title: Alveolar macrophages as accessory cells for human gamma delta T cells activated by Mycobacterium tuberculosis.
abstract: Alveolar macrophages form the first line of defense against inhaled droplets containing Mycobacterium tuberculosis by controlling mycobacterial growth and regulating T cell responses. CD4+ and gamma delta T cells, two major T cell subsets activated by M. tuberculosis, require accessory cells for activation. However, the ability of alveolar macrophages to function as accessory cells for T cell activation remains controversial. We sought to determine the ability of alveolar macrophages to serve as accessory cells for resting (HLA-DR-, IL-2R-) and activated (HLA-DR+, IL-2R+) gamma delta T cells in response to M. tuberculosis and its Ag, and to compare accessory cell function for gamma delta T cells of alveolar macrophages and blood monocytes obtained from the same donor. Alveolar macrophages were found to serve as accessory cells for both resting and activated gamma delta T cells in response to M. tuberculosis Ag. At high alveolar macrophage to T cell ratios (> 3:1), however, expansion of resting gamma delta T cells was inhibited by alveolar macrophages. The inhibition of resting gamma delta T cells by alveolar macrophages was dose-dependent, required their presence during the first 24 h, and was partially overcome by IL-2. Alveolar macrophages did not inhibit activated gamma delta T cells even at high accessory cell to T cell ratios, and alveolar macrophages functioned as well as monocytes as accessory cells. Monocytes were not inhibitory for either resting or activated gamma delta T cells. These findings support the following model. In the normal alveolus the alveolar macrophage to T cell ratio is > or = 9:1, and therefore the threshold for resting gamma delta T cell activation is likely to be high. Once a nonspecific inflammatory response occurs, such as after invasion by M. tuberculosis, this ratio is altered, favoring gamma delta T cell activation by alveolar macrophages.
authors.affiliation: Department of Medicine, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA.
authors.forename: K N
authors.initials: KN
authors.lastname: Balaji
authors.forename: S K
authors.initials: SK
authors.lastname: Schwander
authors.forename: E A
authors.initials: EA
authors.lastname: Rich
authors.forename: W H
authors.initials: WH
authors.lastname: Boom
mesh_terms: D000938:Antigen-Presenting Cells / Q000502:physiology*; D005434:Flow Cytometry; D006801:Humans; D008213:Lymphocyte Activation / Q000276:immunology; D016676:Macrophages, Alveolar / Q000276:immunology*; D009000:Monocytes / Q000276:immunology; D009169:Mycobacterium tuberculosis / Q000276:immunology*; D016692:Receptors, Antigen, T-Cell, gamma-delta / Q000276:immunology*; D016176:T-Lymphocyte Subsets / Q000276:immunology*
chemical_list: D016692:Receptors, Antigen, T-Cell, gamma-delta