Antibody-based molecules that combine two different binding sites — one targeted at a cancer antigen and the other at CD3, which is part of the T cell receptor complex — have shown efficacy as anticancer therapeutics in the clinic. Now, reporting in Nature Cancer, Zhi-yong Yang, Gary Nabel and colleagues take the concept of multi-targeting one step further and show that adding a third functionality to a bispecific antibody can significantly enhance its potency.
The authors had previously established a trispecific antibody platform and constructed antibodies that bind to three different sites on the HIV viral envelope. Now they test whether this platform could be used to combine binding to a cancer antigen with engaging both CD3 and the co-stimulatory receptor CD28 on T cells. The simultaneous activation of these receptors is known to induce efficient stimulation and is thought to promote T cell expansion and survival.
Wu et al. engineered dual T cell-binding specificity into one arm of the antibody and targeted the other arm at CD38, an antigen often highly expressed by malignant cells in myeloma as well as some leukaemias and lymphomas. In vitro testing of the resulting CD38/CD3xCD28 trispecific antibody showed that it preferentially stimulated the proliferation of CD4+ T helper 1 cells as well as central memory and effector CD8+ T cells.
Using trispecific antibodies in which the different binding sites were mutated, the authors demonstrated that each component contributes to optimal T cell activation. In particular, CD28 engagement promoted T cell proliferation and the expression of the pro-survival protein BCL-XL.
Compared with a CD38-targeted monoclonal antibody used in the clinic (daratumumab), the trispecific antibody induced 3- to 4-log higher killing of both CD38hi and CD38low multiple myeloma cells co-cultured with human peripheral blood mononuclear cells (PBMCs). The authors also compared the trispecific antibody to the CD28-targeted superagonist antibody TGN1412, which induced a life-threatening cytokine release syndrome when tested in clinical trials in 2006.
In vitro assays with PBMCs showed that the propensity of TGN1412 to induce cytokine release is due to its bivalent binding to CD28, as a monovalent version of the antibody did not induce cytokine release. The trispecific antibody is monovalent for CD28 binding, and safety testing in non-human primates showed that it was well tolerated at doses that stimulated T cell immunity.
In a humanized mouse model of multiple myeloma, the trispecific antibody induced a potent dose-dependent reduction in tumour burden, with efficient tumour cell regression observed even at low doses. By contrast, a corresponding antibody in which the CD28-binding site was mutated had minimal activity.
“each component contributes to optimal T cell activation”
The authors point out that the trispecific antibody represents a first-in-class T cell activation molecule that delivers two signals to T cells while simultaneously directing them to cancer cells. Moreover, the flexibility of the format should allow the targeting of any cancer surface antigen or a combination of antigens, as well as the precise stimulation of immunomodulatory signals.
Wu, L. et al. Trispecific antibodies enhance the therapeutic efficacy of tumour-directed T cells through T cell receptor co-stimulation. Nat. Cancer https://doi.org/10.1038/s43018-019-0004-z (2019)
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Flemming, A. Cancer immunotherapy goes trispecific. Nat Rev Immunol 20, 4–5 (2020). https://doi.org/10.1038/s41577-019-0255-z
European Journal of Immunology (2020)