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Bridging Innate and Adaptive Immunity

William E. Paul1,* 1Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-1892, USA *Correspondence: [email protected] DOI 10.1016/j.cell.2011.11.036

The Nobel Prize in Physiology or Medicine for 2011 to Jules Hoffmann, Bruce Beutler, and the late Ralph Steinman recognizes accomplishments in understanding and unifying the two strands of immunology, the evolutionarily ancient innate immune response and modern adaptive immunity.

Among the 15 Nobel prizes given for siveness. The celebration of immunolo- the eponymous complete Freund’s adju- discoveries in immunology, including the gists everywhere in response to this vant, consisting of killed Mycobacteria very first, the 2011 prize can be best well-deserved prize is tempered with tuberculosis organisms in a water-in-oil compared to the 1908 prize, shared by sadness, with Ralph Steinman having emulsion. two giants of modern science, Paul died on the Friday preceding the Monday Although this need to use adjuvants to Ehrlich and Ilya Metchnikoff. In awarding announcement of the award. obtain a robust immune response to that prize, the Nobel committee attemp- protein antigens was widely appreciated, ted to grapple with the divide that had Exploring Immunology’s ‘‘Dirty it was papered over as immunologists’ already arisen in the then infant science Little Secret’’ ‘‘dirty little secret.’’ In a remarkably of immunology, using the simple award The narrative underlying the award to prescient and influential lecture at the statement, ‘‘in recognition of their work Hoffmann and Beutler, representing opening of the 1989 Cold Spring Harbor on immunity.’’ innate immunity, and Steinman, very Symposium on Quantitative Biology In fact, Ehrlich (Ehrlich, 1900) and much on the interface between innate (Janeway, 1989), Charles Janeway moti- Metchnikoff (Metchnikoff, 1891) were the and adaptive responses, begins with two vated immunologists everywhere to think champions of two divergent views of how independent themes. Although Stein- about this problem. Janeway argued the body protects itself against foreign man’s key discovery was made earlier, I that the immune system required not invaders. The one concentrated on the start with a description of the reawaken- only an antigen/receptor interaction to exquisitely specific adaptive immune re- ing of the interest of immunologists in initiate a response, but a parallel recog- sponse that exhibited the features of innate immunity. It would be presump- nition of structures that pathogens ex- learning and recall and was the basis tuous to imply that the study of the innate pressed (pathogen-associated molecular of the remarkably successful procedure immune system had ceased in the years patterns [PAMPs]), which would be rec- of vaccination, the only medical interven- after Metchnikoff’s work. But there can ognized by receptors broadly expressed tion that has eradicated diseases (two so be little doubt that, following the proposal by cells of the immune and inflammatory far, smallpox in humans and rinderpest by David Talmage (Talmage, 1957) and systems (pattern recognition receptors in cattle). By contrast, the adherents of by Macfarlane Burnet (Burnet, 1957)of [PRRs]). He posited that these interac- innate immunity emphasized the near the clonal selection theory of immunity, tions stimulated cellular events that were universal capacity of multicellular life the attention of the majority of immunolo- essential for the activation, expansion, forms, including plants, to protect them- gists was riveted on the adaptive system, and differentiation of T and B lympho- selves against foreign invaders with ex- and most of the other Nobel prizes in cytes that would eventually result in elim- tremely effective tools to eliminate or immunology (although not all) were for ination or control of the pathogen. control the invader utilizing the inflamma- advances in understanding adaptive Janeway’s proposal of a microbial tory system. immunity. sensor eliciting an innate immune re- We return to this divide in 2011, but not Yet despite this great attention, all was sponse that was subsequently interpreted in the stark terms that separated Ehrlich not well in the understanding of how by lymphocytes, the key cells of the adap- and Metchnikoff. For the work of the adaptive immune responses were initi- tive immune system, as ‘‘permission’’ to new Nobel Laureates—Jules Hoffmann ated. A great challenge was to determine mount a response when they recognized and Bruce Beutler, ‘‘for their discoveries why the simple introduction of an anti- an antigenic substance was enormously concerning the activation of innate immu- genic protein led to a very weak response influential. Although there was ‘‘no flesh nity’’ and the late Ralph Steinman, ‘‘for or tolerance unless an adjuvant was on the bones,’’ as I will recount later, the his discovery of the dendritic cell and its co-injected to enhance the response. cell that interpreted the innate response role in adaptive immunity’’ —to a very Perhaps the most famous of these was to the adaptive system, the dendritic large degree represents the merging of introduced by my predecessor twice cell, had already been discovered by the two strands and the recognition of removed as chief of the NIAID Laboratory Ralph Steinman. The key step, the finding the fundamental unity of immune respon- of Immunology, Jules Freund, known for of molecular entities that represented the

1212 Cell 147, December 9, 2011 ª2011 Elsevier Inc. PRRs and the PAMPs, was what truly that activated the system in humans, C57BL/10ScCr/ C57BL/10ScSn, one of galvanized the immunological community there is an important intermediate finding. which could respond to LPS and one and led to a revolution in how we under- Charles Janeway and his then postdoc- that failed to respond. To type these stand host-pathogen interactions. toral fellow Ruslan Medzhitov, working at mice, he made use of the capacity of In 1996, Jules Hoffmann, with his Yale University, immediately recognized macrophages from wild-type mice to pro- colleagues Lemaitre, Nicolas, Michaut, that the finding by Hoffmann in flies duce TNFa upon stimulation with LPS and Reichhart, working at the Institut de provided a powerful tool that might aid and the failure of macrophages from Biologie Mole´ culaire et Cellulaire in Stras- in determining the nature of microbial mutant mice to do so. Using a positional bourg, reported that, in Drosophila, an sensors in humans. Medzhitov and Jane- cloning effort over a period of several intact gene cassette consisting of spa¨ t- way scanned the expressed sequence years, he reported in 1998 (Poltorak zle/Toll/cactus/dorsal was essential if tag database and found a transcript that et al., 1998) that the gene that was mutant the fly was to mount a potent antifungal encoded a homolog of Drosophila Toll. in the LPS unresponsive mice was Tlr4, response (Lemaitre et al., 1996). It had They wished to determine whether this which coded for the very TLR that Medz- already been shown that fly resistance human Toll homolog initiated activation hitov and Janeway had shown could acti- to certain bacterial infections was con- of NF-kB, as Drosophila Toll did, and led vate NF-kB and cause proinflammatory trolled by the gene designated immune to the release of key proinflammatory cytokine production when crosslinked. deficiency (imd), but it was clear that cytokines. Because Medzhitov and Jane- This indicated that LPS was a direct or many other infections were not controlled way did not know the ligand for their indirect ligand for TLR4 and showed by the action of imd alone. Hoffmann and ‘‘Toll-like receptor’’ (TLR), they prepared that precisely the type of molecule that his colleagues became interested in the a molecular chimera in which the extra- would fit into Janeway’s postulated set system that activated the morphogen cellular domain of the T cell differentiation of PAMPs could activate production of dorsal because its signaling pathway, marker CD4 was linked to the cytosolic potent mediators of inflammation. initiated by Toll, showed a striking domain of the TLR that they had identi- Of course, as is true of all seminal similarity to components of a key activator fied. They expressed this chimera in a discoveries, what I have described is of vertebrate immune/inflammatory re- human monocyte cell line and showed merely the thin edge of the wedge. The sponses, the interleukin 1 (IL-1)/NF-kB that, when crosslinked with an anti-CD4 study of the TLRs as microbial sensors signaling pathway. The cytosolic portion antibody, NF-kB was activated and a has truly burgeoned, enlisting a legion of of Toll exhibits homology to the compa- series of proinflammatory cytokines extremely gifted investigators who have rable region of the IL-1 receptor, and were produced, including IL-1, IL-6, and identified ligands for each of the TLRs. cactus is homologous to a key regulator IL-8. This work, published in 1997 (Medz- Of these, Shizuo Akira of Osaka University of NF-kB activation, I-kB, and likewise hitov et al., 1997), established that is the generally acknowledged leader, dorsal to NF-kB. Hoffmann and his humans had at least one Toll homolog combining gene knockout technology colleagues showed that the spa¨ tzle/Toll/ (this proved to be TLR4) and that its and elegant biochemistry to determine cactus/dorsal signaling pathway con- signaling properties resembled those of which of the microbial products activate trols the production of the antifungal Drosophila Toll and of human IL-1. This which TLRs. The ligands include double- peptide drosomycin and that flies with still left unknown what TLRs recognized stranded RNA, single-stranded RNA, mutations in this pathway have a dramatic and thus whether the TLRs were the CpG oligodeoxynucleotides, bacterial reduction of survival following fungal microbial sensors/PRRs that Janeway flagellin, lipopeptides, and zymosan, a infection. had postulated. Tragically, Janeway died list that is precisely one that Janeway This finding was enormously exciting in 2003. By way of disclosure, I had the would have constructed as among his not only for the insight that it gave into good fortune to be his postdoctoral PAMPs. These scientists have worked how flies (and presumably other inverte- mentor. out TLR subcellular localization (some brates) protected themselves against The illumination of TLR recognition are in endocytic vesicles, others on the key pathogens, but also because its specificity came from the efforts of cell surface) and have determined in detail homology to components of the NF-kB Bruce Beutler. Beutler had been a post- the signaling mechanisms and the prod- system implied that it was likely that doctoral fellow at the Rockefeller Univer- ucts of these signaling pathways. Indeed, humans and other vertebrates might use sity working with Anthony Cerami, where shortly after Beutler’s publication that a related system. However, there was they codiscovered the proinflammatory TLR4 controlled LPS responses, Akira one important point that led to some cytokine TNFa (Beutler et al., 1985). He and his colleagues reported that deleting uncertainty. The activator spa¨ tzle, like then joined the faculty of the University Myd88, a key signaling intermediate in human IL-1, was an endogenous extra- of Texas Southwestern Medical Center the IL-1 pathway, blocked much of the cellular protein, so if this pathway was in Dallas, where he set himself the task action of LPS (Kawai et al., 1999). indeed Janeway’s PRR-signaling path- of determining the genetic defect that Equally important has been the rec- way, how exactly did it sense the path- rendered some mice unresponsive to ognition that the TLRs are but one system ogen, and how would pathogen sensing lipopolysaccharide (LPS), a bacterial en- of microbial sensors. Other PRRs exist— occur in vertebrates? dotoxin. He took advantage of the exis- some that have been long-known and Before describing Beutler’s contribu- tence of two sets of closely related others, particularly those found in the tion, which gave us insight into the ligands strains of mice, C3H/HeJ/C3H/HeN and cytosol, that have been recently identified.

Cell 147, December 9, 2011 ª2011 Elsevier Inc. 1213 In some cases, a recognition element is on the widely held view that the major antibody complexes, receptors for com- linked through an intermediate to an APCs were the far more numerous mac- plement, mannose receptors and lectins, effector, often by assembly into a molec- rophages and on the uncertainty that and receptors for dying cells, among ular complex, an inflammasome. Often, many immunologists had about the many others. They also express TLRs. the key effector event is the activation assay that Steinman and Cohn used to Thus, as sentinels in the tissues, Langer- of caspase 1 so that it can cleave its establish the function of their dendritic hans cells (and other tissue dendritic targets, one of the most important being cells. cells) can capture antigen from patho- pro-IL-1b, giving rise to biologically active One major function mediated by T cells genic microbes or from other cells in- IL-1b. is the response to the major histocom- fected by those organisms. They possess patibility antigens expressed by animals the microbial sensors that determine how The Unification of Innate and of distinct MHC type. The in vitro assay they should behave. If appropriately Adaptive Immunity that was used to measure this was the stimulated (for example, by LPS acting The adaptive system consists of two ‘‘mixed lymphocyte response’’ (MLR) in on TLR4), they change their phenotype broad sets of antigen-responsive cells, which T cells from mice of one MHC from cells specialized for antigen capture the B and T lymphocytes. B lymphocytes type would proliferate vigorously when to cells adapted for antigen presentation, are the precursors of antibody-producing stimulated by APCs from mice of another and they migrate from the tissue into the cells, and they use antibody in a mem- MHC type. Given that the nature of the draining lymph node. These stimulated brane protein from as their antigen- actual entity recognized in the MLR was dendritic cells acquire enhanced capacity binding receptors. Antibodies are capable not clear and no T cell priming was to process and present antigen, including of recognizing three-dimensional struc- required to obtain a vigorous response, the striking upregulation of class II MHC tures and thus can interact with and many immunologists who studied T cell molecules; they increase their expression lead to the neutralization of pathogens in responses to conventional antigens and of potent costimulatory molecules such extracellular fluid. B cell receptors recog- measured in vitro T cell proliferation as CD80 and CD86 as well as Notch nize the same structures, and though it when primed cells were challenged with ligands that allow them to efficiently acti- now appears that this recognition often antigen were uncertain as to the proper vate those CD4 or CD8 T cells that have occurs on cell surfaces, soluble mole- interpretation of the MLR data. However, recognized antigen on their surface, and cules can certainly bind to B cell receptors this uncertainty was resolved in a series they secrete cytokines important in the and, when in a multivalent conformation, of experiments, beginning with a key differentiation of the activated T cells, can elicit stimulatory signals in the B cell. study done by Michel Nussenzweig (then such as IL-6, IL-12, and IL-23. Indeed, By contrast, the T cell antigen recogni- a Ph.D. student) with Steinman and the pattern of cytokines that the dendritic tion system is not adapted to the rec- Cohn (Nussenzweig et al., 1980), in which cells produce and their efficiency in ognition of three-dimensional structures it was shown that dendritic cells were processing antigen to a large extent on the surface of pathogens. Rather, the key APCs for the development of determine the phenotype that the differ- T cell receptors recognize a complex antigen-specific cytotoxic T cells. This entiating T cells will adopt. Among CD4 consisting of an antigen-derived peptide series of studies demonstrated the critical T cells, this would be whether they de- bound into a specialized groove in class role of dendritic cells in nearly all aspects velop into Th1, Th2, Th17, or induced T I and class II major histocompatibility of T cell activation and established that regulatory cells and thus regulate immune complex (MHC) molecules. As such, their potency as APCs far exceeded that responses adapted to control distinct T cell recognition of antigen occurs on of other cell types that expressed MHC types of infections. the surface of cells expressing these molecules, most particularly of macro- Steinman and Nussenzweig also pio- peptide/MHC complexes, often referred phages and B cells. Indeed, modern neered the idea that a population of den- to as antigen-presenting cells (APCs). In techniques that led to the deletion of dritic cells resident in lymphoid tissues the late 1960s and early 1970s, when the dendritic cells result in a profound inability continuously capture self-antigens and requirement for APCs was first appreci- to mount adaptive immune responses. present these antigens to circulating ated, it was generally believed that mac- Dendritic cells consist of many T cells that are specific for them in inter- rophages and possibly B cells were the subtypes, but the principles underlying actions that, rather than activating the major APCs. In 1973, Ralph Steinman their function, largely established by cells, lead either to their elimination or to and his mentor at the Rockefeller Univer- Steinman and his colleagues, are quite their development into regulatory T cells sity, Zanvil Cohn (who died in 1993), iden- similar. They are found in nearly every (Tregs), effectively enforcing peripheral tified a rare cell in spleen and lymph nodes organ and initially exist in a state in which immunologic tolerance (Hawiger et al., of mice (Steinman and Cohn, 1973). they are adapted to the capture of antigen 2001). These cells had a stellate morphology; and to the receipt of activating signals. One of Steinman’s visions was the Steinman and Cohn designated them An illustrative example is a population of use of dendritic cells to develop a new dendritic cells. In 1974, they published skin cells known as epidermal Langer- generation of vaccines in which antigens evidence that these cells had potent hans cells. These skin cells express cell were loaded into dendritic cells in vitro immunostimulatory activity (Steinman surface molecules that aid in the capture and reinjected or vaccine antigens would and Cohn, 1974). This report was initially of antigen. They include Fc receptors, be targeted to them and efficiently received with some skepticism, based which are effective in binding of antigen- loaded in vivo. Major efforts at developing

1214 Cell 147, December 9, 2011 ª2011 Elsevier Inc. dendritic cell-based vaccines are under- Hoffmann and Akira. Thus, all of the prin- Lemaitre, B., Nicolas, E., Michaut, L., Reichhart, way (Steinman, 2008). Indeed, during his cipal living players in this story received J.M., and Hoffmann, J.A. (1996). Cell 86, 973–983. long bout with pancreatic cancer, Stein- recognition. Medzhitov, R., Preston-Hurlburt, P., and Janeway, man was treated with a dendritic cell C.A., Jr. (1997). Nature 388, 394–397. vaccine prepared with antigens derived REFERENCES Metchnikoff, E. (1891). Phagocytosis and Immunity from his tumor cells. (London: British Medical Association). The 2011 Nobel Prize in Physiology or Beutler, B., Greenwald, D., Hulmes, J.D., Chang, Nussenzweig, M.C., Steinman, R.M., Gutchinov, Medicine honors three remarkable scien- M., Pan, Y.C., Mathison, J., Ulevitch, R., and B., and Cohn, Z.A. (1980). J. Exp. Med. 152, Cerami, A. (1985). Nature 316, 552–554. tists and recognizes the effort to unite 1070–1084. Burnet, F.M. (1957). Aust. J. Sci. 20, 67–69. the disparate strands of immunology by Poltorak, A., He, X., Smirnova, I., Liu, M.Y., Van the merging of the innate and adaptive Ehrlich, P. (1900). Proceedings of the Royal Society Huffel, C., Du, X., Birdwell, D., Alejos, E., Silva, 66, 424–488. 282 systems into a coherent system. M., Galanos, C., et al. (1998). Science , 2085– Hawiger, D., Inaba, K., Dorsett, Y., Guo, M., 2088. Interestingly, two other prizes have Mahnke, K., Rivera, M., Ravetch, J.V., Steinman, Steinman, R.M. (2008). Immunity 29, 319–324. been awarded for innate immunity almost R.M., and Nussenzweig, M.C. (2001). J. Exp. simultaneously with the Nobel Prize. The Med. 194, 769–779. Steinman, R.M., and Cohn, Z.A. (1973). J. Exp. Med. 137, 1142–1162. Shaw Prize, sometimes described as Janeway, C.A., Jr. (1989). Cold Spring Harb. Symp. the ‘‘Asian Nobel,’’ went to Hoffmann, Quant. Biol. 54, 1–13. Steinman, R.M., and Cohn, Z.A. (1974). J. Exp. Med. 139, 380–397. Medzhitov, and Beutler, and the Canadian Kawai, T., Adachi, O., Ogawa, T., Takeda, K., and Gairdner International Award went to Akira, S. (1999). Immunity 11, 115–122. Talmage, D.W. (1957). Annu. Rev. Med. 8, 239–256.

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