Formation and Aggregation of Lipid Rafts in γδ T Cells Following Stimulation with Mycobacterium tuberculosis Antigens

Lipid rafts are plasma membrane microdomains that are implicated in diverse signaling pathways in immune cells. Based on the distinct types of T-cell receptors, two T-cell subpopulations have been identified: alpha beta and gamma delta T cells. In humans, gamma delta T cells represent a relatively rare T lymphocyte population but play a critical role in the immune response to infection by Mycobacterium tuberculosis. It has been demonstrated that Mycobacterium tuberculosis antigens (Mtb-Ag) preferentially activate gamma delta T cells. Thus, we investigated whether lipid rafts are involved in the Mtb-Ag-mediated activation of gamma delta T cells. Human peripheral blood mononuclear cells (PBMCs) were stimulated with Mtb-Ag, and expression of a lipid raft marker ganglioside GM1 (GM1) was determined by flow cytometry. The aggregation of lipid rafts was evaluated by laser confocal microscopy. Non-stimulated fresh PBMCs minimally expressed GM1 (6.55 +/- 2.01%) and had no aggregated rafts in gamma delta T cells. Mtb-Ag stimulation gradually increased the expression of GM1 in a time-dependent manner. At 72 h, the majority of gamma delta T cells expressed GM1 (88.69 +/- 7.55%). Furthermore, accompanied with the increased expression of GM1, aggregation of lipid rafts became gradually visible in gamma delta T cells. The aggregated rafts, however, were not evenly distributed and only occurred over a small portion of GM1-positive cells. Pretreatment with methyl-beta-cyclodextrin, a cholesterol-depleting reagent, completely inhibited the Mtb-Ag-mediated aggregation of lipid rafts. These results demonstrate that lipid raft aggregation occurs in Mtb-Ag-activated gamma delta T cells, suggesting that lipid rafts are involved in activation of gamma delta T cells.