Interleukin 1 receptor antagonist. A new member of the interleukin 1 family.

This review has summarized recent information derived from many laboratories on the discovery, characteristics, and properties of a new member of the IL-1 family, IL-1 receptor antagonist. In addition to information, an emphasis has been placed on unanswered questions and new concepts. The existence of this first-described naturally occurring specific cytokine receptor antagonist may lead to a different perspective on the cytokine network. A major unanswered question emphasized throughout this review, that now can be addressed more directly, concerns what are the physiological roles of members of the IL-1 family. Although IL-1 beta is presumed to function primarily as an extracellular cytokine, this molecule lacks a leader peptide, is synthesized and handled by the cells in a manner suggestive of a cytoplasmic (not secretory) protein, and may only be released after cellular injury. Furthermore, although IL-1ra possesses a leader sequence, 50% or more of this protein remains cell associated. Do these observations suggest that members of the IL-1 family possess important intracellular functions, as yet undetermined? IL-1 alpha may play an intracellular role in regulating senescence; an IL-1 alpha antisense oligodeoxynucleotide was shown to prolong the life span of cultured human endothelial cells. Whether intracellular IL-1ra plays a role in influencing life span has not been determined. The discovery of IL-1ra has led to a first level of assumptions that this molecule may be functioning in vivo to regulate the pleiotropic extracellular effects of IL-1 in physiological or pathophysiological processes. Although enticing, these assumptions have not yet been proven to be true. Perhaps we need to look beyond, or within, and consider that IL-1ra and other members of the IL-1 family may have additional roles in normal or abnormal cell growth and development.