Cell surface expression of an endoplasmic reticulum resident heat shock protein gp96 triggers MyD88-dependent systemic autoimmune diseases.

Heat shock proteins have been implicated as endogenous activators for dendritic cells (DCs). Without tissue distress or death, these intracellular molecules are inaccessible to surface receptor(s) on DCs, possibly to avoid uncontrolled DC activation and breakdown of immunologic tolerance. We herein addressed this hypothesis in transgenic mice by enforcing cell surface expression of gp96, a ubiquitous heat shock protein of the endoplasmic reticulum. Although a pan-specific promoter is used for transgene expression, neither the expression level nor the tissue distribution of the endogenous gp96 was altered by this maneuver. However, cell surface gp96 induced significant DC activations and spontaneous lupus-like autoimmune diseases, even though the development/functions of lymphocytic compartments were unaltered. Using a bone marrow chimera approach, we further demonstrated that both DC activation and autoimmunity elicited by cell surface gp96 are dependent on the downstream adaptor protein MyD88 for signaling by Toll/IL-1 receptor family. Our study not only established the proinflammatory property of cell surface gp96 in vivo, but also suggested a chronic stimulation of DCs by gp96 as a pathway to initiate spontaneous autoimmune diseases.