Immunosuppressive activity of an aqueous Viola tricolor herbal extract

Heartsease (Viola tricolor L.), a member of the Violaceae family, has a long history as a medicinal plant and has been documented in the Pharmacopoeia of Europe. Due to its anti-inflammatory properties it is regarded as a traditional remedy against skin diseases, for example for the treatment of scabs, itching, ulcers, eczema or psoriasis, and it is also used in the treatment of inflammation of the lungs and chest such as bronchitis or asthma. Because T-cells play an important role in the pathological process of inflammatory diseases we investigated the effect of an aqueous Viola extract on lymphocyte functions and explored the 'active' principle of the extract using bioactivity-guided fractionation.

An aqueous Viola extract was prepared by C18 solid-phase extraction. Effects on proliferation of activated lymphocytes (using the cell membrane permeable fluorescein dye CFSE), apoptosis and necrosis (using annexin V and propidium iodide staining), interleukin-2 (IL-2) receptor expression (using fluorochrome-conjugated antibodies) and IL-2 cytokine secretion (using an ELISA-based bead array system) were measured by flow cytometry. Influence on lymphocyte polyfunctionality was characterized by Viola extract-induced production of IFN-γ and TNF-α, as well as its influence on lymphocyte degranulation activity. Fractionation and phytochemical analysis of the extract were performed by RP-HPLC and mass spectrometry.

The aqueous Viola extract inhibited proliferation of activated lymphocytes by reducing IL-2 cytokine secretion without affecting IL-2 receptor expression. Similarly, effector functions were affected as indicated by the reduction of IFN-γ and TNF-α production; degranulation capacity of activated lymphocytes remained unaffected. Bioassay-guided fractionation and phytochemical analysis of the extract led to identification of circular plant peptides, so called cyclotides, as bioactive components.

An aqueous Viola extract contains bioactive cyclotides, which inhibit proliferation of activated lymphocytes in an IL-2 dependent manner. The findings provide a rationale for use of herbal Viola preparations in the therapy of disorders related to an overactive immune system. However, further studies to evaluate its clinical potency and potential risks have to be performed.