Before leaving innate sensing, we shall look briefly at how Toll, TLRs, and NODs are used in invertebrate innate immunity. Although Toll is central to defense against both bacterial and fungal pathogens in Drosophila, Toll itself is not a pattern recognition receptor, but is downstream of other proteins that detect pathogens (Fig. 3.24). In Drosophila, there are 13 genes encoding peptidoglycan-recognition proteins (PGRPs) that bind the peptidoglycan components of bacterial cell walls. Another family, the Gram-negative binding proteins (GNBPs), recognizes LPS and β-1,3-linked glucans. GNBPs recognize Gram-negative bacteria and, unexpectedly, fungi, rather than Gram-positive bacteria. The family members GNBP1 and PGRP-SA cooperate in the recognition of peptidoglycan from Gram-positive bacteria. They interact with a serine protease called Grass, which initiates a proteolytic cascade that terminates in the cleavage of the protein Spätzle. One of the cleaved fragments forms a homodimer that binds to Toll and induces its dimerization, which in turn stimulates the antimicrobial response. A fungus-specific recognition protein, GNBP3, also activates the proteolytic cascade, causing cleavage of Spätzle and activation of Toll.
In Drosophila, fat-body cells and hemocytes are phagocytic cells that act as part of the fly’s immune system. When the Spätzle dimer binds to Toll, hemocytes synthesize and secrete antimicrobial peptides. The Toll signaling pathway in Drosophila activates a transcription factor called DIF, which is related to mammalian NFκB. DIF enters the nucleus and induces the transcription of genes for antimicrobial peptides such as drosomycin. Another Drosophila factor in the NFκB family, Relish, induces the production of antimicrobial peptides in response to the Imd (immunodeficiency) signaling pathway, which is triggered in Drosophila by particular PGRPs that recognize Gram-negative bacteria. Relish induces expression of the antimicrobial peptides diptericin, attacin, and cecropin, which are distinct from the peptides induced by Toll signaling. Thus, the Toll and Imd pathways activate effector mechanisms to eliminate infection by different kinds of pathogens. Four mammalian PGRP homologs have been identified, but act differently than in Drosophila. One, PGLYRP-2, is secreted and functions as an amidase to hydrolyze bacterial peptidoglycans. The others are present in neutrophil granules and exert a bacteriostatic action through interactions with bacterial cell-wall peptidoglycan.
Fig. 3.23 Bacterial LPS induces changes in dendritic cells, stimulating them to migrate and to initiate adaptive immunity by activating T cells. Top panel: immature dendritic cells in the skin are highly phagocytic and macropinocytic, but lack the ability to activate T lymphocytes. Dendritic cells residing in the skin ingest microbes and their products and degrade them. During a bacterial infection, the dendritic cells are activated by various innate sensors and the activation induces two types of changes. Second panel: the dendritic cells migrate out of the tissues and enter the lymphatic system and begin to mature. They lose the ability to ingest antigen but gain the ability to stimulate T cells. Third panel: in the regional lymph nodes, they become mature dendritic cells. They change the character of their cell-surface molecules, increasing the number of MHC molecules on their surface and the expression of the co-stimulatory molecules CD80 (B7.1) and CD86 (B7.2).