CD34 + fibrocytes have been implicated in development of thyroid-associated ophthalmopathy (TAO), a consequential autoimmune manifestation of Graves' disease (GD). In TAO, CD34 + fibrocytes appear to masquerade as CD34 + orbital fibroblasts mixed with CD34 - OF (collectively, GD-OF). Slit2, an axon guidance glycoprotein, is expressed by CD34 - OF and attenuates GD-OF gene expression. Cardinal features of TAO include hyaluronan (HA) accumulation and cytokine-driven inflammation.Compare expression of HA synthase isoenzymes (HAS1-3), UDP glucose dehydrogenase (UDPGD), synthesis of HA, IL-6 and TNF-α in fibrocytes and GD-OF. Determine whether Slit2 alters gene expression patterns.Patients with TAO and healthy donors were recruited from an academic practice.Real-time PCR, HA, IL-6 and TNF-α immunoassays.HA synthesis and release from fibrocytes is substantially lower than in GD-OF. HAS1 expression dominates in fibrocytes while HAS2 in GD-OF. In contrast, HAS2 and UDPGD expression dominate GD-OF and localize to CD34 - OF. rhSlit2 substantially upregulates HA synthesis and HAS2 expression in fibrocytes but attenuates IL-6 and TNF-α production in these cells. In contrast, knocking-down Slit2 in GD-OF reduces HA synthesis, HAS2 and UDPGD expression while upregulating IL-6 and TNF-α.The dramatic differences in HA, IL-6 and TNF-α production, HAS and UDPGD expression found in fibrocytes and GD-OF appear, at least in part, to be attributable to Slit2. These findings provide novel insight into the differences in gene expression exhibited by CD34 + fibrocytes and CD34 + OF and therefore reveal important aspects of disease pathogenesis.