Approaches and challenges in targeting memory T cells in transplant tolerance

Memory T cells are an important cell type in the immune system and are vital to protective immunity against invading pathogens. However, a significant fraction of memory T cells is found to be alloreactive in transplant models, i.e. they can readily attack and dismantle allografts in transplant models. As memory T cells are not as easily amenable as naive T cells, memory T cells constitute a potent barrier to the induction of transplant tolerance. The key issues concerning memory T cells in transplantation are related to the tolerability of alloreactive memory T cells and the effects of commonly used immunosuppressive drugs on the memory response in transplant recipients. The real challenge in the future is to selectively tolerize alloreactive memory T cells but spare those involved in protective immunity following organ transplantation. This review will discuss recent advances in our understating of memory T cells in transplant models, with specific emphasis on the problems and challenges in targeting memory T cells in the induction of transplant tolerance.