Evolving Understanding of T-cell Cosignaling Pathways.

We read with great interest the previously published article “Costimulation Blockade in Kidney Transplantation: An Update” written by Malvezzi et al.[1] In the setting of solid-organ transplantation, it represents a great progress to explore the blockade of some costimulation pathways as an efficient immunosuppressive tool instead of using calcineurin inhibitors. In the section of targeting costimulation pathways, Malvezzi et al[1] summarized main T-cell surface signaling molecules, including costimulation and coinhibition. Cosignaling molecules are controllers of T-cell responses to antigens. By selectively turning on and/or off costimulatory and coinhibitory pathways, the T-cell receptor (TCR) signal could be transiently regulated in positive and negative directions. cluster of differentiation (CD), CD28/CD154(CD40L)/inducible costimulator had a positive regulatory effect on T-cell activation, whereas cytotoxic T-lymphocyte-associated antigen-4/programmed death ligand-1 was negative. Based on this article, we will provide some progress in costimulation pathways over the past few years, as shown in Figure 1.

Figure 1.

Major T-cell cosignaling pathways. Black lines indicate stimulating signals on T cells, whereas red lines indicate inhibitory signals. CD, cluster of differentiation; CTLA-4, cytotoxic t-lymphocyte-associated antigen-4; ICOS, inducible costimulator; ICOS-L, ICOS ligand; MHC, major histocompatibility complex; PD-1, programmed death 1; PD-L1, programmed death ligand-1; TCR, T-cell receptor.

First, as we all know, programmed death 1 (PD-1) and its ligands, mainly PD-L1 and PD-L2, deliver inhibitory signals that regulate the balance among T-cell activation, tolerance, and exhaustion. PD-1 is a CD28 family member. PD-L1 (B7-H1) and PD-L2 (B7-DC) are B7 family members. Several previous findings pointed out a significant bidirectional inhibitory interaction between B7-1 (CD80) and PD-L1, which added an additional dimension to immunoregulatory functions of the B7:CD28 family.[2-5] The competition assays in vitro showed that PD-1 and B7-1 competed for binding to PD-L1 and block each other’s binding.[6,7] Extremely low levels of PD-1 are sufficient for potent inhibition in the earliest stages of T-cell activation.[8] However, this potent inhibition effect of PD-1 depends on the colocalization of TCR and CD28, which initiate the T-cell activation. Therefore, PD-1 plays an extremely important role in the negative regulation of T cells.[9,10]

Second, the expression of PD-1 and its ligands is widespread. PD-1 can be expressed on T cells, B cells, natural killer T cells, activated monocytes, and dendritic cells (DCs). PD-1 is not expressed on resting T cells. It is induced to express after activation.[3-5,11] The expression of PD-L1 and PD-L2 differ in patterns and vary across species. Research data on PD-L2 exhibited a smaller interspecies gap, whereas PD-L1 had a larger expression gap between mice and humans. PD-L1 is constitutively expressed on mouse T and B cells, DCs, macrophages, mesenchymal stem cells, and bone marrow–derived mast cells. However, it is minimally expressed on resting naive human CD4 and CD8 T cells.[12] It is upregulated on several cell types after activation.[2,7] Compared with PD-L1, the expression of PD-L2 is increasingly restricted. It is induced to express on DCs, macrophages, and bone marrow–derived mast cells.[2,5,9,11] Although PD-1 and PD-L1 are simultaneously expressed on T cells and PD-L1 has a high expression on activated T cells in mouse, and studies revealed that PD-L1 could attenuate TCR-mediated T-cell stimulation, PD-1 is the principal inhibitory molecule after T-cell activation, just like CTLA-4. Binding of PD-1 to its ligand PD-L1 or PD-L2 delivers a negative signal by the recruitment of Src homology 2-domain–containing tyrosine phosphatase 2 to the phosphorylated tyrosine residue in the cytoplasmic region.[4] Moreover, the result that PD-L1 fails to inhibit PD-1 deficient T cells indicates that PD-L1 conveys suppression signal via its interaction with PD-1.[3] Consequently, PD-1 plays a major role in activated T-cell inhibition.

Third, the interaction on T-cell responses is comprehensive complex. It is also related to binding affinity, the expression position, spatial conformation, etc. As far as PD-1 and its ligands are concerned, although PD-1 seems to be an exclusive coinhibitor, PD-L1 and PD-L2 have dual functions of coinhibitors and costimulators. A series of in vivo studies indicate that endogenous PD-L1 could act as coinhibitor[2,3,13] or costimulatory,[6,14-17] whereas PD-L2 mainly acts as costimulatory.[18,19] Some observations strongly suggested that PD-L1 and PD-L2 costimulated T-cell growth through a non–PD-1 receptor.[20,21] From an affinity point of view, molecular pairs were ranked according to their binding affinity as B7-1: CTLA-4 > PD-1: PD-L1/PD-L2 > PD-L1: B7-1 > B7-1: CD28,[2,7] which implies that interactions between cosignaling pathway participants on T cells are sequential. Between T cells and antigen-presenting cells (APCs), it is possible that PD-L1 mainly on APC cell preferentially binds PD-1 on T cell, whereas B7-1 (mainly on APC cell) preferentially binds CTLA-4 on T cell. When PD-L1 and B7-1 are simultaneously expressed on the same cell or expressed on 2 different cells, their interactions are different in the aspect of spatial conformation. The interaction between PD-L1 and B7-1 occurs in cis when they are located in the same cell.[7,22,23] However, binding between PD-L1 and B7-1 on different cells still remains controversial.[24] Some reports suggested that PD-L1 and B7-1 on different cells bound in trans,[2,23,25] whereas others indicated that the interaction occurred not in trans.[7,23]

Apart from cosignaling pathway participants (CD28, CD80/86, CTLA-4, PD-1, PD-1 L, and inducible costimulator ligand) mentioned above, several novel negative immune regulation molecules have been introduced over the past few years, such as lymphocyte activation gene-3 (LAG-3 or CD223), T-cell immunoglobulin and mucin-domain–containing-3, T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain, V-type immunoglobulin domain-containing suppressor of T-cell activation, and so on.[26] These negative regulatory molecules provide new targets for inhibiting T-cell activation and are worth our concern.

In summary, there is a dynamic and complex interaction network among PD-1, PD-L1/PD-L2/, B7-1 and CTLA-4. PD-L1 has dual functions as coinhibitor and costimulator.