylaxis, in mice lacking either the Fc(epsilon)RI alpha chain or the FcR gamma chain common to Fc(epsilon)RI and Fc gamma RI/III, or in mice lacking mast cells (Kit(W)/Kit(W-v) mice), and compared the responses to those in the corresponding wild-type mice. We found that the FcR gamma chain is required for the death, as well as for most of the pathophysiological changes, associated with active anaphylaxis or IgE- or IgG(1)-dependent passive anaphylaxis. Moreover, some of the physiological changes associated with either active, or IgG(1)-dependent passive, anaphylactic responses were significantly greater in Fc(epsilon)RI alpha chain -/- mice than in the corresponding normal mice. Finally, while both Kit(W)/Kit(W-v) and congenic +/+ mice exhibited fatal active anaphylaxis, mast cell-deficient mice exhibited weaker physiological responses than the corresponding wild-type mice in both active and IgG(1)-dependent passive systemic anaphylaxis. Our findings strongly suggest that while IgE antibodies and Fc(epsilon)RI may influence the intensity and/or kinetics of some of the pathophysiological changes associated with active anaphylaxis in the mouse, the mortality associated with this response can be mediated largely by IgG(1) antibodies and Fc gamma-RIII.

The IgE-mediated immune system activation can be redirected to combat tumors. Mouse and human IgE have been shown to provide a potent adjuvant effect in antitumor vaccination, with a crucial role played by FcεRI. This effect results from T cell-mediated adaptive immune response. Modified vaccinia virus Ankara (MVA) has been used to infect IgE-loaded tumor cells. These results led to a shift toward a highly safe protocol employing membrane IgE (mIgE), thus eliminating any possible anaphylactogenicity caused by circulating IgE. Evidence that human mIgE and a truncated version lacking IgE Fabs (tmIgE) bind and activate FcεRI has been fundamental and forms the core of this report. Human tmIgE has been engineered into a recombinant MVA (rMVA-tmIgE), and the expression of tmIgE and its transport to the surface of rMVA-tmIgE–infected cells has been detected by Western blot and cytofluorimetry, respectively. FcεRI activation by tmIgE has been confirmed by the release of β-hexosaminidase in a cell-to-cell contact assay using human FcεRI-transfected RBL-SX38 cells. The rMVA-tmIgE antitumor vaccination strategy has been investigated in FcεRIα−/− human FcεRIα+ mice, with results indicating a level of protection comparable to that obtained using soluble human IgE tumor cell loading. The rMVA-tmIgE vector represents a device that suits safe IgE-based antitumor vaccines, harboring the possibility to couple tmIgE with other gene insertions that might enhance the antitumor effect, thus bringing the field closer to the clinics.

Working with C57BL/6 mouse tumor models, we had previously demonstrated that vaccination with IgE-coated tumor cells can protect against tumor challenge, an observation that supports the involvement of IgE in antitumor immunity. The adjuvant effect of IgE was shown to result from eosinophil-dependent priming of the T cell-mediated adaptive immune response. The protective effect is likely to be mediated by the interaction of tumor cell-bound IgE with receptors, which then trigger the release of mediators, recruitment of effector cells, cell killing and tumor Ag cross-priming. It was therefore of utmost importance to demonstrate the strict dependence of the protective effect on IgE receptor activation. First, the protective effect of IgE was confirmed in a BALB/c tumor model, in which IgE-loaded modified VV Ankara-infected tumor cells proved to be an effective cellular vaccine. However, the protective effect was lost in FcεRIα−/− (but not in CD23−/−) knockout mice, showing the IgE-FcεRI interaction to be essential. Moreover, human IgE (not effective in BALB/c mice) had a protective effect in the humanized knockin mouse (FcεRIα−/− hFcεRIα+). This finding suggests that the adjuvant effect of IgE could be exploited for human therapeutics.