Accessory signals for growth and differentiation of human T lymphocytes

Accessory signals represent the level at which functional immune responses are regulated. Impairment of ligand-receptor interactions or, alternatively, intracellular signalling processes plays a pivotal part in vivo. Thus, tumour tolerance might result from missing co-stimulatory signals, which is supported by numerous recent data. Conversely, blocking accessory receptors might provide the means to influence allograft rejection. Upregulation of accessory receptors and their ligands is a hallmark of inflammatory reactions. Finally, given that particular microenvironments are characterized by the presence of particular combinations of ligand molecules for critical accessory receptors on T lymphocytes, one could envisage novel approaches to local immune intervention. All these perspectives, however, are based on our understanding of how accessory receptors are composed and interact with intracellular enzymes, which all together transmit critical signals to the nucleus, and on an understanding of how these signals contribute to functional T cell behaviour.