Circulating heat shock protein (HSP)-27 is associated with tumor progression and increased post-injury infection. Extracellular HSP-27 might alter monocyte (MO)-derived DC and/or MPhi function to mediate immunosuppression. HSP-27 treatment inhibited expression of CD1a and CD1b/c, antigen uptake, and allogeneic T cell induction (MLR) by IL-4 + GM-CSF-differentiated human DC while increasing some MPhi characteristics ( upward arrowCD14, upward arrowCD16, upward arrowCD163). MO cytokine receptor profiles elicited by 24-h exogenous HSP-27 treatment remained supportive of immature DC (iDC) emergence ( upward arrowIL-4R, downward arrowIL-6R, downward arrowM-CSFR). IL-10, IL-6, and M-CSF (which promote MPhi differentiation) were significantly increased in IL-4 + GM-CSF + HSP-27 MO-->iDC differentiation cultures. However, HSP-27 treatment during MO differentiation to DC increased programmed cell death ligand 1 coinhibitor and depressed CD86 costimulator expression in parallel to decreased iDC MLR activity. This suggested that increased MPhi differentiation was not solely responsible for HSP-27 reduction of differentiating DC activity. HSP-27 treatment actually depressed the phagocytic capacity of MO differentiated to MPhi by IL-10 or M-CSF culture. CD163 (hemoglobin receptor) expression was depressed on M-CSF + HSP-27 MO-derived MPhi. HSP-27-mediated inhibition of MO-->iDC differentiation was reversed by p38alpha & beta inhibitor (SB202190) addition or TLR4 receptor modulation. HSP-27 impaired appropriate MO-->iDC and MO-->MPhi differentiation modulating expression of receptors necessary for their proper functions. This suggests that endogenous HSP-27 has immunoregulatory activities which could contribute to immunopathology.