Langerhans cells and chemical allergy

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Langerhans cells and chemical allergy lan Kimber*, Rebecca J Dearman, Marie Cumberbatch and Russell JD Huby Epidermal Langerhans cells (LCs) play a pivotal role in the induction of cutaneous immune responses, including those provoked by chemical allergens. The delivery by LCs of allergen to draining lymph nodes requires cell migration from the skin, a process that is dependent upon the availability of epidermal cytokines - particularly TNF-(~ and IL-1 [3. Here we consider the ways in which these cytokines interact with LCs to both induce and regulate their mobilization in response to skin sensitization. In addition, the effects of these cytokines on both the selectivity of LC migration from the skin and protection of LCs from cell death are considered. Finally, the possible counter-regulatory activity of other cutaneous cytokines and the influence of LCs on the development of selective T lymphocyte responses are explored.

Addresses Zeneca Central Toxicology Laboratory,Alderley Park, Macclesfield, Cheshire SK10 4TJ, UK *e-mail: [email protected] ECA.com Correspondence: lan Kimber Current Opinion in Immunology 1998, 10:614-619

http:/Ibiomednet.comlelecref10952791501000614 © Current Biology Ltd ISSN 0952-7915

Abbreviations DC dendritic cell ICE IL-1 [3-convertingenzyme LC Langerhans cell mATPase membraneATPase MDR multidrug resistance TN F-~. turnout necrosis factor et TNFR TNF-~ receptor TRAF TN FR-associated factor

Introduction Langerhans cells (LCs), members of the wider family of dendritic cells (DCs), form a semi-contiguous network within the epidermis; here their primary functions are the recognition, internalization, processing, transport and (eventually) presentation of antigen that is encountered in the skin. As such, LCs play pivotal roles in the induction of cutaneous immune responses - - including those provoked following topical exposure to chemical allergens [1]. Although sensitization to chemicals via the skin is associated usually with the development of allergic contact dermatitis, there is increasing evidence that other forms of allergic response - - including those resulting in IgE antibody production and immediate-type hypersensitivity reactions - - may also be stimulated by topical exposure to certain chemical allergens [2]. To deliver antigen to responsive T lymphocytes in skin-draining lymph nodes and to initiate allergic sensitization to chemicals, it is necessary that LCs are stimulated to migrate from the epidermis and travel from the skin. In this article we will

consider the molecular events that serve to initiate and regulate LC migration and the processes which may determine the vigour and quality of immune responses induced in the draining lymph nodes. L a n g e r h a n s cell m i g r a t i o n a n d m a t u r a t i o n : roles of epidermal cytokines With respect to antigen presentation, LCs within the epidermis are considered to be relatively immature dendritic cells; their attribute being instead the capacity to internalize and process exogenous antigen [3]. Following topical exposure to chemical allergens or upon receipt of other appropriate stimuli, a proportion of local LCs is induced to leave the epidermis and migrate - - via afferent lymphatics - - to draining lymph nodes [1]. Many of these cells bear high levels of antigen and during migration they are subject to flmctional maturation, losing their ability to process antigen and acquiring instead the immunostimulatory properties of lymphoid DCs which are able to present antigen effectively to responsive T lymphocytes. By analogy with in vitro studies, the differentiation of LCs in vivo is effected by granulocyte-macrophage colony-stimulating factor (GM-CSF), TNF-ot, IL-1 and possibly other epidermal cytokines; it is associated with the t, pregulated expression of membrane determinants required for effective antigen presentation (including M H C class II, B7-1 and B7-2 [CD80 and CD861 and intercellular adhesion molecule 1 [ICAM-1]) [4-7].

In addition to causing LC maturation, epidermal cytokines also stimulate the migration of LCs from the epidermis. One mandatory signal is provided by TNF-ot, an inducible product of keratinocytes (as well as other cell types in other tissues). The intradermal administration to mice of homok)gous recombinant TNF-o~ causes the rapid (within 30 minutes) loss of a proportion of epidermal LCs local to the site of injection; this is associated somewhat later (by 2 hours) with an accumulation of DCs within draining lymph nodes [8,9], Systemic treatment of mice, via intraperitoneal injection, with neutralizing anti-TNF-o~ antibody inhibits the I,C migration and DC accumulation that normally results from topical exposure to a chemical allergen such as oxazolone; the antibody also impairs significantly the development of skin sensitization [10]. T h e available evidence clearly points to the signal provided by TNF-0t being delivered through the type 2 (p75) TNF-cx receptor (TNFR2; CD120b): l~Cs express only T N F R 2 (and not rI'NFR1; CD120a) [11,12]; LC migration is significantly impaired in mice that lack "FNFR2, but not in those lacking T N F R I [13"]; finally, homologous TNF-0t will induce LC migration in mice but, in contrast, the human cytokine will not (the significance of this being that unlike TNFR1, T N F R 2 is largely speciesspecific) [8,9]. The relevance of selective signalling through T N F R 2 will be considered later.

Langerhans cells and chemical allergy Kimber et aL

Figure 1 (a) Skin sensitization

(b)

M~grat~on CurrentOpinionin Immunology Cytokine signal requirements for the initiation of LC migration. Two signals are required - one provided by IL-1 J], the other by TNF-o~. In murine epidermis, IL-1 J3is a constitutive and inducible product of LCs themselves. (a) Skin sensitization is associated with the induced expression of IL-1 [3, which (b) provides one (autocrine) signal (via LC I1_-1RI) for migration and (c) also induces (in paracrine fashion) the production (d) of TNF-o~ by keratinocytes. The second signal for (e) LC migration is provided by TNF-c~ acting through membrane TNFR2.

"FNF-ot is, however, not the only stimulus required for the mobilization of LCs. It is clear that, in addition, a second independent signal is necessary; this is provided by IL-1[3, a cytokine that in murine epidermis is produced only by LCs themselves. In common with TNF-o~, the intradermal injection of recombinant IL-I[~ stimulates both LC migration and DC accumulation, although with a somewhat slower tempo than is observed with TNF-(x [14]; treatment with neutralizing anti-IL-1]~ antibody inhibits both processes [15"]. It is relevant also that mice deficient in II,-113 display impaired contact sensitivity to picryl chloride [16]. With respect to LC migration it is believed that the sequence of events is as follows. ~lbpical exposure to chemical allergen stimulates the increased production by LCs of I L - I ~ [17]. This cytokine performs two functions. First it provides, in autocrine fashion, one stimulus for LC migration acting through the type I IL-1 receptor (IL-1RI). Second, 1I,-113 induces the production by keratinocytes of TNF-ot [18] - - this cytokine acting on adjacent LCs to deliver the second signal for migration. Such is consistent with the observation that a lack of either cytokine effectively inhibits the mobilization of LCs. T h e induction of migration by exogenous IL-113 ahme is easily reconciled by the ability of this cytokine to provoke TNF-o~ production [18]. In contrast, the mobilization of LCs by intradermal injection

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of TNF-c~ alone is dependent presumably on the availability of sufficient constitutive [L-1[~ to deliver the autocrine signal to LCs. A schema representing these interactions is illustrated in Figure 1. It should be noted, however, that other dermal stimuli that are known to cause LC migration - - such as topical exposure to nonsensitizing skin irritants or to ultraviolet B irradiation [19,20] - - may induce mobilization through a similar, although not necessarily identical, sequence of events; these are perhaps associated with alternative and/or additional interactions between cytokines and cytokine receptors. T h e interpretation is that LCs move from the epidermis in response to any trauma, damage or danger of sufficient magnitude to provoke, or increase, the production of relevant epidermal cvtokines. This presumably reflects a sentinel activity fi~r LCs wherein their function is to survey and sample the external environment and to traffic to draining lymph nodes bearing information on the changing antigenic environment at skin surfaces. It may be that the nature of the initial stimulus for LC migration provided by chemical allergens is rather different from those associated with other dermal insults (such as skin irritation or ultraviolet-B irradiation) that are known to result in LC mobilization. T h e tmique feature of the migration that is induced by chemical allergens may prove to be the rapid upregulation of IL-113 expression by LCs. Based upon analyses of induced changes in epidermal cell mRNA expression it has been shown that chemical sensitizers, but not skin irritants, stimulate the rapid (within 15 minutes) transcriptional upregulation of IL-I[3 expression by LCs [17]. One question that needs to be addressed regards the nature of changes induced by IL-113 and TNF-oc that permit LCs to leave the skin and travel to regional lymph nodes. Of critical importance appears to be the altered expression of adhesion molecules. There is a reduced expression by LCs of E-cadherin, which allows their disassociation from surrounding keratinocytes as a first step in migration [21"]. There is evidence also that effective migration is dependent upon intercellular adhesion molecule 1, or6 integrin and certain isoforms of CD44; the expression of these membrane determinants is believed to be maintained or upregulated by proinflammatory cytokines [22,23",24]. In addition, LC activation is associated with the increased production by these cells of matrix metalloproteinase 9 [25]. These changes, in concert, permit the cell--cell and cell-tissue matrix interactions that are necessary for the migration of LCs though cutaneous tissues, access to and across the basement membrane and successful traffic to the paracortical regions of draining lymph nodes.

Langerhans cell selectivity and survival Two issues of particular interest have yet to be clarified. T h e first of these is why only a proportion of LCs local to the site of an appropriate stimulus is mobilized to migrate away from the skin. T h e second is what significance

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signalling solely through T N F R 2 has for the integrity of LC migration and function. T h e selective responsiveness of LCs to the signals provided by II,-113, T N F - ~ or skin sensitization itself may be attributable to phenotypic heterogeneity reflecting variable differentiation status within the epidermis, which may in turn be dependent upon the duration of residence from the time of arrival of LC precursors in the skin. It is possible, for instance, that epidermal LCs vary with respect to their level of expression of T N F R 2 or IL-1RI. There are, however, other alternatives. For example many, but not all, LCs have demonstrable membrane ATPase (mATPase). In fact the proportion of LCs that lacks mATPase (approximately 20%) is similar to the fraction of cells that becomes mobilized [26]. T h e LCs that possess mATPase will be comparatively resistant to the effects of extracelhdar ATP (ATPe) generated and released as a result of cutaneous trauma [26]. It is known that, in macrophages at least, ATPe causes both the post-translational modification of p r o - I L - l ~ and the release of the bioactive cytokine [27,28] - - a process that is mediated via activation of purinergic P2Z receptors [29]. LCs express these receptors [30], so in theory it is possible that the epidermal subpopulation that lacks demonstrable mATPase will respond to ATP by the release of II,-ll~ - - the precursor having been processed either by IL-113-converting enzyme (ICE; caspase 1) or by non-ICE proteases [31]. Such a mechanism would permit the conversion of a damage (or danger) signal into a stimulus for LC migration, via the induction and/or upregulation of cutaneous cytokine production. Expanding on this theme, it is possible that a role is played by p-glycoprotein (or multidrug resistance [MDR]-I), a plasma-membrane protein known to be an ATP-dependent drug-efflux pump that confers multidrug resistance on cells [32]. It has been shown recently that neutralizing anti-MDR-1 antibodies inhibit the migration of DCs from skin explants and cause the retention of LCs within the epidermis [33"]. Similar effects were seen with the MDR-1 antagonist verapamil [33"]. One suggestion is that MDR-1 activity is required for the release by LCs of IL-113 and/or other cytokines [33°°]. T h e above provides one plausible, but unproven, explanation for the selective loss from the epidermis of only a proportion of LCs; the basis for such selectivity being the preferential production of I L - I ~ by LCs lacking mATPase in response to local trauma, This explanation may resolve an interesting potential paradox - - why LCs should express P2Z receptors which appear in many cells to be a potent stimulators of cytotoxicity. If the purpose of expression is to permit the rapid processing and release of IL-113 (and possibly other cytokines) in response to appropriate stimuli, then obvious questions relate to whether (and how) LCs are protected from cell death. These in turn raise the issue of whether the exclusive expression by LCs of T N F R 2 confers upon them protection from TNF-mediated death and other cell death signals. T N F R 2 differs from T N F R 1 insofar as it

has no death-domain and does not directly bind T R A D D (TNFR-activated death-domain-containing protein), that associates selectively with T N F R 1 [34]. T N F R 2 binds to two proteins, T R A F (TNFR-associated factor)l and T R A F 2 [35], and recently it has been found that the T N F R 2 - T R A F signalling complex contains two additional proteins - - designated cellular inhibitor of apoptosis protein (c-IAP)I and c-lAP2; these are closely related mammalian members of the IAP family which was identified first in Baculovirt, ses [36]. T h e hypothesis is that the expression by LCs of only T N F R 2 confers a selective advantage, providing protection against apoptotic signals in a potentially hostile environment and allowing them to respond to T N F - ~ with a reduced likelihood of cell death. Consistent with this are results of investigations conducted by Chainy et al. [.37] using T N F muteins that interact only with either T N F R 1 or T N F R 2 . Using human myeloid leukaemia cells known to express approximately equal numbers of these receptors it was found that T N F molecules specific for T N F R 1 induced cell killing, whereas muteins reactive only with T N F R 2 were totally ineffective in this respect. It must be assumed that the initiation of cutaneous immune responses by LCs would be favoured by their sustained protection from cell death, not only in the skin but also while in transit via afferent lymphatics and during their residence in the paracortical regions of lymph nodes. T h e interaction of DCs with responsive T lymphocytes requires the engagement of several pairs of counter-receptors, including interaction of CD40 expressed on DCs with the appropriate ligand (CD40 ligand) on T cells. It has been shown that both the spontaneous [38] and the Fas (CD95)-induced [39 °] apoptosis of DCs are inhibited by ligation of CD40. Such would serve to prolong the immunostimulatory signals supplied by antigen-bearing DCs. Of relevance also may be the fact that, in certain cells at least, intracellular IL-113 inhibits Fas-mediated apoptosis; the presumption being that precursor II.-113 competitively inhibits the cleavage by ICE of other substrates required for Fas-induced cell death [40]. Finally, a new T N F family member, "FRANCE (TNFR-activationinduced cytokine), has been described which may selectively inhibit the death of DCs. T R A N C E is produced by T lymphocytes following engagement of the T cell receptor; it interacts with a receptor that appears to be selectively expressed by DCs, resulting in increased cell survival [41].

Counter-regulation T h e ways in which the functional integrity and safety of LCs and DCs are secured have been considered but there exist also counter-regulatory processes that serve to constrain their activity. An important inhibitory influence is provided by IL-10, another epidermal cytokine. T h e spontaneous apoptosis of DCs that is inhibited by CD40 ligation (and TNF-00 is markedly enhanced by IL-10 [38]. T h e potential effects of IL-10 on LCs are far-reach-

Langerhans cells and chemical allergy Kimber et aL

ing inasmuch as this cytokine appears to inhibit the normal development of immature cells into immunostimulatory DCs; this inhibition is in many instances associated either with the reduced expression of certain membrane determinants (eg CD44, CD83 and CD86) or the failure to upregulate such molecules in response to appropriate stimuli such as TNF-ct [42-45]. One action of IL-10 may be to prevent (or inhibit) immune activation or to cause the delivery of a tolerogenic, rather than immunogenic, signal [45]. Another possibility is that, under conditions where the influence of IL-10 predominates, the development of LCs may be directed in such a way that the DCs into which they mature preferentially stimulate type 2 immtme responses.

Langerhans cells and immune selectivity and deviation As alluded to above, there is evidence that topical exposure to chemicals may result in the development of qualitatively discrete immtme responses. It has been shown in mice that contact allergens provoke selective type 1 immune responses whereas under conditions of comparable immunogenicity, chemicals known in humans to cause allergic sensitization of the respiratory tract stimulate the preferential development of type 2 immune responses [2,46]. An intriguing, but as yet unresolved, question is why cutaneous exposure to haptens may result in the generation of different qualities of immune response. T h e characteristics of the relevant antigen-presenting cells themselves may be among the critical determinants. In this context it is interesting that DCs deficient in IL-12 have been found preferentially to promote the maturation of Th2-type CD4 + T cells [47] and that DCs that have developed from precursors in the presence of IL-10 have a impaired capacity to induce T h l - t y p e responses secondary to a decreased production of IL-12 [48"']. T h e interpretation is that, under those conditions where high levels of IL-10 are available, LC maturation will follow a course where the stimulation of Th2 responses will be favoured. This of course, in turn, raises questions regarding the nature of events in the skin that determine the extent to which IL-10 will be produced following chemical exposure. It is possible, of course, that DC cytokines other than IL12 may influence the selectivity of induced T lymphocyte responses. One candidate is IL-6. There is evidence that in addition to IL-12, LCs and DCs synthesize and secrete IL-6 [49,50]. Recently Rincon et al. [51] have shown that IL-6 deriving from antigen-presenting cells is able to polarize T h cell responses to those of Th2-type, probably by stimulating the early production of IL-4 by CD4 + T lymphocytes. On this basis an alternative hypothesis is that the selective immunostimulatory properties of DCs are dictated by their relative production of IL-6 and IL-12, which in turn is determined by the conditions under which LCs mature and the presence of immunoregulatory cytokines such as

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IL-10. T h e selectivity of T lymphocyte responses provoked by DCs may be influenced additionally by the nature of the antigenic stimulus delivered, with highdose antigen and/or strong and high-affinity signals to the T cell receptor favouring the development of type 1 immune responses [52-54,55°].

Conclusions LCs, and the DCs into which they develop, play pivotal roles in the initiation of cutaneous immune responses; this includes the induction of allergic sensitization to chemicals. With respect to chemical sensitization through the skin it is assumed that exposure results in the production, or the increased production, of relevant cytokines that together initiate the functional maturation and migration of LCs. In this context the importance of LC migration is that antigen encountered at skin surfaces is delivered in an immunogenic form to the peripheral lymphoid system. What is not yet certain is whether exposure to chemical allergens per se is sufficient for induction of sensitization. One view is that chemical allergens are able to cause the increased expression of IL-l]3 by LCs themselves and that this is sufficient to stimulate the other downstream changes necessary for migration and maturation; however an alternative view is that, in theory at least, encounter with chemical allergens in the absence of sufficient trauma ('danger' signals) will be insufficient for effective sensitization, particularly in those instances where the allergen itself appears not to induce significant irritation. T h e argument is that sentinel LCs are actually programmed to respond to local damage or trauma, rather than to antigen itself; in practice, however, exposure to antigen will normallv be associated with sufficient disruption to provide the necessary danger signals.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • of special interest °" of outstanding interest 1.

Kimber I, Cumberbatch M: Dendritic cells and cutaneous immune responses to chemical allergens. Toxicol Appl Pharmacol 1992, 117:137-146.

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Kimber I, Dearman RJ: Cell and molecular biology of chemical allergy. Clin Rev Allergy Immunol 1997, 15:145-168.

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Streilein JW, Grammer SF: In vitro evidence that Langerhans cells can adopt two functionally distinct forms capable of antigen presentation to T lymphocytes. J Immunol 1989, 143:3925-3933.

4.

Heufler C, Koch F, SchuJer G: Granulocyte/macrophage colonystimulating factor and interleukin 1 mediate the maturation of epidermal Langerhans cells into potent immunosUmulatory dendritic cells. J Exp Med 1988, 167:700-705.

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Picut CA, Lee CS, Dougherty EP, Anderson KL, Lewis RM: Immunostimulatory capabilities of highly enriched Langerhans cells in vitro. J Invest Dermato/1988, 90:201-206.

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FurueM, Chang CH, Tamaki K: Interleukin-1 but not tumour necrosis factor a synergistically upregulates the granulocytemacrophage colony-sUmulating factor-induced B7-1 expression of murine Langerhans cells. Brit J Dermato/1996, 135:194-198.

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Ozawa H, Nakagawa S, Tagami H, Aiba S: Interleukin-1J~ and granulocyte-macrophage colony-stimulating factor mediate

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Langerhans cell maturation differently. J Invest Dermatol 1996, 106:441-445. 8.

Cumberbatch M, Kimber I: Dermal tumour necrosis factor-~

induces dendritic cell migration to draining lymph nodes and possibly provides one stimulus for Langerhans cell migration. /mmuno/ogy 1992, 75:257-263. 9.

Cumberbatch M, Fielding I, Kimber I: Modulation of epidermal Langerhans cell frequency by tumour necrosis factor-0~ Immuno/ogy 1994, 81:395-401.

10. Cumberbatch M, Kimber I: Tumour necrosis factor-a is required for accumulation of dendritic cells in draining lymph nodes and for optimal contact sensitization./mrnuno/ogy 1995, 84:31-35. 11. Ryffel B, Brockhaus M, Greiner B, Mihatsch MJ, Gudat F: Tumour necrosis factor receptor distribution in human lymphoid tissue. /mmuno/ogy 1991,74:446-452. 12. Larre9ina A, Morelli A, Kolkowski E, Fainboim L: Flow cytometric analysis of cytokine receptors on human Langerhans cells. Changes observed after short term culture./mmuno/ogy 1996, 87:317-325. 13. Wang B, Fujisawa H, Zhuang L, Kondo S, Shivji GM, Kim CS, Mak TW, Sauder DN: Depressed Langerhans cell migration and reduced contact hypersensitivity response in mice lacking TNF receptor p75. J/rnmuno/1997, 159:6148-6155. This paper, together with others [8,9,11,12], provides evidence that TNF-(z signalling in LCs is effected exclusively via TNFR2. LC migration was normal in mice lacking TNFR1 but was inhibited in TNFR2-deficient mice. •

14. Cumberbatch M, Dearman RJ, Kimber h Interleukin 1~ and the stimulation of Langerhans cell migration: comparisons with tumour necrosis factor ~. Arch Dermato/Res 199?, 289:277-284. 15. Cumberbatch M, Dearman RJ, Kimber I: Langerhans cells require • signals from both tumour necrosis factor-o~ and interleukin 1~ for migration./mmuno/ogy 1997, 92:388-395. Together with other references [10,13°], this paper provides evidence that the effective stimulation of LC migration requires two discrete signals - one provided by IL-1 ~, the other by TNF-c~. 16. Shornick LP, De Togni P, Mariathasan S, Goellner .I, StraussSohoenberger J, Karr RW, Ferguson TA, Chaplin DO: Mice deficient in IL-I~ manifest impaired contact hypersensitivity to trinitrochlorobenzene..I Exp Med 1996, 183:1427-1436.

24. Weiss JM, Sleeman J, Renkl AC, Dittmar H, Termeer CC, Taxis S, Howells N, Hofmann M, Kohler G, Schopf E et aL: An essential role for CD44 variant isoforms in epidermal Langerhans cell and blood dendritic cell function. J Cell Biol 1997, 137:1137-114?. 25. Kobayashi Y: Langerhans cells produce type IV collagenase (MMP-9) following epicutaneous stimulation with haptens. Immunology 199"7, 90:496-501. 26, Girolomoni G, Santantonio ML, Pastore S, Bergstresser PR, Gianetti A, Cruz PD Jr: Epidermal Langerhans cells are resistant to the permeabilizing effects of extracellular ATP: in vitro evidence supporting a protective role of membrane ATPase. J Invest Dermatol 1993, 100:282-287. 2?.

PerregauzD, Gabel CA: Interleukin-1 ~ maturation and release in response to ATP and nigericin. J Biol Chem 1994, 269:15195-15203.

28. Griffiths RJ, Stam EJ, Downs JT, Otterness IG: ATP induces the release of IL-1 from LPS-primed cells in vivo. J Immunol 1995, 154:2821-2828. 29. Ferrari D, Chiozzi P, Falzoni S, Del Susino M, Melchiorri L, Baricordi OR, Di Virgilio F: Extracellular ATP triggers IL-11~ release by activating the purinergic P2Z receptor of human macrophages. J Immuno11997, 159:1451-1458. 30. Di Virgilio F: The P2Z purinoreceptor: an intriguing role in immunity, inflammation and cell death. Immunol Today 1995, 16:524-528. 31. FantuzziG, Ku G, Harding MW, Livingston DJ, Sipe JD, Kuida K, Flavell RA, Dinarello CA: Response to local inflammation of ILl ~-converting enzyme-deficient mice. J/mmunol 199"7, 158:1818-1824. 32. Dong M, Ladaviere L, Penin F, Deleage G, Baggetto LG: Secondary structure of p-glycoprotein investigated by circular dichroism and amino acid sequence analysis. Biochim Biophys Acta 1998, 1371:317-334. 33. Randolph GJ, Beaulieu S, Pope M, Sugawara I, Hoffman L, Steinman o. RM, Muller WA: A physiologic function for p-glycoprotein (MDR-1) during the migration of dendritic cells from skin via afferent lymphatic vessels. Proc Nat/Acad Sci USA 1998, 95:6924-6929. In this report, inhibition of MDR-1 - using either neutralizing antibodies or an antagonist - was found to reduce the migration of LCs from skin explants. The speculation is that MDR-1 may be required for the export of cytokines that are required for LC mobilization.

17. Enk AH, Katz Sh Early molecular events in the induction phase of contact sensitivity. Proc Nat/Acad Sci USA 1992, 89:1398-1402.

34. BarinagaM: Forging a path to cell death. Science 1996, 273:735-737.

18. Enk AH, Angeloni VL, Udey MC, Katz Sh An essential role for Langerhans cell-derived IL-1 ~ in the initiation of primary immune responses in the skin../Immune/1993, 150:3698-3704.

35. Rothe M, Wong SC, Henzel WJ, Goeddel DV: A novel family of putative signal transducers associated with the cytoplasmic domain of the 75 kDa tumor necrosis factor receptor. Cell 1994, 78:681-692.

19. Cumberbatch M, Scott RC, Basketter DA, Scholes EW, Hilton .I, Dearman RJ, Kimber I: Influence of sodium lauryl sulphate on 2,4dinitrochlorobenzene-induced lymph node activation. Toxico/egy 1993, 77:181-191. 20. Moodycliffe AM, Kimber I, Norval M: The effect of ultraviolet B irradiation and urocanic acid isomers on dendritic cell migration. /mmunofogy 1992, 77:394-399. 21. Jakob T, Udey MC: Regulation of E-cadherin-mediated adhesion in • • Langerhans cell-like dendritic cells by inflammatory mediators that mobilize Langerhans cells in vivo. J Immune/1998, 160:4067-4073. E-cadherin is a homophilic adhesion molecule expressed in the epidermis by both LCs and keratinocytes. It is believed that this molecule maintains the integrity of interaction between LCs and keratinocytes and that successful LC migration requires downregulated expression of E-cadherin. This paper reveals that II--1~ and TNF-c~ induce a rapid reduction in E-cadherin mRNA expression by LC-like cells. 22, Ma J, Wing .I-H, Guo Y-J, Sy M-S, Bigby M: In vivo treatment with anti-ICAM-1 and anti-LFA-1 antibodies inhibits contact sensitization-induced migration of epidermal Langerhans cells to regional lymph nodes. Cell Immune/1994, 158:389-399. 23. Price AA, Cumberbatch M, Kimber I, Ager A: o~6 integrins are • required for Langerhans cell migration from the epidermis../Exp Med 1997, 186:1725-1735. Evidence is presented here that the stimulation of LC migration, both in vitro and in vivo, is dependent upon the integrity of c~6 integrin expression. Other references [21 "°,22,24] define the role of other adhesion molecules in LC migration.

36. Rothe M, Pan M-G, Henzel WJ, Ayres TM, Goedell DV: The TNFR2-TRAF signalling complex contains two novel proteins related to Baculoviral inhibitor of apoptosis proteins. Cell 1995, 83:1243-1 252. 37

Chainy GBN, Singh S, Raju U, Aggarwal BB: Differential activation of the nuclear factor-KB by TNF muteins specific for the p60 and p80 TNF receptors. J Immune/1996, 157:2410-2417.

38. Ludewig B, Graf D, Gelderblom HR, Becker Y, Kroczek RA, Pauli G: Spontaneous apoptosis of dendritic cells is efficiently inhibited by TRAP (CD40-1igand) and TNF-~, but strongly enhanced by interleukin-10. Eur J Immune/1995, 25:1943-1950. 39. Bjorck P, Banchereau J, Flores-Romo L: CD40 ligation counteracts • Fas-induced apoptosis of human dendritic cells./nt Immune/199?, 9:365-372. In this paper, evidence is presented that ligation of CD40 - during interaction with antigen-responsive T lymphocytes - protects DCs from Fas (CD95)-induced apoptosis, possibly via mechanisms related to the increased expression of bcl-2. This is one o1 several processes that facilitate the survival of LCs and the DCs into which they mature. See also references [37,38,40,41]. 40. Tatsuta T, Cheng J, Mountz JD: Intracellular IL-1 ~ is an inhibitor of Fas-mediated apoptosis. J Immune/1996, 157:3949-3957. 41. Wong BR, Josien R, Lee SY, Sauter B, Li H-L, Steinman RM, Choi Y: TRANCE (tumour necrosis factor [TNF]-related activation-induced cytokine), a new TN F family member predominantly expressed in T cells, is a dendritic cell specific survival factor. J Exp Med 199?, 186:2075-2080.

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42. Osada A, Nakashima H, Furue M, Tamaki K: Up-regulation of CD44 expression by tumor necrosis factor-Q is neutralized by interleukin-10 in Langerhans cells. J/nvest Dermato/1995, 105:124-127.

49. Hope J, Cumberbatch M, Fielding I, Dearman RJ, Kimber I, Hopkins SJ: Identification of dendritic cells as a major source of interleukin-6 in draining lymph nodes following skin sensitization of mice. Immunology 1995, 86:441-447.

43. Buelens C, Willems F, Delvaux A, Pierard G, Delville J-P, Velu T, Goldman M: Interleukin-10 differentially regulates B7-1 (CD80) and B7-2 (CD86) expression on human peripheral blood dendritic cells. Eur J Immuno/1995, 25:2668-2672.

50. Curnberbatch M, Dearrnan RJ, Kirnber I: Constitutive and inducible expression of interleukin 6 by Langerhans cells and lymph node dendritic cells./mmuno/ogy 1996, 87:513-518.

44. Morel A-S, Quaratino S, Douek DC, Londei M: Split activity of interleukin-10 on antigen capture and antigen presentation by human dendritic cells: definition of a maturative step. Eur J /mrnuno/199?, 27:26-34. 45. Steinbrink K, Wolff M, Jonuleit H, Knop J, Enk AH: Induction of tolerance by IL-I O-treated dendritic cells. J/mmuno/199"7, 159:47"72-4780. 46. Dearrnan RJ, Moussavi A, Kerneny DM, Kirnber I: Contributions of CD4 + and CD8 + T lymphocyte subsets to the cytokine secretion patterns induced in mice during sensitization to contact and respiratory chemical allergens. Immuno/ogy 1996, 89:502-510. 4?.

KalinskiP, Hilkens CMU, Snijders A, Snijdewint FGM, Kapsenberg ML: IL-12 deficient dendritic cells, generated in the presence of prostoglandin E2, promote type 2 cytokine production in maturing human naive T helper cells. J/mmuno/199"7, 159:28-35.

48. De Srnedt T, Van Mechelen M, De Becker G, Urbain J, Leo O, Moser == M: Effect of interleukin-10 on dendritic cell maturation and function. Eur J/mrnuno/199?, 27:1229-1235. Maturation of DCs in the presence of IL-10 was found to inhibit the subsequent stimulation by these ceils of IFN-7 production (and thereby their ability to induce Thl-type responses) secondary to a decreased production of IL-12. See also reference [51] for the impact of LC/DC cytokines on CD4 + T lymphocyte selectivity.

51. Rincon M, Anguita J, Nakarnura T, Fikrig E, Flavell RA: Interleukin (IL)-6 directs the differentiation of IL-4 producing CD4 + T cells. J Exp Med 1997, 185:461-469. 52. Pfeiffer C, Stein J, Southwood S, Ketelaar H, Sette A, Bottomly K: Altered peptide ligands can control CD4 T lymphocyte differentiation in vivo. J Exp Med 1995, 181:1569-1574. 53. Constant S, Pfeiffer C, Woodard A, Pasqualini T, Bottomly K: Extent of T cell receptor ligation can determine the functional differentiation of naive CD4 + T ceils. J Exp Med 1995, 182:1591-1596. 54. Secrist H, De Kruyff RH, Umetsu DT: Interleukin 4 production by CD4 + T cells from allergic individuals is modulated by antigen concentration and antigen-presenting cell type. J Exp Med 1995, 81:1081-1089. 55. Tao X, Constant S, Jorritsrna P, Bottornly K: Strength of TCR signal • determines the costimulatory requirements for Thl and Th2 CD4 + T cell differentiation. J/mrnuno/1997, 159:5956-5963. In this paper, na'(ve CD4 + T lyrnphocytes were primed with either weak or strong TCR signals. Weak signals generated IL-4-producing Th2-type cells (requiring CD28-B7 interactions), whereas stronger signals were associated with the generation of Thl-type cells. These data (together with other reports, references [52-54]) provide evidence for the influence of both the antigen dose and the affinity of interaction with the TCR on the quality of induced immune responses.