The TNF receptor superfamily members are all type I membrane glycoproteins with typical homology in the extracellular domain of variable numbers of cysteine-rich repeats (overall homologies, 25% to 30%). In contrast, the TNF ligand superfamily members (with the exception of LT alpha) are type II membrane glycoproteins with homology to TNF in the extracellular domain (overall homologies, 20%). TNF and LT alpha are trimeric proteins and are composed of beta-strands forming a beta-jellyroll. The homology of the beta-strand regions for the TNF ligand superfamily members suggest a similar beta-sandwich structure and possible trimeric or multimeric complex formation for most or all members. A genetic linkage, as evidence for evolutionary relatedness, is found by chromosomal cluster of TNFR p80, CD30, 4–1BB, and OX40 for 1p36; TNFR p60, TNFR-RP, and CD27 for 12p13; TNF, LT alpha, and LT beta for 6 (MHC locus); CD27L and 4–1BBL for 19p13; and FASL and OX40L for 1q25. Of the TNF ligand superfamily, TNF, LT alpha, and LT beta and their receptors (TNFR p60, TNFR p80, and TNFR-RP) interact in a complex fashion of cross-binding. However, the other family members presently have a one ligand/one receptor binding principle (CD27/CD27L, CD30/CD30L, CD40/CD40L, 4–1BB/4–1BBL, OX40/gp34, and FAS/FASL). In general, the members of the TNF ligand superfamily mediate interaction between different hematopoietic cells, such as T cell/B cell, T cell/monocyte, and T cell/T cell. Signals can be transduced not only through the receptors but also through at least some of the ligands. The transduced signals can be stimulatory or inhibitory depending on the target cell or the activation state. Taken together, TNF superfamily ligands show for the immune response an involvement in the induction of cytokine secretion and the upregulation of adhesion molecules, activation antigens, and costimulatory proteins, all known to amplify stimulatory and regulatory signals. On the other hand, differences in the distribution, kinetics of induction, and requirements for induction support a defined role for each of the ligands for T-cell-mediated immune responses. The shedding of members of the TNF receptor superfamily could limit the signals mediated by the corresponding ligands as a functional regulatory mechanism. Induction of cytotoxic cell death, observed for TNF, LT alpha, CD30L, CD95L, and 4–1BBL, is another common functional feature of this cytokine family. Further studies have to identify unique versus redundant biologic and physiologic functions for each of the TNF superfamily ligands.(ABSTRACT TRUNCATED AT 400 WORDS)