429
DIAGN MICROBIOL INFECT DIS 1990;13:429-433
Interleukins, Mycobacteria, and Listeriae Stefan H.E. Kaufmann
INTRODUCTION
GAMMA INTERFERON
The term intracellular bacteria is commonly used to describe bacterial pathogens with the capacity to survive inside mononuclear phagocytes (MPs) and certain other host cells (Hahn and Kaufmann, 1981). These pathogens include the etiologic agents of tuberchlosis (Mycobacterium tuberculosis and Mycobacteriam bovis ), leprosy ( Mycobacterium leprae), listeriosis (Listeria monocytogenes), Legionnaire's disease (Legionella pneumophilia), and many others, with tubercle and leprosy bacilli being the most important ones medically. The relationship between intracellular bacteria and MPs is an interesting one: In their resting state, MPs provide a major habitat, whereas in their activated state, they become the principal effectors of host defense. The signals that induce transition from the resting to the activated state are provided by lymphokines produced by specific T lymphocytes after antigen-specific stimulation. Mediators of similar type, so-called monokines, are produced by MPs after appropriate stimulation. Lymphokines and monokines, which are often called interleukins, are polypeptides that orchestrate a broad spectrum of immune functions. Major interleukins and their prime biological functions are indicated in Figure 1. In this short treatise, the role of interleukins in the host response to mycobacteria and listeriae will be discussed. Emphasis will be given to studies utilizing recombinant molecules.
Application of recombinant (r) gamma interferon (IFN-'y) protects mice against subsequent infection with L. monocytogenes, after both local and systemic infection (Kiderlen et al., 1984). Conversely, in vivo neutralization of IFN-'y with appropriate antibodies exacerbates listeriosis in mice (Buchmeier and Schreiber, 1985). Although T cells are the major source of IFN-~, recent studies with severe combined immuno-deficiency (scid) mice that lack T and B cells show that IFN-'y is produced and contributes to protection against listeriosis even in these mice (Bancroft et al., 1987). Human macrophages can be activated by IFN-~ to kill L. monocytogenes (Peck, 1985); however, evidence against the role of IFN-~ in antilisterial immunity has also been published (Van Dissel et al., 1987; Campbell et al., 1988). Recent trials in lepromatous leprosy patients indicate that intradermal application of IFN-~ causes a decrease of acid fast bacilli at the site of application (Nathan et al., 1986). In vitro, rIFN-~/activates tuberculostatic activities in murine macrophages (Flesch and Kaufmann, 1987); however, it appears that certain strains of M. tuberculosis are resistant to IFN-~/activated macrophage functions. On the other hand, in vitro studies with blood monocytes indicate that IFN-~/ fails to activate tuberculostatic functions in human macrophages (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch, 1988).
From the Department of Medical Microbiology and Immunology, University of Ulm, Ulm, Federal Republic of Germany. Received May 10, 1990; revised and accepted June 10, 1990. Address reprint requests to: Dr. S.H.E. Kaufmann, Department of Medical Microbiology and Immunology, University of Ulm, Albert-Einstein-AUee 22, D-7900 Ulm, FRG. © 1990 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/90/3.50
1,25-DIHYDROXYVITAMIN D3 Several groups have experienced that in vitro IFN~/fails to activate tuberculostasis in human MPs (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch,
430
S.H.E. Kaufmann
1988). Rather, 1,25-dihydroxyvitamin D3, the biologically active form of vitamin D3, seems to fulfill this function (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch, 1988). This metabolite is formed by loPhydroxylation of 25-hydroxyvitamin D3, the circulating product of vitamin D3. In h u m a n MPs, loPhydroxylase activity can be induced by IFN~/(Adams and Gacad, 1985). It is therefore possible that at the site of microbial growth, IFN--y activates MPs to metabolize 25-hydroxyvitamin D3 into 1,25dihydroxyvitamin D3, which then activates antimycobacterial functions. INTERLEUKIN-1
velops. In many experimental animal systems this unresponsiveness can be reversed by application of exogenous IL-2 (Colizzi, 1984). T cells from lepromatous leprosy patients often show selective unresponsiveness to M. leprae antigens, which can be restored by addition of exogenous IL-2 (Haregewoin et al., 1983). Also, application of rIL-2 into lesions of lepromatous leprosy patients leads to partial reconstitution of effective cellular immunity at the site of administration (Kaplan et al., 1989). This is probably caused by freshly immigrant T cells and monocytes.
T U M O R N E C R O S I S FACTOR
Interleukin-1 (IL-1) is produced by MPs and several other cells in response to various stimuli. Administration of rIL-1 increases resistance of mice to listeriosis (Czuprynski et al., 1988). Although its mode of action is not fully understood, it is most probable that IL-1 attracts inflammatory phagocytes, which are better equipped for microbial elimination than are local tissue macrophages. INTERLEUKIN-2
Interleukin-2 (IL-2) is a major T-cell growth and differentiafion factor that is also involved in antimicrobial immunity. During many intracellular bacterial infections, unresponsiveness of varying severity de-
Tumor necrosis factor (TNF) is a macrophage product that shares several similarities with the T-cellderived lymphotoxin also termed TNF-~. Mice treated with specific anti-TNF antibodies suffer from severe listeriosis (Havell, 1987). Conversely, application of TNF has a beneficial effect on subsequent infection with L. monocytogenes (Desiderio et al., 1989). Antilisterial resistance seems to d e p e n d on TNF in scid as well as in normal mice (Bancroft et al., 1989). In mice suffering from M. bovis infection, granuloma formation is parallelled by local TNF synthesis, and application of anti-TNF antibodies markedly affects granuloma formation and host resistance in these mice (Kindler et al., 1989). Concomitant injection of TNF and mycobacterial products into nude
Inflammation I
l
[Cell destruction I TNF-~
IT
-cell
IL-1 I
-,,,
/"
IFN-a
.,' I / I Cell destructlonl Tm Products
IL-3
1
[Virus
inhibition
]
B-cell stimulation, tumor growth [B-cell stimulation, ] eoslnophl] differentiation
stimulation I
TNF-~
-~
~'~
B-cell stimulation, mast cel! differentiation
Stem cell differentiation
TH2 Products
FIGURE 1. Major interleukins and their principal function.
Interleukins, Mycobacteria, and Listeriae
431
mice induces marked necrotic reactions, further substantiating the notion that TNF participates in necrotic granulomatous lesions (Kaufmann et al., 1989). TNF is identical with cachectin and, hence, may also be involved in detrimental consequences of chronic bacterial infections.
INTERLEUKIN-4 A N D INTERLEUKIN-6 Interleukin-4 (IL-4) is a major B-cell growth and differentiation factor. Recent studies revealed that it also acts on MPs (Crawford et al., 1987). Importantly, IL-4 has been shown to induce the influx of blood monocytes into local tissue sites in vivo and the formation of multinucleated giant cells from MPs in vitro (McInnes and Rennick, 1988; Tepper et al., 1989). Both events are important steps in granuloma development. Interleukin-6 (IL-6) is another B-cell stimulatory factor that induces B-cell maturation. Activation of bone marrow-derived macrophages with IL-4 or IL-6 is without effect on subsequent infection with mycobacteria (Kaufmann and Flesch, 1990). On the contrary, previously infected macrophages can be activated for tuberculostasis by rIL-4 and rIL-6 (Kaufmann and Flesch, 1990). These findings indicate that IL-4 and IL-6 can activate antimycobacterial functions in macrophages dependent on a second stimulus that can be provided by viable mycobacteria.
S O U R C E OF I N T E R L E U K I N S The major sources of interleukins are MPs and T lymphocytes. IL-1 and TNF are products of MPs, whereas IL-2, IL-4, IFN-~, and TNFq3 are T-cell products. IL-6 is secreted by many cells, including MP and T lymphocytes. Upon stimulation with certain microbial products, MPs secrete IL-1 and TNF and, hence, may participate in the initiation and maintenance of antimicrobial resistance independent of the specific immune response. Still, their secretion is also controlled by specific T lymphocytes. T cells are characterized by the CD3 surface molecule. T lymphocytes in the periphery segregate into two major populations, which utilize different receptors for antigen recognition. The more frequent and better characterized T-cell set uses a receptor composed of an alpha and a beta chain, whereas the less frequent and less well-understood T-cell set expresses a receptor made up of a gamma and a delta chain. Alpha/beta T cells can be further divided into two subpopulations, namely T cells, which express the CD4 molecule and recognize antigenic peptides in the context of major histocompatibility complex (MHC) class II molecules, and CD8 T cells, which
D(8~
_-4,UL-6) )
,2)
FIGURE 2. Possible role of different interleukins and T lymphocytes in granulomatous lesions. (1) Development of the granulomatous lesion crucially depends on TNF (Kindler et al., 1989). (2) The B-cell stimulatory factors IL4 and IL-6 induce a certain degree of antimycobacterial activity in previously infected macrophages (Kaufmann and Flesch, 1990). (3) IFN-~ acts similarly (Kaufmann and Flesch, 1990). Though not sufficient for sterile eradication of pathogens, this mechanism contributes to microbial containment in the lesion. (4) The effects of IFN-~ on noninfected MPs are stronger, indicating that IFN-~/is particularly involved in the activation of immigrant blood monocytes (Flesch and Kaufmann, 1987). (5) TNF synergizes with IFN-~ in the activation of antimycobacterial activities (Kaufmann and Flesch, 1990). (6) For freshly immigrant phagocytes to be capable of engulfing microbial organisms in the granuloma, cytolysis of infected MPs is required (Kaufmann, 1988). This event allows transmission of mycobacteria from a more protective to a more aggressive environment. (7) IFN-~/and IL-2 induce the influx of fresh blood monocytes and T lymphocytes, thus facilitating entry of new effector cells (Nathan et al., 1986; Kaplan et al., 1989). (8) Recent evidence suggests that IL-4 also participates in this step (Tepper et al., 1989). The events described thus far are all functions of conventional alpha/beta T lymphocytes, either of CD4 or of CD8 phenotype. (9) At certain stages of infection (for example, reactional stages of leprosy), a novel T-ceU type bearing a gamma/delta receptor may be involved (Modlin et al., 1989).
432
respond to antigenic peptides plus MHC class I products. Although CD4 cells are the major source of interleukins, in several systems CD8 cells were found to secrete certain interleukins as well. In particular, T-cell clones with specificity to L. monocytogenes, M. tuberculosis, M. bovis, and M leprae have been shown ot secrete IFN-~ upon restimulation with appropriate antigens, accessory cells, and exogenous IL-2 (Kaufmann, 1988). In the mouse, the CD4 T-cell set has been divided even further into two subsets designated TH1 and TH2 cells. Although this segregation is not strict, B-cell-stimulatory factors (IL-4, IL-5, and IL-6) are primarily products of TH2 cells, whereas interleukins involved in cell-mediated immunity (IL-2, IFN-~/, and TNF-[3) represent products of TH1 cells. Less is known about gamma/delta T cells. Recent observations by several groups that mycobacterial components represent preferred ligands for gamma/delta T cells point to a major role of these cells in mycobacterial infections (Modlin et al., 1989; O'Brien et al., 1989; Janis et al., 1990). These T lymphocytes produce IL-2 and other interleukins, which may play a role in granulomatous lesions.
ARE ONLY INTERLEUKINS INVOLVED IN ANTIMICROBIAL IMMUNITY? Evidence is accumulating that in addition to interleukin-mediated immune mechanisms, cytolytic mechanisms are also relevant to the host response against intracellular bacteria (Kaufmann, 1988). This holds true for both the murine and the human system. Although in most systems, target cell lysis is a function of CD8 cells, in mycobacterial and listerial infections CD4 cells with cytolytic activity are rapidly activated (Kaufmann et al., 1987; Kaufmann, 1988; Hancock et al., 1989). Experimental animal studies have shown that both CD4 and CD8 cells are required for optimum protection against listeriae and mycobacteria (Kaufmann et al, 1985; Muller et al., 1987). T cells from mice immunized with L. monocytogenes, M. bovis, M. tuberculosis, and M. leprae have been isolated and cloned, and these clones express cytolytic activity against macrophages presenting the homologous antigen (Kaufmann, 1988). Following local application of IL-2 into lesions of lepromatous leprosy patients, a rapid influx of CD4
S.H.E. Kaufmann
and CD8 T cells, as well as blood monocytes, has been observed, and evidence for destruction of local tissue macrophages harboring mycobacteria was obtained (Kaplan et al., 1989). Mycobacteria seem to be degraded more effectively as a consequence of interactions between newly arriving and local cells. CD4 T cells from leprosy patients treated in this way express cytolytic activity against mycobacteria-primed targets (Hancock et al., 1989). In addition, evidence has been presented that NK cells and gamma/delta T cells contribute to cytolytic events in granulomatous lesions (Hancock et al., 1989; Modlin et al., 1989).
AN OVERALL VIEW From the data discussed, the following conclusions and speculations can be drawn. Both interleukinmediated helper activities and cytolytic mechanisms participate in the immune response against mycobacteria, listeriae, and many other intracellular pathogens, and both events occur in granulomatous lesions. The concerted action of different interleukins leads to granuloma formation and maintains containment of bacteria within the lesion. Although interleukin-activated MPs gain the capacity to inhibit their bacterial predators, this activity is often not sufficient for complete elimination. It may therefore become necessary for the bacteria to be released from their protective environment, and that step requires cytolysis. Under the influence of different interleukins, new T cells, blood monocytes, and granulocytes arrive at the lesion. Freshly immigrant blood monocytes and/or granulocytes with high antibacterial potential can take up bacteria released through target cell lysis and, upon appropriate activation by T-cell-derived interleukins, achieve microbial elimination more effectively. These events are illustrated in Figure 2. This work received financial support from UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases, WHO as part of its program for vaccine development, German Leprosy Relief Association, EC-India Science and Technology Cooperation Program, SFB 322, Landesschwegpunkt 30, and A. Krupp Award for young professors. The superb help of R. Mahmoudi is gratefully acknowledged.
REFERENCES Adams JS, Gacad ME (1985) Characterization of lc~-hydroxylation of vitamin D3 sterols by cultured alveolar macrophages from patients with sarcoidosis. J Exp Med 161:755-765. Bancroft GJ, Schreiber RD, Bosma GC, Bosma MJ, Unanue
ER (1987) A T cell-independent mechanism of macrophage activation by interferon-~/. J Immunol 139:11041107. Bancroft GH, Sheehan KCF, Schreiber RD, Unanue ER (1989) Tumor necrosis factor is involved in the T cell-
Interleukins, Mycobacteria, and Listeriae
independent pathway of macrophage in scid mice. J Immunol 143:127-130. Buchmeier NA, Schreiber RD (1985) Requirement of endogenous interferon-~ production for resolution of Listeria monocytogenes infection. Proc NatI Acad Sci USA 82:7404-7408. Campbell PA, Canono BP, Cook JL (1988) Mouse macrophages stimulated by recombinant gamma interferon to kill tumor cells are not bactericidal for the facultative intracellular bacterium Listeria monocytogenes. Infect Immun 56:1371-1375. Colizzi V (1984) In vivo and in vitro administration of interleukin 2-containing preparation reverses T-cell unresponsiveness in Mycobacteriumbovis BCG-infected mice. Infect Immunol 45:25-28. Crawford RM, Finbloom DS, Ohara J, Paul WE, Meltzer, MS (1987) B cell stimulatory factor-1 (interleukin 4) activates macrophages for increased tumoricidal activity and expression of Ia antigens. J Immunol 139:135-141. Crowle AJ, Ross EJ, May MH (1987) Inhibition by 1,25 (OH)2-vitamin D3 of the multiplication of virulent tubercle bacilli in cultured human macrophages. Infect Immun 55:2945-2950. Czuprynski CJ, Brown JF, Young KM, Cooley AJ, Kurtz RS (1988) Effects of murine recombinant interleukin 1 on the host response to bacterial infection. J Immunol 140:962-968. Desiderio JV, Kiener PA, Lin P-F, Warr GA (1989) Protection of mice against Listeria Monocytogenes infection by recombinant human tumor necrosis factor alpha. Infect Immun 57:1615-1617. Flesch I, Kaufmann SHE (1987) Mycobacterial growth inhibition by interferon-~-activated bone marrow macrophages and differential susceptibility among strains of Mycobacterium tuberculosis. J Immunol 138:4408--4413. Hahn H, Kaufmann SHE (1981) The role of cell-mediated immunity in bacterial infections. Rev Infect Dis 3:12211250. Hancock GE, Cohn ZA, Kaplan G (1989) The generation of antigen-specific major histocompatibility complexrestricted cytotoxic T lymphocytes of the CD4 ÷ phenotype. ] Exp Med 169:909-919. Haregewoin A, Godal T, Mustafa AS, Belehu A, Yemaneberhan T (1983) T-cell conditioned media reverse Tcell unresponsiveness in lepromatous leprosy. Nature 303:342-344. Havell EA (1987)Production of tumor necrosis factor during murine listeriosis. J Immunol 139:4225-4231. Janis E, Kaufmann, SHE, Schwartz RH, Pardoll DM (1990) Activation of ~/8 + T cells in the primary immune response to Mycobacterium tuberculosis. Science 244:713716. Kaplan G, Kiessline R, Tekelmariam S, Hancock G, Sheftel G, Job CK, Converse P, Ottenhoff THM, Becx-Bleumink M, Dietz M, Cohn ZA (1989) The reconstitution of cell-mediated immunity in the cutaneous lesions of lepromatous leprosy by recombinant interleukin 2. J Exp Med 169:893-907. Kaufmann SHJE, Hug E, V/ith U, Miiller I (1985) Effective protection against Listeria monocytogenes and delayedtype hypersensitivity to listerial antigens depend on
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cooperation between specific L3T4 + and Lyt2 + T cells. Infect Immunol 48:263-266. Kaufmann SHE, Hug E, V~ith U, De Libero D (1987) Specific lysis of Listeria monocytogenes-infected macrophages by class II-restricted L3T4 + T cells. Eur J Immunol 17:237-246. Kaufmann SHE (1988) CD8 T lymphocytes in intracellular microbial infections. Immunol Today 9:168--173. Kaufmann SHE, Flesch IEA (1988) Cell-mediated immunity, immunodeficiency, and microbial infections. In Perspectives in Antiinfective Therapy. Eds, GG Jackson, HD Schlumberger, and HJ Zeiler, pp 165-73. Kaufmann SHE, Golecki J, Kazda J, Steinhoff U (1989) T lymphocytes, macrophages, Schwann cells, and Mycobacterium leprae. Acta Leprologica 7:$141-$148. Kaufrnann SHE, Flesch IEA (1990) Antimycobacterial functions in bone marrow derived macrophages. Res Microbiol 141:244-248. Kiderlen AJ, Kaufmann SHE, Lohmann-Matthes ML (1984) Protection of mice against the intracellular bacterium L. monocytogenes by recombinant immune interferon. Eur J Immunol 14:964-967. Kindler V, Sappino A-P, Grau GE, Pignet P-F, Vassalli P (1989) The inducing role of tumor necrosis factor in the development of bactericidal granulomas during BCG infection. Cell 56:731-740. McInnes A, Rennick DM (1988) Interleukin 4 induces cultured monocytes/macrophages to form giant multinucleated cells. J Exp Med 167:598-611. Modlin RL, Pirmez C, Hofman FM, Torigian V, Uyemura K, Rea TH, Bloom BR, Brenner MB (1989) Antigenspecific T cell receptor ~//8 bearing lymphocytes accumulate in human infectious disease lesions. Nature 339:5~4 584. M~iller I, Cobbold SP, Waldmann H, Kaufmann SHE (1987) Impaired resistance against Mycobacterium tuberculosis infection after selective in-vivo depletion of L3T4 + and Lyt2 + T cells. Infect ImmunoI 55:2037-2041. Nathan CF, Kaplan G, Levis WR, Nusrat A, Witmer MD, Sherwin SA, Job CK, Path FRC, Horowitz CR, Steinman RM, Cohn ZA (1986) Local and systemic effects of intradermal recombinant interferon-~/in patients with lepromatous leprosy. N Engl J Med 3:6-14. O'Brien RL, Happ MP, Dallas A, Palmer E, Kubo R, Born W (1989) Stimulation of a major subset of lymphocytes expressing T cell receptor ~//8 by an antigen-derived Mycobacterium tuberculosis. Cell 57:667-674. Peck R (1985) A one plate assay for macrophage bactericidal activity. J Immunol Methods 82:131-138. Rook GAW, Steele J, Fraher L, Barker S, Karmali R, O'Riordan J, Stanford J (1986) Vitamin D3, gamma interferon, and control of proliferation of Mycobacterium tuberculosis by human monocytes. Immunology 57:159163. Tepper RI, Pattengale PK, Leder P (1989) Murine interleukin-4 displays potent anti-tumor activity in vivo. Cell 57:503-512. Van Dissel JT, Stikkelbroeck JJM, Michel BC, van den Barselaar M Th, Leijh PCJ, van Furth R (1987) Inability of recombinant interferon-~/ to activate the antibacterial activity of mouse peritoneal macrophages against Listeria monocytogenes and Salmonella typhimurium. J Immunol 139:1673-1677.
DIAGN MICROBIOL INFECT DIS 1990;13:429-433
Interleukins, Mycobacteria, and Listeriae Stefan H.E. Kaufmann
INTRODUCTION
GAMMA INTERFERON
The term intracellular bacteria is commonly used to describe bacterial pathogens with the capacity to survive inside mononuclear phagocytes (MPs) and certain other host cells (Hahn and Kaufmann, 1981). These pathogens include the etiologic agents of tuberchlosis (Mycobacterium tuberculosis and Mycobacteriam bovis ), leprosy ( Mycobacterium leprae), listeriosis (Listeria monocytogenes), Legionnaire's disease (Legionella pneumophilia), and many others, with tubercle and leprosy bacilli being the most important ones medically. The relationship between intracellular bacteria and MPs is an interesting one: In their resting state, MPs provide a major habitat, whereas in their activated state, they become the principal effectors of host defense. The signals that induce transition from the resting to the activated state are provided by lymphokines produced by specific T lymphocytes after antigen-specific stimulation. Mediators of similar type, so-called monokines, are produced by MPs after appropriate stimulation. Lymphokines and monokines, which are often called interleukins, are polypeptides that orchestrate a broad spectrum of immune functions. Major interleukins and their prime biological functions are indicated in Figure 1. In this short treatise, the role of interleukins in the host response to mycobacteria and listeriae will be discussed. Emphasis will be given to studies utilizing recombinant molecules.
Application of recombinant (r) gamma interferon (IFN-'y) protects mice against subsequent infection with L. monocytogenes, after both local and systemic infection (Kiderlen et al., 1984). Conversely, in vivo neutralization of IFN-'y with appropriate antibodies exacerbates listeriosis in mice (Buchmeier and Schreiber, 1985). Although T cells are the major source of IFN-~, recent studies with severe combined immuno-deficiency (scid) mice that lack T and B cells show that IFN-'y is produced and contributes to protection against listeriosis even in these mice (Bancroft et al., 1987). Human macrophages can be activated by IFN-~ to kill L. monocytogenes (Peck, 1985); however, evidence against the role of IFN-~ in antilisterial immunity has also been published (Van Dissel et al., 1987; Campbell et al., 1988). Recent trials in lepromatous leprosy patients indicate that intradermal application of IFN-~ causes a decrease of acid fast bacilli at the site of application (Nathan et al., 1986). In vitro, rIFN-~/activates tuberculostatic activities in murine macrophages (Flesch and Kaufmann, 1987); however, it appears that certain strains of M. tuberculosis are resistant to IFN-~/activated macrophage functions. On the other hand, in vitro studies with blood monocytes indicate that IFN-~/ fails to activate tuberculostatic functions in human macrophages (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch, 1988).
From the Department of Medical Microbiology and Immunology, University of Ulm, Ulm, Federal Republic of Germany. Received May 10, 1990; revised and accepted June 10, 1990. Address reprint requests to: Dr. S.H.E. Kaufmann, Department of Medical Microbiology and Immunology, University of Ulm, Albert-Einstein-AUee 22, D-7900 Ulm, FRG. © 1990 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/90/3.50
1,25-DIHYDROXYVITAMIN D3 Several groups have experienced that in vitro IFN~/fails to activate tuberculostasis in human MPs (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch,
430
S.H.E. Kaufmann
1988). Rather, 1,25-dihydroxyvitamin D3, the biologically active form of vitamin D3, seems to fulfill this function (Rook et al., 1986; Crowle et al., 1987; Kaufmann and Flesch, 1988). This metabolite is formed by loPhydroxylation of 25-hydroxyvitamin D3, the circulating product of vitamin D3. In h u m a n MPs, loPhydroxylase activity can be induced by IFN~/(Adams and Gacad, 1985). It is therefore possible that at the site of microbial growth, IFN--y activates MPs to metabolize 25-hydroxyvitamin D3 into 1,25dihydroxyvitamin D3, which then activates antimycobacterial functions. INTERLEUKIN-1
velops. In many experimental animal systems this unresponsiveness can be reversed by application of exogenous IL-2 (Colizzi, 1984). T cells from lepromatous leprosy patients often show selective unresponsiveness to M. leprae antigens, which can be restored by addition of exogenous IL-2 (Haregewoin et al., 1983). Also, application of rIL-2 into lesions of lepromatous leprosy patients leads to partial reconstitution of effective cellular immunity at the site of administration (Kaplan et al., 1989). This is probably caused by freshly immigrant T cells and monocytes.
T U M O R N E C R O S I S FACTOR
Interleukin-1 (IL-1) is produced by MPs and several other cells in response to various stimuli. Administration of rIL-1 increases resistance of mice to listeriosis (Czuprynski et al., 1988). Although its mode of action is not fully understood, it is most probable that IL-1 attracts inflammatory phagocytes, which are better equipped for microbial elimination than are local tissue macrophages. INTERLEUKIN-2
Interleukin-2 (IL-2) is a major T-cell growth and differentiafion factor that is also involved in antimicrobial immunity. During many intracellular bacterial infections, unresponsiveness of varying severity de-
Tumor necrosis factor (TNF) is a macrophage product that shares several similarities with the T-cellderived lymphotoxin also termed TNF-~. Mice treated with specific anti-TNF antibodies suffer from severe listeriosis (Havell, 1987). Conversely, application of TNF has a beneficial effect on subsequent infection with L. monocytogenes (Desiderio et al., 1989). Antilisterial resistance seems to d e p e n d on TNF in scid as well as in normal mice (Bancroft et al., 1989). In mice suffering from M. bovis infection, granuloma formation is parallelled by local TNF synthesis, and application of anti-TNF antibodies markedly affects granuloma formation and host resistance in these mice (Kindler et al., 1989). Concomitant injection of TNF and mycobacterial products into nude
Inflammation I
l
[Cell destruction I TNF-~
IT
-cell
IL-1 I
-,,,
/"
IFN-a
.,' I / I Cell destructlonl Tm Products
IL-3
1
[Virus
inhibition
]
B-cell stimulation, tumor growth [B-cell stimulation, ] eoslnophl] differentiation
stimulation I
TNF-~
-~
~'~
B-cell stimulation, mast cel! differentiation
Stem cell differentiation
TH2 Products
FIGURE 1. Major interleukins and their principal function.
Interleukins, Mycobacteria, and Listeriae
431
mice induces marked necrotic reactions, further substantiating the notion that TNF participates in necrotic granulomatous lesions (Kaufmann et al., 1989). TNF is identical with cachectin and, hence, may also be involved in detrimental consequences of chronic bacterial infections.
INTERLEUKIN-4 A N D INTERLEUKIN-6 Interleukin-4 (IL-4) is a major B-cell growth and differentiation factor. Recent studies revealed that it also acts on MPs (Crawford et al., 1987). Importantly, IL-4 has been shown to induce the influx of blood monocytes into local tissue sites in vivo and the formation of multinucleated giant cells from MPs in vitro (McInnes and Rennick, 1988; Tepper et al., 1989). Both events are important steps in granuloma development. Interleukin-6 (IL-6) is another B-cell stimulatory factor that induces B-cell maturation. Activation of bone marrow-derived macrophages with IL-4 or IL-6 is without effect on subsequent infection with mycobacteria (Kaufmann and Flesch, 1990). On the contrary, previously infected macrophages can be activated for tuberculostasis by rIL-4 and rIL-6 (Kaufmann and Flesch, 1990). These findings indicate that IL-4 and IL-6 can activate antimycobacterial functions in macrophages dependent on a second stimulus that can be provided by viable mycobacteria.
S O U R C E OF I N T E R L E U K I N S The major sources of interleukins are MPs and T lymphocytes. IL-1 and TNF are products of MPs, whereas IL-2, IL-4, IFN-~, and TNFq3 are T-cell products. IL-6 is secreted by many cells, including MP and T lymphocytes. Upon stimulation with certain microbial products, MPs secrete IL-1 and TNF and, hence, may participate in the initiation and maintenance of antimicrobial resistance independent of the specific immune response. Still, their secretion is also controlled by specific T lymphocytes. T cells are characterized by the CD3 surface molecule. T lymphocytes in the periphery segregate into two major populations, which utilize different receptors for antigen recognition. The more frequent and better characterized T-cell set uses a receptor composed of an alpha and a beta chain, whereas the less frequent and less well-understood T-cell set expresses a receptor made up of a gamma and a delta chain. Alpha/beta T cells can be further divided into two subpopulations, namely T cells, which express the CD4 molecule and recognize antigenic peptides in the context of major histocompatibility complex (MHC) class II molecules, and CD8 T cells, which
D(8~
_-4,UL-6) )
,2)
FIGURE 2. Possible role of different interleukins and T lymphocytes in granulomatous lesions. (1) Development of the granulomatous lesion crucially depends on TNF (Kindler et al., 1989). (2) The B-cell stimulatory factors IL4 and IL-6 induce a certain degree of antimycobacterial activity in previously infected macrophages (Kaufmann and Flesch, 1990). (3) IFN-~ acts similarly (Kaufmann and Flesch, 1990). Though not sufficient for sterile eradication of pathogens, this mechanism contributes to microbial containment in the lesion. (4) The effects of IFN-~ on noninfected MPs are stronger, indicating that IFN-~/is particularly involved in the activation of immigrant blood monocytes (Flesch and Kaufmann, 1987). (5) TNF synergizes with IFN-~ in the activation of antimycobacterial activities (Kaufmann and Flesch, 1990). (6) For freshly immigrant phagocytes to be capable of engulfing microbial organisms in the granuloma, cytolysis of infected MPs is required (Kaufmann, 1988). This event allows transmission of mycobacteria from a more protective to a more aggressive environment. (7) IFN-~/and IL-2 induce the influx of fresh blood monocytes and T lymphocytes, thus facilitating entry of new effector cells (Nathan et al., 1986; Kaplan et al., 1989). (8) Recent evidence suggests that IL-4 also participates in this step (Tepper et al., 1989). The events described thus far are all functions of conventional alpha/beta T lymphocytes, either of CD4 or of CD8 phenotype. (9) At certain stages of infection (for example, reactional stages of leprosy), a novel T-ceU type bearing a gamma/delta receptor may be involved (Modlin et al., 1989).
432
respond to antigenic peptides plus MHC class I products. Although CD4 cells are the major source of interleukins, in several systems CD8 cells were found to secrete certain interleukins as well. In particular, T-cell clones with specificity to L. monocytogenes, M. tuberculosis, M. bovis, and M leprae have been shown ot secrete IFN-~ upon restimulation with appropriate antigens, accessory cells, and exogenous IL-2 (Kaufmann, 1988). In the mouse, the CD4 T-cell set has been divided even further into two subsets designated TH1 and TH2 cells. Although this segregation is not strict, B-cell-stimulatory factors (IL-4, IL-5, and IL-6) are primarily products of TH2 cells, whereas interleukins involved in cell-mediated immunity (IL-2, IFN-~/, and TNF-[3) represent products of TH1 cells. Less is known about gamma/delta T cells. Recent observations by several groups that mycobacterial components represent preferred ligands for gamma/delta T cells point to a major role of these cells in mycobacterial infections (Modlin et al., 1989; O'Brien et al., 1989; Janis et al., 1990). These T lymphocytes produce IL-2 and other interleukins, which may play a role in granulomatous lesions.
ARE ONLY INTERLEUKINS INVOLVED IN ANTIMICROBIAL IMMUNITY? Evidence is accumulating that in addition to interleukin-mediated immune mechanisms, cytolytic mechanisms are also relevant to the host response against intracellular bacteria (Kaufmann, 1988). This holds true for both the murine and the human system. Although in most systems, target cell lysis is a function of CD8 cells, in mycobacterial and listerial infections CD4 cells with cytolytic activity are rapidly activated (Kaufmann et al., 1987; Kaufmann, 1988; Hancock et al., 1989). Experimental animal studies have shown that both CD4 and CD8 cells are required for optimum protection against listeriae and mycobacteria (Kaufmann et al, 1985; Muller et al., 1987). T cells from mice immunized with L. monocytogenes, M. bovis, M. tuberculosis, and M. leprae have been isolated and cloned, and these clones express cytolytic activity against macrophages presenting the homologous antigen (Kaufmann, 1988). Following local application of IL-2 into lesions of lepromatous leprosy patients, a rapid influx of CD4
S.H.E. Kaufmann
and CD8 T cells, as well as blood monocytes, has been observed, and evidence for destruction of local tissue macrophages harboring mycobacteria was obtained (Kaplan et al., 1989). Mycobacteria seem to be degraded more effectively as a consequence of interactions between newly arriving and local cells. CD4 T cells from leprosy patients treated in this way express cytolytic activity against mycobacteria-primed targets (Hancock et al., 1989). In addition, evidence has been presented that NK cells and gamma/delta T cells contribute to cytolytic events in granulomatous lesions (Hancock et al., 1989; Modlin et al., 1989).
AN OVERALL VIEW From the data discussed, the following conclusions and speculations can be drawn. Both interleukinmediated helper activities and cytolytic mechanisms participate in the immune response against mycobacteria, listeriae, and many other intracellular pathogens, and both events occur in granulomatous lesions. The concerted action of different interleukins leads to granuloma formation and maintains containment of bacteria within the lesion. Although interleukin-activated MPs gain the capacity to inhibit their bacterial predators, this activity is often not sufficient for complete elimination. It may therefore become necessary for the bacteria to be released from their protective environment, and that step requires cytolysis. Under the influence of different interleukins, new T cells, blood monocytes, and granulocytes arrive at the lesion. Freshly immigrant blood monocytes and/or granulocytes with high antibacterial potential can take up bacteria released through target cell lysis and, upon appropriate activation by T-cell-derived interleukins, achieve microbial elimination more effectively. These events are illustrated in Figure 2. This work received financial support from UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases, WHO as part of its program for vaccine development, German Leprosy Relief Association, EC-India Science and Technology Cooperation Program, SFB 322, Landesschwegpunkt 30, and A. Krupp Award for young professors. The superb help of R. Mahmoudi is gratefully acknowledged.
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