Echinacea purpurea extracts modulate murine dendritic cell fate and function

Abstract

Echinacea is a top-selling herbal remedy that purportedly acts as an immunostimulant. However, the specific immunomodulatory effects of Echinacea remain to be elucidated. We focused on defining the effects of Echinacea purpurea extracts in dendritic cells (DCs), which generate innate and adaptive immune responses. We hypothesized that E. purpurea extracts would enhance murine bone marrow-derived DC (BMDC) activation leading to increased immune responses. The fate and function of DCs from C57Bl/6 mice was evaluated following 48h exposure to E. purpurea root and leaf extracts. Flow cytometry revealed that the polysaccharide-rich root extract increased the expression of MHC class II, CD86, and CD54 surface biomarkers whereas the alkylamide-rich leaf extract inhibited expression of these molecules. Production of IL-6 and TNF-alpha increased in a concentration-dependent manner with exposure to the root, but not leaf, extract. In contrast, the leaf but not root extract inhibited the enzymatic activity of cyclooxygenase-2. While both extracts decreased the uptake of ovalbumin by BMDCs, the leaf but not root extract inhibited the antigen-specific activation of naïve CD4(+) T cells from OT II/Thy1.1 mice. Collectively, these results suggest that E. purpurea can be immunostimulatory, immunosuppressive, and/or anti-inflammatory depending on the portion of the plant and extraction method.

Figure 1

BMDC expression of accessory molecules is differentially affected by E. purpurea leaf and root extracts. Flt3-ligand-derived BMDCs (10⁶) were treated with extracts or the ethanol (0.5%) controls for 48 h at the designated concentrations. The percentage positive (A) and Mean Channel Fluorescence (B) profiles were determined by FACS analysis. Results are shown as mean ± SEM (n=6) and are representative of two separate experiments. (B) Histograms represent BMDCs treated with the ethanol vehicle (black line) or E. purpurea leaf extracts (gray). Dashed lines indicate isotype controls. L4 corresponds to our leaf extract while R1 corresponds to our root extract. *p<0.05 and **p<0.01 compared to control.

Figure 2

BMDCs secrete TNF-α and IL-6 following exposure to root but not leaf extracts. Flt3-ligand-derived BMDCs were treated as described in Fig. 2. Supernatants were collected and analyzed by ELISA. Results are mean ± SEM (n=6) and are representative of two separate experiments. **p<0.01 compared to control.

Figure 3

Phagocytosis of FITC-OVA by BMDCs is decreased following exposure to E. purpurea extracts. BMDCs were treated with ethanol (0.5%) or extracts (150 μg/ml) for 48 h. Samples were washed and FITC-OVA added for 2 h. Cells were then harvested and analyzed by flow cytometry (A) or fluorescence microscopy (B). Results are mean ± SEM (n=4) and are representative of two separate experiments. ***p<0.001 compared to control.

Figure 4

OVA-loaded BMDCs treated with E. purpurea extracts modulate clonal expansion of CD4⁺ OT II T cells. BMDCs were pre-treated with leaf or root extract (150 μg/ml) for 48 h prior to overnight addition of OVA_323-229 peptide. BMDCs were added at various ratios to CFSE-labeled OT II T cells. After 4 days, clonal expansion of OVA-specific T cells was determined by FACS analysis. Results are mean ± SEM (n=4). **p<0.005 or ***p<0.0001 compared to control.