Courtesy of Luke A. O'Neill
Luke A. O'Neill
The march to demystify mammalian immunity has been long and arduous. At the frontlines we face a dizzying array of biochemicals and interactions between multiple cell types aimed at detecting, eliminating, and remembering intruders. The regulation of this system often appears impenetrable.
But recent advances in our understanding of innate immunity – that hard-wired, first line of defense that doesn't appear to adapt during infection – have served as a signal flare, rallying those discouraged by the system's complexity. As an initiating factor in the immune response, innate immunity offers an inroad to the entire system. Cracking its code may provide the tools necessary to decipher and define interactions throughout the chain of command.
Much of the activity is now focused on Toll-like receptors (TLRs), proteins on leukocytes that hook up with specific microbial products recognized as foreign.1 Once engaged they...
THE CALL TO ARMS
TLRs capture the essence of what an immune system needs to do. Host immunity needs to sense the structure of foreign microbial products and then elicit an appropriate response. Broadly speaking, we now know of several families of such host molecules: Antibodies and T-cell receptors show mind-boggling diversity to recognize microbial peptides; the complement components in biological fluids bind microbial membranes; antimicrobial peptides slot into microbial membranes causing lysis; and now the TLRs are known to detect an array of foreign biomolecules and to trigger various response pathways.
Textbooks still separate these families of host molecules into those belonging to adaptive immunity (antibodies and T-cell receptors) and innate immunity (the rest). Both types of immunity are intimately linked, however, as was first reported almost 100 years ago when Irishman Almroth Wright described opsonization, the process whereby antibodies can coat microbes, enhancing the ability of macrophages, the key cells of innate immunity, to take up the microbes and destroy them.
Ten TLRs are known in humans, including TLR4, which senses the highly toxic lipopolysaccharide, found in the walls of gram-negative bacteria, and TLR3, which senses viral nucleic acids. Their discovery is a testament to serendipity and researchers doggedly following their instinct. It was research into how fruit flies fight infection (where Toll was first described) and the power of the bioinformatics methods that prompted us to search the human genome for Toll homologues.2 Plus, of course, the hunch that TLRs would be conserved across nature as key molecules of innate immunity.
Because the components they recognize are common to different classes of pathogens, TLRs allow your immune system to sense all bacteria, viruses, fungi, and probably parasites that infect you. Once they bind the particular microbial product, they activate signals inside cells that lead to enhanced expression of virtually all genes implicated in immunity and inflammation.
They achieve this via a set of cytosolic adapter proteins that presumably act as on/off switches. When flipped by the TLRs, the adapters launch the signals that lead to enhanced gene expression. TLRs use the four adapters, MyD88, Mal, Trif, and Tram in different combinations. This, in turn, drives signaling pathways in a direction specific to the threat being sensed.
TLR3, for example, boosts the expression of virus-busting interferons; Trif is needed for this response. Such subtlety was somewhat unexpected for the innate response, which was previously thought of as somewhat crude, a mere foot soldier in immune response.
We can conclude that TLRs, by regulating gene expression, have the capacity to launch all effector systems of the immune response. The genes they induce encode antibodies, MHC (major histocompatibility complex) molecules needed to present antigen from microbes, cytokines that activate all classes of immune cells, and chemokines that recruit immune cells to the infected site. Thus a relatively simple and limited set of proteins which interpret, at first glance, the apparently limitless complexity of the microbial world, leading to an equally complex array of immune response mechanisms.
GRUNTS NO MORE
© 2004 AAAS
TLRs sense microbial products and engage four adapter molecules. Signaling pathways are then launched, which culminate in enhanced expression of immune and inflammatory genes, whose products trigger the range of complex effector mechanisms required to eliminate the provoking pathogen. (Reprinted with permission from L. O'Neill, Science, 303:1481–2, 2004.)
TLRs were first described in 1998 and since then, over 2,000 papers have been published on the subject. Emphasizing their importance, mice deficient in certain TLRs or adapter molecules mount puny immune responses to certain infections.
Thus the adaptive immune response, historically considered the more sophisticated, has revealed an intimate reliance on innate immunity's grunt molecules, best represented by the TLRs.
Other evidence can be seen in the growing list of mechanisms used by pathogens to dodge or manipulate TLRs for their own ends. The 300-gene Vaccinia genome codes at least two proteins, A46R and A52R, that disable TLR signaling pathways and are used by the virus to limit TLR action.4 The impact of malfunctioning TLRs in autoimmune disease is also being revealed. For example, in a substantial subset of patients with Crohn disease it has been shown that TLR2 is hyperactive because of a mutation in a protein called NOD2, which normally keeps TLR2 in check.5
TLRs may be important drivers of inflammation in diseases such as rheumatoid arthritis, where a root cause is still lacking. And there is evidence that they may also detect the products of damaged tissues, although how this would lead to the propagation of a chronic inflammatory disease is not clear. Nevertheless, these examples suggest possible therapeutic strategies.
Drugs that target TLRs are already on the market. Imiquimod, a treatment for genital warts, acts by stimulating TLR7, a key antiviral TLR that drives interferons. Several other companies are pursuing their own TLR activators for use in boosting vaccines or promoting antitumor immunity.
There are additional components of the sensor network too. One is the complement system. Another is made up of nucleotide oligomerization domain (NOD) proteins, another emerging family, which also bind microbial products in the cytosol of cells3 and trigger similar pathways as TLRs. More are likely to emerge given the upsurge of interest in innate sensing.
We have worked to profile many of the key players in immunity, but now with the emerging picture of the agents responsible for immune initiation, we can draw a much better battle plan for deciphering the entire army. This might make immunity a fully tractable system. Enhanced detail will allow us to precisely determine the requirements for effective host defense and also to identify aberrations in the proteins and genes that lead to disease. This will give rise to new treatments for infectious and inflammatory diseases and also cancer.
The discovery of TLRs has lifted the fog of war, and so we might claim to have reached the end of the beginning in our understanding.
Luke A. O'Neill is research professor of biochemistry in the Department of Biochemistry, Trinity College Dublin. His work focuses on signal transduction mechanisms in innate immunity and the mechanism of action of Toll-like receptors.
He can be contacted at