The B7/CD28 costimulatory family in autoimmunity

Translating the concept of suppressor/regulatory T cells to clinical applications

The in vivo expansion of suppressor/regulatory T cells (Tregs) is a desirable event in autoimmunity and transplantation. Here we summarize the general rules involved in antigen recognition by T cells and describe Tregs and their requirements, discussing different levels of immune intervention.

Tissue expression of PD-L1 mediates peripheral T cell tolerance

Programmed death 1 (PD-1), an inhibitory receptor expressed on activated lymphocytes, regulates tolerance and autoimmunity. PD-1 has two ligands: PD-1 ligand 1 (PD-L1), which is expressed broadly on hematopoietic and parenchymal cells, including pancreatic islet cells; and PD-L2, which is restricted to macrophages and dendritic cells. To investigate whether PD-L1 and PD-L2 have synergistic or unique roles in regulating T cell activation and tolerance, we generated mice lacking PD-L1 and PD-L2 (PD-L1/PD-L2^−/− mice) and compared them to mice lacking either PD-L. PD-L1 and PD-L2 have overlapping functions in inhibiting interleukin-2 and interferon-γ production during T cell activation. However, PD-L1 has a unique and critical role in controlling self-reactive T cells in the pancreas. Our studies with bone marrow chimeras demonstrate that PD-L1/PD-L2 expression only on antigen-presenting cells is insufficient to prevent the early onset diabetes that develops in PD-L1/PD-L2^−/− non-obese diabetic mice. PD-L1 expression in islets protects against immunopathology after transplantation of syngeneic islets into diabetic recipients. PD-L1 inhibits pathogenic self-reactive CD4⁺ T cell–mediated tissue destruction and effector cytokine production. These data provide evidence that PD-L1 expression on parenchymal cells rather than hematopoietic cells protects against autoimmune diabetes and point to a novel role for PD-1–PD-L1 interactions in mediating tissue tolerance.

Are regulatory T-cells linked with aging?

There is increasing evidence for an active and 'dominant' tolerance mediated by regulatory T-cells. Out of these CD4+ 'naturally occurring' regulatory T-cells (TREGs) are currently the main research focus in this field. TREGs exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and the forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3). Age-related increment of the prevalences of CD4+ CD25(hi) TREGs were described controversially, and whether such changes explain immune dysfunction in the elderly is still unclear. During aging thymic TREG output may decrease with significant loss of thymic capacity to generate new T-cells, and TREG homeostasis has been shown to be sustained by alternative pathways like peripheral generation of TREGs. An imbalance of TREG homeostasis would then predispose to immune dysfunction in aged individuals explaining their higher risk of immune-mediated diseases, cancer or infections.

Imbalance of regulatory T cells in human autoimmune diseases

The breakdown of mechanisms assuring the recognition of self and non-self is a hallmark feature of autoimmune diseases. In the past 10 years, there has been a steadily increasing interest in a subpopulation of regulatory T cells, which exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3) (TREGs). Recent findings of changed prevalences and functional efficiencies indicate that these TREGs play a unique role in autoimmune diseases. Clinical findings in patients with mutated FOXP3 genes and a specific polymorphism in the promotor region of FOXP3 also support the role of FOXP3 as a 'master control gene' in the development and functioning of TREGs. Both altered generation of TREGs and insufficient suppression of inflammation in autoimmune diseases are considered to be crucial for the initiation and perpetuation of disease. TREG-related somatic cell therapy is considered as an intriguing new intervention to approach autoimmune diseases.

Analysis of a functional BTNL2 polymorphism in type 1 diabetes, rheumatoid arthritis, and systemic lupus erythematosus

The aim of this study was to test whether the functional variant rs2076530 of the BTNL2 gene confers susceptibility to the autoimmune diseases type 1 diabetes (T1D), rheumatoid arthritis (RA), and systemic lupus erythematosus (SLE). Our study populations consisted of 326 patients with T1D and 351 healthy subjects, 808 patients with RA and 1137 healthy controls, and 372 patients with SLE and 280 healthy controls. Genotyping of the BTNL2 gene rs2076530 polymorphism was performed by real-time polymerase chain reaction technology, using the TaqMan 5'-allele discrimination assay. We observed statistically significant differences in the distribution of BTNL2rs2076530 alleles between patients with T1D, RA, and SLE and healthy controls (p=0.0035, 0.000003, and 0.00002, respectively), but in two divergent ways: the G allele was associated with T1D and RA, and the A allele was associated with SLE. However, the polymorphism exhibited strong linkage disequilibrium with HLA DQB1-DRB1 haplotypes previously identified as predisposing to the diseases. When the BTNL2 polymorphism was tested conditional on HLA DQB1-DRB1haplotypes, the BTNL2 effect was no longer significant in all three study populations. The BTNL2 rs2076530 polymorphism is associated with T1D, RA, and SLE because of its strong linkage disequalibrium with predisposing HLA DQB1-DRB1 haplotypes in Caucasian populations.

Evolutionarily divergent herpesviruses modulate T cell activation by targeting the herpesvirus entry mediator cosignaling pathway

The herpesvirus entry mediator (HVEM), a member of the TNF receptor (TNFR) superfamily, can act as a molecular switch that modulates T cell activation by propagating positive signals from the TNF-related ligand LIGHT (TNFR superfamily 14), or inhibitory signals through the Ig superfamily member B and T lymphocyte attenuator (BTLA). Competitive binding analysis and mutagenesis reveals a unique BTLA binding site centered on a critical lysine residue in cysteine-rich domain 1 of HVEM. The BTLA binding site on HVEM overlaps with the binding site for the herpes simplex virus 1 envelope glycoprotein D, but is distinct from where LIGHT binds, yet glycoprotein D inhibits the binding of both ligands, potentially nullifying the pathway. The binding site on HVEM for BTLA is conserved in the orphan TNFR, UL144, present in human CMV. UL144 binds BTLA, but not LIGHT, and inhibits T cell proliferation, selectively mimicking the inhibitory cosignaling function of HVEM. The demonstration that distinct herpesviruses target the HVEM-BTLA cosignaling pathway suggests the importance of this pathway in regulating T cell activation during host defenses.