T cells in ITP: Focus on T follicular helper cells and cytotoxic T cells.

Immune thrombocytopenia (ITP) is an autoimmune disease where premature destruction of platelets as well as inhibition of platelet production leads to thrombocytopenia and associated bleeding. It has long been considered a disease primarily caused by B cells, but the role of T lymphocytes in its pathogenesis is now better understood and deserves elucidation. Two types of T cells will be discussed: (1) splenic T follicular helper cells (TFH) that participate in differentiation of B cells within germinal centres (GC) and stimulate the production of antiplatelet antibodies, thus supporting the humoral autoimmune response; and (2) antibody-independent mechanisms of action of cytotoxic T lymphocytes (CTL) that may directly participate in platelet destruction as well as inhibit their production by targeting megakaryocytes. To date, most novel therapies target antibody-mediated disease, but targeting either TFH or CTL may provide new therapeutic opportunities.