Host-derived CD8⁺ dendritic cells protect against acute graft-versus-host disease after experimental allogeneic bone marrow transplantation.

Graft-versus-host disease (GVHD) is a frequent life-threatening complication after allogeneic hematopoietic stem cell transplantation (HSCT) and induced by donor-derived T cells that become activated by host antigen-presenting cells. To address the relevance of host dendritic cell (DC) populations in this disease, we used mouse strains deficient in CD11c(+) or CD8α(+) DC populations in a model of acute GVHD where bone marrow and T cells from BALB/c donors were transplanted into C57BL/6 hosts. Surprisingly, a strong increase in GVHD-related mortality was observed in the absence of CD11c(+) cells. Likewise, Batf3-deficient (Batf3(-/-)) mice that lack CD8α(+) DCs also displayed a strongly increased GVHD-related mortality. In the absence of CD8α(+) DCs, we detected an increased activation of the remaining DC populations after HSCT, leading to an enhanced priming of allogeneic T cells. Importantly, this was associated with reduced numbers of regulatory T cells and transforming growth factor-ß levels, indicating an aggravated failure of peripheral tolerance mechanisms after HSCT in the absence of CD8α(+) DCs. In summary, our results indicate a critical role of CD8α(+) DCs as important inducers of regulatory T cell-mediated tolerance to control DC activation and T cell priming in the initiation phase of GVHD.

Donor and host B cell-derived IL-10 contributes to suppression of graft-versus-host disease.

Graft-versus-host disease (GvHD) is a frequent life-threatening complication following allogeneic HSC transplantation (HSCT). IL-10 is a regulatory cytokine with important roles during GvHD, yet its relevant sources, and mode of action, remain incompletely defined in this disease. Using IL-10-deficient donor or host mice (BALB/c or C57BL/6, respectively) in a MHC-mismatched model for acute GvHD, we found a strongly aggravated course of the disease with increased mortality when either donor or host cells could not produce this cytokine. A lack of IL-10 resulted in increased allogeneic T-cell responses and enhanced activation of host DCs in spleen and MLNs. Remarkably, IL-10 was prominently produced by host- and donor-derived CD5(int) CD1d(int) TIM-1(int) B cells in this disease, and consistent with this, allogeneic HSCT resulted in exacerbated GvHD when mice lacking IL-10 expression in B cells were used as donor or host, compared with controls. Taken together, this study demonstrates that host and donor B cell-derived IL-10 provides a unique mechanism of suppression of acute GvHD, and suggests that DCs are the targets of this B cell-mediated suppressive effect. These findings open novel therapeutic possibilities based on the use of B cells to increase the feasibility of allogeneic HSCT.

Increased frequencies of CD11b(+) CD33(+) CD14(+) HLA-DR(low) myeloid-derived suppressor cells are an early event in melanoma patients.

Myeloid-derived suppressor cells (MDSC) are a heterogeneous cell population characterized by immunosuppressive activity. Elevated levels of MDSC in peripheral blood are found in inflammatory diseases as well as in malignant tumors where they are supposed to be major contributors to mechanisms of tumor-associated tolerance. We investigated the frequency and function of MDSC in peripheral blood of melanoma patients and observed an accumulation of CD11b(+) CD33(+) CD14(+) HLA-DR(low) MDSC in all stages of disease (I-IV), including early stage I patients. Disease progression and enhanced tumor burden did not result in a further increase in frequencies or change in phenotype of MDSC. By investigation of specific MDSC-associated cytokines in patients' sera, we found an accumulation of IL-8 in all stages of disease. T-cell proliferation assays revealed that MDSC critically contribute to suppressed antigen-specific T-cell reactivity and thus might explain the frequently observed transient effects of immunotherapeutic strategies in melanoma patients.

Blue light irradiation suppresses dendritic cells activation in vitro.

Blue light is a UV-free irradiation suitable for treating chronic skin inflammation, for example, atopic dermatitis, psoriasis, and hand- and foot eczema. However, a better understanding of the mode of action is still missing. For this reason, we investigated whether dendritic cells (DC) are directly affected by blue light irradiation in vitro. Here, we report that irradiation neither induced apoptosis nor maturation of monocyte-derived and myeloid DC. However, subsequent DC maturation upon LPS/IFNγ stimulation was impaired in a dose-dependent manner as assessed by maturation markers and cytokine release. Moreover, the potential of this DC to induce cytokine secretion from allogeneic CD4 T cells was reduced. In conclusion, unlike UV irradiation, blue light irradiation at high and low doses only resulted in impaired DC maturation upon activation and a reduced subsequent stimulatory capacity in allogeneic MLRs with strongest effects at higher doses.

The N-terminal region of acyl-CoA synthetase 3 is essential for both the localization on lipid droplets and the function in fatty acid uptake.

Cytosolic lipid droplets (LDs) are storage organelles for neutral lipids derived from endogenous metabolism. Acyl-CoA synthetase family proteins are essential enzymes in this biosynthetic pathway, contributing activated fatty acids. Fluorescence microscopy showed that ACSL3 is localized to the endoplasmic reticulum (ER) and LDs, with the distribution dependent on the cell type and the supply of fatty acids. The N-terminus of ACSL3 was necessary and sufficient for targeting reporter proteins correctly, as demonstrated by subcellular fractionation and confocal microscopy. The N-terminal region of ACSL3 was also found to be functionally required for the enzyme activity. Selective permeabilization and in silico analysis suggest that ACSL3 assumes a hairpin membrane topology, with the N-terminal hydrophobic amino acids forming an amphipathic helix restricted to the cytosolic leaflet of the ER membrane. ACSL3 was effectively translocated from the ER to nascent LDs when neutral lipid synthesis was stimulated by the external addition of fatty acids. Cellular fatty acid uptake was increased by overexpression and reduced by RNA interference of ACSL3. In conclusion, the structural organization of ACSL3 allows the fast and efficient movement from the ER to emerging LDs. ACSL3 not only esterifies fatty acids with CoA but is also involved in the cellular uptake of fatty acids, presumably indirectly by metabolic trapping. The unique localization of the acyl-CoA synthetase ACSL3 on LDs suggests a function in the local synthesis of lipids.