David M. Underhill

The Toll-like receptor 2 is recruited to macrophage phagosomes and discriminates between pathogens

Macrophages orchestrate innate immunity by phagocytosing pathogens and coordinating inflammatory responses. Effective defence requires the host to discriminate between different pathogens. The specificity of innate immune recognition in Drosophila is mediated by the Toll family of receptors; Toll mediates anti-fungal responses, whereas 18-wheeler mediates anti-bacterial defence. A large number of Toll homologues have been identified in mammals, and Toll-like receptor 4 is critical in responses to Gram-negative bacteria. Here we show that Toll-like receptor 2 is recruited specifically to macrophage phagosomes containing yeast, and that a point mutation in the receptor abrogates inflammatory responses to yeast and Gram-positive bacteria, but not to Gram-negative bacteria. Thus, during the phagocytosis of pathogens, two classes of innate immune receptors cooperate to mediate host defence: phagocytic receptors, such as the mannose receptor, signal particle internalization, and the Toll-like receptors sample the contents of the vacuole and trigger an inflammatory response appropriate to defence against the specific organism.

Leptospiral lipopolysaccharide activates cells through a TLR2-dependent mechanism

Leptospira interrogans are zoonotic pathogens that have been linked to a recent increased incidence of morbidity and mortality in highly populated tropical urban centers. They are unique among invasive spirochetes in that they contain outer membrane lipopolysaccharide (LPS) as well as lipoproteins. Here we show that both these leptospiral outer membrane constituents activate macrophages through CD14 and the Toll-like receptor 2 (TLR2). Conversely, it seems that TLR4, a central component for recognition of Gram-negative LPS, is not involved in cellular responses to L. interrogans. We also show that for intact L. interrogans, it is LPS, not lipoprotein, that constitutes the predominant signaling component for macrophages through a TLR2 pathway. These data provide a basis for understanding the innate immune response caused by leptospirosis and demonstrate a new ligand specificity for TLR2.

Amphiphysin IIm, a Novel Amphiphysin II Isoform, Is Required for Macrophage Phagocytosis

Phagocytosis of pathogens by macrophages initiates the innate immune response, which in turn orchestrates the adaptive immune response. Amphiphysin II participates in receptor-mediated endocytosis, in part, by recruiting the GTPase dynamin to the nascent endosome. We demonstrate here that a novel isoform of amphiphysin II associates with early phagosomes in macrophages. We have ablated the dynamin-binding site of this protein and shown that this mutant form of amphiphysin II inhibits phagocytosis at the stage of membrane extension around the bound particles. We define a signaling cascade in which PI3K is required to recruit amphiphysin II to the phagosome, and amphiphysin II in turn recruits dynamin. Thus, amphiphysin II facilitates a critical initial step in host response to infection.

Heterogeneity in macrophage phagocytosis

Publisher Summary This chapter discusses the heterogeneity in macrophage phagocytosis. It is important to recognize that the formation of the phagosome can be a heterogenous process even when a single cell ingests two identical particles. While all phagocytosis involves actin remodeling around the phagocytic cup, it has been observed that certain cytoskeletal proteins that decorate a particular phagosome are absent from otherwise identical phagosomes in the same cell. These differences are not temporal, but are likely to arise from stochastic, or even chaotic, processes. It is clear that the molecular dissection of phagocytosis represents a daunting task; more than 100 actin-binding proteins have been identified, and many of these are expressed in the same cell. Two common approaches that have been used to establish the role of specific gene products in a particular biological phenomenon have been to express dominant negative forms of the protein, or to delete the gene encoding the protein. However, the interpretation of the results is complicated by the observation that these two approaches do not always yield the same phenotype. Ultimately, phagocytosis, like most other problems in biology, will have to be analyzed as a complex system rather than a linear series of isolated enzymatic reactions. In this guise, phagocytosis provides a window into the coordinate functioning of the actin and tubulin based cytoskeletons, and could serve as a model system for analyzing diverse biological phenomena including synaptic transmission, mitogenesis, and morphogenesis.