CTLA-4: a moving target in immunotherapy

Abstract

CD28 and CTLA-4 are members of a family of immunoglobulin-related receptors that are responsible for various aspects of T-cell immune regulation. The family includes CD28, CTLA-4, and ICOS as well as other proteins, including PD-1, BTLA, and TIGIT. These receptors have both stimulatory (CD28, ICOS) and inhibitory roles (CTLA-4, PD-1, BTLA, and TIGIT) in T-cell function. Increasingly, these pathways are targeted as part of immune modulatory strategies to treat cancers, referred to generically as immune checkpoint blockade, and conversely to treat autoimmunity and CTLA-4 deficiency. Here, we focus on the biology of the CD28/CTLA-4 pathway as a framework for understanding the impacts of therapeutic manipulation of this pathway.

Figure 1. Schematic of CTLA-4 cell biology

(A) CTLA-4 and CD28 receptors share two ligands CD80 and CD86. CD80 is a dimeric high affinity ligand and CD86 is a monomeric lower affinity ligand for both receptors. CTLA-4 has a higher affinity and avidity for CD80 than CD86. The relative affinities go from high to low from left to right. (B) CTLA-4 expressed in T-cells is highly endocytic. CTLA-4 is constitutively expressed in Treg or induced following T cell activation via CD28 and TCR signaling. In the absence of the ligand, CTLA-4 is mainly found in intracellular compartments following clathrin-mediated endocytosis mediated through CTLA-4 interaction with the AP2 molecule. The AP2 (μ2 subunit) binds to the tyrosine-based (YVKM) motif of the cytoplasmic domain of CTLA-4 and mediates rapid internalization. LRBA and AP1 proteins have also been found to bind to the YVKM motif on CTLA-4, which appear to impose different fates on CTLA-4. LRBA may mediate recycling of CTLA-4 to the plasma membrane, whereas AP1 may mediate CTLA-4 trafficking to lysosomal compartments resulting in subsequent degradation.

Figure 2. CTLA-4 function and the impact of immune checkpoint blockade

(A) In health, regulatory T cells express CTLA-4, which binds CD80 and CD86 expressed on antigen presenting cells (APCs). CTLA-4 binds to CD80 and CD86 with higher affinity and avidity than does CD28, preventing Tcon stimulation through CD80/CD86 interaction with CD28. Removal of CD80/CD86 ligands by trans-endocytosis results in impaired costimulation of T cells via CD28, resulting in immune regulation. (B) When the immune system is stimulated in the presence of inflammatory/innate immune stimuli, APCs up-regulate the expression of CD80/CD86 overcoming their control by Treg, enabling co-stimulation and the proliferation of T cells. (C) In the tumour microenvironment, CD80/CD86 are controlled by Treg and the abundance of Treg leads to the suppression of immune responses. (D) Anti-CTLA-4 antibodies bind to CTLA-4 molecules with high affinity leading to Treg depletion or functional blockade resulting in enhanced T cell activation and immunological responses to cancer. The impacts of CTLA-4 blockade can be mediated by a variety of mechanisms: prevention of trans-endocytosis increasing CD80/CD86 levels on APCs, direct Treg cytotoxicity and antibody dependent cellular cytotoxicity mediated by FcR-IV expressing intra-tumoural macrophages. NB only CD80 is shown here for clarity.