A number of thiol-disulfide oxidoreductases are known to be secreted and to act on the cell surface. One of these redox catalysts is protein disulfide isomerase (PDI), a member of the thioredoxin (Trx) superfamily. Cell surface-PDI has been found to act on transmembrane and surface-associated proteins, including the envelope protein of HIV-1, to cause its fusogenic conformation (Markovic et al, 2004) and integrins, to mediate platelet adhesion (Lahav et al, 2003) . Another thiol-disulfide oxidoreductase associated with extracellular functions is Trx1. Best known for its intracellular roles, Trx1 reduces transiently formed disulfide bonds of cytosolic and nuclear target proteins and thereby participates in a multitude of fundamental processes, ranging from oxidant scavenging and DNA synthesis to regulation of apoptosis and cell proliferation (Powis and Montfort, 2001) . In addition, Trx1 is released to the extracellular environment by a variety of normal and neoplastic cells (Rubartelli et al, 1992) . Human Trx1 was first purified as a cytokine-like factor from supernatants of virally transformed lymphocytes and initially named adult T-cell leukemia-derived factor (Tagaya et al, 1988) , Tac-inducing factor (Tagaya et al, 1989) , B-cell stimulatory factor or 'B cell IL-1' (Wakasugi et al, 1990) . Extracellular Trx1 is present in the circulation of healthy subjects and its levels increase under inflammatory conditions, including viral infection (Nakamura et al, 2001a) . Circulatory Trx1 acts as a chemoattractant for monocytes, neutrophils and lymphocytes (Bertini et al, 1999) , and inhibits neutrophil migration into inflammatory sites both in vitro and in vivo (Nakamura et al, 2001b) . More recently, Trx1 was found to be secreted by dendritic cells upon cognate T-cell recognition and to contribute to subsequent T-cell activation (Angelini et al, 2002) .