Interestingly, HO-1 expression can modulate monocyte function by regulating the production

of pro-inflammatory cytokines.32 Accordingly, during acute inflammatory states there is an increase in HO-1 expression on monocytes, leading to an anti-inflammatory FDA-approved Drug Library response.32 It is likely that a reduction in HO-1 expression in monocytes from patients with SLE compared with healthy controls could trigger an aberrant function in this population, contributing to the inflammation occurring in this disease. Consistent with this notion is the observation that monocytes from patients with SLE are less responsive to the immunosuppressive effect of IL-10 in the presence of immune complexes.44 As the mechanisms involved in the IL-10 response by monocytes depend on HO-1 activity,46 our results could in part explain why monocytes from patients with SLE are resistant to IL-10. Further research is necessary to conclusively address this question. In spite of the differences in HO-1 expression found in monocytes, we could not find differences in HO-1 levels from monocytes-derived

DCs of patients with SLE compared with healthy MG-132 in vivo controls. Because DCs were generated after 5 days of differentiation with GM-CSF and IL-4, it is possible that during this time, normal HO-1 levels could be re-established on these cells. One possible explanation for the reduced expression of HO-1 found in patients Interleukin-2 receptor with SLE could be the presence of high circulating levels of type 1 interferon (IFN) and IFN-γ in the blood of patients with SLE.47,48 There is evidence suggesting that HO-1 expression could be repressed by IFN-γ.49 Although no evidence

suggests a similar effect of IFN-α on HO-1 expression, we could speculate that after 5 days of culture in media without these cytokines, HO-1 expression could be restored in DCs from patients with SLE. Further experiments would be needed to test this possibility. Monocytes from patients with SLE have been shown to be impaired at clearing apoptotic cells.50 Reduced clearance of apoptotic cells might represent an important source of autoantigens with the potential of promoting the autoimmune process associated with SLE.51 In addition, a defective clearance of immune complexes could lead to their deposition in different organs triggering tissue damage.52 Remarkably, it has been demonstrated that increased HO-1 expression in circulating inflammatory cells enhances their phagocytic capacity.53 We can therefore speculate that the defect in the clearance of apoptotic cells by monocytes from patients with SLE could be in part explained by the reduced levels of HO-1, which could contribute to the initiation and maintenance of an immune response against autoantigens. Several studies support the notion that HO-1 expression can be controlled at a transcriptional level.