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. 2001 Mar;102(3):323–330. doi: 10.1046/j.1365-2567.2001.01187.x

Interleukin (IL)-18 induces Langerhans cell migration by a tumour necrosis factor-α- and IL-1β-dependent mechanism

M Cumberbatch *, R J Dearman *, C Antonopoulos †, R W Groves †, I Kimber *
PMCID: PMC1783183  PMID: 11298831

Abstract

Following skin sensitization a proportion of epidermal Langerhans cells (LC) are stimulated to leave the skin and to migrate, via afferent lymphatics, to draining lymph nodes where they accumulate as immunostimulatory dendritic cells (DC). It has been demonstrated previously that tumour necrosis factor-α (TNF-α), an inducible product of epidermal keratinocytes, and interleukin (IL)-1β, produced exclusively by LC in murine epidermis, provide important signals for the initiation of this response. Recently, it has been demonstrated that IL-18, a cytokine produced by both LC and keratinocytes within the epidermis, may also participate in immune responses induced following skin sensitization. In the present investigations, the ability of IL-18 to contribute to the regulation of LC migration and the accumulation of DC in draining lymph nodes has been examined. It was found that, like IL-1β, IL-18 administered intradermally to mice resulted in a significant reduction in epidermal major histocompatibility complex (MHC) class II+ LC densities and a marked increase in lymph node DC numbers. Using neutralizing anti-TNF-α and blocking anti-type I IL-1 receptor (IL-1RI) antibodies, it was shown also that the induction by IL-18 of both LC mobilization and DC accumulation in regional lymph nodes was dependent upon availability of TNF-α and the integrity of IL-1RI signalling. Furthermore, using IL-1β converting enzyme (caspase-1) knockout mice, IL-18-induced LC migration was found to have a mandatory requirement for active IL-1β. Importantly, not only was IL-18 able to contribute to the regulation of LC migration, it was found to be essential for the manifestation of these processes in response to topical sensitization with the contact allergen oxazolone.

Introduction

The induced migration of epidermal Langerhans cells (LC) from skin, and their arrival as immunostimulatory dendritic cells (DC) in regional lymph nodes, play pivotal roles in the initiation of cutaneous immune responses, most notably contact sensitization.1–3 The mobilization of LC, their directed movement through afferent lymphatics and ultimate localization within the paracortical regions of draining lymph nodes are processes stimulated and regulated by cytokines and chemokines.2,3 Of particular importance for the initiation of migration are the epidermal cytokines interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α).4–12 The available evidence indicates that LC require at least two independent cytokine signals for mobilization, one being supplied by TNF-α (probably derived from keratinocytes) acting through TNF-R2 receptors,3,14–16

An epidermal cytokine that has not been considered previously in the context of LC migration is IL-18; a molecule first described as possessing IFN-γ-inducing properties, but which has since been found to influence a variety of immune and inflammatory responses.17–19 There are several reasons why it is relevant to question the potential influence of IL-18 on LC mobilization. First, IL-1β and IL-18 are structurally similar and although they signal through separate receptors, these cytokines induce virtually identical signal transduction pathways.17,18 Second, both dendritic cells (DC; including LC) and keratinocytes have been shown to express IL-18.20–23 Third, there is evidence that chemical allergens can induce the secretion by keratinocytes of IL-1822 and that there is elevated expression of this cytokine in contact hypersensitivity reactions.24

The purpose of the investigations described here was to examine in mice the ability of IL-18 to stimulate LC migration and the accumulation of DC in draining lymph nodes and whether this cytokine plays a mandatory role in LC migration stimulated by skin sensitization. In addition, we have sought to define the requirements for IL-1β and TNF-α in IL-18-induced LC mobilization.

Materials and methods

Animals

Young adult (6–8 weeks old) BALB/c strain mice, obtained from the Specific Pathogen Free Breeding Unit (Alderley Park, Macclesfield, UK), were used throughout these studies. In one series of experiments, caspase-1 knockout (KO) and wild-type (WT) littermate control mice (a gift of Dr W. Wong, BASF Corporation, Worcester, MA) were used which have been described previously.25

Cytokines and antibodies

Recombinant murine IL-18 (endotoxin content: < 0·1 ng/µg of IL-18) and recombinant murine IL-1β (endotoxin content: < 0·1 ng/µg of IL-1β) were purchased from PeproTech EC Ltd. (London, UK) and from R & D Systems (Abingdon, UK), respectively. Cytokines were diluted in sterile phosphate-buffered saline (PBS) containing 0·1% bovine serum albumin (BSA) as carrier protein and were administered locally by intradermal injection into both ear pinnae (30 µl) using 1 ml syringes with 30-gauge stainless steel needles. Control mice received an equivalent volume of carrier protein alone or were untreated. Polyclonal rabbit anti-mouse TNF-α (Genzyme Diagnostics, West Malling, UK) was supplied as a neat hyperimmune serum and was diluted 1 : 5 in sterile PBS prior to administration by intraperitoneal injection (100 µl). Control mice were treated concurrently with sterile normal rabbit serum (NRS) diluted to the same extent in sterile PBS. Monoclonal rat anti-mouse CD121a (IL-1RI/p80, rat immunoglobulin G1κ (IgG1κ); endotoxin content: < 0·01 ng/µg) or purified rat IgG1κ isotype control (endotoxin content: < 0·01 ng/µg; both from Pharmingen, San Diego, CA), polyclonal goat anti-mouse IL-18 (purified goat IgG; endotoxin content: < 0·01 ng/µg), polyclonal goat anti-mouse IL-1β (purified goat IgG; endotoxin content: < 0·01 ng/µg) or purified normal goat IgG (endotoxin content: < 0·01 ng/µg) (all from R & D Systems) were diluted in sterile PBS containing 0·1% BSA and were administered locally by intradermal injection into both ear pinnae (30 µl) using 1 ml syringes with 30-gauge stainless steel needles.

Chemicals and exposure

The skin-sensitizing chemical 4-ethoxy-2-phenyloxazol-5-one (oxazolone; Sigma Chemical Co., St Louis, MO) was dissolved in 4 : 1 acetone : olive oil (AOO). Groups of mice received 25 µl of 0·5% oxazolone, or AOO vehicle alone, on the dorsum of both ears.

Preparation and analysis of epidermal sheets

Ears were split with the aid of forceps into dorsal and ventral halves. Dorsal ear halves were incubated for 90 min at 37° with 0·02 m ethylenediamine tetra-acetic acid (EDTA; Sigma) dissolved in PBS. The epidermis was separated from the dermis using forceps and was washed in PBS. Epidermal sheets were fixed in acetone for 20 min at −20°. Following fixation, epidermal sheets were washed in PBS and then incubated at room temperature for 30 min with rat anti-mouse I-Ad/I-Ed monoclonal antibody (clone 2G9, rat IgG2aκ; Pharmingen), or purified rat IgG2aκ (Pharmingen), each diluted to 5 µg/ml in 0·1% BSA/PBS. Sheets were washed in PBS prior to incubation for a further 30 min with fluorescein isothiocyanate (FITC)-conjugated F(ab)2 goat anti-rat IgG (Serotec, Oxford, UK) diluted 1 : 100 in 0·1% BSA/PBS. Finally, sheets were washed in PBS and mounted on microscope slides in Citifluor (Citifluor Ltd, London, UK) and sealed with nail varnish. Samples were examined in a blinded fashion by fluorescence microscopy and the frequency of stained cells assessed using an eye piece with a calibrated grid (0·32 × 0·213 at × 40 magnification). Four epidermal sheets from each experimental group were incubated with anti-I-Ad/I-Ed antibody and two epidermal sheets from each experimental group were treated with isotype-matched control antibody. For each sample 10 consecutive fields in the central portion of the ear were examined. Results are expressed as the mean number of cells/mm2 (± se) derived from examination of 10 fields/sample for each of four samples. In no instance was any fluorescence detected following treatment with isotype-matched control antibody. The statistical significance of differences between experimental groups was calculated using Student's 2-sided t-test.

Isolation and enumeration of lymph node dendritic cells

Draining (auricular) lymph nodes were excised at various periods following treatment. Nodes were pooled for each experimental group, and single-cell suspensions of lymph node cells (LNC) prepared by mechanical disaggregation through 200-mesh stainless steel gauze. LNC were washed with, and suspended in, RPMI-1640 growth medium (Gibco, Paisley, UK) supplemented with 25 mm HEPES, 400 µg/ml streptomycin, 400 µg/ml ampicillin and 10% heat-inactivated fetal calf serum (RPMI–FCS). Viable cell counts were performed by exclusion of 0·5% trypan blue and the cell concentration adjusted to 5 × 106 cells/ml in RPMI–FCS. Dendritic cell-enriched populations were prepared as described previously.26 Briefly, 2 ml of Metrizamide (Sigma; 14·5% in RPMI–FCS) was layered gently under 8 ml of the cell suspension, and tubes centrifuged for 15min (600 g) at room temperature. Cells accumulating at the interface were collected, washed once, and resuspended in RPMI–FCS. The frequency of DC in such low buoyant density fractions was assessed routinely by direct morphological examination using phase contrast microscopy. Results are expressed as DC/node.

Results

IL-18 stimulates epidermal LC migration and draining lymph node DC accumulation

We have reported previously that intradermal administration to mice of homologous recombinant IL-1β stimulates a time-dependent reduction in epidermal MHC class II+ LC density reaching approximately 68% of control levels found in BSA-treated mice within 4 hr of exposure.8 This reduction in epidermal LC frequency was accompanied by a marked increase in draining lymph node DC numbers measured 17 hr later.8 Given the similarities between IL-18 and IL-1β, we compared directly the ability of these two cytokines to stimulate LC migration and the subsequent accumulation of DC in lymph nodes draining the site of exposure, each measured 4 hr and 17 hr following cytokine administration, respectively (Fig. 1). As shown previously, injection of carrier protein alone (0·1% BSA/PBS) was without effect on control epidermal LC numbers measured 4 hr following exposure. In contrast, a significant reduction in MHC class II+ LC frequency was detected following similar treatment of mice with IL-18 or IL-1β. Thus, in the experiments illustrated in Fig. 1 (ai) and (aii), IL-18 induced a 26·1% reduction (P < 0·01) and a 23·6% reduction (P < 0·001) in LC numbers, respectively. By comparison, IL-1β caused a 21·5% reduction (P < 0·005) and a 26·4% reduction (P < 0·005) in the same experiments. Importantly, under the same conditions of exposure, IL-18 induced not only a significant loss of detectable MHC class II+ LC from the epidermis but also a corresponding increase in the number of DC arriving in regional lymph nodes. In draining lymph nodes excised 17 hr after administration of cytokine, IL-18 and IL-1β were found to induce similar increases in lymph node DC numbers of 3·14 fold and 3·17 fold, respectively, when compared with groups of mice that had been exposed to carrier protein alone (Fig. 1b).

Figure 1.

Figure 1

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IL-18 induces epidermal MHC class II+ LC migration and draining lymph node DC accumulation. Groups of mice (a: n = 2/group, b: n = 10/group) received 30 µl intradermal injections into both ear pinnae of 50 ng of IL-18 or IL-1β each suspended in BSA, or of BSA alone. Control mice were untreated (–). (ai & aii) Ears (n = 4/group) were removed 4 hr later and epidermal sheets were prepared for analysis of MHC class II+ LC frequency (two independent experiments). (b) Draining auricular lymph nodes were excised 17 hr later and the number of DC/node was measured (a representative experiment).

IL-18 stimulates LC migration and lymph node DC accumulation in a TNF-α-dependent manner

The evidence available suggests that LC migration provoked by IL-1β requires receipt by LC of a second cytokine signal and that this is provided by TNF-α.9 Thus, mobilization of LC from the epidermis and their influx into draining lymph nodes provoked by local administration of IL-1β are compromised in mice pretreated systemically with neutralizing antibodies to TNF-α.9 To investigate whether IL-18-induced responses are similarly dependent upon TNF-α, groups of mice were exposed systemically (by intraperitoneal injection) to neutralizing anti-TNF-α antibody 2 hr prior to local intradermal injection of IL-18. Epidermal LC density and draining lymph node DC numbers were then assessed. In mice pretreated with normal rabbit serum, injection of IL-18 caused a significant decrease in epidermal LC numbers of 20·6% (Fig. 2ai; P < 0·05) and 26·9% (Fig. 2aii; P < 0·005) compared with mice that had received BSA in place of IL-18. Epidermal LC frequencies in control mice (NRS/BSA treatment) were not significantly different from LC values obtained for concurrent naive mice. Importantly, the reduction in LC numbers provoked following intradermal administration of IL-18 was prevented completely by systemic pretreatment of mice with neutralizing anti-TNF-α antibody. LC densities derived from these mice did not differ significantly from naive LC values.

Figure 2.

Figure 2

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IL-18-induced responses require TNF-α. Groups of mice (a: n = 2/group, b: n = 10/group) received a single intraperitoneal injection (100 µl) of anti-TNF-α antibody diluted 1 : 5 in PBS, or 100 µl of normal rabbit serum (NRS) diluted to an equivalent extent in PBS, 2 hr prior to intradermal administration of 30 µl (50 ng) of IL-18 suspended in BSA or of BSA alone. Control mice were untreated (–). (ai & aii) Ears (n = 4/group) were removed 4 hr later and epidermal sheets were prepared for analysis of MHC class II+ LC frequency (two independent experiments). (b) Draining auricular lymph nodes were excised 17 hr later and the number of DC/node was measured (a representative experiment).

That IL-18-induced LC migration is dependent upon TNF-α was reflected also in the pattern of DC accumulation observed in draining lymph nodes. Thus, in mice pretreated systemically with NRS, intradermal administration of IL-18 resulted in a 3·4-fold increase in lymph node DC number examined 17 hr following injection of cytokine compared with mice that had received BSA alone (Fig. 2b). Prior exposure of mice to neutralizing anti-TNF-α antibody prevented completely the influx of DC into draining lymph nodes provoked by injection of IL-18.

IL-18-induced responses require signalling through the type I IL-1 receptor

Since IL-18 and IL-1β share many common elements with respect to their signal transduction pathways, we speculated that either IL-18 was providing an alternative signal to IL-1β for the initiation of LC migration or possibly that IL-18 was providing an additional signal essential for the mobilization of LC during immune responses. To investigate whether migration provoked by IL-18 was dependent upon IL-1, blocking anti-IL-1RI antibodies were administered intradermally to mice prior to injection into the same site of IL-18 (Fig. 3). In mice pretreated with control immunoglobulin (IgG), IL-18 induced a significant reduction in LC numbers measured in epidermal sheets prepared 4 hr following administration of cytokine (28·9% reduction compared with BSA-treated control mice; P < 0·001, Fig. 3a). Similarly, under identical conditions of exposure, intradermal administration of IL-18 caused a marked influx of DC into draining lymph nodes measured 17 hr after cytokine treatment (5·4 fold increase compared with BSA-treated control mice, Fig. 3b). Importantly, prior exposure of mice to blocking anti-IL-1RI antibodies abrogated completely the IL-18 induced decrease in epidermal LC densities and resulted in a 74% inhibition of subsequent increases in lymph node DC numbers.

Figure 3.

Figure 3

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IL-18-induced responses are dependent upon IL-1. Groups of mice (a: n = 2/group, b: n = 10/group) received 30 µl intradermal injections into both ear pinnae of 3 µg of anti-IL-1RI or IgG1κ isotype control (IgG) 90 min prior to receipt of a second injection (30 µl) into the same site of IL-18 (50 ng) suspended in BSA or of BSA alone. Control mice were untreated (–). (a) Ears (n = 4/group) were removed 4 hr later and epidermal sheets were prepared for analysis of MHC class II+ LC frequency. (b) Draining auricular lymph nodes were excised 17 hr later and the number of DC/node was measured. Individual experiments for a and b are shown.

LC migration provoked by IL-18 requires functional caspase-1

To investigate further the apparent dependence of IL-18-induced responses on IL-1, LC migration in mice lacking the IL-1β converting enzyme (caspase-1) was examined. In three independent experiments, caspase-1 KO and WT littermate control mice were exposed intradermally to either IL-18, IL-1β or carrier protein (BSA) alone and epidermal LC frequencies were determined 4 hr later. As the representative experiment shown in Fig. 4 illustrates, administration to WT control mice of either IL-18 or IL-1β caused a similar decline in LC numbers of 26·4% for IL-18 (P < 0·005) and 30·2% for IL-1β (P < 0·001), each compared with BSA-treated WT control mice. A similar decrease in LC numbers was observed for all three independent experiments resulting in a mean (± sd) percentage reduction in LC frequencies of 25·1 ± 1·2% for IL-18 and 25·8 ± 4·2% for IL-1β. In caspase-1 KO mice treated concurrently with these cytokines, although IL-1β provoked a comparable reduction in LC frequencies to that obtained for WT mice (24·7% decrease compared with BSA-treated caspase-1 KO mice; P < 0·005), IL-18 failed to initiate LC migration in these mice. Thus, the mean (± sd) LC frequency following administration of IL-18 to caspase-1 KO mice, across all three experiments, was 688·6 ± 35·5 cells/mm2 compared with 698·1 ± 41·3 cells/mm2 for BSA-treated controls, representing a mean (± sd) percentage decline of 1·3 ± 1·8% (not significant). The mean LC value across all three experiments following treatment of caspase-1 KO mice with IL-1β was 534·5 ± 25·5 cells/mm2; a mean (± sd) percentage reduction of 23·4 ± 1·3% compared with BSA-treated KO mice.

Figure 4.

Figure 4

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Epidermal LC migration stimulated by IL-18 is absent in caspase-1 KO mice. Groups of mice (caspase-1 KO or WT controls, n = 2/group) received 30 µl intradermal injections into both ear pinnae of 50 ng of IL-18 or IL-1β each suspended in BSA, or of BSA alone. Control mice were untreated (–). Ears (n = 4/group) were removed 4 hr later and epidermal sheets were prepared for analysis of MHC class II+ LC frequency. One experiment, representative of three independent experiments, is shown.

Allergen-induced LC migration and draining lymph node DC accumulation are dependent upon IL-18

Taken together, the results described in Figs 1, 2, 3 and 4 demonstrate that IL-18 is able to initiate epidermal LC migration and that this response requires the additional presence of TNF-α and IL-1β. To investigate whether IL-18 is important for the stimulation of LC migration in response to an external stimulus, the influence of a neutralizing anti-IL-18 antibody on LC migration during allergic sensitization was determined. Mice were treated locally, by intradermal injection into both ear pinnae, with anti-IL-18 antibody or goat IgG, prior to topical application to the same site of oxazolone (Ox; 0·5%). Epidermal LC numbers were determined 4 hr later (Fig. 5a) and lymph node DC accumulation was measured after 18 hr (Fig. 5b). In mice treated previously with control goat IgG, topical application of Ox was associated with a 24·2% (Fig. 5ai; P < 0·05) and a 29·4% (Fig. 5aii; P < 0·005) loss of MHC class II+ LC from the epidermis compared with mice that had received vehicle (AOO) alone in place of Ox. In contrast, similar treatment of mice with anti-IL-18 antibody abrogated completely the decline in LC numbers normally detected after 4 hr of exposure to this allergen.27 A similar inhibition of Ox-induced DC accumulation was also observed in the presence of anti-IL-18 antibody (Fig. 5b). Following exposure to Ox, in mice that had been pretreated with IgG, 5·2-fold and 5·5-fold increases in draining lymph node DC numbers compared with naive mice were achieved in the experiments shown in Fig. 5 (bi) and (bii), respectively. In Fig. 5 (bii), the influence of intradermal administration of IgG itself on Ox-induced DC accumulation was also examined and found not to affect adversely the ability of DC to reach draining lymph nodes (5·9-fold increase compared with naive controls). Mice that received anti-IL-18 locally prior to treatment with Ox yielded a 64·1% inhibition (Fig. 5 bi) and an 86·0% inhibition (Fig. 5 bii) of the number of DC accumulating in draining lymph nodes. The inhibition of Ox-induced DC accumulation was comparable to that achieved in the presence of anti-IL-1β antibody (69·3% compared with IgG/Ox group); an observation that has been reported previously.9

Figure 5.

Figure 5

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Influence of anti-IL-18 antibody on oxazolone-induced epidermal LC migration and lymph node DC accumulation. Groups of mice (a: n = 2/group, b: n = 10/group) received 30 µl intradermal injections into both ear pinnae of (a) and (bi) 1 µg of anti-IL-18 antibody or goat IgG, or in the case of the experiment shown in (bii) an additional group received 1 µg of anti-IL-1β antibody, 30 min prior to exposure on the dorsum of both ears to 25 µl of 0·5% oxazolone (Ox). Control mice were untreated (–). (ai & aii) Ears (n = 4/group) were removed 4 hr later and epidermal sheets were prepared for analysis of MHC class II+ LC frequency (two independent experiments). (b) Draining auricular lymph nodes were excised 18 hr later and the number of DC/node was measured (two separate experiments).

Discussion

The results presented here demonstrate that IL-18 when administered intradermally to mice is able to stimulate both the migration away from the epidermis of a proportion of LC local to the site of exposure and, somewhat later, the accumulation of DC in draining lymph nodes. In these respects IL-18 has activities comparable with IL-1β and TNF-α, both of which are able to induce such responses when given intradermally.4,8,28 Importantly, not only was IL-18 able to induce LC migration and DC accumulation, it was found to be essential for the manifestation of these processes in response to topical sensitization with the contact allergen oxazolone. These observations prompted additional experiments to determine what requirements for IL-1β and TNF-α exist for IL-18-induced LC migration. It is clear from the results of these experiments that the induction by IL-18 of both LC mobilization and DC accumulation in regional nodes is dependent upon availability of TNF-α and the integrity of IL-1RI receptor signalling. The mandatory requirement for IL-1β in IL-18-induced LC migration was confirmed in experiments conducted using caspase-1 KO mice and their WT counterparts. As expected, both IL-1β and IL-18 were able to provoke LC migration in intact animals. However, in mice lacking caspase-1, and as a consequence lacking bioactive IL-1β, IL-18 was unable to effect migration.

Taken together these data reveal an important, and possibly obligatory, role for IL-18 in the stimulation of LC migration by chemical allergens during contact sensitization. In the light of previous investigations9 the conclusion drawn is that IL-1β, TNF-α and IL-18 all represent essential elements for normal LC mobilization following cutaneous sensitization.

The theory previously has been that topical sensitization causes the up-regulated expression of IL-1β by LC29 and that this cytokine, as well as delivering one signal for migration, acting in an autocrine fashion through IL-1RI receptors, also stimulates the production by keratinocytes of TNF-α. The latter cytokine provides a second signal to adjacent LC acting via TNF-R2 receptors.1,3 This sequence of events now has to be modified and extended to accommodate the need for IL-18. One possibility is that the availability of bioactive IL-18 is an early event during skin sensitization and acts as the initial trigger for IL-1β production, which in turn stimulates the production of TNF-α. Such a hypothesis is supported by the finding that IL-18 fails to stimulate LC migration in caspase-1 KO mice. Alternatively, or additionally, IL-18 might serve as the direct stimulus for the induction or up-regulation of both IL-1β and TNF-α. The precise contribution of IL-18 to IL-1β and/or TNF-α production will be dependent upon whether LC and/or keratinocytes prove to express the functional IL-18R complex. In either case an early inducing role for IL-18 is consistent with our observation that the ability of this cytokine to provoke LC migration is entirely dependent upon the availability of both IL-1β and TNF-α. Consistent with these observations also is evidence that IL-18 is able to induce the production of both IL-1β and TNF-α in other cell types such as peripheral blood mononuclear cells, in this instance the induction of IL-1β being secondary to TNF-α.30

If IL-18 is required for the elaboration of bioactive IL-1β and TNF-α following sensitization then it may be presumed that encounter with chemical allergen causes release by keratinocytes and/or LC of bioactive IL-18.22 There is no doubt that keratinocytes express IL-18,20,22,23 but there appears to be no clear consensus regarding the ability of these cells to process the precursor molecule to the bioactive cytokine. Recent studies of human keratinocytes suggest that they constitutively produce pro IL-18, but are unable to process it.23 If this is the case then one possibility is that cleavage of pro-IL-18 to the active cytokine early in the response to skin sensitization is effected by caspase-1 of LC.31 Alternatively, the generation of bioactive IL-18 could be achieved by the induction in keratinocytes of converting enzyme activity32 and/or through caspase-1-independent mechanisms.33 Irrespective of the provenance of IL-18 and the mechanism(s) through which it is processed, the available evidence points to IL-18 having a critical role in the generation of IL-1β and TNF-α. However, this does not necessarily exclude the possibilities that IL-18 itself delivers a signal directly to LC and/or that IL-18 mediates some of the other changes that facilitate the movement of LC from the skin to draining lymph nodes.

One interesting issue that arises from these data is the fact that there appears to be a specific need for IL-1β that cannot be accommodated by the availability of IL-18. Although the primary structure of IL-18 suggests a relationship with the IL-1 cytokine family, IL-18 does not bind to either the type I or type II IL-1R, and IL-1 does not bind to the IL-18R.34,35 However, the signalling pathways activated by IL-18 do share some common elements with those stimulated by IL-1. Thus, both cytokines induce the formation of the IL-1R-associated kinase (IRAK)/TNF-receptor associated factor-6 (TRAF-6) complex, subsequently activating nuclear factor κB (NFκB).34,35 Despite these similarities, it is clear from the present series of experiments that IL-18 is unable to replace the need for an IL-1β signal to LC and/or keratinocytes for the initiation of LC migration, and that the signals delivered by IL-18 and IL-1β in this respect are different. Furthermore these data suggest that whatever the mechanism of IL-18-induced migration, this response is dependent upon functional caspase-1, the interpretation being that IL-18 is required, either directly or indirectly, for IL-1β production.

In conclusion, the data presented here reveal that IL-18 is able to induce in mice LC migration through a mechanism that is dependent upon the availability of IL-1β and TNF-α. Moreover, the evidence indicates that contact allergen-induced LC migration and DC accumulation in draining lymph nodes require IL-18. Whatever the exact roles played by IL-18 in initiating and regulating LC mobilization it is clear that this cytokine is a critical mediator of cutaneous immune responses.

References

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