How do we "know" when we have an infection? What are the receptors that alert us? How do we discriminate self from nonself, and why does the immune system sometimes attack our own cells and tissues? Why does it sometimes fail to eradicate infectious microbes? The tools of genetics have been used to address these questions, and answers have begun to emerge.
For more than a century, and in fact, since microbes were recognized as the cause of infections, it has been clear that mammals are genetically programmed to recognize them. Moreover, it has long been an obvious corollary that certain molecules of microbial origin must trigger a host response, and that specialized receptors of the host must mediate recognition of these molecules. This, after all, is how biological systems operate. But what were these receptors? A genetic approach was required to answer the question.
Because the innate immune system must act promptly to contain an infection, mammals respond violently to purified molecules of microbial origin such as endotoxin (e.g. lipopolysaccharide; LPS). LPS has been investigated for many decades as a prototypic inducer of innate responses. (Figure 1) And it has long been known that sensing LPS is required for a mouse to overcome a Gram-negative infection (1, 2). It has also been clear that cytokines, produced by mononuclear phagocytes in response to LPS, orchestrate the innate response and can be highly toxic when produced in large amounts (3-5). But the nature of the LPS receptor, which ignites the entire process, was long elusive.
The present activities of the Beutler laboratory stem from a longstanding interest in innate immune sensing and response, and involve the use of positional cloning as a method to decipher it. In 1998, the laboratory identified the mammalian LPS receptor as Toll-like receptor 4 (TLR4) (Figure 2) by genetically mapping and then cloning a mutant allele known as Lpsd, which in homozygous form caused unresponsiveness to LPS in mice (6).
This discovery—the first assignment of function to a TLR—led directly to the present concept that the mammalian TLRs serve as sensors of microbial infection (7). It is now believed that each of the 12 mouse TLRs and 10 human TLRs detect a limited number of the signature molecules that herald infection (e.g. LPS, lipopeptides, flagellin, unmethylated DNA, dsRNA, and ssRNA). Under some conditions, they may also detect molecular ligands of host origin and participate in sterile inflammation, a condition observed in autoimmune diseases. The TLRs are the gatekeepers of the most powerful inflammatory responses known, and as such, are probably important in a wide range of diseases. Without TLR signaling, a state of severe immunocompromise exists (8). The discovery of the TLRs was recognized by the Nobel Prize in Physiology or Medicine in 2011.
The Beutler laboratory systematically employs a forward genetic approach to identify genes that are essential for the mammalian innate immune response, and to determine their functions relative to one another. The forward genetic approach entails: (i) the induction of thousands of random germline point mutations on a defined genetic background (C57BL/6J) using N-ethyl-N-nitrosourea (ENU), (ii) the phenotypic screening of many thousands of mice for specific defects of immunity, and (iii) the positional cloning of those transmissible mutations that are detected. This classical genetic method does not depend upon hypotheses, nor upon assumptions about how innate immunity "should" work. Hence, it is unbiased, and errors of interpretation are extremely rare.