Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity

18 Pages·2000·0.184 MB·English

by  Oppenheim J.J., Feldmann M.

Introduction to the Role
of Cytokines in Innate
Host Defense and
Adaptive Immunity
Joost J. Oppenheim1,* and Marc Feldmann2
1Laboratory of Molecular Immunoregulation, Division of Basic Sciences, National Cancer Institute,
Frederick, MD 21702-1201, USA
2Cytokine and Cellular Immunology Division, Kennedy Institute of Rheumatology,
1 Aspenlea Road, Hammersmith, London, W6 8LH, UK
*corresponding author tel: [phone], fax: [phone], e-mail: [email protected]
DOI: 10.1006/rwcy.2000.01001.
HISTORICAL OVERVIEW cells to become inhospitable to viruses and inhibit
viral replication.
Although the methods available prior to 1970
permitted only phenomenological detection of biolo-
The era of ‘lymphodrek’
gical activities, the possibility that host factors were
responsibleforfeverwassuggestedover50yearsago.
The first to propose that soluble factors modulated Immunologists, however, should be credited for the
host reactions was Menkin, who ‘purified’ fever- enthusiastic pursuit of studies of mediators of host
inducing activities from inflammatory exudates and defense. The concept that ‘humoral factors’ might be
called them ‘pyrexin’ (Menkin, 1944). These factors responsible for cell-mediated host defenses could be
weresubsequentlyshowntosurviveboilingandtobe readily understood by immunologists who were
contaminated by bacterial pyrogens (endotoxin). accustomed to the concept of specific antibody-
Bennett and Beeson in 1953 were able to separate mediatedreactions.Theidentificationoflymphocytes
an endogenous pyrogen (EP) from endotoxin present as the principal immunocompetent cells by Gowans
in acute inflammatory exudates and were also able to (1959) and the development of tissue culture tech-
extract EP from peripheral blood leukocytes. niques for studies of in vitro lymphoproliferative
Analogous intercellular signals, namely nerve growth reactions to a polyclonal stimulant, namely phytohe-
factors, were discovered by Levi-Montalcini and magglutinin, by Nowell (1960), laid the groundwork
Hamburger (1953). They observed that implanted for the detection of lymphocyte-derived soluble
mouse sarcoma cells produced an agent that induced mediators by immunologists. These critical observa-
marked growth and differentiation of distant sympa- tions were reinforced by the demonstrations by
thetic ganglia of chick embryos. Interferons were Pearmain et al. (1963) that only lymphocytes from
discovered by Isaacs and Lindenmann (1957), as tuberculin-sensitive donors could undergo blastogen-
nonantibody cell-derived factors that induced host esis in response to tubercle antigens. The idea that
4 Joost J. Oppenheim and Marc Feldmann
antigens induced specific in vitro proliferative repeti- reviewedbyBernhagenetal.,1993).Studiesoffactors
tionswasreinforcedbytheobservationsofBainetal. acting on macrophages led to the more readily inter-
(1964) who showed that allogeneic mixed leukocyte pretableobservation that antigen-stimulatedlympho-
cultures also resulted in lymphocyte blastogenesis. cyte cultures also produce a macrophage-activating
Three successive discoveries between 1964 and 1967 factor (MAF), which nonspecifically arms macro-
by independent laboratories initiated the studies of phages to kill intracellular bacteria (Nathan et al.,
the effects of ‘factors in culture supernatants’ by 1971). Immune interferon (IFN(cid:13)) was subsequently
immunologists.KasakuraandLowenstein(1965)were reportedtohavepotentMAFactivity(asreviewedby
the first to detect the presence in supernatants of Schreiber and Celada, 1985). MAF was considered a
antigen- or alloantigen-stimulated leukocyte cultures major lymphocyte-derived nonspecific mediator of
of mitogenic or blastogenic factors (BFs) that were host defense. Studies of lymphotoxin were amplified
not attributable to antibodies. This was rapidly by the subsequent discovery by Carswell et al. (1975)
followed by the detection in such supernatants of of serum factors with in vitro cytotoxic effects that
immunologically nonspecific macrophage migration inducedinvivotumornecrosis(tumornecrosisfactor,
inhibitoryfactors(MIFs)byDavid(1966)andBloom TNF). TNF production was induced by endotoxin
and Bennett (1966) and of a cytotoxic factor called and, in contrast to lymphocyte-derived LT, was
lymphotoxin (LT) by Ruddle and Waksman (1967) predominantly a macrophage-derived product. Both
and Granger and Williams (1968). These three of these factors were considered to contribute to host
activities were considered to be in vitro ‘correlates’ defense against infectious and neoplastic diseases.
or indicators of cellular immunity that could be
induced by specific in vitro immune responses.
Immunologists became fascinated with these non- The advent of nonlymphocytic
specific lymphoproliferative factors and effectors of
cytokines
host defense and were very intrigued by mitogenic
growth factors as mediators of lymphocyte replica-
tion. Kasakura and Lowenstein (1965) reported that In an independent but convergent line of research,
leukocytes stimulated by bidirectional allogeneic Gery and colleagues (Gery et al., 1971; Gery and
mixed leukocyte reactions (MLRs) secreted BFs. Waksman,1972)reportedthatactivatedmacrophages
The supernatants of MLRs were considerably more secreted a mitogenic factor for thymocytes called
mitogenic than those from unstimulated leukocytes, lymphocyte-activatingfactor(LAF).Thisrepresented
while cell extracts were completely inactive. Gordon the first report that nonlymphocytes could also
and MacLean (1965) demonstrated that inhibition of producegrowthfactorsactingonlymphocytes.Based
theblastogenicresponseinanMLRbypuromycinor on the overlap in the biochemical and biological acti-
5-fluorouracil also blocked the production of BFs, vities of LAF and endogenous pyrogens (EPs had
suggesting that BF was newly synthesized by lym- mitogenic effects on thymocytes, while LAF was
phoblasts. Since unstimulated leukocytes from auto- pyrogenic),Rosenwasseretal.(1979)werethefirstto
logousaswellashomologousdonorswereinducedby propose that these activities might be attributable to
BFs to synthesize DNA and RNA to enter the cell the same molecule. This observation permitted rapid
cycle,KasakuraandLowenstein(1967)demonstrated progress in the subsequence purification and identi-
for the first time that specific alloantigen-stimulated fication of endogenous pyrogen by substituting a
leukocyte cultures could generate immunologically simple quantitative LAF assay for in vivo fever
nonspecific mitogenic factors. Dumonde et al. (1969) assays.
reported that these nonantibody secretory mitogenic Cohen was the first to observe that MIF-like acti-
factors were products of antigen-activated lym- vities could be produced by a variety of nonmacro-
phocytes and coined the term ‘lymphokines’ for phageandnonlymphocyticcelllines(e.g.fibroblasts).
lymphocyte-derived mediators. This observation and the recognition by Gery that
Althoughmanylaboratoriesinthe1970sattempted macrophages produced ‘monokines’ such as LAF led
to characterize MIFs, in retrospect this indirect assay Cohen to propose the more inclusive term of ‘cyto-
of induction of macrophage adhesion proteins led up kines’ for the family of polypeptides secreted by a
ablindalley anddidnotresult inanyprogress inour variety of cell types that engage in immunologically
understanding of the processes of host defense. In mediatedinflammatoryreactions(Cohenetal.,1974).
fact, a number of cytokines such as interferon have Cytokines at present are defined as soluble, extra-
been shown to have MIF activity and the physiolo- cellularproteinsthatregulateinnateaswellasimmu-
gical role of the recently cloned cytokine with ‘MIF nologically regulated inflammatory reactions, cell
activity’ remains unclear (Weiser et al., 1989 and as growth, differentiation, development, and repair
Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity 5
processes culminating in the restoration of cell-activating factor and differentiation factor be
homeostasis. renamedinterleukin1(IL-1).Thelymphocyte-derived
By 1978 a review by Waksman listed almost 100 mitogenic factors such as lymphocyte mitogenic
apparently distinct cytokine activities, many more factor (LMF)/BF, killer helper factor (KHF) and T
than can be listed in our chronological review cell growth factor (TCGF) were renamed IL-2 (Mizel
(Table 1). At the second International Lymphokine and Farrar, 1970). The discoverers of TCGF
Workshop held in Ermatingen, Switzerland in 1979, (Morgan et al., 1976) resisted this change in ter-
advances in techniques for characterizing the biolo- minology on the grounds that their descriptive term
gicalandbiochemicalpropertiesofanumberofthese accurately reflected the activity of this lymphokine.
cytokine activities fostered the mistaken belief that However, the controversy was resolved by reports
most of these activities could be attributed to only a that IL-2 is also a proliferative signal for B cells
few molecules and culminated in their renaming as (Zubler et al., 1984) and natural killer (NK) cells
either interleukin 1 or interleukin 2 (see review by (Ortaldo et al., 1984). Despite the fact that many of
Oppenheim and Gery, 1993). This idea led to the the cytokines, such as IL-1, can be produced by and
substitution of a more ‘generic’ interleukin terminol- also act on nonleukocytic somatic cells as well as
ogy for the numerous confusing acronyms based on leukocytes (Oppenheim and Gery, 1982), the inter-
the activities of these cytokines. It was proposed that leukin nomenclature has been accepted; we are now
monocyte/macrophage-derived mitogenic factors up to IL-18. A number of the other cytokines,
such as LAF/EP, T cell-replacing factor III, B includingTNF,LT(cid:11)(nowalsoknownasTNF(cid:12)),the
Table 1 Chronology of cytokine discoveries
Date Discoverers Mediators
1944 Menkin Fever-inducing ‘pyrexin’ in exudates
1953 Bennett and Beeson Endogenous pyrogen in exudates
1953 Levi-Montalcini and Hamburger Nerve growth factor
1957 Isaacs and Lindenmann Nonantibody interferons (IFN)
1965 Kasakura and Lowenstein Blastogenic factors for lymphocytes
1966 David, and Bloom and Bennett Macrophage migration inhibitory factor
1967 Ruddle and Waksman Lymphotoxin
1969 Dumonde et al. Named lymphocyte-derived factors ‘lymphokines’
1969 Ward et al. Chemoattractants for monocytes and neutrophils
1971 Nathan et al. Macrophage-activating factor
1971 Gery et al. Macrophages – source of
‘lymphocyte-activating factor’ (LAF)
1974 Cohen et al. Nonleukocytes also produce ‘cytokines’
1975 Carswell et al. Tumor necrosis factors
1976 Morgan et al. T cell growth factor
1979 2nd International Lymphokine Nonlymphocyte- and lymphocyte-derived
Workshop (reviewed by Mizel and Farrar, 1970) mitogenic factors renamed interleukin 1
and 2 (IL-1 and IL-2) respectively
1980 Taniguchi et al., Nagata et al. Cloning of IFN(cid:12)1 and IFN(cid:11)1 respectively
1981 Ihle et al. IL-3 growth factor for hematopoietic progenitors
1982 Gray et al. Cloning of IFN(cid:13)
1984 Leonard et al. Cloning of IL-2 receptor (cid:11) chain
1985 Derynck et al. Cloning of transforming growth factor (cid:12) (TGF(cid:12))
1986 Mosmann et al. Identification of CSIF/IL-10
1986 Zilberstein et al. Sequenced IFN(cid:12)2 and renamed it IL-6
6 Joost J. Oppenheim and Marc Feldmann
IFNs and colony-stimulating factors (CSF) escaped receptor’. During 1984 the two cytotoxic factors LT
being renamed. The initial intent to restrict the inter- (renamed TNF(cid:12)) and TNF (designated TNF(cid:11)) were
leukin terminology to lymphocyte-targeted cytokines alsoclonedandexpressedbyPennicaetal.(1984)and
was very short-lived, however, with the naming of a Gray et al. (1984), respectively. The development of
lymphokine that acts predominantly as a multi-CSF new technologies permitting the generation of mice
and is a growth factor for hematopoietic progenitor with targeted deletion of TNF or LT genes has
cellsratherthanlymphocytesasIL-3(Ihleetal.,1981). drastically altered our perception of reality (Durum
and Muegge, 1998). Rather than being defined as a
cytotoxic factor, this revealed that mice lacking LT
The molecular period
fail to develop peripheral lymph nodes, and have
disorganized splenic tissue deficient in germinal
The molecular cytokine era began soon after the centers and as a result are immunodeficient. These
Ermatingen meeting with the development of newer studies showed for the first time that LT plays a
technologies such as high-performance liquid chro- pivotal role in the development of peripheral lym-
matography (HPLC), microsequencing, and the pro- phoid tissues. In contrast, TNF depletion generated
ductionofmonoclonalantibodiestocytokines.These mice with only limited disorganization of the peri-
developments permitted the purification and amino pheral lymphoid tissues, but they exhibited reduced
acid sequencing of the minuscule quantities of cyto- resistance to infectious challenges. These studies
kines secreted into culture supernatants. Application revealed TNF to be a key regulator of inflammation
of molecular biological techniques revolutionized the in host defense rather than a cytotoxic antitumor
cytokine field further by making available larger factor. The antitumor effects of TNF are not based
quantities of cloned and expressed recombinant cyto- on tumor cytotoxic effects, but are actually largely
kines and also resulted in the identification of more due to the capacity of TNF to stimulate endothelial
cytokines by direct expression gene cloning. These cells of newly formed blood vessels to produce
new technologies have led to the discovery of numer- clotting factors which results in their occlusion and
ous new cytokines and have modified our concepts thus the central necrosis of tumors. The differences
concerning the spectrum of activities ascribed to between TNF and LT also led to the prediction that
previously described cytokines. The first cytokine to they must use receptors in addition to TNFRI and
be cloned in 1980 by Taniguchi and his colleagues – TNFRII. Consequently, receptors specific for LT
IFN(cid:12)1 – was rapidly followed by the cloning of have been identified as LT(cid:12)R, because it binds a
IFN(cid:11)1by Nagataet al. (1980). To date, about 16–20 heterotrimer of LT(cid:11)/LT(cid:12)2. Another member of the
variants of IFN(cid:11) have been identified, all of which TNF family, Fas, has actually been identified as a
interact with the same cell surface receptor and pro- potent cytotoxic factor (Nagata and Golstein, 1995;
mote antiviral resistance. This was followed by the Nagata, 1997). Mice with defective gld gene products
cloning of IFN(cid:13), by Gray and his colleagues at that lack a functional Fas ligand as well as mice who
Genentech(Grayetal.,1982).Thefollowingyearsaw have homolozygous defective lpr genes for the Fas
thecloningofIL-2byTaniguchiwithcollaboratorsat receptor are unable to eliminate lymphocytes and
AjinimotoCorp.(Taniguchietal.,1983).Availability develop marked lymphoid hyperplasia and severe
of the recombinant form of IL-2 enabled many autoimmune syndromes. The TNF family continues
investigatorstoconfirmthislymphokinetobeamajor to expand at an alarming rate and novel members of
lymphoproliferative cytokine for T cells, B cells, and the TNF family are still being identified to date.
NK cells. IL-2 also enhances the activities of lym-
phoidcellseitherdirectlyorindirectlybyinducingthe The identification of suppressive
production of a variety of other immunostimulating
cytokines
cytokines such as IFN(cid:13), TNF, and IL-1. Additional
lymphoproliferative growth factors in BF/LMF pre-
parationssuchasIL-4,IL-6,IL-7,IL-9,IL-10,IL-13, A number of cytokines have been identified that act
and IL-15 have since been identified. predominantly as downregulators of inflammatory
and immune responses. The first of the cytokines
whichprovedsubsequentlytohavepotentnonspecific
The cytokine receptor phase
immunosuppressive and anti-inflammatory effects,
namely transforming growth factor (cid:12) (TGF(cid:12)) was
In 1984, the first of the three chains in the IL-2 cloned in 1985 by Derynck with his colleagues at
receptor, IL-2R(cid:11), was cloned by Leonard and colle- Genentech and with collaborators at the NIH
agues,thususheringinthepresenteraofthe‘cytokine (Derynck et al., 1985). In addition, a ‘cytokine
Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity 7
synthesisinhibitoryfactor’(CSIF)initiallydiscovered of T cells and B cells to their proper locations in
by Mosmann and colleagues (Moore et al., 1990) lymphoid tissues.
functions as a pivotal immunomodulator. CSIF, now
renamed IL-10, has immunoenhancing effects on
The IL-6 family
humoral immunity as well as considerable immuno-
suppressive effects on cell-mediated immune res-
ponses. It should be noted that some viruses subvert The amino acid sequence of IL-6 was actually first
the immune response by producing homologs of reported as a so-called 26kDa protein by Content
mammalian cytokines or their receptors. This is etal.in1982.Thisproductoffibroblastswasthought
exemplified by the Epstein–Barr virus (EBV) which to have antiviral activity and was therefore mis-
producesahomolog‘virokine’versionofIL-10which identified as IFN-B2. A protein termed B cell dif-
has only the immunosuppressive and B cell-stimulat- ferentiation factor (BCDF) or B cell-stimulating
ing, but lacks the T cell-stimulating capacities of the factor 2 (BSF-2) based on its capacity to induce
mammalian cytokine. EBV has therefore cleverly antibody secretion by B cells was cloned by Hirano
mutated the cytokine to suppress host resistance and et al. in 1986. BCDF/BSF-2 was renamed IL-6 and
promote the growth of its target B cells. It is the first found to be identical to IFN-B2 (Zilberstein et al.,
ofmanyvirallyencodedcytokineligandandreceptor 1986; Poupart et al., 1987). Although IL-6 does not
homologs that act as agonists or antagonists to have any antiviral activity, it may induce cells to
subvert host defenses. produce IFN. IL-6 also was identified as an
‘hepatocyte-stimulating factor’ based on its capacity
to induce acute phase protein production by cultured
Chemotactic cytokines hepatocytes (Andus et al., 1988). IL-6 also plays an
important role in early hematopoietic cell prolifera-
(‘chemokines’)
tionanddifferentiation.Anumberofligandsthatare
closely related to IL-6 in using a common receptor
The interest in chemotactic cytokines began with the chain were subsequently discovered as will be
report of Ward and colleagues showing that antigen discussed. It is beyond the scope of this chapter to
induced lymphocytes to produce chemoattractants discuss the discovery ofall the remaininginterleukins
formonocytes(Wardetal.,1969).Investigatorsinthe in detail, but we will discuss the discovery of some of
1970s identified mononuclear cells as a source of the receptors with which the interleukins interact.
neutrophil chemoattractants and these were subse-
quently attributed to partially purified preparations
Cytokines today
of natural IL-1 (Luger et al., 1983; Sauder et al.,
1984).LeonardandOppenheimsubsequentlyrealized
that more purified preparations of recombinant IL-1 Today, we are again confronted by considerably
failedtoactaschemoattractants.ThisledYoshimura more than 100 cytokines, but this time they are
et al. (1987) to purify these ‘contaminant’ chemoat- structurally identified molecules that exhibit unique
tractant cytokines in their laboratories and these as well as many redundant activities (as reviewed by
efforts culminated in the cloning of a monocyte- Oppenheim and Saklatvala, 1993). The observations
derived neutrophil chemotactic factor (MDNCF), that some of these cytokines, although themselves
also known as neutrophil-attracting protein 1 structurally distinct, share receptors or receptor
(NAP-1), by Matsushima et al. (1988). MDNCF chains may account for some of this redundancy.
also was observed to chemoattract T cells as well as The identification of cytokine receptor families has
neutrophils and was therefore renamed IL-8 (Larsen established relationships between groups of structu-
et al., 1989). To date a large superfamily of cytokines rally independent cytokines that by sharing receptors
with more than 50 distinct members of structurally presumablygoontousethesamesignaltransduction
related chemoattractants acting on every inflamma- pathways. This is amply illustrated by the 20 IFN(cid:11)
tory cell type has been cloned. These ‘chemoattrac- variants as well as IFN(cid:12) and IFN! which all share
tant cytokines’ are now called chemokines for short. the same receptor, nuclear-binding proteins, and
Chemokines have been shown to regulate the adhe- antiviralactivities.TheyaredistinguishedfromIFN(cid:13)
sion of leukocytes to endothelial cells, to promote whichbindstoadifferentreceptorandpossessesboth
diapedesis and migration of leukocytes into inflam- antibacterial and antiviral activities. However, recep-
matory sites, to costimulate immune responses, tors can have disparate functions. Two receptors for
enhance allergic reactions, to regulate angiogenesis, IL-1 have been identified which belong to the
influence hematopoiesis and to promote the homing immunoglobulin superfamily. Both IL-1(cid:11) and IL-1(cid:12)
8 Joost J. Oppenheim and Marc Feldmann
bind to each of these receptors with equal affinity. (e.g. TNF(cid:11), IL-1(cid:11), TGF(cid:11)) are also functional as cell
ThetypeIreceptor,however,issolelyresponsiblefor surface signaling molecules (Kriegler et al., 1988).
signal transduction while the nonsignaling type II Gene cloning has revealed that cytokines with
receptor sequestrates both IL-1(cid:11) and IL-1(cid:12) on the varying degrees of amino acid homology belong to a
cell surface, or when shed in soluble form is able to number of cytokine families (Smith et al., 1994;
selectivelybindIL-1(cid:12) andcanserveasaninhibitorof Dinarello, 1998). Some members of these families are
IL-1(cid:12) activities. TNF(cid:11) and LT, although only 28% predominantly, if not exclusively, active as the cell
homologous in amino acid sequence, share the same surface form (e.g. CD40 ligands), while others act
two TNF receptors, each of which activates some exclusively extracellularly. As these molecules share
distinct cellular activities. Binding of TNF or LT to the same signaling pathways as secreted forms, these
the smaller p55 receptor results in cytotoxic events, are now also considered cytokines.
while binding to the p75 TNF receptor favors lym- All cytokines are proteins, but their physical
phoproliferative responses. These two TNF receptors characteristics vary. The largest family, the chemo-
exhibitunexpectedhomologytoreceptorsfordistinct kines, are low molecular weight 8–10kDa polypep-
cytokines with unrelated activities such as nerve tides. Some are single-chain proteins (e.g. IL-1),
growth factor (NGF), to cytokine receptors resulting others, such as IFN(cid:13), form homodimers, while mem-
in apoptotic consequences such as Fas, or with costi- bers of the TNF(cid:11) family are trimers, usually homo-
mulatory lymphoproliferative effects as for CD27, trimers except for lymphotoxin which can be a
CD30, and CD40. Observations relating these cyto- heterotrimer.Manyofthecytokinesareglycosylated.
kinestooneanotherbasedonreceptorhomologyand A distinctive feature of cytokines is that they are
sharing has accelerated the characterization of newly usuallynotconstitutivelyproduced,butaregenerated
identified members of the TNF family. inresponsetostimulation.Typicallytheirproduction
Most of the receptors for the colony-stimulating cycle lasts a few hours to a few days in the normal
factors (e.g. IL-3, GM-CSF, and G-CSF) as well as state,butifthestimuluspersists,asinadiseasestate,
for many of the lymphoproliferative interleukins are then it is possible for cytokine production to be
members of the hematopoietin receptor family. A prolonged (Feldmann et al., 1996a,b).
numberofthesecytokinesshareareceptorchain.The Virtuallyallcellscanproducecytokines,inresponse
receptors for IL-2, IL-4, IL-7, IL-9, and IL-15 all todiverse stimuli. Which cytokinesa cell makesfrom
appear to share a (cid:13) chain, which presumably con- its potential repertoire depends on the stimulus, its
tributes to the signal transduction by these lympho- nature, duration, intensity, as well as the presence of
proliferativecytokines.Thismayalsoaccountforthe otherfactors–othercytokines,hormones,cellcontact
recent observation that defects in the IL-2 receptor (cid:13) interaction, etc.
chain result in a much more severe human immu- A key feature of cytokines is their ‘potency’ –
nodeficiency state (Schorle et al., 1991) than is meaning that they are bioactive at very low con-
obtainedbyadefectinIL-2asinIL-2knockoutmice centrations, often in range 10(cid:255)10 to 10(cid:255)13mol/L,
(DiSanto et al., 1995). Similar results reviewed by roughly 1 ng(cid:255)1 pg/mL. This potency is linked to the
Miyajima et al. (1992) indicate that three of the high affinity of their receptors, usually 10(cid:255)9 to 10(cid:255)12
hematopoietic growth factors (GM-CSF, IL-3, and mol/L, and also that signaling does not require high
G-CSF) share a (cid:12) signal transducing chain. Finally, receptor occupancy, often (cid:24)10% will suffice.
the discovery that the IL-6 receptor pl30 signal The generic classification ‘cytokine’ is now used to
transducer is shared by leukemia inhibitory factor include interferons, discovered as antiviral proteins,
(LIF),oncostatinM,IL-11,ciliaryneurotropicfactor interleukins, molecules initially described as media-
(CNTF) and cardiotropin 1 (CT-1) has accelerated tors between leukocytes, chemokines (chemotactic
characterizationofIL-6 familymembers (asreviewed cytokines), hematopoietic factors and other growth
by Taga and Kishimoto, 1997). factors.Theterm‘lymphokine’,denotingaproductof
lymphocytes,isnolongercommonlyused,asmostof
the molecules produced by lymphocytes are also
WHAT ARE CYTOKINES?
produced by other cells.
Othergeneraldiscussionsofcytokinescanbefound
Theoriginalviewof‘cytokines’describedaboverested in The Cytokine Handbook, Chapter 1 by Jan Vilcek
on the concept that these were extracellular protein (Vilcek,1998),orSectionI–IntroductiontoCytokine
messenger molecules produced by cells involved in Biology in Clinical Applications of Cytokines
inflammation,immunity,differentiation,celldivision, (Oppenheim et al., 1993).
fibrosis repair, etc. However, molecular character- Alistingofsomecytokinesgroupedintofamiliesis
ization of these proteins has revealed that many shown in Table 2.
Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity 9
Table 2 Cytokine families: examples grouped by structural similarity of ligands and/or receptors
Family Abbreviation Name
Hematopoietins IL-2 Interleukin 2
IL-3 Interleukin 3
IL-4 Interleukin 4
IL-5 Interleukin 5
IL-6 Interleukin 6
IL-7 Interleukin 7
IL-9 Interleukin 9
IL-11 Interleukin 11
IL-12 Interleukin 12
EPO Erythropoietin
LIF Leukemia inhibitory factor
GM-CSF Granulocyte– macrophage
colony-stimulating factor
G-CSF Granulocyte colony-stimulating factor
OSM Oncostatin M
CNTF Ciliary neurotropic factor
GH Growth hormone
TPO Thrombopoietin
TNF family TNF(cid:11) Tumor necrosis factor (cid:11)
LT(cid:11) Lymphotoxin (cid:11)
LT(cid:12) Lymphotoxin (cid:12)
CD40L CD40 ligand
CD30L CD30 ligand
CD27L CD27 ligand
4-1BBL
FasL Fas ligand
IL-1 family IL-1(cid:11) Interleukin 1(cid:11)
IL-1(cid:12) Interleukin 1(cid:12)
IL-1Ra Interleukin 1 receptor
antagonist
bFGF Basic fibroblast growth factor
aFGF Acidic fibroblast growth factor
ECGF Endothelial cell growth factor
(cid:11) (CXC) family IL-8 Interleukin 8
GRO(cid:11)/(cid:12)/(cid:13) Melanocyte growth-stimulating factor
NAP-2 Neutrophil-activating protein
ENA 78 Epithelial neutrophil-activating peptide
GCP-2 Granulocyte chemotactic protein
PF4 Platelet factor 4
CTAP-3 Connective tissue-activating peptide 3
MIG Monokine induced by IFN(cid:13)
10 Joost J. Oppenheim and Marc Feldmann
Table 2 (Continued)
Family Abbreviation Name
(cid:13)IP-10 IFN(cid:13)-inducible protein 10
I-TAC
(cid:12) (CC) family MCP-1 Monocyte chemoattractant protein 1
MCP-2 Monocyte chemoattractant protein 2
MCP-3 Monocyte chemoattractant protein 3
MIP-1(cid:12) Macrophage inflammatory protein 1(cid:12)
MIP-1(cid:11) Macrophage inflammatory protein 1(cid:11)
RANTES Regulated upon activation normal T cell
expressed and secreted
PDGF family PDGF A Platelet-derived growth factor A
PDGF B Platelet-derived growth factor B
CSF Macrophage colony-stimulating factor
SCF Stem cell factor
TGF(cid:12) family TGF(cid:12) Transforming growth factor (cid:12)
Inhibin (cid:12) 2A, 2B
Activin 1, 2A, 2b, 4
BMP 1, Bone morphogenetic protein
2A, 2B, 4
RELATIONSHIP OF CYTOKINES The properties of growth factors are intermediate
between hormones and the cytokines; these are the
TO HORMONES AND GROWTH
ones most likely to be found in the blood in the
FACTORS
absence of acute stimulation. Many of the receptors
for growth factors like those for many hormones are
Although cytokines share properties in common with kinases (e.g. PDGF, VEGF, FGF), tyrosine kinases
hormones and interact with them, there are also except for the receptors for the TGF(cid:12) family, which
notable distinctions. There are differences between are serine/threonine kinases.
inducible cytokines, and hormones which are pro-
duced continuously, and growth factors. Hormones
HOW DO CYTOKINES ACT?
are typically produced by specialized cells and
released into the bloodstream and so can act at a
distance from their source, in an ‘endocrine fashion’. Cytokines are produced by most cells. Each cell has
In contrast, cytokines typically act at short range, a its program of cytokines it can synthesize. Some cell
fewcelldiametersapart,asina‘paracrine’(actingon typessuchasmacrophages,Tcells,andmastcellsmake
neighboringcell)or‘autocrine’(actingonself)manner. a very wide spectrum of cytokines. Whereas most
While levels of hormones are easily measurable cytokines are produced and soon released, in some
in the serum or plasma, only a few cytokines are circumstancescytokinesaretemporarilystoredbythe
regularly detected there. These include macrophage cell. These can be cell surface-bound cytokines which
colony-stimulating factor (M-CSF), erythropoietin may (e.g. TNF(cid:11)) or may not (e.g. TGF(cid:12)) be bio-
(EPO), thrombopoietin (TPO), stem cell factor logically active. Cytokines may be stored intracyto-
(SCF), the latent form of TGF(cid:12), and a chemokine, plasmically(e.g.IL-1(cid:11)),andmaynothaveanobvious
SDF1. A summary of the similarities and differ- form of release, or in granules as in mast cells or
ences between hormones and cytokines is shown in platelets.Theexportintothesupernatantisnolonger
Figure 1. an obligatory part of cytokine physiology.
Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity 11
Figure 1 What are the differences between cytokines and hormones?
Gradient
Cytokine IL-6, M-CSF, EPO Hormone
Acts locally Acts at distance
Made by many cells, e.g. IL-1, IL-6 almost ubiquitous Made by specialized cells and organs, e.g. pituitary, adrenal
Synthesized transiently after cell activation Produced constitutively and continuously
Usually inactive in serum/plasma Bioactive in serum/plasma
Common properties: Receptors often homologous (e.g. hematopoietin)
Potent signals
The usual mechanism of action of cytokines is on fewest possible genes (Feldmannet al., 1996a; Vilcek,
neighboring cells, and there are multiple mechanisms 1998).
to restrict the diffusion of cytokines. The most wide- For certain important functions, such as macro-
spread mechanism is the presence of receptors on the phage activation, there appear to be multiple cyto-
cell surface which bind, signal and then usually inter- kines capable of this function, all of which may be
nalize the cytokine and lead to its degradation. There present in apparently active concentration at times of
are also, for those receptors with a single transmem- activation.Thisis knownas‘redundancy’incytokine
brane spanning section (which means all except the function. An even more marked example is the
chemokine receptors), soluble receptors consisting of numerous chemokines that can attract monocytes for
the extracellular domain (Fernandez-Botran, 1991). example. It is likely that this functional redundancy
Theseretaincytokine-bindingactivityandacttolimit based on in vitro assays is more apparent than real.
thebioavailabilityofthecytokine,whichwhenbound Gene targeting (knockout) experiments have shown
to soluble receptor is not free to bind to cell surface unexpecteddifferencesbetweencertaincytokineswith
signaling receptor. There are other lower affinity overlapping in vitro activities. This is presumably
cytokine inhibitors in the blood, such as (cid:11) -macro- based on in vivo compartmentalization, cytokine
2
globulin which binds IL-1. So far there is only one interactions, and differences in the kinetics of
describedcompetitiveinhibitorofcytokinesactingon cytokine production and effects.
receptors, the IL-1 receptor antagonist (IL-1Ra)
(Arend, 1993). This gene is alternatively spliced to
generate a number of isoforms, two of which remain
SOURCES OF CYTOKINES
intracytoplasmic. The function of intracytoplasmic
IL-1Raisnotclear.ExtracellularIL-1Raispresentin
normal body fluids at appreciable concentrations, Essentially all cells make cytokines, when stimulated.
(cid:24)300pg/mL,aconcentrationprobablynotsufficient Macrophages, T cells, and mast cells are among the
to block IL-1 signaling but sufficient to limit its most abundant sources, but other cells of hemato-
spreadfromthesource.Asummaryofcytokineinhib- poietic linage such as B cells, dendritic cells, and NK
itors is shown in Table 3. The interactions of cyto- cells are all important sources, as are nonhemato-
kines with their receptors is discussed in the chapter poieticcellssuchasfibroblasts,chondrocytes,hepato-
on Cytokine and receptor paradigm. Families of cytes, epithelial cells, etc.
receptors are illustrated in Figure 2. For a period of time in the 1980s it was considered
The human genome comprises (cid:24)3(cid:2)109 bases of thatgranulocytesdidnotmakeanycytokinesnordid
DNA and estimates of the number genes range dendriticcells.However,morerefinedtechnologyhas
between 50,000 and 100,000 (Schuler et al., 1996). shownthatthecytokineproductionfoundingranulo-
Thusitisnotsurprisingthatthereisatendencytouse cytepreparationswasnotduetocontaminantmacro-
the same messenger molecule (e.g. cytokine) and phages, and even though each granulocytemay make
receptormolecules,withtheircomplexsignalinghook- little compared with a macrophage, their huge
ups for mediation of distinct messages in different numbers in certain lesions means that the cytokines
cells. Thus IL-1 was reported to be a ‘lymphocyte released bygranulocytesare ofimportance.Thismay
activating factor’ on T cells, as well as ‘catabolin’, especially be the case for their release of CC chemo-
a degradation-inducing factor on cartilage. The kines, which helps to orchestrate the subsequent
different actions of a single cytokine on distinct cells ingress of macrophages and T cells. The current con-
are sometimes known as ‘pleiotropy’. This process sensusisthusthatallnucleatedcells,includingneurons
maximizes information transfer while utilizing the make cytokines. In the on-line version, cytokines
12 Joost J. Oppenheim and Marc Feldmann
Table 3 Cytokine inhibitors
Inihibitors Examples
Soluble, extracellular domain of cytokine receptor sTNFR p55 and p75, sIL-1R types I and II,
sIFN(cid:13)R, sIL-6R, etc.
IL-1 receptor antagonist (IL-1Ra)
Cytokines with mostly inhibitory properties TGF(cid:12), IL-10, IL-4, IL-13, IFN(cid:13), and IFN(cid:12)
Low affinity (cid:11) -Macroglobulin, ? albumin
2
Figure 2 Cytokine receptor families are grouped broadly into six categories
based on their molecular structure.
TNF/NGF-R
Ig family
Hematopoietic family
GF-R family Ig IFN-R Type II
Ig family TGFb R Transmembrane
Ig
IL-2Rb IL-1R IFNa R P55 TNFR Chemokine receptor
IL-3R CSF-1R IFNb R p75 TNFR e.g. IL-8RA
IL-4Ra PDGFR IFNg R CD30 IL-8RB
IL-5Ra SCFR IL-10R CD27 RANTES
IL-6R CD40
IL-7R OX40
IL-9 NGF R
IL-3b , IL-5 Fas
gp130 4-1BB
GM-CSFR
EPOR
G-CSFR
LIFR
CNTFR
producedbydifferentcelltypesarelistedbychoosing will then become involved. Under the stimulation of
the ‘cell type’ view. the entities (and probably others not yet known) a
single stem cell can repopulate the hematopoietic
system of a mouse, yielding millions of progeny in a
few weeks. This topic is discussed in the chapter on
CYTOKINES IN BIOLOGICAL
Hematopoietic growth factors.
PROCESSES
Numerous cytokines are also involved in wound
healing and repair. For example repair of skin
Cytokines are involved in growth, differentiation, epitheliumgrowthprobablyinvolvesEGFandKGF,
cell division, apoptosis, inflammation, immunity, the dermal tissue growth probably involves PDGF,
repair, fibrosis, etc. This is perhaps not surprising TGF(cid:12), and FGF, new blood vessel formation and
considering the numbers of cytokines and growth repair probably involves VEGF and basic FGF, etc.
factors which have been described. Thusinanybiologicalprocess,anumberofcytokines
Hematopoiesis is a good example, with numerous are usually involved.
cytokines and hemotopoietic growth factors inter-
playing to permit the highly regulated and flexible
Inflammation
pattern of cell growth and differentiation, both in
health and in response to stress. To activate stem
cells, IL-1, IL-6, stem cell factor, and Flt-3 ligand Cytokines are critical molecules in the induction and
interact. If these cells are differentiating towards the resolution of inflammatory responses. There are
myeloidlineage,IL-3,G-CSF,M-CSF,andGM-CSF numerous so-called ‘proinflammatory cytokines’, but