Scand. J. Immunol. 10, 285-290, 1979
EDITORIAL
Protective Immunity to Chronic Bacterial Infection An established chronic bacterial infection involves a delicate balance between host defence mechanisms and manoeuvres made by the microorganism to evade these mechanisms. To become established, the infection must pass through an initial stage that may or may not give rise to clinical symptoms. During this initial stage the infection may either be terminated owing to development of effective protective immunity, or the bacteria may survive and continue to multiply within the host tissues and thus establish a chronic infection. Symptoms of chronic infections reflect the reaction of the host to the multiplication and persistence of microorganisms. It is important to realize that in some instances our recognition of a chronic infection depends as much on the development of host reaction as on the persistence of living microorganisms. There is considerable variation in the clinical course of many chronic bacterial infections, and studying the basis of this variation could provide us with clues to some of the mechanisms involved in host protection. The variation is determined by both microbial and host factors. An individual who is able to eliminate an infection with one strain of bacteria may be susceptible to a more virulent strain, to a higher number of infecting organisms, or even to the same number of the same bacterial strain when it is introduced by a different route. Less is known about the mechanisms that may generate variation of host resistance in normal individuals. The ability of the host to combat the infection depends on both natural and acquired resistance. In several animal models, host resistance has been shown to be genetically controlled [6, 8, 16, 34], but it may also be influenced by environmental factors such as state of nutrition or intercurrent disease [4]. The relative importance of each of these factors has yet to be determined. Most of the chronic bacterial infections are characterized by intracellular survival of the microorganisms, and host protection appears to be closely correlated with the ability to mount and maintain an effective cell-mediated immune response against microbial antigens [27]. Still, it would be an oversimplification to regard heterogeneity in clinical forms of a particular chronic infection as a direct result of individual variation in primary immune responsiveness; there must be a multifactorial correlation between the clinical forms of the disease and aetiopathological factors. Moreover, during the course of the disease, additional control or regulatory mechanisms may modulate the initial immunological events. Thus, the final outcome of the infection may not necessarily have any direct relationship to the primary immune responsiveness of the patient. Protective immunity confers upon the individual the ability to stop the multiplication of the microorganisms before the infection has reached the chronic stage. The ability to prevent the multiplication of virulent pathogens is generally mediated by effector mechanisms that are focused at the sites of infection after recognition of specific microbial antigens [11]. However, enhancement of certain non-specific mechanisms may also produce a state of increased resistance. Cell wall components of Gram-negative bacteria, mycobacteria, and brucellae are known to stimulate the resistance of the host to a variety of antigenically 0300-9475/79/1000-0285 $02.00 © 1979 Blackwell Scientific Publications
285
286
P. H. Lagrange & O. Closs
unrelated microorganisms. Even if it were possible to accomplish a stimulatory effect on non-specific immunity, it would be of short duration, and thus we are left with promotioti of specific acquired immunity as the main protective measure against chronic bacterial infection. It has repeatedly been observed that certain immunizing procedures that protect the great majority of the population still remain ineffective in certain normal individuals even though they do not present any obvious immunodeficiency. For instance, after vaccination with bacille Calmette-Guerin (BCG) a certain percentage of the vaccinees were insufficiently protected [30]. Furthermore, in several studies, the degree of protection against tuberculosis did not show any direct correlation with the rate of conversion to tuberculin positivity after vaccination [10]. The last point may serve as an illustration of the general lack of adequate immunological tests to measure protective immunity in these infections. The basic mechanisms of protective immunity have been well characterized in many infections in which protection is mediated by humoral antibodies. For instance, the protective effect of vaccination against tetanus, pneumococcal infections, or measles and rubella is easily assessed by measuring the antibody activity against tetanus toxin, pneumococcal capsular polysaccharides, or certain virus-neutralizing antibodies. Much less is known about the protective mechanisms involved in the defence against chronic bacterial infections, such as brucellosis, salmonellosis, syphilis, tuberculosis, and leprosy. Since the nature of protective immunity is not yet known in detail at the cellular and molecular levels in any of these infections, it is also difficult to select the appropriate immunological parameters to measure acquired protective immunity. Apparently, for intracellularly growing parasites, immune activation of macrophages is connected with limitation of growth in vivo [29] and with the presence of some expressions of T-cell-dependent responses [2, 21], such as delayed-type hypersensitivity (DTH) [26]. On the other hand, protective immunity has been demonstrated in the absence of any measurable T-cell activity [42]. Moreover, specific T-cell activity, as measured by in vitro or in vivo tests, may be present without any evidence of acquired resistance [42]. Most likely, interaction between certain specific cell populations is required to produce adequate protection. Although some information is available concerning which subpopulations of T cells are involved in DTH [18, 39], the specific role of these cell populations in the generation of protective immunity is not known at present. Besides, different subpopulations of mononuclear phagocytes have been described [12], and conceivably there is a difference in their killing potential [33]. Furthermore, differentiation of mononuclear phagocytes seems to be regulated by T-cell activity [2, 21] but may also, to some extent, take place in the absence of any T-cell function [24, 36]. This emphasizes the necessity of having appropriate experimental models to determine the exact populations of cells involved at various levels in the cooperation between T cells and mononuclear phagocytes in order to produce effective killing of intracellular bacteria. The elusive protective antigen in cell-mediated immunity (CMl) has still not been found. We do not know whether it is one or several antigens, or whether it is located at the surface of the microorganism or internally. Furthermore, we do not know whether the key to its action as inductor of protective immunity is found in the location and physieochemical properties of the antigen or in the type of immune response it induces. There is evidence that in some infections the antigenic stimulus for protective immunity is produced only by growing microorganisms [3, 28]. If the induction of protective immunity requires interaction between macrophages and living microorganisms, any purified bacterial antigen would fail to induce protection. This might substantially complicate the analysis of the antigens involved
Protective Immunity to Chronic Bacterial Infection
287
in protective immunity, especially if living bacteria are also needed to elicit the protective effector mechanism. The requirements for a protective antigen are as follows: first, to be produced by the intracellular microorganism; second, to be presented by the macrophage; third, to promote the activation of the appropriate subset of T cells, A defect in the processing and presentation of this antigen may produce an altered stimulus that will not trigger the right sequence of events and therefore will not induce an adequate immune response. Even with an appropriate presentation ofthe antigen, additional regulator mechanisms may be able to modulate, and even to obviate, the final response at the site ofthe infection. We may consider leprosy as a typical chronic infection in which all ofthe above-described factors may come into play. The classical description of leprosy involves a spectrum of clinical forms, ranging from the polar tuberculoid form (TT) to the polar lepromatous form (LL) [35], Apparently, an inverse relationship exists between specific CM[ and bacterial multiplication in the tissues [15, 38], Every clinical form of leprosy is the result of a sequence of events, any of which may have modified the functional capacity of the patient's immune system. Patients with lepromatous leprosy, who are the main source of infective bacilli, are known to lack specific CMI, Whether this defect is due to a genetic predisposition, is developed as a consequence of the infection, is caused by environmental factors, or is caused by a combination of the three, we do not know. To clarify this point and the mechanisms involved is of paramount importance to the strategy of immune prevention of leprosy. There is evidence that only a small proportion of those who become exposed to the leprosy bacillus develop clinical disease. Furthermore, in most cases in the tuberculoid end of the spectrum, the infection is strictly localized, and these patients probably are not an important factor in the transmission of the disease. From the preventive point of view, the need to enhance protective immunity concerns in particular a small proportion ofthe population; those who are or will become defective in handling the bacilli and thus may develop into new infectious cases. It is essential to be able to identify this high-risk group. To define the requirements of a vaccine and to be able to test its efficacy directly on the high-risk group, we need to know what the crucial defects are and their relationship to known immunological parameters. Several hypotheses have been put forward to explain the nature of the defect in lepromatous leprosy, including lack of circulating Mycobacterium leprae-reacti\e T lymphocytes [14], activation of suppressor cells [13, 31] and defective macrophage function [1]. At present insufficient data are available for a critical evaluation of the various alternatives. What we have to do now is to put forward hypotheses and test them in experimental models. Thereby the complex multifactorial events, acting simultaneously and for long periods of time, may be dissociated and provide the key to the identification ofthe high-risk group. M. leprae has long been thought to be a natural pathogen only for man. Recently, the armadillo has proved to be susceptible to experimental inoculation [19], and there have been some reports of a leprosy-like infection in wild armadillos [40], However, since little is known about the immune system ofthe armadillo, basic research is needed before this model can be used to dissect the mechanisms of protective immunity in mycobacterial infection [41], In mice, on the other hand, many immunological parameters are well characterized and controllable, and therefore this species should be used as the preferential model. Since M. leprae exhibits only limited multiplication in mice, experimental work would have to include other bacteria that are mouse pathogens, such a M. lepraemurium, other mycobacteria, or Listerla
288
P. H. Lagrange & O. Closs
monocytogenes. Because they do not conform identically to the course of the human disease, these models can be used only to explore some aspects of the protective mechanism. In mice infected with M. lepraemurium, variations in protective immunity have been observed which appear to be genetically controlled. Outbred mice vary in their susceptibility to infection [9], and some selected inbred strains of mice (for example, C57BL) have been found to be able to mount a specific CMI that enables them to control the multiplication of bacteria, wheras other mice (C3H) are unable to do so at any stage of the infection [7]. Some controversy concerning the resistance pattern exists in the literature, and C57BL mice have been claimed to be susceptible to and C3H highly resistant to M. lepraemurium infection [23]. There may be several explanations for this discrepancy, the most important in this context being the use of different parameters of resistance—that is, survival time versus multiplication of bacteria. In acute infections, an increased survival time is well correlated with protective immunity. This may not always be the case in chronic infection. Lymphocyte choriomeningitis virus infection in mice may serve as an example of this point. Mice who become infected with the virus in utero develop a carrier state with no clinical symptoms, and they completely lack an effective protective immune response [17, 37]. If the virus is injected into normal adult mice, the mice will die from the intracranial immune reaction. Thus it is not the multiplication of the infective agent but the immune response of the host that is the fatal event. This may be true also in certain chronic bacterial infections when the multiplication of bacteria per se is well tolerated by the host. In response to a large inoculum of M. lepraemurium, C3H mice may survive for a longer time than, for instance, C57BL mice because the former do not develop an immune response and therefore tolerate the infection better. It is not sufficient to use prolongation of survival time as a criterion of protective immunity in chronic bacterial infection unless it has been shown to correlate with a limitation of bacterial multiplication. It has been demonstrated experimentally that mechanisms exist for trapping lymphocytes during M. lepraemurium infection in rats and mice [5], which may limit the expression of the specific subset of T cells at the periphery. Yet, we do not know whether this is the basis of the natural defect in the non-responsive C3H mice. Moreover, attempts to restore the immune response by eliminating the trapping mechanism have been unsuccessful [22]. Furthermore, DTH reactions induced by M. lepraemurium antigen in cyclophosphamide-pretreated C3H mice and elicited by injecting the same antigen repeatedly at the site of infection did not affect the multiplication of bacilli [25]. Therefore, lack of induction of a specific subset of T cells seems to occur. These mice have also been shown to be low responders to BCG immunization [32] and may therefore be unable to properly handle certain mycobacteria. As has recently been shown in Biozzi-selected lines of mice, BCG immunization is effective only in the line of high antibody producers (H/Ab) and not in the line of low antibody producers (L/Ab) [20]. The two lines differ in their natural macrophage antibacterial capacity, the (L/Ab) mice being able to more effectively limit the growth of L. monocytogenes and M. bovis, strain BCG. It was concluded that L/Ab mice were deficient in some aspects of their acquired protective mechanism because of the inability of their macrophages to recruit the right subset of T cells. These mice seem to be different from C3H mice, so it could be worthwhile to compare these two mechanisms of low responsiveness to M. lepraemurium. In both man and mice, the resistance to chronic bacterial infection is characterized by great individual variation. Our understanding of the mechanism by which this variation may be generated is at present very incomplete. We know that certain hereditary diseases and certain experimental manipulations of the immune system may create a state of immunodeficiency leading to increased susceptibility to a variety of infections. There is at present no
Protective Immunity to Chronic Bacterial Infection
289
direct relationship between such gross immunodeficiencies and the more subtle defects responsible for the increased susceptibility that some normal individuals have to a certain infection. An important aspect of experimental models such as the M. lepraemurium models mentioned above is that they allow variation in infection resistance to be studied in unmanipulated animals. Thereby they provide new tools for analysing the mechanisms involved in protective immunity and may help us to understand the basis of natural variation in host resistance. Laboratoire d'Immunobiologie du BCG, Institute Pasteur, 25 rue du Docteur Roux, 75724 Paris Cedex 15, France. Institute for Experimental Medical Research, University of Oslo, Ullevdl Hospital, Oslo 1, Norway
REFERENCES 1 Beiguelman, B. Leprosy and genetics. £«//. Wld Hlth Org. 37,461, 1967. 2 Blanden, R.V. & Langman, R.E. Cell mediated immunity to bacterial infection in the mouse. Thymus-derived cells as effectors of acquired resistance to Listeria monocytogenes. Scand. J. Immunol. 1, 379, 1972. 3 Blanden, R.V., Lefford, M.J. & Mackaness, G.B. The host response to Calmette-Gu6rin bacillus infection in mice. / . exp. Med. 129, 1079, 1969. 4 Bryceson, A. Anergy and suppressed cellular immunity in disease, pp. 161-170 in Brent, L. & Holborrow, J. (eds.) Progress in Immunology II, Vol. 3. Elsevier, Amsterdam, 1974. 5 Bullock, W.E. Leprosy: a model of immunological perturbations in chronic infection. / . infect. Dis. 137, 341, 1978. 6 Cheers, C , McKenzie, I.F.C., Pavlov, H., Waid, C. & York, ]. Resistance and susceptibility of mice to bacterial infection. Infect. Immunity, 19, 755, 1977. 7 Closs, O. Experimental murine leprosy: growth of Mycobacterium lepraemurium in C3H and C57BL mice after foot pad inoculation. Infect. Immunity, 12, 480, 1975. 8 Closs, O. Experimental murine leprosy. Thesis, Oslo, 1975. 9 Closs, O. & Haugen, O.A. Experimental murine leprosy. II. Further evidence for varying susceptibility of outbred mice and evaluation of the response of 5 inbred mouse strains to infection with Mycobacterium lepraemurium. Acta path, microbiol. scand. Seer./4. 82, 459-474, 1974. 10 Comstock, G.W. & Edwards, P.Q. An american view of BCG vaccination, illustrated by results of a controlled trial in Puerto Rico. Scand. J. resp. Dis. S3, 207, 1972. 11 Dannenberg, A.M., Jr, Meyer, O.T., Esterly, J.R. & Kambara, T. The local nature of immunity in tuberculosis, illustrated histochemically in dermal BCG lesions. / . Immunol. 100, 931, 1968. 12 Furth, R. van, Langevoort, H.L. & Shaberg, A. Mononuclear phagocytes in human pathology— proposal for an approach to improved classification, pp. 1-15 in van Furth, R. (ed.) Mononuclear Phago-
13 14
15
16
17
18
19
20
21
22
23
24
P. H. LAGRANGE
O. CLOSS
cytes in Immunity, Infection and Pathology. Blackwell Scientific Publications, Oxford, 1975. Godal, T. Immunological aspects of leprosy— present status. Prog. Allergy, IS, 211, 1978. Godal, T., Myklestad, B., Samuel, D.R. & Myrvang, B. Characterization of the cellular immune defect in lepromatous leprosy: a specific lack of circulating Mycobacterium leprae-reaclive lymphocytes. Clin. exp. Immunol. 9, 821, 1971. Godal, T., Myrvang, B., Stanford, J.L. & Samuel, D.R. Recent advances in the immunology of leprosy with special reference to new approaches in immunoprophylaxis. Butt. Inst. Pasteur, 72, 273, 1974. Gray, D.F., Graham-Smith, H. & Noble, J.L. Variations in natural resistance to tuberculosis. / . Hyg. (Camb.), 58, 215, 1960. Hotchin, J. The biology of lymphocytic choriomeningitis infection: virus-induced immune disease. Symp. quant. Biol. 27, 479, 1962. Huber, B., Devinsky, O., Gershon, R.K. & Cantor, H. Cell mediated immunity: delayed type hypersensitivity and cytotoxic responses are mediated by difierent T-cell subclasses. / . exp. Med. 143, 1534, 1976. Kirchheimer, W.F. & Storrs, E.E. Attempts to establish the Armadillo {Dasypus novemcinctus Linn.) as a model for the study of leprosy. Int. J. Leprosy, 39, 693, 1971. Lagrange, P.H., Hurtel, B. & Thickstun, P.M. Immunological behaviour after mycobacterial infection in selected lines of mice with high or low antibody responses. Infect. Immunity, 25, 39, 1979. Lane, F.C. & Unanue, E.R. Requirement of thymus (T) lymphocytes for resistance to listeriosis. / . exp. Med. 135, 1104, 1972. Lefford, M.J. & Mackaness, G.B. Suppression of immunity to Mycobacterium lepraemurium infection. Infect. Immunity, 18, 363, 1977. Lefford, M.J., Patel, P.J., Poulter, L.W. & Mackaness, G.B. Induction of cell-mediated immunity to Mycobacterium lepraemurium in susceptible mice. Infect. Immunity, 18, 654, 1977. Lodmell, D.L. & Ewalt, L.C. Induction of enhanced resistance against encephalomyocarditis virus infection of mice by nonviable Mycobacterium
290
25
26
27 28
29 30
31
32
33
P. H. Lagrange & O. Chss
tuberculosis: mechanisms of protection. Infect. Immunity, 22, 740, 1978. Lovik, M. & Closs, O. Delayed type hypersensitivity to mycobacterial antigens without protective immunity: a failure to produce the right specificity or the right type of immune reaction? Scand. J. infect. Dis., in press. Mackaness, G.B. The infltience of immunologically committed iymphoid cells on macrophage activity in vitro. / . exp. Med. 129, 973, 1969. Mackaness, G.B. Resistance to intracellular infection. / . infect. Dis. 123, 439, t971. Mackaness, G.B., Auclair, D.J. & Lagrange, P.H. Immunopotentiation with BCG: the immune response to different strains and preparations. / . nat. Cancer Inst. 51, 1655, 1973. Mackaness, G.B. & Blanden, R.V. Cellular immunity. Progr. Allergy, 11, 89, 1967. Medical Research Council. BCG and vole bacillus vaccines in the prevention of tuberculosis in adolescence and early adult life. Brit. med. J. i, 973, 1963. Mehra, V., Bloom, B.R., Mason, L. & Fields, J. Studies on Mycobacterium leprae-iniuced suppression of in vitro response in leprosy patients. Abstracts, XI International Leprosy Congress, Mexico City, 1978, p. 59. Nakamura, R.M. & Tokunga, T. Strain difference of delayed-type hypersensitivity to BCG and its genetic control in mice. Infect. Immunity, 22, 657, 1978. North, R.J. The concept of the activated macrophage. / . Immunol. 121, 806, 1978.
34 Plant, J. & Glynn, A.A. Natural resistance to salmonella infection, delayed hypersensitivity and Ir genes in different strains of mice. Nature, 248, 345, 1974. 35 Ridley, D.S. & Jopling, W.H. Classification of leprosy according to immunity. A five-group system. Int. J. Leprosy, 34, 255, 1963. 36 Ruitenberg, E.J. & van Noorle-Jansen, L.M. Effect of Corynebacterium parvum on the course of a Listeria monocytogenes infection in normal and congenitally athymic (nude mice). Zbl. Bakt., I. Abt. Orig. Heihe A, 232, 197, 1975. 37 Traub, E. Persistence of lymphocytic choriomeningitis virus in immune animals and its relation to immunity. / . exp. Med. 63, 847, 1936. 38 Turk, J.L. & Bryceson, A.D.M. Immunological phenomena in leprosy and related diseases. Adv. Immunol 13, 209, 1971. 39 Vadas, M.A., Miller, J.F.A.P., Mackenzie, I.F.C., Chism, S.E., Shen, F.-W., Boyse, E.A., Gamble, J.R. & Whitelaw, A.M. Ly and Ia antigen phenotypes of T-cells involved in delayed-type hypersensitivity and in suppression. / . exp. Med. 144, 10, 1976. 40 Walsh, G.P., Storrs, E.E., Burchfield, H.P., Cottrell, E.H., Vidrine, M.F. & Binford, C.H. Leprosy-like disease occurring naturally in armadillos. / . Retieuloendothelial Sac. 18, 347, 1975. 41 World Health Organisation. Report of the third IMMLEP Scientific Working Group Meeting. Protocol, 6, 1977. 42 Youmans, G.P. Relation between delayed hypersensitivity and immunity in tuberculosis. Am. Rev. resp. Dis. I l l , 109, 1975.
EDITORIAL
Protective Immunity to Chronic Bacterial Infection An established chronic bacterial infection involves a delicate balance between host defence mechanisms and manoeuvres made by the microorganism to evade these mechanisms. To become established, the infection must pass through an initial stage that may or may not give rise to clinical symptoms. During this initial stage the infection may either be terminated owing to development of effective protective immunity, or the bacteria may survive and continue to multiply within the host tissues and thus establish a chronic infection. Symptoms of chronic infections reflect the reaction of the host to the multiplication and persistence of microorganisms. It is important to realize that in some instances our recognition of a chronic infection depends as much on the development of host reaction as on the persistence of living microorganisms. There is considerable variation in the clinical course of many chronic bacterial infections, and studying the basis of this variation could provide us with clues to some of the mechanisms involved in host protection. The variation is determined by both microbial and host factors. An individual who is able to eliminate an infection with one strain of bacteria may be susceptible to a more virulent strain, to a higher number of infecting organisms, or even to the same number of the same bacterial strain when it is introduced by a different route. Less is known about the mechanisms that may generate variation of host resistance in normal individuals. The ability of the host to combat the infection depends on both natural and acquired resistance. In several animal models, host resistance has been shown to be genetically controlled [6, 8, 16, 34], but it may also be influenced by environmental factors such as state of nutrition or intercurrent disease [4]. The relative importance of each of these factors has yet to be determined. Most of the chronic bacterial infections are characterized by intracellular survival of the microorganisms, and host protection appears to be closely correlated with the ability to mount and maintain an effective cell-mediated immune response against microbial antigens [27]. Still, it would be an oversimplification to regard heterogeneity in clinical forms of a particular chronic infection as a direct result of individual variation in primary immune responsiveness; there must be a multifactorial correlation between the clinical forms of the disease and aetiopathological factors. Moreover, during the course of the disease, additional control or regulatory mechanisms may modulate the initial immunological events. Thus, the final outcome of the infection may not necessarily have any direct relationship to the primary immune responsiveness of the patient. Protective immunity confers upon the individual the ability to stop the multiplication of the microorganisms before the infection has reached the chronic stage. The ability to prevent the multiplication of virulent pathogens is generally mediated by effector mechanisms that are focused at the sites of infection after recognition of specific microbial antigens [11]. However, enhancement of certain non-specific mechanisms may also produce a state of increased resistance. Cell wall components of Gram-negative bacteria, mycobacteria, and brucellae are known to stimulate the resistance of the host to a variety of antigenically 0300-9475/79/1000-0285 $02.00 © 1979 Blackwell Scientific Publications
285
286
P. H. Lagrange & O. Closs
unrelated microorganisms. Even if it were possible to accomplish a stimulatory effect on non-specific immunity, it would be of short duration, and thus we are left with promotioti of specific acquired immunity as the main protective measure against chronic bacterial infection. It has repeatedly been observed that certain immunizing procedures that protect the great majority of the population still remain ineffective in certain normal individuals even though they do not present any obvious immunodeficiency. For instance, after vaccination with bacille Calmette-Guerin (BCG) a certain percentage of the vaccinees were insufficiently protected [30]. Furthermore, in several studies, the degree of protection against tuberculosis did not show any direct correlation with the rate of conversion to tuberculin positivity after vaccination [10]. The last point may serve as an illustration of the general lack of adequate immunological tests to measure protective immunity in these infections. The basic mechanisms of protective immunity have been well characterized in many infections in which protection is mediated by humoral antibodies. For instance, the protective effect of vaccination against tetanus, pneumococcal infections, or measles and rubella is easily assessed by measuring the antibody activity against tetanus toxin, pneumococcal capsular polysaccharides, or certain virus-neutralizing antibodies. Much less is known about the protective mechanisms involved in the defence against chronic bacterial infections, such as brucellosis, salmonellosis, syphilis, tuberculosis, and leprosy. Since the nature of protective immunity is not yet known in detail at the cellular and molecular levels in any of these infections, it is also difficult to select the appropriate immunological parameters to measure acquired protective immunity. Apparently, for intracellularly growing parasites, immune activation of macrophages is connected with limitation of growth in vivo [29] and with the presence of some expressions of T-cell-dependent responses [2, 21], such as delayed-type hypersensitivity (DTH) [26]. On the other hand, protective immunity has been demonstrated in the absence of any measurable T-cell activity [42]. Moreover, specific T-cell activity, as measured by in vitro or in vivo tests, may be present without any evidence of acquired resistance [42]. Most likely, interaction between certain specific cell populations is required to produce adequate protection. Although some information is available concerning which subpopulations of T cells are involved in DTH [18, 39], the specific role of these cell populations in the generation of protective immunity is not known at present. Besides, different subpopulations of mononuclear phagocytes have been described [12], and conceivably there is a difference in their killing potential [33]. Furthermore, differentiation of mononuclear phagocytes seems to be regulated by T-cell activity [2, 21] but may also, to some extent, take place in the absence of any T-cell function [24, 36]. This emphasizes the necessity of having appropriate experimental models to determine the exact populations of cells involved at various levels in the cooperation between T cells and mononuclear phagocytes in order to produce effective killing of intracellular bacteria. The elusive protective antigen in cell-mediated immunity (CMl) has still not been found. We do not know whether it is one or several antigens, or whether it is located at the surface of the microorganism or internally. Furthermore, we do not know whether the key to its action as inductor of protective immunity is found in the location and physieochemical properties of the antigen or in the type of immune response it induces. There is evidence that in some infections the antigenic stimulus for protective immunity is produced only by growing microorganisms [3, 28]. If the induction of protective immunity requires interaction between macrophages and living microorganisms, any purified bacterial antigen would fail to induce protection. This might substantially complicate the analysis of the antigens involved
Protective Immunity to Chronic Bacterial Infection
287
in protective immunity, especially if living bacteria are also needed to elicit the protective effector mechanism. The requirements for a protective antigen are as follows: first, to be produced by the intracellular microorganism; second, to be presented by the macrophage; third, to promote the activation of the appropriate subset of T cells, A defect in the processing and presentation of this antigen may produce an altered stimulus that will not trigger the right sequence of events and therefore will not induce an adequate immune response. Even with an appropriate presentation ofthe antigen, additional regulator mechanisms may be able to modulate, and even to obviate, the final response at the site ofthe infection. We may consider leprosy as a typical chronic infection in which all ofthe above-described factors may come into play. The classical description of leprosy involves a spectrum of clinical forms, ranging from the polar tuberculoid form (TT) to the polar lepromatous form (LL) [35], Apparently, an inverse relationship exists between specific CM[ and bacterial multiplication in the tissues [15, 38], Every clinical form of leprosy is the result of a sequence of events, any of which may have modified the functional capacity of the patient's immune system. Patients with lepromatous leprosy, who are the main source of infective bacilli, are known to lack specific CMI, Whether this defect is due to a genetic predisposition, is developed as a consequence of the infection, is caused by environmental factors, or is caused by a combination of the three, we do not know. To clarify this point and the mechanisms involved is of paramount importance to the strategy of immune prevention of leprosy. There is evidence that only a small proportion of those who become exposed to the leprosy bacillus develop clinical disease. Furthermore, in most cases in the tuberculoid end of the spectrum, the infection is strictly localized, and these patients probably are not an important factor in the transmission of the disease. From the preventive point of view, the need to enhance protective immunity concerns in particular a small proportion ofthe population; those who are or will become defective in handling the bacilli and thus may develop into new infectious cases. It is essential to be able to identify this high-risk group. To define the requirements of a vaccine and to be able to test its efficacy directly on the high-risk group, we need to know what the crucial defects are and their relationship to known immunological parameters. Several hypotheses have been put forward to explain the nature of the defect in lepromatous leprosy, including lack of circulating Mycobacterium leprae-reacti\e T lymphocytes [14], activation of suppressor cells [13, 31] and defective macrophage function [1]. At present insufficient data are available for a critical evaluation of the various alternatives. What we have to do now is to put forward hypotheses and test them in experimental models. Thereby the complex multifactorial events, acting simultaneously and for long periods of time, may be dissociated and provide the key to the identification ofthe high-risk group. M. leprae has long been thought to be a natural pathogen only for man. Recently, the armadillo has proved to be susceptible to experimental inoculation [19], and there have been some reports of a leprosy-like infection in wild armadillos [40], However, since little is known about the immune system ofthe armadillo, basic research is needed before this model can be used to dissect the mechanisms of protective immunity in mycobacterial infection [41], In mice, on the other hand, many immunological parameters are well characterized and controllable, and therefore this species should be used as the preferential model. Since M. leprae exhibits only limited multiplication in mice, experimental work would have to include other bacteria that are mouse pathogens, such a M. lepraemurium, other mycobacteria, or Listerla
288
P. H. Lagrange & O. Closs
monocytogenes. Because they do not conform identically to the course of the human disease, these models can be used only to explore some aspects of the protective mechanism. In mice infected with M. lepraemurium, variations in protective immunity have been observed which appear to be genetically controlled. Outbred mice vary in their susceptibility to infection [9], and some selected inbred strains of mice (for example, C57BL) have been found to be able to mount a specific CMI that enables them to control the multiplication of bacteria, wheras other mice (C3H) are unable to do so at any stage of the infection [7]. Some controversy concerning the resistance pattern exists in the literature, and C57BL mice have been claimed to be susceptible to and C3H highly resistant to M. lepraemurium infection [23]. There may be several explanations for this discrepancy, the most important in this context being the use of different parameters of resistance—that is, survival time versus multiplication of bacteria. In acute infections, an increased survival time is well correlated with protective immunity. This may not always be the case in chronic infection. Lymphocyte choriomeningitis virus infection in mice may serve as an example of this point. Mice who become infected with the virus in utero develop a carrier state with no clinical symptoms, and they completely lack an effective protective immune response [17, 37]. If the virus is injected into normal adult mice, the mice will die from the intracranial immune reaction. Thus it is not the multiplication of the infective agent but the immune response of the host that is the fatal event. This may be true also in certain chronic bacterial infections when the multiplication of bacteria per se is well tolerated by the host. In response to a large inoculum of M. lepraemurium, C3H mice may survive for a longer time than, for instance, C57BL mice because the former do not develop an immune response and therefore tolerate the infection better. It is not sufficient to use prolongation of survival time as a criterion of protective immunity in chronic bacterial infection unless it has been shown to correlate with a limitation of bacterial multiplication. It has been demonstrated experimentally that mechanisms exist for trapping lymphocytes during M. lepraemurium infection in rats and mice [5], which may limit the expression of the specific subset of T cells at the periphery. Yet, we do not know whether this is the basis of the natural defect in the non-responsive C3H mice. Moreover, attempts to restore the immune response by eliminating the trapping mechanism have been unsuccessful [22]. Furthermore, DTH reactions induced by M. lepraemurium antigen in cyclophosphamide-pretreated C3H mice and elicited by injecting the same antigen repeatedly at the site of infection did not affect the multiplication of bacilli [25]. Therefore, lack of induction of a specific subset of T cells seems to occur. These mice have also been shown to be low responders to BCG immunization [32] and may therefore be unable to properly handle certain mycobacteria. As has recently been shown in Biozzi-selected lines of mice, BCG immunization is effective only in the line of high antibody producers (H/Ab) and not in the line of low antibody producers (L/Ab) [20]. The two lines differ in their natural macrophage antibacterial capacity, the (L/Ab) mice being able to more effectively limit the growth of L. monocytogenes and M. bovis, strain BCG. It was concluded that L/Ab mice were deficient in some aspects of their acquired protective mechanism because of the inability of their macrophages to recruit the right subset of T cells. These mice seem to be different from C3H mice, so it could be worthwhile to compare these two mechanisms of low responsiveness to M. lepraemurium. In both man and mice, the resistance to chronic bacterial infection is characterized by great individual variation. Our understanding of the mechanism by which this variation may be generated is at present very incomplete. We know that certain hereditary diseases and certain experimental manipulations of the immune system may create a state of immunodeficiency leading to increased susceptibility to a variety of infections. There is at present no
Protective Immunity to Chronic Bacterial Infection
289
direct relationship between such gross immunodeficiencies and the more subtle defects responsible for the increased susceptibility that some normal individuals have to a certain infection. An important aspect of experimental models such as the M. lepraemurium models mentioned above is that they allow variation in infection resistance to be studied in unmanipulated animals. Thereby they provide new tools for analysing the mechanisms involved in protective immunity and may help us to understand the basis of natural variation in host resistance. Laboratoire d'Immunobiologie du BCG, Institute Pasteur, 25 rue du Docteur Roux, 75724 Paris Cedex 15, France. Institute for Experimental Medical Research, University of Oslo, Ullevdl Hospital, Oslo 1, Norway
REFERENCES 1 Beiguelman, B. Leprosy and genetics. £«//. Wld Hlth Org. 37,461, 1967. 2 Blanden, R.V. & Langman, R.E. Cell mediated immunity to bacterial infection in the mouse. Thymus-derived cells as effectors of acquired resistance to Listeria monocytogenes. Scand. J. Immunol. 1, 379, 1972. 3 Blanden, R.V., Lefford, M.J. & Mackaness, G.B. The host response to Calmette-Gu6rin bacillus infection in mice. / . exp. Med. 129, 1079, 1969. 4 Bryceson, A. Anergy and suppressed cellular immunity in disease, pp. 161-170 in Brent, L. & Holborrow, J. (eds.) Progress in Immunology II, Vol. 3. Elsevier, Amsterdam, 1974. 5 Bullock, W.E. Leprosy: a model of immunological perturbations in chronic infection. / . infect. Dis. 137, 341, 1978. 6 Cheers, C , McKenzie, I.F.C., Pavlov, H., Waid, C. & York, ]. Resistance and susceptibility of mice to bacterial infection. Infect. Immunity, 19, 755, 1977. 7 Closs, O. Experimental murine leprosy: growth of Mycobacterium lepraemurium in C3H and C57BL mice after foot pad inoculation. Infect. Immunity, 12, 480, 1975. 8 Closs, O. Experimental murine leprosy. Thesis, Oslo, 1975. 9 Closs, O. & Haugen, O.A. Experimental murine leprosy. II. Further evidence for varying susceptibility of outbred mice and evaluation of the response of 5 inbred mouse strains to infection with Mycobacterium lepraemurium. Acta path, microbiol. scand. Seer./4. 82, 459-474, 1974. 10 Comstock, G.W. & Edwards, P.Q. An american view of BCG vaccination, illustrated by results of a controlled trial in Puerto Rico. Scand. J. resp. Dis. S3, 207, 1972. 11 Dannenberg, A.M., Jr, Meyer, O.T., Esterly, J.R. & Kambara, T. The local nature of immunity in tuberculosis, illustrated histochemically in dermal BCG lesions. / . Immunol. 100, 931, 1968. 12 Furth, R. van, Langevoort, H.L. & Shaberg, A. Mononuclear phagocytes in human pathology— proposal for an approach to improved classification, pp. 1-15 in van Furth, R. (ed.) Mononuclear Phago-
13 14
15
16
17
18
19
20
21
22
23
24
P. H. LAGRANGE
O. CLOSS
cytes in Immunity, Infection and Pathology. Blackwell Scientific Publications, Oxford, 1975. Godal, T. Immunological aspects of leprosy— present status. Prog. Allergy, IS, 211, 1978. Godal, T., Myklestad, B., Samuel, D.R. & Myrvang, B. Characterization of the cellular immune defect in lepromatous leprosy: a specific lack of circulating Mycobacterium leprae-reaclive lymphocytes. Clin. exp. Immunol. 9, 821, 1971. Godal, T., Myrvang, B., Stanford, J.L. & Samuel, D.R. Recent advances in the immunology of leprosy with special reference to new approaches in immunoprophylaxis. Butt. Inst. Pasteur, 72, 273, 1974. Gray, D.F., Graham-Smith, H. & Noble, J.L. Variations in natural resistance to tuberculosis. / . Hyg. (Camb.), 58, 215, 1960. Hotchin, J. The biology of lymphocytic choriomeningitis infection: virus-induced immune disease. Symp. quant. Biol. 27, 479, 1962. Huber, B., Devinsky, O., Gershon, R.K. & Cantor, H. Cell mediated immunity: delayed type hypersensitivity and cytotoxic responses are mediated by difierent T-cell subclasses. / . exp. Med. 143, 1534, 1976. Kirchheimer, W.F. & Storrs, E.E. Attempts to establish the Armadillo {Dasypus novemcinctus Linn.) as a model for the study of leprosy. Int. J. Leprosy, 39, 693, 1971. Lagrange, P.H., Hurtel, B. & Thickstun, P.M. Immunological behaviour after mycobacterial infection in selected lines of mice with high or low antibody responses. Infect. Immunity, 25, 39, 1979. Lane, F.C. & Unanue, E.R. Requirement of thymus (T) lymphocytes for resistance to listeriosis. / . exp. Med. 135, 1104, 1972. Lefford, M.J. & Mackaness, G.B. Suppression of immunity to Mycobacterium lepraemurium infection. Infect. Immunity, 18, 363, 1977. Lefford, M.J., Patel, P.J., Poulter, L.W. & Mackaness, G.B. Induction of cell-mediated immunity to Mycobacterium lepraemurium in susceptible mice. Infect. Immunity, 18, 654, 1977. Lodmell, D.L. & Ewalt, L.C. Induction of enhanced resistance against encephalomyocarditis virus infection of mice by nonviable Mycobacterium
290
25
26
27 28
29 30
31
32
33
P. H. Lagrange & O. Chss
tuberculosis: mechanisms of protection. Infect. Immunity, 22, 740, 1978. Lovik, M. & Closs, O. Delayed type hypersensitivity to mycobacterial antigens without protective immunity: a failure to produce the right specificity or the right type of immune reaction? Scand. J. infect. Dis., in press. Mackaness, G.B. The infltience of immunologically committed iymphoid cells on macrophage activity in vitro. / . exp. Med. 129, 973, 1969. Mackaness, G.B. Resistance to intracellular infection. / . infect. Dis. 123, 439, t971. Mackaness, G.B., Auclair, D.J. & Lagrange, P.H. Immunopotentiation with BCG: the immune response to different strains and preparations. / . nat. Cancer Inst. 51, 1655, 1973. Mackaness, G.B. & Blanden, R.V. Cellular immunity. Progr. Allergy, 11, 89, 1967. Medical Research Council. BCG and vole bacillus vaccines in the prevention of tuberculosis in adolescence and early adult life. Brit. med. J. i, 973, 1963. Mehra, V., Bloom, B.R., Mason, L. & Fields, J. Studies on Mycobacterium leprae-iniuced suppression of in vitro response in leprosy patients. Abstracts, XI International Leprosy Congress, Mexico City, 1978, p. 59. Nakamura, R.M. & Tokunga, T. Strain difference of delayed-type hypersensitivity to BCG and its genetic control in mice. Infect. Immunity, 22, 657, 1978. North, R.J. The concept of the activated macrophage. / . Immunol. 121, 806, 1978.
34 Plant, J. & Glynn, A.A. Natural resistance to salmonella infection, delayed hypersensitivity and Ir genes in different strains of mice. Nature, 248, 345, 1974. 35 Ridley, D.S. & Jopling, W.H. Classification of leprosy according to immunity. A five-group system. Int. J. Leprosy, 34, 255, 1963. 36 Ruitenberg, E.J. & van Noorle-Jansen, L.M. Effect of Corynebacterium parvum on the course of a Listeria monocytogenes infection in normal and congenitally athymic (nude mice). Zbl. Bakt., I. Abt. Orig. Heihe A, 232, 197, 1975. 37 Traub, E. Persistence of lymphocytic choriomeningitis virus in immune animals and its relation to immunity. / . exp. Med. 63, 847, 1936. 38 Turk, J.L. & Bryceson, A.D.M. Immunological phenomena in leprosy and related diseases. Adv. Immunol 13, 209, 1971. 39 Vadas, M.A., Miller, J.F.A.P., Mackenzie, I.F.C., Chism, S.E., Shen, F.-W., Boyse, E.A., Gamble, J.R. & Whitelaw, A.M. Ly and Ia antigen phenotypes of T-cells involved in delayed-type hypersensitivity and in suppression. / . exp. Med. 144, 10, 1976. 40 Walsh, G.P., Storrs, E.E., Burchfield, H.P., Cottrell, E.H., Vidrine, M.F. & Binford, C.H. Leprosy-like disease occurring naturally in armadillos. / . Retieuloendothelial Sac. 18, 347, 1975. 41 World Health Organisation. Report of the third IMMLEP Scientific Working Group Meeting. Protocol, 6, 1977. 42 Youmans, G.P. Relation between delayed hypersensitivity and immunity in tuberculosis. Am. Rev. resp. Dis. I l l , 109, 1975.
