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Presidential Address to The American
Association of Immunologists: The Road Less
Traveled by: The Role of Innate Immunity in the
Adaptive Immune Response
Charles A. Janeway Jr.
J Immunol 1998; 161:539-544; ;
http://www.jimmunol.org/content/161/2/539
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Copyright © 1998 by The American Association of
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References
Presidential Address to The American Association
of Immunologists
The Road Less Traveled by: The Role of Innate Immunity in the Adaptive
Immune Response1,2
Charles A. Janeway, Jr.
Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same.
And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.
I shall be telling this with a sigh,
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
—Robert Frost
T
he title for my talk was chosen from a poem by Robert
Frost called “The Road Not Taken.” I think this poem is
a wonderful metaphor for the choices we make all the
time in our lives. Choices that define our paths at all forks in the
road, which have irrevocable consequences for all of us, and which
are often made on the merest of whims. These decisions have a
profound impact on our lives, but we are usually too busy to notice
this. So in my address to the members of this association, I want
to review with you my life in science, especially those individuals
who have influenced my choices over the years, and how I decided
eventually to go into the study of the innate immune response to
infection.
Looking backwards is always a temptation in these talks, and I
would like to share with you my own roots in medicine and science, because these also focused my attention on the role of the
immune system in protecting the body from infection.
Section of Immunobiology, Yale University School of Medicine and Howard Hughes
Medical Institute, New Haven, CT 06520
1
Presented at the Annual Meeting of The American Association of Immunologists,
April 18 –22, 1998, San Francisco, CA.
2
This work was supported in part by a grant from the Human Frontiers of Science
Program; by National Institutes of Health; Grant AI-26810, National Institute of Allergy and Infectious Diseases; and by the Howard Hughes Medical Institute.
Copyright © 1998 by The American Association of Immunologists
Charles A. Janeway, Jr.
My great-grandfather, Edward Gamaliel Janeway (Fig. 1, left),
was the health commisioner of the City of New York and a professor of medicine and pathology at Bellvue Hospital on Roosevelt
Island. Here he is standing beside a patient, but we don’t know if
the patient is alive or dead, as he took care of live patients and their
corpses! Note the formal dress of the medical students in the audience; I would like to see a class at Yale with such dignity! (Of
course, I would myself have to dress up similarly to my great
grandfather!) I think I inherited my interest in medicine and research from my great-grandfather because we both have such
broad-ranging interests in science.
My grandfather, Theodore Caldwell Janeway (Fig. 1, center),
was a professor of medicine with interests in cardiology and infectious disease. The eponymous Janeway’s lesions in subacute
bacterial endocarditits are named for him. He first worked at Columbia’s College of Physicians and Surgeons, and later at John’s
Hopkins University School of Medicine, where he served as the
first full-time professor on the staff of the Johns Hopkins Hospital.
In late 1917 he was asked by the United States Army to visit the
encampments of soldiers, as the rate of mortality in these camps
was very high. He did this, contracted the same pneumococcal
pneumonia that accounted for the high mortality rate among the
troops, and died within a week in December 1917; my father was
8 at the time. From this family tragedy, I learned that in the preantibiotic era, infectious disease posed a real threat to life. Fortunately, I have lived in an era that is relatively free of infection, as
key advances in antibiotic therapy of infection and in vaccines
occurred within my lifetime. If penicillin had been available at the
time of my grandfather’s illness, he could have lived much longer;
0022-1767/98/$02.00
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The Road Not Taken
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
540
PRESIDENTIAL ADDRESS
unfortunately, it was not available until 1945, and so my grandfather died as a relatively young and otherwise healthy man.
My father, Charles A. Janeway (Fig. 1, right), was a professor
of pediatrics and department chairman for about 30 years at the
Children’s Hospital Medical Center in Boston. During the World
War II, he worked with Dr. Edwin Cohn on the fractionation of
human plasma into various protein solutions. One of these, which
I think was called fraction V, contained a set of proteins called
gammaglobulins that had been demonstrated by Kabat and Tiselius
to have antibody function. My father studied this fraction of proteins extensively over the ensuing years, using protein electrophoresis to define patients that lacked gammaglobulins. He had
accumulated four such patients by 1953, when Colonel Oswald
Bruton reported one case in a military recruit. My father then reported his four cases and pointed out that they had an unusual
susceptibility to infection. He used injections of pooled gammaglobulin to treat these patients, which protected them from infection; this treatment is still used today. He later noted that several
distinct proteins were missing from the plasma of this patient, thus
discovering that several distinct proteins had antibody activity; this
led directly to experiments that defined the various isotypes of
human immunoglobulin. From my father, I learned that the presence of a functioning immune system in man was an essential
characteristic for a healthy life. It should be noted that these agammaglobulinemic individuals were only discovered once effective
antibiotic therapy for infection was available.
Thus, from many observations, and from my own personal history, it was clear that the main purpose of the adaptive immune
response was resistance to infectious disease, and that, in its absence, death would occur early in life. Thus this function of the
immune system was clearly of paramount evolutionary importance. These observations, as well as many chance decisions along
the way, led to my initial interest in the adaptive immune system.
Several events during my early training had a great impact on
my thinking. I was fortunate to train with a series of great thinkers
in immunology. My first mentor, who loves to disclaim me, was
Hugh McDevitt. Hugh introduced me to the subject of immunology and taught me how to read the literature. He told me that one
should always attempt to construct an internal image of the universe of knowledge and then ask if a given publication can, in any
significant way, modify that image. I still use this system to read
papers and evaluate research proposals, although I admit that it
works less well in the molecular era, as new genes define new
proteins that have to be added to the list at an ever-accelerating
rate. One paper that we read in class was by Marion “Bunny”
Koshland on the analysis of the amino acid composition of antibodies purified from rabbits immunized with distinct antigens. I
remember very well reading this paper, as it contained the first
evidence of variability in protein structure in the specific case of
antibody molecules. This led to a debate among my fellow students
about the origin of this diversity, which was later solved by the
finding made by Susumu Tonegawa that antibodies are encoded in
gene segments, and that these gene segments are present in the
germline in large numbers and undergo rearrangement to generate
specific antibody molecules.
Hugh arranged for me to spend 2 years with John Humphrey at
Mill Hill in northern London. Again, this choice had three major
impacts on my life. First, it raised my interest in immunity to
infection and in the science of immunology to a high level. Second, it further fed my craving for variety in my interests, as John
was interested not only in all of immunology, but in country walks,
in politics, and in public service. He remains a principle inspiration
to me in all of these areas. Third, it got me interested in writing, as
John had, together with Bob White, written one of the first textbooks of immunology, entitled “Immunology for Medical Students.” I also met many of the world’s top scientists, especially in
immunology, at the early age of 22. Finally, I learned from John
the approach to training young scientists that I still apply today:
Leave them to their own devices; if they are good, they will take
care of themselves and become better scientists than they would if
excessively monitored. I truly believe that this is the best way to
train young scientists; it has certainly benefited me.
After working in an immunology lab for 2 years, I returned
home to complete my medical studies, but with a new attitude
toward them; I was more focused and more critical, which did not
serve me well in clinical medicine. I realize now that people who
take care of other people do not have the luxury of testing out their
ideas on their patients; they simply have to make the best decision
based often on the flimsiest of evidence. This bothered me, and
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FIGURE 1. Photographs of my forbears: Edward Gamaliael Janeway (left), Theodore Caldwell Janeway (center), and Charles A. Janeway (right).
The Journal of Immunology
answered that it was undoubtedly due to infectious agents. “But
how do infectious agents activate naive T cells,” Kim asked? To
which I blithely answered: “They do so via their effects on the
innate immune system.”
Once I had this idea in my head, it was impossible to let go of
it, and I quickly assembled my thoughts and tried them out on
many colleagues. By the time June rolled around, I could tell Adrian Hayday about my ideas in some detail as I drove him to the
1989 Cold Spring Harbor meeting on immune recognition. While
at the meeting, I was asked by John Ingliss if I would be willing
to write the introductory chapter to the book that is traditionally
prepared from the meeting proceedings. I asked if I could write it
on anything that interested me and was told yes. Meanwhile, on the
last morning of the meeting, I remember Len Herzenberg, who
received the American Association of Immunologists’ lifetime
achievement award this year, asking me over breakfast if I could
sum up the meeting in one sentence. I made an off-the-cuff comment that I thought that we were approaching the asymptote in our
understanding of the central processes of adaptive immunity, but
we weren’t. Although this turned out to be true, it was for different
reasons. What I meant was that we needed to invest more time in
work on innate immunity, which at that time was barely mentioned. So that was how I came to prepare the paper entitled: “Approaching the Asymptote: Evolution and Revolution in Immunology” (1). I always say it was the best talk I never gave. The
implication of this article can best be summed up by the statement
that the immune system does not just discriminate self from nonself, as Jerne, Talmage, Burnet, and many others believed, but
rather that it could discriminate infectious non-self from
noninfectious self.
Having a powerful idea is one thing: proving it is entirely different, and I would say that we are about one percent of the way
to demonstrating that the idea has merit, but that it still represents
a major advance in my knowledge, if not that of the audience. But
I would like to revisit this hypothesis to explain what we now
accept as true, and what is still at issue.
Let me start from the point of view of adaptive or acquired
immunity. The clonal selection hypothesis, while modified in detail over the years, has been and remains a robust explanation for
most of the phenomena of adaptive immunity. In fact, at the time
of the first Cold Spring Harbor Symposium on immunology in
1967, Nils Jerne’s summary of the meeting was entitled: “Waiting
for the end.” I was tempted by this to call my introductory article
at the meeting held 22 years later in 1989: “Still waiting for the
end.” The clonal selection hypothesis states that each lymphocyte
is equipped with many identical copies of an antigen-specific receptor, and when this receptor binds a ligand with sufficient avidity, the lymphocyte is activated to undergo clonal expansion and
differentiation to effector cell function. This is true of both T cells
and B cells. However, for naive T cells to become activated, an
additional requirement for a second or costimulatory signal was
later proposed and demonstrated by many groups. In my laboratory, Yang Liu demonstrated that not only did costimulation have
to be present to activate naive T cells, but that naive T cells at least
needed to see the costimulatory signal on the same cell as the
specific antigenic peptide to clonally expand. In this experiment,
Yang used anti-CD3 antibody as a surrogate TCR ligand that has
to bind to FcR, and a B7.1 gene transfected into the same cells (2).
Yang also devised a system for quantitating this signal, and
showed that it was induced by a variety of ligands that derived
from pathogenic microbes. It is this costimulatory signal that we
believe is regulated by the innate immune system via so-called
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eventually played a part in my going into science. But the immediate force that made me choose science over medicine was getting
into the Public Health Service as a “yellow beret,” our joking name
for the group that came of age at the end of the 1960s and did not
want to serve in the army in Vietnam in what we regarded as an
unjust war.
I was lucky enough to be accepted into the Laboratory of Immunology at the National Institutes of Health; at the time of the
decision, the head of the laboratory was Baruj Benacerraf, but on
the day I arrived in Bethesda, he departed to assume the Fabyan
Professorship of Pathology at Harvard Medical School. In his
stead, he wisely left Bill Paul, first as an acting head of the laboratory and eventually, as the wisdom of the choice became selfevident, as lab chief. Bill became a most important mentor to me:
he taught me how to write better and more carefully reasoned
papers, and he, like John Humphrey, basically left me alone to
follow my own interests. After 5 happy and productive years at the
National Institutes of Health, I left to accept a position at Yale, but,
as my laboratory was not going to be finished for a year, I obtained
a fellowship from Harvard Medical School which allowed me to
travel to Sweden and work with Hans Wigzell. There, I worked on
cultures of mouse T cells, learning how to isolate responding T
cells, which led directly to my ability to isolate cloned T cell lines.
Hans often asked me, in his Swedish-accented English, if I would
like it “In Jail.” I had to tell him that I was not going to Jail, but
to be on the faculty at Yale. Finally, I returned to Yale and set up
my own group, entered the peer review system in which I met
many of you, either on paper or in person, and learned how the
world works.
One of the distressing things about Yale Medical School, as I
learned to my amazement, was that they were one of the few medical schools in the country that had decided that they could abandon the study of infectious disease because antibiotics would soon
have cleansed the earth’s surface of pathogens. This decision was
made before my arrival, and has just now been reversed with the
establishment of a new section of microbial pathogenesis under the
leadership of Jorge Galan. In the absence of collaborators who
were working on mouse or human pathogens, I turned my attention
to the analysis of T cell responses to protein antigens, especially
the analysis of cloned T cell lines, which we produced in large
numbers. At that time, one of my collaborators said that my lab
reminded him of a farm in his native Italy because we were always
growing things. In any case, I worked productively in this area for
some years, and at the same time taught Yale medical students,
graduate students, and undergraduates. The medical students at
Yale are a unique breed, full of genuine curiosity, and they were
always asking naive but provocative questions about health and the
immune system. In attempting to answer these questions, I was
forced more and more to fall back on the primary function of the
immune system, which is to fight infection. I would like to dedicate this talk to the many medical student classes that I have taught
over the years for forcing this realization on me.
My personal moment of understanding of the importance of innate immunity happened in a casual conversation with my wife,
Kim Bottomly, who is also a distinguished immunologist. Kim has
often inspired me to think more deeply and more freely than I
thought I was capable of doing. In this case, we were attending a
meeting that I had organized in Steamboat Springs in January
1989. There were many issues discussed at this meeting, but one of
the central issues was the regulation of the immune response by
signals in the environment of the cells themselves. These signals
were thought to be mediated by cytokines, and the cytokine network was a hot topic of discussion. But Kim wondered what initiated cytokine secretion in the absence of activated T cells. I glibly
541
542
PRESIDENTIAL ADDRESS
Table I. Innate and adaptive immunity
Property
Receptors
Distribution
Recognition
Self–non-self
discrimination
Action time
Response
Innate Immune System
Adaptive Immune System
Fixed in genome; rearrangement not necessary
Nonclonal; all cells of a class identical
Conserved molecular patterns (LPS, LTA,
mannans, glycans)
Perfect: selected over evolutionary time
Encoded in gene segments; rearrangement necessary
Clonal; all cells of a class distinct
Details of molecular structure (proteins, peptides,
carbohydrates)
Imperfect: selected in individual somatic cells
Immediate activation of effectors
Costimulatory molecules; cytokines (IL-1b,
IL-6); chemokines (IL-8)
Delayed activation of effectors
Clonal expansion or anergy; IL-2; effector cytokines
(IL-4, IFN-g)
NFkB pathway using homologous components to tube and pelle.
Furthermore, we have data that Toll also activates the AP-1 promotor binding proteins Fos and Jun. Finally, although the Toll
ligand in the fruit fly is known to be Spaetzle, such proteins have
not been identified in mammalian systems. However, we have recently identified candidate activators of these pathways in both
Drosophila and humans, which have scavenger receptor domains
in their N termini, and pro-serine protease activity in their C-terminal domains.
To return to more conventional aspects of host defense for a
minute, Table I lists several characteristics of innate immunity and
contrasts them with the adaptive immune system. The main point
of this table is to contrast these two systems of host defense. The
innate immune system is ancient, being found in all multicellular
organisms down to Caenorhabditis elegans, whereas the adaptive
immune system exists only in vertebrates. The receptors of the two
systems are their biggest point of difference. Receptors for innate
immunity have evolved over an evolutionary time scale, whereas
receptors for adaptive immunity undergo gene rearrangement and
thus evolve in individual members of a vertebrate species such as
humans or the mouse. For this reason, clonal selection of lymphocytes is a specialization unique to vertebrates. This specialization
allows us to remember those pathogens that we have resisted before, or to vaccinate against such pathogens, so that the initial
infection that used to be required to establish protective immunity
can now be done painlessly and with few symptoms. However,
adaptive immunity can also create problems through defects in
self–non-self discrimination. As all vertebrates have some form of
adaptive immunity, the existence of immunologic memory was
obviously very important in vertebrate evolution.
However, accumulating evidence that innate immunity uses the
same building blocks in organisms as diverse as the worm C. elegans, plants, insects, and mammals suggests that this form of
immunity is fundamental and primordial, and therefore worthy of
the intense study that has hitherto only been lavished on the adaptive immune response (Fig. 2) (4).
During the rest of this talk, when I say we I mean Ruslan
Medzhitov, who has performed all of the studies I am about to
describe. Ruslan came to me over the internet, first from Tashkent,
Uzbekistan, and then from the University of California in San Diego, where he went to study protein evolution with Russ Doolittle.
While working with Russ, Ruslan made contact with Dick Dutton,
last years past-president of the American Association of Immunologists and currently the husband of the director of the Trudeau
Labs in Saranac Lake, NY, Dr. Susie Swain. Dick did me a real
kindness by calling me and insisting that I take Ruslan on, as he
was, in Dick’s words, brilliant. Dick was correct, as always, in his
assessment of character and intelligence, and I immediately offered
Ruslan a position in my lab. He has subsequently identified over 25
unknown genes which are candidates for pattern recognition receptors, one of which was Toll. He also identified several different
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pattern recognition receptors (or PRRs), which recognize pathogen-associated molecular patterns (or PAMPs).
What roles do we envision for innate immunity? We believe that
innate immunity, by definition, accounts for host defense in the
early phases of an infection. This occurs by processes that act
immediately on the infectious agent, such as the alternative pathway of complement activation, NK cells, and the epithelial barriers
of the body, as well as later when antibacterial peptides are secreted, cytokines are produced by phagocytes, and NK cells are
activated by type I interferons. Second, it plays a key role in the
adaptive immune response by inducing costimulatory molecules
on cells that take up the pathogen. These costimulatory molecules
are absolutely essential to the activation of naive T cells, as shown
by Yang Liu and many others. Third, we believe that the production of cytokines by cells of the innate immune system also contributes to the type of T cell activated.
In the fruitfly Drosophila melanogaster there is clearly no adaptive immune system; Drosophila relies solely on the mechanisms
of innate immunity to protect itself from infection. Little was
known about the mechanisms of host defense in Drosophila until
Jules Hoffman and colleagues in France began studying it, following on from the pioneering work of Hans G. Boman of the University of Stockholm. However, developmental mutants had been
isolated by Dr. Christine Nusslein-Volhard, and these turned out to
use many of the same elements as the immune response. When
Hoffman’s group pricked adult flies with various pathogenic microbes, they found that flies with mutations in the Toll signaling
pathway from the gene for pro-Spaetzle to the gene for cactus
could not resist infection with fungal spores (3). Pricking adult
fruit flies with fungal spores leads to the production of the antifungal peptide called drosomycin, the only antifungal peptide
known in Drosophila. The fungal spores activate a cascade of proteases that ends up creating the ligand for Toll, called Spaetzle.
The upstream members of the dorsoventral pattern formation pathway that lead to Spaetzle cleavage were tested and found not to be
essential for the activation of Toll by fungal spores, so presumably
there is an independent mechanism to generate Spaetzle from proSpaetzle in host defense in adult fruit flies. Ligation of Toll by
Spaetzle leads to activation of a series of proteins, some of which,
called tube and pelle, are defined, which eventually leads to phosphorylation of cactus, degradation of the cactus protein, and release of a rel protein that is a key promoter-binding protein for the
activation of the gene encoding drosomycin. The mammalian homologues of cactus and rel are IkB and NFkB.
Thus, as we can see from studies in Drosophila, Toll is a key
mediator of innate immunity. At the time that Hoffman’s results
were published, we had already cloned the mammalian homologue
of the Drosophila Toll protein, which we called hToll. We showed,
as I will tell you in a few minutes, that a dominant active form of
hToll induced the synthesis of B7 costimulators and pro-inflammatory cytokines. We have also shown that Toll activates via the
The Journal of Immunology
543
C-type lectins, several scavenger receptors, and several undefined
members of the complement control proteins. Once having done
this, I insisted that he complete the work-up of one of these genes,
and he wisely chose hToll for his initial focus.
I will now outline the published work on hToll (5) before turning to three related issues: an adaptor protein that was initially
cloned as an inducer of monocyte differentiation, a TRAF homologue in Drosophila that Ruslan has recently identified, and a
novel serine/threonine innate immunity kinase. Finally, I will mention briefly how we believe the upstream elements detect infection
and activate a cascade of proteases that transmit the signal to Toll
and hence to the nucleus.
The first issue we faced in the cloning of homologues of the
dToll sequence was whether the homology was along the full
length of the protein or, as in the IL-1R, only extended to the
cytoplasmic domain. The sequence of hToll shows that the two
genes from the fly and the human are homologous over their entire
length. Thus, hToll is truly a mammalian homologue of this protein in the fly.
The ectodomain is made up of leucine-rich repeats; only one
structure of a LRR domain has been solved, that of the pancreatic
RNase inhibitor. This structure was determined by Johann Deisenhofer of Dallas. We don’t yet know what the hToll ectodomain will
look like, but we are collaborating with Dr. Deisenhofer and we
hope eventually to know its structure.
The cytoplasmic domain of both dToll and hToll are homologous to the cytoplasmic domain of the human IL-1R, forming a
so-called TIR domain (Toll-IL-1R), which was originally thought
to mediate signaling from these receptors. However, it is now
known to interact via an adaptor protein that we have shown binds
to Toll as well as to the IL-1R, as shown earlier by Dixit’s group
at Michigan and by Cao’s team at Tularik. However, the two TIR
domains signal differentially, in that the IL-1R signals via NFkB,
whereas hToll signals both via NFkB and AP-1. A fourth protein
that is shown in Figure 2 has leucine-rich repeats joined to a TIR
domain and is involved in host defense in plants. This suggests a
common set of building blocks in host defense in plants, insects,
and mammals, and we have also identified genes encoding similar
proteins in the worm C. elegans. The cytoplasmic domain of hToll
is more homologous to other Toll proteins than any of the IL-1R
cytoplasmic domains. The TIR domain of N protein, as might be
expected for genes that are separated by a billion years of evolution, is distinct from both the IL-1R family and the Toll family of
cytoplasmic TIR domains.
Finally, we analyzed the cystein-rich membrane proximal domain and found close homology with dToll in this region of hToll.
It was determined previously by Kathryn Anderson that mutation
in any one of the conserved cysteins would activate Toll signaling
in Drosophila. We took advantage of this to prepare dominantly
active hToll mutant proteins by making chimeric proteins between
the ectodomain of mouse CD4, against which many antibodies are
available, and the transmembrane and cytoplasmic domains of
hToll, to produce a construct in which three of the four conserved
cysteins were replaced. This construct was transfected into Jurkat
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FIGURE 2. Ancient pathways of host defense, shared between plants, insects, and vertebrates. See text for details.
544
regulate the expression of a variety of anti-microbial peptides, as
shown by RT-PCR and by activation of reporter constructs driven
by the promoter regions of several of these antimicrobial peptides.
The role of dTRAF in host defense in Drosophila thus appears to
be analogous to the role of TRAF6 in mammalian cells. Thus, we
have identified a TRAF protein in Drosophila, which appears to
have similar activities to mammalian TRAF6.
We have also looked for other molecules with the structural
signature of serine/threonine innate immunity kinases, similar to
IRAK, pelle, and pto (see Fig. 2). We have recently identified one
such gene which we have tentatively called CCK. When this kinase is overexpressed, it activates the Toll signaling cascade, as
shown by activating the same NFkB reporter construct. Moreover,
dominant negative forms of TRAF6 inhibit NFkB activation by
this kinase. This positions this CCK on the same level as all of the
other serine/threonine innate immunity kinases (see Fig. 2).
Finally, we feel that the Toll proteins are signaling molecules
whose ligand is generated by a protease cascade triggered by one
of many different pattern recognition receptors. We have recently
identified two proteins with a similar architecture, one in Drosophila and one in mammals. These proteins have three scavenger repeats in their N-terminal halves and a pro-serine protease domain
in their C-terminal halves. We suspect that proteins such as this
will bind to PAMPs to initiate the cleavage of proteins that eventually generate Spaetzle in Drosophila and the Spaetzle-like ligand
that we suspect will soon be defined in mammals.
In conclusion, I would like to leave you with four statements
about innate immunity. It is clear to me that the innate immune
system should be taken as seriously as the adaptive immune system, because it has several important functions in host defense.
First, innate immunity is an essential component of host defenses
against infection, and it is always on the scene when needed. Second, innate immunity can control infection until the adaptive immune response can take over. Third, innate immunity discriminates between self and non-self perfectly, as all of its components
are hard-wired in the genome. Finally, innate immunity is critical
for the induction and direction of the adaptive immune response, as
originally discussed in my article for the 1989 Cold Spring Harbor
meeting, and updated by Doug Fearon and Richard Locksley in a
recent article published in Science (6). As the area of innate immunity is largely unexplored, we can look forward to many novel
findings in this area in the immediate future.
References
1. Janeway, C. A., Jr. 1989. Approaching the asymptote?: evolution and revolution
in immunology. Cold Spring Harbor Symp. Quant. Biol. 54:1.
2. Liu, Y., and C. A. Janeway, Jr. 1992. Cells that present both specific ligand and
costimulatory activity are the most efficient inducers of clonal expansion of normal CD4 T cells. Proc. Natl. Acad. Sci. USA 89:3845.
3. Lemaitre, B., E. Nicolas, L. Michaut, J. M. Reichart, and J. A. Hoffmann. 1996.
The dorsoventral regulatory gene casette spatzle/Toll/cactus controls the potent
antifungal response in Drosophila adults. Cell 86:973.
4. Medzhitov, R. M., and C. A. Janeway, Jr. 1998. An ancient system of host
defense. Curr. Opin. Immunol. 10:12.
5. Medzhitov, R. M., P. Preston-Hurlburt, and C. A. Janeway, Jr. 1997. A human
homologue of the Drosophila Toll protein signals activation of adaptive immunity. Nature 388:394.
6. Fearon, D. T., and R. M. Locksley. 1996. The instructive role of innate immunity
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cells. These transfectants were tested for the induction of NFkB
activity using an NFkB-driven luciferase reporter gene. The signal
given by the hToll dominantly active protein is almost as strong as
the positive control of PHA plus PMA in the activation of NFkB
in Jurkat. We also transfected the human monocytic cell line
THP-1. We detected the transfectants by FACS analysis with antimouse CD4. The existence of positive and active transfectants allowed us to examine the production of costimulators and pro-inflammatory cytokines by THP-1 cells transfected with the
dominantly active form of hToll, and we found that IL-1, IL-8,
and, in the presence of interferon-g, IL-6 are produced, as is IL-12.
Most importantly to our working hypothesis, we found clear induction of B7.1, which was present at undetectable levels in parental, nontransfected THP-1 cells, and B7.2, which was present at
a low level in THP-1 cells and was strongly induced by transfection with a dominantly active form of hToll. Thus, many of the
features predicted for pattern recognition receptors were met by
hToll. However, two things are clear: First, the mechanism by
which hToll recognizes pathogens remains to be worked out, and
second, the protease product that forms the ligand for Toll in Drosophila is not yet defined in mammalian systems.
We next investigated the proximal part of the hToll signaling
pathway, and determined that it involved an adapter protein called
MyD88. Overexpression of MyD88 signaled expression of an
NFkB reporter gene construct that was dose-dependent. The induction of NFkB by hToll was dependent upon the intact MyD88
construct, as neither the N-terminal death domain nor the C-terminal TIR domain could activate NFkB. Because the construct
consisting only of the C-terminal domain of MyD88, called
MyDC, did not activate NFkB even when it was overexpressed,
we asked whether this construct could act as a dominant negative
inhibitor of NFkB activation by hToll. In this experiment, the
MyDC construct expressing only the TIR domain inhibited activation of NFkB by the dominantly active form of hToll, showing
that MyD88 was downstream of hToll and upstream of NFkB. Our
next experiments were designed to position IRAK and TRAF6 in
the pathway, and these showed that IRAK dominant negative mutants as well as a dominant negative mutant of TRAF6 inhibited
signals from hToll and from MyD88, positioning IRAK downstream from MyD88 and TRAF6 downstream of IRAK. A dominant negative mutant of TRAF2, which signals activation of NFkB
by TNF-a, does not interfere with NFkB activation by hToll or
IL-1. Finally, we showed that both hToll and MyD88 could induce
the transcription factor called AP-1, which is a heterodimer of Fos
and Jun. Surprisingly, in the same experiment, we failed to observe
AP-1 signals in the presence of IL-1, suggesting that there is a
branch-point in this pathway downstream of TRAF6. Thus, we
have defined another member of the hToll signaling pathway as the
adapter protein MyD88.
Recently we have cloned another member of this ancient pathway of host defense in Drosophila, which is a homologue of the
mammalian TRAF proteins and appears to function in the dToll
signaling pathway in a way analogous to the function of TRAF6 in
the mammalian host defense system. From the sequence of this
protein it is not clear which mammalian TRAF protein it most
closely resembles, so we call it simply dTRAF. dTRAF appears to
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