PERSPECTIVE
Could the Risk of Asthma and Atopy Be Reduced by a Vaccine That Induces a Strong T-helper Type 1 Response?

Leena C. von Hertzen and Tari Haahtela

The Finnish Lung Health Association; and Helsinki University Central Hospital, Skin and Allergy Hospital, Helsinki, Finland

A serious concern has been raised in the western world over the substantial increase in asthma and atopy prevalence over the last few decades. Changes in diagnostic procedures, genetic composition, or exposure to the conventional etiologic factors of asthma and atopy appear to be unable to explain most of this increase (1). Several lines of evidence point to childhood infections providing significant protection from atopy (4); the lack of microbial stimulation of sufficient intensity in early life may, paradoxically, affect the maturation of the immune system, leading to the predominance of a biased T-helper (Th)2 cell subtype in genetically susceptible individuals (8). Intense childhood infections in western societies have become uncommon, and therefore the immune system may remain relatively unoccupied, a situation that may result in increased susceptibility to sensitization with otherwise harmless allergens in the environment (11). In contrast to certain natural infections, Calmette-Guerin bacillus (BCG) vaccination has not had any effect on the subsequent development of asthma or atopy either in genetically predisposed (12) or in unselected Swedish (13) children. Probably only those infections that are able to mount a strong cell-mediated immune response and a long-living memory immunity may play a role here. Such infections are able to mobilize and activate tissue macrophages that migrate to regional lymph nodes and secrete cytokines that inhibit Th2 cell differentiation (10). On the basis of evidence obtained from humans (5) and animal models (14, 15), we hypothesize that a vaccine that induces a strong Th1 immune response and a long-living memory immunity might in early life prevent the establishment of a biased Th2 cytokine milieu in the lungs of genetically predisposed children. This article briefly outlines the theoretical basis of the hypothesis and the possible role of mycobacterial vaccines in this immunomodulation.

	Background

The recognition of the divergence of Th cells into predominantly two arms, Th1 and Th2 subsets that are largely mutually exclusive and reciprocally regulated (16, 17), has decisively improved our understanding of the mechanisms that contribute to the pathogenensis of many chronic diseases. An increasing body of evidence from mice and humans now supports the concept that the predominance of either reciprocally regulated subset may result in certain chronic diseases (18).

It is now well established that atopy is a disorder characterized by the predominance of Th2 cells that secrete mainly interleukin (IL)-4 and IL-5 (9, 18). Such T cells displaying messenger RNA for Th2 cytokines but not for Th1 cytokines have been demonstrated in mucosal bronchial biopsies or bronchoalveolar lavage (BAL) from patients with atopic diseases (reviewed in Reference 9). There is compelling evidence that allergen-specific Th2 cells accumulate in the target organs of atopic patients and play a crucial role in the pathogenesis of allergy (9, 21). Thus, preventing (or reversing) the differentation of Th cells into Th2 cells appears a logical therapeutic approach to atopic asthma (21). Inasmuch as IL-12 is the key cytokine for the induction of Th1 response (22), inheritance of an impaired capacity to produce this cytokine could lead to biased Th2 development. Indeed, it has recently been demonstrated that stimulated mononuclear blood cells obtained from patients with atopic asthma produced highly reduced levels of IL-12 and IL-12-dependent interferon (IFN)- compared with control subjects. It was suggested that the diminished production of IL-12 may be the primary inherited defect in patients with atopic asthma. This reduced production of IL-12 favors the development of Th2 cells, resulting in elevated immunoglobulin (Ig)E levels, and, independently of IgE, may lead to more severe airway hyperresponsiveness (AHR) (23).

Reversal of the Local Th1/Th2 Balance

Whether the biased cytokine milieu in asthma could be reversed by persistent immunomodulation is not known. Evidence from animal studies (24) suggests that reversal might be possible even after Th2 clones have been established, although data on the duration of this effect are scanty. However, chronic antigen stimulation has been found to render the T-cell populations more homogenic and resistant to change (28).

Data obtained from sensitized mice suggested that IL-12 given locally or intraperitoneally was able to enhance IFN- and decrease IL-4 and IL-5 production in the lung and prevent AHR as well as reduce lung eosinophilia (24, 27). Further, transfer of IFN- gene into the airway mucosa of sensitized mice has been demonstrated to result in significant expression of IFN- in the pulmonary epithelium and inhibition of pulmonary eosinophilia and AHR (25). Because injected cytokines are often short lived in vivo, the gene-based delivery of IFN- was proposed to provide stable therapeutic local concentrations (25). Nebulized recombinant-IFN- has already been used in a few patients with mild atopic asthma, most of whom showed a decrease in BAL eosinophil percentages (29). Moreover, nebulized but not parenteral IFN- has significantly inhibited immediate hypersensitivity and normalized airway responsiveness in a murine model of allergen sensitization (30). In humans, both locally administered IL-12 and IFN- still need to be evaluated. Data on proper doses, treatment schedules, and the duration of the effect of these cytokines are almost completely lacking. For systemically administered IL-12, the effects have shown schedule-dependent toxicity, particularly leukopenia and hyperbilirubinemia (31).

However, cytokines are not the only possible approach to immunomodulation of lung disease. The idea that the Th1/Th2 balance could be affected by vaccination with relevant micro-organisms or microbial components is another logical consequence of the current knowledge of the Th1/Th2 paradigm. In that regard, Mycobacterium tuberculosis is one of the most potent immunomodulatory micro-organisms and could be expected to strongly effect the cytokine milieu in the lung. This intracellular pathogen is known to induce a vigorous Th1 immune response, especially at the site of infection, and long-lived immunity (32, 33). This bacterium has also been extensively studied, and its molecular structure and the mechanisms in the host-pathogen interactions are well characterized. An improved mycobacterial vaccine might be an alternative to direct cytokine treatment in this immunomodulation.

A Novel Vaccine against M. tuberculosis

Tuberculosis remains a major international health problem with high and still increasing mortality rates in many countries (34). To date, the only antituberculosis vaccine available is the BCG vaccine based on live, attenuated Bacillus Calmette-Guerin-Mycobacterium bovis. The vaccine was developed more than 60 years ago and has not been significantly improved since then (32). In animal models, BCG has been shown to elicit high IFN- levels and marked cell-mediated immune responses, but its efficacy in human populations remains controversial. Although BCG appears to provide good protection when given in infancy, no protective efficacy has been found in some adult populations (reviewed in Reference 34). Thus, the development of an improved vaccine against tuberculosis remains a high priority for health care (33).

An increasing body of evidence now supports the view that the generation of protective immunity in M. tuberculosis is mediated primarily by Th1 cells that are preferentially directed to the secreted proteins of the organism (33, 35). During the early phases of infection, the bacteria secrete proteins that are processed and presented in the context of major histocompatibility complex class II and sensitized CD4⁺ T cells start to accumulate in the infected organs. These CD4⁺ T cells secrete high levels of IFN- (33). Encouraging results have been obtained in animal studies using M. tuberculosis subunit vaccines based on proteins secreted during early phases of infection by live bacteria. The dose of antigen has proved to play a crucial role in vaccine efficacy (36). Similarly, adjuvants markedly modulate immune responses to M. tuberculosis (37). Both issues still need careful investigation (33). The impact of different adjuvants on the efficacy of tuberculosis subunit vaccines has been reported elsewhere (37).

Other possibilities for antituberculosis vaccines include improving the present BCG vaccine by gene engineering (33, 38) or using BCG together with another mycobacteria. There is some evidence to suggest that a fast-growing mycobacterium, Mycobacterium vaccae, that induces a vigorous cell-mediated immune response and shares several immunodominant epitopes with other mycobacteria might be a potential candidate (39, 40). Heat-killed M. vaccae has been used as a powerful bacterial adjuvant and Th1 driver (41), and the mixture of live BCG and killed M. vaccae, or even the latter alone, may provide better protection against tuberculosis than does BCG alone (40). This finding may relate to the observation that BCG is a poor inducer of IL-12 (42), the key cytokine in the induction of the Th1 response.

Route of Vaccination

Evidence obtained from animal models shows that vaccines given via the mucosa are able to elicit neutralizing serum antibodies and cell-mediated immune responses as well as mucosal secretory IgA antibodies (43). Thus, it is possible that vaccines delivered by mucosal surfaces would stimulate vigorous cell-mediated immune responses similarly in humans and efficaciously protect against diseases such as tuberculosis and measles (43). Relatively little attention has been paid so far to vaccination via the mucosal route. Oral polio vaccine (44) was the first commercially available preparation, and to date there is another oral preparation, live Salmonella typhi vaccine, that has shown to be highly effective against typhoid fever (45, 46). Protection by oral polio vaccine is considered to be permanent and by S. typhi vaccine is thought to be valid for several years (47), indicating that long-lived immunologic memory is achievable also by vaccines administered via noninvasive routes.

In animal models with another intracellular bacterium, Chlamydia trachomatis, the intranasal route has proved to be particularly effective in terms of inducing long-lasting protective immunity (48). However, contrary to most pathogens invading the body via mucosal surfaces, M. tuberculosis reaches the lower respiratory tract through inhalation of droplet nuclei and is ingested primarily by alveolar macrophages. Alternative noninvasive routes, such as intranasal or bronchial, should nonetheless be thoroughly considered as possible routes for a novel M. tuberculosis vaccine. In support of this possibility, Erb and coworkers (14) showed that in a murine model of allergen-induced eosinophilia, infection with M. bovis-BCG suppressed the accumulation of eosinophils into the airways. The inhibitory effect of BCG infection on eosinophil accumulation depended on the route of infection. Intranasally induced infection proved to be superior to intraperitoneal or subcutaneous routes of infection. In addition, the amount of BCG inoculum significantly affected the reduction of airway eosinophilia in a dose-dependent manner. These authors suggested that antituberculosis vaccination in children may be helpful in reducing the risk of developing severe asthma that is closely associated with inflammation by eosinophils. Importantly, the results of the study indicated that the vaccine should be administrated directly into the respiratory tract to obtain the most pronounced effect (14). As noted above, the currently available BCG vaccination given intradermally has not shown any protective effect against atopic diseases.

Compartmentalization of Immune Responses to M. tuberculosis

Local administration of vaccine might be an especially attractive alternative in the case of M. tuberculosis. Immune responses to this intracellular bacterium appear to be largely compartmentalized, with the strongest immune reponse being found at the site of infection. A vaccine administered locally would ideally induce a strong and long-lived immunity, particularly at the site of the early invasion of M. tuberculosis. For example, in patients with tuberculoid pleurisy, Barnes and coworkers (49) found that the mean concentration of Th1 cytokines IFN- and IL-2 produced by M. tuberculosis-stimulated pleural fluid cells were significantly higher than corresponding concentrations produced by peripheral blood mononuclear cells. This difference was also found for the anti-inflammatory cytokine IL-10, which is likely produced by macrophages in later phases of the infection to ameliorate the inflammatory response that otherwise could be vigorous due to increased production of IFN- and tumor necrosis factor- by Th1 cells and macrophages, respectively (42, 50, 51). Th2 cytokine production (using IL-4 as a marker) was decreased in pleural fluid compared with blood. These data thus provide strong evidence for compartmentalization of Th1 cytokines and IL-10 at the site of the disease in humans. Further, Zhan and coworkers (52) found that in patients with tuberculoid pleurisy, highly elevated levels of IL-12 could be found at the site of disease as compared with blood, indicating that immune responses were compartmentalized into the pleural space.

	Summary and Conclusion

Various environmental factors may be involved in the substantial increase in asthma and atopy prevalence, but it has recently been proposed that the western lifestyle with its relatively high standards of hygiene may play a crucial role. In particular, the immune system in early life is rather unstimulated with intense infections that elicit a Th1 response, and is therefore susceptible to sensitization with otherwise harmless allergens in the environment (11), leading to a biased Th2 response and atopy. This view is based on the concept that Th1 and Th2 responses are largely mutually inhibitory and reciprocally regulated. This Th1/Th2 balance might be affected by specific immunomodulatory interventions in infancy. Several species of mycobacteria are known to induce strong Th1 responses and a long-lived immunologic memory (32, 33, 40). In addition, atopic asthma and pulmonary tuberculosis appear to be inversely related disorders. A predominance of Th2 cytokine cells has been demonstrated in BAL of patients with atopic asthma, whereas in BAL from patients with pulmonary tuberculosis, a predominance of Th1 cells has been found (53, 54). Immune responses in both diseases are also largely compartmentalized to the lungs. It is evident that the currently used vaccine against tuberculosis, the BCG vaccine, neither is satisfactorily effective against M. tuberculosis, nor seems to be able to affect the local Th1/Th2 balance. Due to the re-emerged threat of tuberculosis and to the unsatisfactory efficacy of BCG vaccine, there is an urgent need for an improved antituberculosis vaccine. On the basis of the evidence (1) that a weak Th1 immune response favors atopy, (2) that intense childhood infections appear to provide significant protection from atopy, and (3) that the type of T cells that dominate immunologic memory is associated with the nature of host responsiveness to inhaled antigens (55), we postulate that an improved mycobacterial vaccine, possibly administered via the mucosa, might in infancy be able to considerably strengthen the memory pool of Th1 cells and prevent the development of a biased cytokine milieu in the lungs of susceptible individuals, leading to reduced risk of subsequent asthma and atopy. An efficacious vaccine should induce a stimulus similar to a natural M. tuberculosis infection in terms of mounting a vigorous cell-mediated (Th1) immune response and long-lived immunity.

	Footnotes

Address correspondence to: Dr. L. von Hertzen, The Finnish Lung Health Association, Sibeliuksenkatu 11 A 1, 00250 Helsinki, Finland.

(Received in original form April 5, 1999 and in revised form August 20, 1999).

	References

1. Hopkin, J. M.. 1997. Mechanisms of enhanced prevalence of asthma and atopy in developed countries. Curr. Opin. Immunol. 9: 788-792 [Medline].

2. Lewis, S.. 1998. ISAAC: a hypothesis generator for asthma? Lancet 351: 1220-1221 [Medline].

3. Lewis, S., B. Butland, D. Strachan, D. Bynner, D. Richards, N. Butler, and J. Britton. 1996. Study of the etiology of wheezing illness at age 16 in two national British birth cohorts. Thorax 51: 670-676 [Abstract].

4. Serafini, U.. 1997. Do infections protect against asthma and atopy? Allergy 52: 955-957 [Medline].

5. Shaheen, S. O., P. Aaby, A. J. Hall, D. J. Barker, C. B. Heyes, A. W. Shiell, and A. Goydiaby. 1996. Measles and atopy in Guinea-Bissau. Lancet 347: 1792-1796 [Medline].

6. Matricardi, P. M., F. Rosmini, L. Ferrigno, R. Nisini, M. Rapicetta, P. Chionne, T. Stroffolini, P. Pasquini, and R. D'Amelio. 1997. Cross sectional retrospective study of prevalence of atopy among Italian military students with antibodies against hepatitis A virus. Br. Med. J. 314: 999-1003 [Abstract/Free Full Text].

7. Shirakawa, T., T. Enomoto, S. I. Shimazu, and J. M. Hopkin. 1997. The inverse association between tuberculin responses and atopic disorder. Science 275: 77-79 [Abstract/Free Full Text].

8. Strachan, D. P.. 1989. Hay fever, hygiene and household size. Br. Med. J. 229: 1259-1260 .

9. Romagnani, S.. 1994. Regulation of the development of type 2 T-helper cells in allergy. Curr. Opin. Immunol. 6: 838-846 [Medline].

10. Holt, P. G.. 1996. Infections and the development of allergy. Toxicol. Lett. 86: 205-210 [Medline].

11. Hogg, C.. 1997. T-helper polarization in atopic disease: how early does it occur? Clin. Exp. Allergy 27: 1237-1239 [Medline].

12. Alm, J. S., G. Lilja, G. Pershagen, and A. Scheynius. 1997. Early BCG vaccination and development of atopy. Lancet 350: 400-403 [Medline].

13. Strannegård, I. L., L. O. Larsson, G. Wennergren, and Ö. Strannegård. 1998. Prevalence of allergy in children in relation to prior BCG vaccination and infection with atypical mycobacteria. Allergy 53: 249-254 [Medline].

14. Erb, K. J., J. W. Holloway, A. Sobeck, H. Moll, and G. L. Gros. 1998. Infection of mice with Mycobacterium bovis-Bacillus Calmette-Guerin (BCG) suppresses allergen-induced airway eosinophilia. J. Exp. Med. 187: 561-569 [Abstract/Free Full Text].

15. Wang, C. C., and G. A. W. Rook. 1998. Inhibition of an established allergic response to ovalbumin in BALB/c mice by killed Mycobacterium vaccae. Immunology 93: 307-313 [Medline].

16. Mosmann, T. R., and R. L. Coffman. 1989. Th1 and Th2 cells: different patterns of lymphokine secretion lead to different functional properties. Annu. Rev. Immunol. 7: 145-173 [Medline].

17. Romagnani, S.. 1994. Lymphokine production by human T cells in disease states. Annu. Rev. Immunol. 12: 227-257 [Medline].

18. Lucey, D. R., M. Clerici, and G. M. Shearer. 1996. Type 1 and type 2 cytokine dysregulation in human infectious, neoplastic and inflammatory diseases. Clin. Microbiol. Rev. 9: 532-562 [Abstract].

19. Mosmann, T. R., and S. Sad. 1996. The expanding universe of T-cell subsets: Th1, Th2 and more. Immunol. Today 17: 138-146 [Medline].

20. Gaston, J. S.. 1998. Will the increasing prevalence of atopy have a favourable impact on rheumatoid arthritis? Ann. Rheum. Dis. 57: 265-267 [Free Full Text].

21. Wills-Karp, M.. 1998. Interleukin 12 as a target for modulation of the inflammatory response in asthma. Allergy 53: 113-119 [Medline].

22. Trinchieri, G., and F. Gerosa. 1996. Immunoregulation by interleukin-12. J. Leukoc. Biol. 59: 505-511 [Abstract].

23. Van der Pouw Kraan, T. C., L. C. Boeije, E. R. de Groot, S. O. Stapel, A. Snijders, M. L. Kapsenberg, J. S. van der Zee, and L. A. Aarden. 1997. Reduced production of IL-12 and IL-12 dependent IFN- release in patients with allergic asthma. J. Immunol. 158: 5560-5565 [Abstract].

24. Gavett, S. H., D. J. O'Hearn, X. Li, S. K. Huang, F. D. Finkelman, and M. Wills-Karp. 1995. Interleukin 12 inhibits antigen-induced airway hyperresponsiveness, inflammation, and Th2 cytokine expression in mice. J. Exp. Med. 182: 1527-1536 [Abstract/Free Full Text].

25. Li, X. M., R. K. Chopra, T. Y. Chou, B. H. Schofield, M. Wills-Karp, and S. K. Huang. 1996. Mucosal IFN- gene transfer inhibits pulmonary allergic responses in mice. J. Immunol. 157: 3216-3219 [Abstract].

26. Scott, D. E., I. Agranovich, J. Inman, M. Gober, and B. Golding. 1997. Inhibition of primary and recall allergen-specific T helper cell type 2-mediated responses by a T helper cell type 1 stimulus. J. Immunol. 159: 107-116 [Abstract].

27. Schwarze, J., E. Hamelmann, G. Cieslewicz, A. Tomkinson, A. Joetham, K. Bradley, and E. W. Gelfand. 1998. Local treatment with IL-12 is an effective inhibitor of airway hyperresponsiveness and lung eosinophilia after airway challenge in sensitized mice. J. Allergy Clin. Immunol. 102: 86-93 [Medline].

28. Murphy, E., K. Shibuya, N. Hosken, P. Openshaw, V. Maino, K. Davis, K. Murphy, and A. O'Garra. 1996. Reversibility of T helper 1 and 2 populations is lost after long-term stimulation. J. Exp. Med. 183: 901-913 [Abstract/Free Full Text].

29. Bogniewicz, M., R. J. Martin, D. Martin, U. Gibson, A. Celniker, M. Williams, and D. Y. Leung. 1995. The effects of nebulized recombinant interferon-gamma in asthmatic airways. J. Allergy Clin. Immunol. 95: 133-135 [Medline].

30. Lack, G., K. L. Bradley, E. Hamelmann, H. Renz, J. Loader, D. Y. Leung, G. Larsen, and E. W. Gelfand. 1996. Nebulized IFN- inhibits the development of secondary allergic responses in mice. J. Immunol. 157: 1432-1439 [Abstract].

31. Leonard, J. P., M. L. Sherman, G. L. Fisher, L. J. Buchanan, G. Larsen, M. B. Atkins, J. A. Sosman, J. P. Dutcher, N. J. Vogelzang, and J. L. Ryan. 1997. Effects of single-dose interleukin 12 exposure on interleukin 12-associated toxicity and interferon- production. Blood 90: 2541-2548 [Abstract/Free Full Text].

32. Orme, I. M., P. Andersen, and W. H. Boom. 1993. T cell response to Mycobacterium tuberculosis. J. Infect. Dis. 167: 1481-1497 [Medline].

33. Andersen, P.. 1997. Host responses and antigens involved in protective immunity to Mycobacterium tuberculosis. Scand. J. Immunol. 45: 115-131 [Medline].

34. Hart, C. A., and C. M. Parry. 1996. BCG: immunisation against tuberculosis? J. Med. Microbiol. 44: 20-24 .

35. Orme, I. M., A. D. Roberts, J. P. Griffin, and J. S. Abrams. 1993. Cytokine secretion by CD4+ T lymphocytes acquired in response to Mycobacterium tuberculosis infection. J. Immunol. 151: 518-525 [Abstract].

36. Andersen, P.. 1994. Effective vaccination of mice against Mycobacterium tuberculosis infection with a soluble mixture of secreted mycobacterial proteins. Infect. Immun. 62: 2536-2544 [Abstract/Free Full Text].

37. Lindblad, E. B., M. J. Elhay, R. Silva, R. Appelberg, and P. Andersen. 1997. Adjuvant modulation of immune responses to tuberculosis subunit vaccines. Infect. Immun. 65: 623-629 [Abstract].

38. Leclerc, C., and J. Ronco. 1998. New approaches in vaccine development. Immunol. Today 19: 300-302 [Medline].

39. Vacirca, A., J. Dominino, E. Valentini, E. Hartopp, and O. A. Bottasso. 1994. A pilot study of immunotherapy with M. vaccae against tuberculosis. Tuber. Lung Dis. 74: 47-48 .

40. Standford, J. L., and C. A. Standford. 1996. Immunotherapy for tuberculosis with M. vaccae. J. Med. Microbiol. 44: 24-34 .

41. Rook, G. A., and J. L. Standford. 1995. Adjuvants, endocrines and conserved epitopes; factors to consider when designing "therapeutic vaccines." Int. J. Immunopharmacol. 17: 91-102 [Medline].

42. Trinchieri, G.. 1997. Cytokines acting on or secreted by macrophages during intracellular infection (IL-10, IL-12, IFN-). Curr. Opin. Immunol. 9: 17-23 [Medline].

43. Levine, M. M., and G. Dougan. 1998. Optimism over vaccines administrated via mucosal surfaces. Lancet 351: 1375-1376 [Medline].

44. Sabin, A. B.. 1985. Oral poliovirus vaccine: history of its development and use and current challenge to eliminate poliomyelitis from the world. J. Infect. Dis. 151: 420-436 [Medline].

45. Wahdan, M. H., C. Serie, Y. Cerisier, S. Sallam, and R. Germanier. 1982. A controlled field trial of live Salmonella typhi strain Ty21a oral vaccine against typhoid: three year results. J. Infect. Dis. 145: 292-295 [Medline].

46. Ferreccio, C., M. M. Levine, H. Rodriguez, R. Contreras, and the Chilean Typhoid Committee. 1989. Comparative efficacy of two, three or four doses of Ty21a live oral typhoid vaccine in enteric-coated capsules: a field trial in an endemic area. J. Infect. Dis. 159: 766-769 [Medline].

47. Grossman, M. 1994. Immunization. In Basic and Clinical Immunology. D. P. Stites, A. I. Terr, and T. G. Parslow, editors. Appleton and Lange, East Norwalk, CT. 717-738.

48. Stagg, A. J.. 1998. Vaccines against Chlamydia: approaches and progress. Mol. Med. Today 4: 166-173 . [Medline]

49. Barnes, P. F., S. Lu, J. S. Abrams, E. Wang, M. Yamamura, and R. L. Modlin. 1993. Cytokine production at the site of disease in human tuberculosis. Infect. Immun. 61: 3482-3489 [Abstract/Free Full Text].

50. D'Andrea, A., M. Aste-Amezaga, N. M. Valiante, X. Ma, M. Kubin, and G. Trinchieri. 1993. Interleukin-10 inhibits human lymphocyte IFN- production by suppressing natural killer cell stimulatory factor/interleukin-12 synthesis in accessory cells. J. Exp. Med. 178: 1041-1048 [Abstract/Free Full Text].

51. Rennik, D., N. Davidson, and D. Berg. 1995. Interleukin 10 gene knock-out mice: a model of chronic inflammation. Clin. Immunol. Immunopathol. 76: S174-S178 [Medline].

52. Zhan, M., M. K. Gately, E. Wang, J. Gong, S. F. Wolf, S. Lu, R. L. Modlin, and P. F. Barnes. 1994. Interleukin 12 at the site of disease in tuberculosis. J. Clin. Invest. 93: 1733-1739 .

53. Robinson, D. S., Q. Hamid, Y. Sun, A. Tsicopoulos, J. Barkans, A. M. Bentley, C. J. Corrigan, S. R. Durham, and A. B. Kay. 1992. Predominant Th2-like bronchoalveolar T lymphocyte population in atopic asthma. N. Engl. J. Med. 326: 298-304 [Abstract].

54. Robinson, D. S., S. Ying, I. K. Taylor, A. Wangoo, D. M. Mitchell, A. B. Kay, Q. Hamid, and R. J. Shaw. 1994. Evidence for a Th1-like bronchoalveolar T cell subset and predominance of interferon-gamma gene activation in pulmonary tuberculosis. Am. J. Respir. Crit. Care Med. 149: 989-993 [Abstract].

55. Holt, P. G. 1996. Immunoregulation of the allergic reaction in the respiratory tract. Eur. Respir. J. 9(Suppl. 22):85s-89s.