Structural and biological features of the TNF receptor and TNF ligand superfamilies: Interactive signals in the pathobiology of Hodgkin's disease

Abstract

Members of the TNF receptor superfamily are type I membrane glycoproteins with limited homology (overall homologies: 25%-30%) in the extracellular domain containing variable numbers of cysteine-rich repeats. In contrast, the TNF ligand superfamily members (with the exception of LT-alpha) are type II membrane glycoproteins with limited homology to TNF (overall homologies: 20%) in the extracellular region. 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 suggests a similar trimeric or multimeric complex formation for the other members. A genetic linkage, as evidence for evolutionary relatedness, is also found by chromosomal cluster for CD30, CD120b, 4-1BB and OX40 to 1p36; CD27, CD120a and TNFR-RP to 12p13; TNF, LT-alpha and LT-beta to 6p21; CD27L and 4-1BBL to 19p13; CD95L and OX40L to 1q25. TNF, LT-alpha and LT-beta and their receptors (CD120a, CD120b, TNFR-RP) interact in a complex fashion. Other family members, however, show a one ligand/one receptor binding principle. Signals can also be transduced through at least some of the ligands. TNF superfamily ligands are involved in induction of cytokine secretion, upregulation of adhesion molecules, activation antigens and costimulatory proteins, all known to amplify stimulatory and regulatory signals that occur during immune responses. On the other hand, differences in the distribution, kinetics of induction and requirements for induction support the view of a defined role for each of the ligands for T-cell-mediated immune activities. 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 is another common functional feature of this cytokine family (TNF, LT-alpha, CD30L, CD95L and 4-1BBL). Further studies have to identify unique versus redundant biological and physiological functions for each of the TNF superfamily ligands. In addition to other cytokines primary H-RS cell frequently express at least TNF, LT-alpha, CD27L and CD30L, but not CD40L. Furthermore, H-RS cells express several TNF receptors, such as CD30, CD40, CD95, CD120a, CD120b and 4-1BB. The TNF-like ligands might support growth and activation of HD-associated tumor cells and/or interact with surrounding reactive bystander cells, particularly T-cells. The different interactions between H-RS cells and surrounding reactive bystander cells are part of the pathobiology of HD. Detailed functional analysis have to confirm the predicted biological activities of TNF, LT-alpha, CD27L, CD30L, CD40L, CD95L, 4-1BBL and gp34/OX40L for the H-RS cell/T-cell interactions with impact on tumor growth and pathogenesis of HD. TNF and LT-alpha/CD120a and CD120b, CD30/CD30L, and CD40/CD40L are clearly critical elements in the deregulated network of interactive signals between H-RS cells and surrounding bystander cells with membrane-associated and cytokine-mediated events. Several TNFR superfamily members are also candidates for novel treatment protocols, including CD30 and CD40.