Abbreviations: Alexa-MIF (Alexa-488-modified MIF); ERK-1/2 (extracellular signal-related kinase); KO (gene knock-out); LPS (lipopolysaccharide); MAPK (mitogen-activated protein kinase); MIF (macrophage migration inhibitory factor); RIP (regulated intramembrane proteolysis); sCD74 (soluble CD74 ectodomain); TLR-4 (Toll-like receptor 4).
IntroductionThe protein mediator known as macrophage migration inhibitory factor (MIF) is considered to be one of the first cytokines to be discovered. In the late 1950s, "MIF" described an activity elaborated by activated lymphocytes that arrested the random movement of monocytes/macrophages [1]. This activity correlates with delayed-type hypersensitivity in vitro, and it induced significant interest among immunologists for representing a soluble, but non-immunoglobulin, factor that could be studied and manipulated in vitro. By 1966, John David and Barry Bloom independently described that MIF was most likely a protein that was secreted into the conditioned medium of activated lymphocytes [2,3], and immunologists soon measured MIF's eponymous activity by a capillary tube assay. However, MIF was difficult to isolate or clone, so as various other lymphokines (T cell growth factor/IL-2), monokines (endogenous pyrogen/IL-1; tumor necrosis factor/lymphotoxin), and interferons were systematically cloned and produced recombinantly and additional proteins, such as interferon-g and IL-4, were also observed to exhibit migration inhibitory activity, immunologic interest in MIF waned. It wasn't until 1989 that a discrete gene sequence was determined to be associated with MIF [4] and in 1993, pure recombinant protein became available for biologic and structure-function studies [5]. The threedimensional crystal structure of MIF was solved by three laboratories in 1996, thus revealing a new protein fold and structural superfamily with MIF as its defining member [6][7][8]. These studies also provided insights into the likely native form of the protein -a homotrimer with a MW of 37.5 kDa (Figure 1). Figure 1 Three-dimensional ribbon diagram of human MIF, revealing its homotrimeric subunit structure. From ref [7]. Each color denotes one monomer.
Human MIF homotrimerCell Research (2006) The recent cloning of MIF receptor fills an important gap in our understanding of the molecular biology and immunology of MIF. The MIF receptor, like MIF, does not fall into any established family of protein mediators, providing both new challenges and opportunities for the structural and functional analysis of MIF signal transduction.