Toll-like receptors (TLRs) are an ancient class of proteins used in antimicrobial defense (see the review by O'Neill). Mammalian TLRs can respond to microbial products by activating immune response genes, including those for cytokines and costimulatory molecules. Unlike Toll receptors from insects, however, a direct anti-microbial function has not been demonstrated for mammalian TLR. Thoma-Uszynski et al. observed that macrophages from humans and mice could directly kill the intracellular bacterium Mycobacterium tuberculosis after stimulation through TLR-2. However, whereas antimicrobial activity in mice was achieved by induction of nitric oxide (NO), killing by human macrophages was mediated by a mechanism independent of NO.
L. A. J. O'Neill, The interleukin-1 receptor/toll-like receptor superfamily: Signal transduction during inflammation and host defense. Science's STKE (2000), http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2000/44/re1. [Abstract] [Full Text]
S. Thoma-Uszynski, S. Stenger, O. Takeuchi, M. T. Ochoa, M. Engele, P. A. Sieling, P. F. Barnes, M. Röllinghoff, P. L. Bölcskei, M. Wagner, S. Akira, M. V. Norgard, J. T. Belisle, P. J. Godowski, B. R. Bloom, R. L. Modlin, Induction of direct antimicrobial activity through mammalian Toll-like receptors. Science 291, 1544-1547 (2001). [Abstract] [Full Text]