The immune regulation of PD-1/PDL-1 axis, a potential biomarker in multiple sclerosis

Multiple sclerosis is an autoimmune disease characterised by a chronic inflammation within the central nervous system. In the last ten years, studies on multiple sclerosis have been concentrated on the discovery of new biomarkers of disease and potential therapeutic targets. In chronic infection or in cancer, the immune system response is faulty and maintained in a condition defined as T-cell exhaustion induced by expression of co-inhibitory receptors. The PD-1/PDL-1 pathway is demonstrated to be the main one responsible for promoting T-cell exhaustion, and immunotherapies targeting PD-1 or PDL-1 have shown beneficial clinical outcomes in several tumours and chronic diseases. Contrarily, transcriptional T-cell exhaustion signature and high expression of co-inhibitor receptor PD-1 are associated with favourable prognosis in multiple sclerosis and other autoimmune diseases. Several studies have clearly demonstrated PD-1 has a dual role in immune self-tolerance: to constrain autoreactive T cells in anergic condition and to protect the tissue from the damage caused by the activation of endogenous autoreactive T cells. Consequently, immune checkpoint inhibitor therapies that target inhibitory receptors in cancer cause an exacerbation of autoimmune diseases. This review describes the roles of the PD-1/ PDL-1 pathway in cancer and autoimmune diseases, especially in multiple sclerosis, and how manipulating PD-1 can be a therapeutic approach in multiple sclerosis.