CD28 Costimulation: From Mechanism to Therapy

Abstract

Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Figure 1. CD28 cytoplasmic motifs and corresponding interacting molecules

Engagement of CD28 initiates signal transduction cascades mediated by specific association of proteins with motifs of the CD28 cytoplasmic tail. Proteins that bind specifically to phosphotyrosine motifs or to unphosphorylated motifs are indicated according the legend. Data are adapted from (Tian et al., 2015).

Figure 2. Signaling pathways downstream of CD28

Lck phosphorylates PDK1, which in turn phosphorylates and activates PKCθ. PKCθ inactivates GSK3β, ultimately leading to enhanced transcription of NFAT-dependent genes. PKCθ also mediates signaling events leading to the activation of the NF-κB, and AP-1 transcription factors. The adaptor proteins GRB2 and GADS bind CD28. GRB2 binds Sos and Vav1 via its SH3 domain. In turn, Sos and Vav1 activate Ras, Rac1, and CDC42, resulting in signaling cascades culminating in JNK activation and formation of the AP-1 transcriptional complex. GRB2 and GADS also mediate the formation of CARMA1-Bcl-10-Malt1 complexes, which contribute to the activation of IKKs that regulate NF-κB activation.

Figure 3. Major CD28 pathways in human T cells

CD4⁺CD45RA⁺ human T cells were stimulated with anti-CD3 antibodies or anti-CD3 and anti-CD28 antibodies for 24 hours before harvest and transcriptome analysis. Differentially regulated genes were mapped to specific pathways (nodes), which are connected to each other based on common function (edges) using the Reactome pathway database (Croft et al., 2014; Milacic et al., 2012). Top-level nodes are categorized by a collection of pathways specific to its category (e.g. Immune System includes CD28 and TCR stimulation and cytokine signaling pathways, among others). The major pathway categories are indicated by pop-out text boxes in the network above. The density of connected nodes indicates the relative enrichment of a given class of pathways in T cells after CD28 stimulation. This figure was created from a re-analysis of data originally published in (Martinez-Llordella et al., 2013).