Han et al. showed that anti-TIGIT antibodies activate myeloid cells to induce an antitumor response via interactions between their Fc domains and FcγRs on myeloid cells. Anti-TIGIT antibodies with intact Fc portions (anti-TIGIT:mIgG2a) inhibited growth in MC38 and CT26 tumors, both as a monotherapy and in combination with anti-PD-1, dependent on FcγRIV. Anti-TIGIT:mIgG2a didn’t deplete intratumoral Tregs and failed to reduce tumor growth in TIGIT knockout mice. Anti-TIGIT:mIgG2a upregulated chemokines, cytokines, and APC activation markers (MHC class II, CD86, or CD40), and increased perforin, granzyme B and INFγ in combination with anti-PD-1.
Contributed by Shishir Pant
ABSTRACT: The molecule "T cell immunoreceptor with immunoglobulin and ITIM domain," or TIGIT, has recently received much attention as a promising target in the treatment of various malignancies. In spite of the quick progression of anti-TIGIT antibodies into clinical testing both as monotherapy and in combination with programmed cell death-1 (PD-1)-directed immune checkpoint blockade, the molecular mechanism behind the observed therapeutic benefits remains poorly understood. Here we demonstrate, using mouse tumor models, that TIGIT blocking antibodies with functional Fc binding potential induce effective anti-tumor response. Our observations reveal that the anti-TIGIT therapeutic effect is not achieved by depletion of intratumoral regulatory T cells (Treg) or any cell population expressing TIGIT, but instead is mediated by possible "reverse activating signals" through FcγRs on myeloid cells, inducing expression of various mediators such as cytokines and chemokines. Furthermore, we discovered an induction of a robust and persistent granzyme B and perforin response, distinct from a predominantly interferon-γ (IFN-γ)-driven anti-PD-1 blockade. Our observations, for the first time, provide the basis for a mechanistic hypothesis involving the requirement of a functional Fc domain of anti-TIGIT monoclonal antibodies, of which various isotypes are currently under intense clinical investigation.