Modulation of in vitro proliferation and cytokine secretion of human lymphocytes by Mentha longifolia extracts

Mentha longifolia L. Hudson has been used in folk medicine for various purposes especially for its anti-inflammatory effects. Lymphocytes play a central role in development of inflammation. In the present study, we investigated the immunomodulatory effects of different extracts of M. longifolia on human peripheral blood lymphocytes (PBLs), as main players in development of inflammation. PBLs stimulated with phytohemagglutinin (PHA) were cultured in the presence of the plant extracts. The effects of the extracts on activation of cells were determined by BrdU assay. The viability of cells was examined by flow cytometry using propidium iodide staining. Also, IFN-γ (T helper 1, TH1) and IL-4 (TH2) secretion was measured by ELISA. Except for the water extract which had a weak inhibitory effect, treatment of cells with more than 1μg/ml of butanol, hexane, ethyl acetate and dichloromethane extracts resulted in strong inhibition of cells proliferation (IC50 4.6-9.9 µg/ml). Flow cytometry analysis showed that these extracts at ≤10μg/ml were non-cytotoxic. Dichloromethane and ethyl acetate extracts at 10 μg/ml decreased IFN-γ production in a dose-dependent manner from 919±91.1 pg/ml in PHA-only-treated cells to 568±22.6 pg/ml (in dichloromethane-treated cells) and 329±12.3 pg/ml (in ethyl acetate-treated cells) (p<0.001). At 10 μg/ml, the ethyl acetate extract increased IL-4 secretion compared to PHA-only-treated cells (p<0.05). The hexane extract decreased IFN-γ level but did not affectIL-4 production. Reduction of IFN-γ and augmentation of IL-4 secretion induced by the extracts suggested the potential of M. longifolia to inhibit TH1 inflammatory responses toward a TH2 dominant response.