Treatment with anti‐cytokine monoclonal antibodies can potentiate the target cytokine rather than neutralize its activity

Airway diseases such as allergic rhinitis and allergic asthma are due to a Th2 response to innocuous environmental antigens. Animal models have shown that the antibody-mediated neutralization of Th2 cytokines can greatly diminish airway inflammation (1–3); these results lead to the development and clinical investigation of humanized anti-Th2 cytokine antibodies for the amelioration of asthma (4–8). Unfortunately, clinical trials of these biologics have not been a resounding success, with inabilities to significantly improve clinical symptoms being common and successful trials requiring strict patient stratification (9). These failures have spawned numerous plausible explanations, such as a requirement to neutralize more than one Th2 cytokine to achieve a diseasemodifying effect or that antibodies with a greater affinity for the target cytokine may be required. Alternatively, a welldocumented phenomenon may be hampering clinical efficacy, namely the potentiation of cytokines through the formation of cytokine/anti-cytokine immune complexes. The formation of cytokine/anti-cytokine immune complexes does not guarantee cytokine neutralization, as these complexes can actually increase the potency of a cytokine. Due to their bivalent nature, antibodies form immune complexes when neither the antibody nor the antigen is in excess. Murine studies have shown that cytokine/anti-cytokine immune complexes can potentiate the activity of numerous cytokines. For instance, IL-4/anti-IL-4 complexes are much more efficient than IL-4 alone at stimulating B cells, increasing their production of IgE and inducing the proliferation of CD8 T cells (10–12). Similarly, IL-2/anti-IL-2 complexes are much more potent in their ability to expand memory CD8 T cells, NK cells or regulatory T cells (Tregs) than IL-2 alone (12–15). An increase in the potency and bioactivity of IL-3, IL-6 and IL-7, when complexed with their respective antibodies, has also been reported (11, 16–22). Furthermore, recent studies have demonstrated the ability of IL-2/anti-IL-2 complexes to cause type 2 innate lymphoid cells (ILC2s) to expand, produce IL-5 and induce airway eosinophilia (23, 24). Together, these studies establish that the formation of immune complexes can potentiate the effects of the cytokine rather than neutralize its activity. The ability of immune complexes to increase the potency of the cytokine has also been described in humans. Stein et al. reported the presence of IL-5/anti-IL-5 complexes in the circulation of subjects treated with mepolizumab and that these immune complexes facilitated increased production of IL-5 by CD4 T cells (25). Indirect evidence that immune complexes can worsen disease comes from a dose-finding study using lebrikizumab, an anti-IL-13 antibody, in subjects with moderate to severe asthma in which there was a direct correlation between the exacerbation rate and the dose of lebrikizumab in periostin-high subjects (26). Although the presence of IL-13/lebrikizumab complexes was not assessed, the seemingly paradoxical dose response of lebrikizumab is readily explained by the hypothesis that higher doses of lebrikizumab led to increased complex formation; this is also in line with the observations from the murine studies outlined above. The potentiation of cytokine activity is not restricted to antibodies but also extends to decoy receptors. For instance, when subjects with ankylosing spondylitis were treated with etanercept, which is a dimerized TNF receptor/Fc fusion protein designed to bind to and neutralize soluble TNF-a, a significant increase in the percentage of CD4 and CD8 T cells producing TNF-a and IFN-c was reported (27). Collectively, these data extend the findings of murine studies to humans and emphasize that attempting to neutralize free cytokine with antibodies or decoy receptors may be an inefficient approach, as complexes can potentiate cytokine activity rather than neutralize it. Treatment of subjects with neutralizing antibodies can not only increase the activity of the cytokine but may also increase its levels in the circulation. For example, subjects treated with mepolizumab displayed greater levels of IL-5 in the circulation with the majority of it bound in IL-5/anti-IL5 complexes (17), while the treatment of asthmatic subjects with anti-IL-13 antibodies directly increased the levels of IL13 in the serum (7). Similarly, the treatment of patients with metastatic breast cancer with etanercept increased TNF-a levels in the plasma, demonstrating that this effect is not restricted to Th2 cytokines and diseases (28). Studies in mice using antibodies to IL-2, 4, 6 and 7 have paralleled these clinical findings as greater bioactive levels of the targeted cytokine have been found following antibody treatment (12, 14, 16). Despite these observations, there appears to be little importance placed upon the formation of cytokine/anti-cytokine complexes in subjects treated with monoclonal antibodies as it is largely assumed that cytokines present in immune complexes are neutralized. However, as we have discussed, studies have demonstrated that the neutralization of a cytokine by its binding to antibody is not a foregone conclusion and further studies in humans are required. The formation of immune complexes is prevented when the neutralizing antibody is present in excess. This concept is very clearly exhibited in early studies on the stimulatory potential of cytokine/anti-cytokine complexes where an