Human Vγ9/Vδ2 T cells: Innate adaptors of the immune system

Abstract

a b s t r a c tUnconventional T cells are gaining center stage as important effector and regulatory cells that orchestrate innate and adaptive immune responses. Human Vc9/Vd2 T cells are amongst the best understood unconventional T cells, as they are easily accessible in peripheral blood, can readily be expanded and manipulated in vitro, respond to microbial infections in vivo and can be exploited for novel tumor immunotherapies. We here review findings that suggest that Vc9/Vd2 T cells, and possibly other unconventional human T cells, play an important role in bridging innate and adaptive immunity by promoting the activation and differentiation of various types of antigen-presenting cells (APCs) and even turning into APCs themselves, and thereby pave the way for antigen-specific effector responses and long-term immunological memory. Although the direct physiological relevance for most of these mechanisms still needs to be demonstrated in vivo, these findings may have implications for novel therapies, diagnostic tests and vaccines.Ó 2015 Published by Elsevier Inc. Introduction cd T cells represent a third lineage of lymphocytes expressingre-arranged antigen receptors that co-evolved with 'classical' B cells and ab T cells over 400 million years, arguing for a crucial and non-redundant contribution of each lymphocyte to effective immune responses, and hence the evolutionary survival of all jawed vertebrate species [1]. 30 years have passed since the accidental and unexpected cloning of the cd T cell receptor (TCR) and the subsequent realization that ab T cells and cd T cells are fundamentally different with respect to the types of antigens they recognize and the distinct effector functions triggered upon such recognition. However, the manifold contributions of cd T cells to homeostasis, inflammation, infection and tumor surveillance have historically been neglected and are only beginning to be integrated into comprehensive models of the immune system [1][2][3][4][5][6][7].1. Innate-like pattern recognition by human Vc9/Vd2 T cells Like other 'unconventional' T cells carrying an ab TCR such as mucosal-associated invariant T (MAIT) cells and natural killer T (NKT) cells, cd T cells are characterized by a markedly restrictedTCR usage that allows them to recognize self and non-self molecules in the absence of classical antigen presentation via MHC I or MHC II. Vc9/Vd2 + cd T cells represent the major cd T cell subset in human peripheral blood where they typically comprise 1-5% in healthy adults [8]. In many microbial infections, Vc9/Vd2 T cells increase locally and/or systemically [9,10] and can reach in excess of 50% of all peripheral T cells within a matter of days [11], revealing a fundamental role of this unconventional T cell population in acute disease and suggesting their exploitation for diagnostic and therapeutic purposes [12][13][14]. Vc9/Vd2 T cells uniformly respond to a class of low molecular weight molecules often referred to as 'phosphoantigens' that represent metabolites of the isoprenoid biosynthesis...

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