The nuclear protein HMGB1 is secreted by monocytes via a non-classical, vesicle-mediated secretory pathway.

HMGB1, a non-histone nuclear factor, acts extracellularly as a mediator of delayed endotoxin lethality, which raises the question of how a nuclear protein can reach the extracellular space. We show that activation of monocytes results in the redistribution of HMGB1 from the nucleus to cytoplasmic organelles, which display ultrastructural features of endolysosomes. HMGB1 secretion is induced by stimuli triggering lysosome exocytosis. The early mediator of inflammation interleukin (IL)-1beta is also secreted by monocytes through a non-classical pathway involving exocytosis of secretory lysosomes. However, in keeping with their respective role of early and late inflammatory factors, IL-1beta and HMGB1 respond at different times to different stimuli: IL-1beta secretion is induced earlier by ATP, autocrinally released by monocytes soon after activation; HMGB1 secretion is triggered by lysophosphatidylcholine, generated later in the inflammation site. Thus, in monocytes, non-classical secretion can occur through vescicle compartments that are at least partially distinct.

Antigen-presenting dendritic cells provide the reducing extracellular microenvironment required for T lymphocyte activation.

T lymphocytes are defective in cystine uptake and thus require exogenous thiols for activation and function. Here we show that monocyte-derived human dendritic cells (DCs) release cysteine in the extracellular space. Cysteine generation is increased by lipopolysaccharide and tumor necrosis factor alpha, and by contact with T cells specifically recognizing soluble or alloantigens. These stimuli also induce thioredoxin (TRX) accumulation in DCs. However, only the contact with antigen-specific T cells triggers TRX secretion by the antigen-presenting cells. Fewer extracellular thiols are recovered after DC-T cell interactions when cystine uptake or TRX activity are inhibited. In addition, glutamate (Glu) and anti-TRX-inactivating antibodies inhibit antigen-dependent T lymphocyte proliferation. These findings indicate that, during antigen presentation, DCs uptake cystine and release cysteine and TRX, thus providing a reducing microenvironment that facilitates immune response.