"IL-2 immunotherapy for targeting regulatory T cells in autoimmunity"

FOXP3⁺ regulatory T cells (T_reg) are indispensable for immune homoeostasis and for the prevention of autoimmune diseases. Interleukin-2 (IL-2) signalling is critical in all aspects of T_reg biology. Consequences of defective IL-2 signalling are insufficient numbers or dysfunction of T_reg and hence autoimmune disorders in human and mouse. The restoration and maintenance of immune homoeostasis remain central therapeutic aims in the field of autoimmunity. Historically, broadly immunosuppressive drugs with serious side-effects have been used for the treatment of autoimmune diseases or prevention of organ-transplant rejection. More recently, ex vivo expanded or in vivo stimulated T_reg have been shown to induce effective tolerance in clinical trials supporting the clinical benefit of targeting natural immunosuppressive mechanisms. Given the central role of exogenous IL-2 in T_reg homoeostasis, a new and promising focus in drug development are IL-2-based approaches for in vivo targeted expansion of T_reg or for enhancement of their suppressive activity. In this review, we summarise the role of IL-2 in T_reg biology and consequences of dysfunctional IL-2 signalling pathways. We then examine evidence of efficacy of IL-2-based biological drugs targeting T_reg with specific focus on therapeutic candidates in clinical trials and discuss their limitations.