TGFβ control of immune responses in cancer: a holistic immuno-oncology perspective

Abstract

The immune system responds to cancer in two main ways. First, there are prewired responses involving myeloid cells, innate lymphocytes and innate-like adaptive lymphocytes that either reside in premalignant tissues or migrate directly to tumours, and second, there are antigen priming-dependent responses, in which adaptive lymphocytes are primed in secondary lymphoid organs before homing to tumours. Transforming growth factor-β (TGFβ) - one of the most potent and pleiotropic regulatory cytokines - controls almost every stage of the tumour-elicited immune response, from leukocyte development in primary lymphoid organs to their priming in secondary lymphoid organs and their effector functions in the tumour itself. The complexity of TGFβ-regulated immune cell circuitries, as well as the contextual roles of TGFβ signalling in cancer cells and tumour stromal cells, necessitates the use of rigorous experimental systems that closely recapitulate human cancer, such as autochthonous tumour models, to uncover the underlying immunobiology. The diverse functions of TGFβ in healthy tissues further complicate the search for effective and safe cancer therapeutics targeting the TGFβ pathway. Here we discuss the contextual complexity of TGFβ signalling in tumour-elicited immune responses and explain how understanding this may guide the development of mechanism-based cancer immunotherapy.

Figure 1.. Molecular modalities of TGFβ activation and signaling

TGFβ is produced in a latent complex involving two copies of latency-associated peptide (LAP) and the active cytokine. This complex can be covalently linked to latent TGFβ-binding proteins (LTBPs) which can be found in the extracellular matrix (ECM), or to leucine-rich repeat-containing proteins such as LRRC32 (also known as GARP), which can be found on the cell surface. The integrin αvβ6, which interacts with the cytoskeleton, and integrin αvβ8 can liberate and expose the receptor-binding site of active TGFβ. TGFβ binds to the heterotetrameric TGFβ type II and type I serine/threonine kinase receptors (TGFBR2 and TGFBR1), triggering TGFBR2 phosphorylation of TGFBR1, which then phosphorylates SMAD2 and SMAD3. These two phosphorylated proteins can modulate gene expression in two ways: (1) through forming a heterotrimeric complex with SMAD4 and translocating to the nucleus, controlling target gene expression, and (2) through binding the SMAD4–SKI–SKIL complex, which normally represses transcription where it binds, and leads to its degradation, thus alleviating transcriptional repression. Phosphorylated SMAD3 also has transcription-independent cell signalling functions including liberation of protein kinase A (PKA) from an inactive PKA complex. Only the mediators of TGFβ signalling discussed in the text are displayed.

Figure 2.. TGFβ control of priming-dependent lymphocyte responses in cancer

a, In secondary lymphoid organs, TGFβ inhibits naive CD4⁺ T cell, regulatory T (Treg) cell, naïve CD8⁺ T cell and memory-phenotype (MP) CD8⁺ T cell priming by dendritic cells (DCs). b, TGFβ also inhibits Th1 and Th2 cell, but promotes Th17 cell and peripheral Treg (pTreg) cell, differentiation by regulating expression of the transcription factors T-bet, GATA3, RORγt, and FOXP3, while its effect on T follicular helper (Tfh) cell differentiation is contextual. Among CD8⁺ T cells, TGFβ promotes differentiation of regulatory CD8⁺ T cells, in part by promoting expression of the transcription factor Helios. These cells reside in the B cell follicle where they can inhibit Tfh cell responses. In addition, TGFβ promotes tissue resident memory (Trm)-like cytotoxic T lymphocytes (CTLs) through expression of the integrin CD103, while also repressing CTL differentiation by suppressing expression of T-bet and Eomes, IFNγ and granzyme B, and promoting expression of the inhibitory receptor PD-1. c, TGFβ inhibits B cell proliferation, and promotes both IgA class-switching and migration from the light zone (LZ) and the dark zone (DZ) in the germinal centre. d-e, In the tumour, TGFβ, in part regulated by Treg cells, inhibits circulating T effector memory (Tem)-like CTL responses against mesenchymal phenotype cancer cells (d), while promoting Trm-like CTL responses, including cytotoxicity against epithelial cancer cells through CD103 interaction with E-cadherin (e). f-g, TGFβ also inhibits both Th1 and Th2 effector states. Should this inhibition be removed, IFNγ-producing Th1 cells can impact angiogenesis (f), ‘M1-like’ macrophage polarization and CTL function (g), while IL-4-producing Th2 cells can influence tissue-level vascularization and tumour tissue healing (f), ‘M2-like’ macrophage polarization and eosinophil responses (g), which collectively suppress tumor development by targeting cancer cells (g) and the cancer environment (f).

Figure 3.. TGFβ control of prewired innate lymphocyte and innate-like T cell responses in cancer

a, In the thymus, TGFβ is required for development of CD1d/lipid agonist antigen-reactive invariant Natural Killer T (iNKT) precursor (iNKTp) and MHC/peptide agonist antigen-reactive intraepithelial lymphocyte (IEL) precursor (IELp), likely through attenuated clonal deletion. In addition, TGFβ is required for the differentiation of IL-17-producing iNKT cells (iNKT17) and IL-17-producing ɣδ lineage T cells. b, In the bone marrow, the innate lymphoid cell progenitor (ILCp) gives rise to ILC1, ILC2, and ILC3, while the natural killer (NK) progenitor (NKp) gives rise to NK cells. TGFβ promotes differentiation of ILC2 in part via ST2 expression. c-d, In the tumor, TGFβ inhibits NK cell activation and effector function against mesenchymal phenotype cancer cells (c), while promoting cytotoxic ILC1s as well as killer innate-like T cells (ILTCKs) of both αβ and γδ T cell lineages that co-localize with epithelial cancer cells in part through CD103 interaction with E-cadherin (d). The cancer surveillance functions of NK cells, ILC1s and ILTCKs are additionally regulated by IL-15 and activation receptor signaling pathways.

Figure 4.. Strategies to target the TGFβ pathway for cancer therapy

Pharmacological interventions of the TGFβ pathway are grouped into two categories: first, systemic blockade that acts on 1) the latency-associated peptide (LAP) and TGFβ complex, 2) the LAP/TGFβ tethering molecule GARP, 3) the LAP/ TGFβ activating integrin, 4) the active form of TGFβ, or 5) the TGFβ receptor with antibodies and the ectodomain of TGFBR2 (TGFβ Trap)-based biologics as well as small-molecule kinase inhibitors; and second, targeted blockade with bispecific molecules to deliver TGFβ Trap to targeted cell populations such as CD4⁺ T cells with 4T-Trap or PD-L1-expressing cancer cells with PD-L1-Trap, or overexpressing a dominant-negative mutant of TGFBR2 (TGFDNR) in tumor antigen-specific T cell receptor (TCR) T cells or cancer cell-reactive chimeric antigen receptor (CAR) T cells for cell therapy.