Impaired resistance to Mycobacterium tuberculosis infection after selective in vivo depletion of L3T4+ and Lyt-2+ T cells

Francisella tularensis-specific T-cell clones are human leukocyte antigen class II restricted, secrete interleukin-2 and gamma interferon, and induce immunoglobulin production

T-cell clones (TLC) were established from a Francisella tularensis-vaccinated subject in order to study the cells responsive for cell-mediated immunity against F. tularensis. All the clones were human leukocyte antigen (HLA) class II restricted to one of the HLA-DR specificities of the original donor. The TLC cells were CD4+ and produced interleukin-2 and gamma interferon after stimulation with specific antigen. Seven of the eight clones tested assisted in the production of immunoglobulin G (IgG), IgA, and IgM antibodies.

T-cell-mediated protection of mice against virulent Mycobacterium tuberculosis

We sought to protect CBA mice against tuberculosis using in vivo transfer of a T-cell line previously shown to be capable of I-A-restricted recognition of peritoneal macrophages infected in vitro with Mycobacterium tuberculosis. This line induces total bacteriostasis in vitro. In mice that received 500 rads of irradiation 48 h before infection, the T-cell line caused significant prolongation of life when given intravenously with a challenge dose of 5 x 10(6) organisms. Similar experiments with two other T-cell lines showed that these lines offered no protection. Bacterial load at the time of death was inversely related to the time of survival. Thus, death occurred at a lower bacterial load in adoptively protected mice, implying the contribution of an immunopathological component in these animals. The protective T-cell line, which was CD4+ CD8-, had no effect on the rate of growth of strain BCG in CBA nu/nu mice or M. tuberculosis in fully T-cell-deprived mice. This could indicate that CD8+ cells play a role in this system or that there is a need for the recruitment of interleukin 2-producing cells in the recipient. Experiments with monoclonal antibodies to selectively deplete T-cell subsets in normal CBA mice showed that depletion of CD4+ cells strikingly shortened survival, whereas depletion of CD8+ cells did not. However, CD8-depleted mice died with a lower bacterial load than those found in nondepleted controls, and the lesions in CD8-depleted mice were histopathologically distinct. These results suggest that the CD8+ cells either down-regulate bacteriostasis or cause immunopathology in this model and that it is the CD4+ cells that are the major protective subset in long-term protection experiments.

Administration of purified anti-L3T4 monoclonal antibody impairs the resistance of mice to Listeria monocytogenes infection

Anti-L3T4 monoclonal antibody (GK1.5) treatment significantly impaired the antilisteria resistance of mice as manifested by increased recovery of listeriae from the spleens and livers of anti-L3T4-treated mice and by greater severity of damage to the liver and other organs. Anti-L3T4-treated mice demonstrated a profound decrease in their T cell-mediated responses to Listeria monocytogenes and its products; they failed to develop delayed type hypersensitivity to soluble listeria antigens in vivo, and their spleen cells proliferated poorly in response to stimulation by either mitogens or listeria antigens in vitro. Spleen cells from control listeria-infected mice produced significant amounts of gamma interferon when stimulated with listeria antigens in vitro, whereas under the same conditions spleen cells from anti-L3T4-treated listeria-infected mice failed to produce detectable gamma interferon. Anti-L3T4 treatment resulted in a slight increase in serum colony-stimulating activity as compared with control listeria-infected mice, probably as a result of the increased bacterial burden in these animals. Despite the dramatic decrease in T-cell activities, anti-L3T4-treated mice succeeded in clearing L. monocytogenes from the spleen and liver in a manner that was only slightly delayed as compared with control listeria-infected mice. In addition, anti-L3T4-treated listeria-immunized mice exhibited a moderate degree of enhanced resistance to rechallenge with L. monocytogenes, and their spleen cells were able to transfer a limited degree of antilisteria resistance to naive recipient mice. Both of these responses, however, were diminished as compared with those of control listeria-immunized mice in the same experiments. Although these observations establish a critical role for L3T4+ cells in the development of maximal resistance to listeriosis, they also suggest that compensatory mechanisms may allow mice to develop considerable antilisteria resistance even when L3T4+ cell activities are substantially reduced.

Immunocytochemical analysis of cellular responses to BCG

The study reported here was performed to find out whether changes in the number of mycobacteria in various organs of BCG-infected mice can be related to changes in the phenotype of monocytes, macrophages and lymphocytes in the blood, various tissues, and peritoneal cavity and to the formation of granulomas in the spleen, liver and lungs. The relative amounts of various antigens on the leukocytes were assessed semi-quantitatively after immunocytochemical detection of the binding of monoclonal antibodies. Granuloma formation was determined after immunocytochemical staining of cells in sections of liver and lung tissue with a monoclonal antibody against the common leukocyte antigen and in sections of the spleen with a monoclonal antibody against the Mac-2 antigen. The results showed that during the first week of infection the number of BCG in spleen, liver and lungs declined considerably. Multiplication of mycobacteria during the second week of infection was associated with decreased expression of antigen F4/80 and increased expression of Ia antigen and Mac-2 antigen by blood monocytes and macrophages. Reduction of the numbers of BCG in the spleen and liver during the third week after i.v. injection of BCG and in lungs during the fourth week of the infection was found to be correlated with the degree of granuloma formation in these organs. After intravenous injection of killed BCG no changes were observed in the phenotype of monocytes and the macrophages in spleen, liver, lungs and peritoneal cavity. These mice showed considerably less granuloma formation than BCG-infected mice. The present results indicate that live but not killed mycobacteria induce macrophage activation.

The role of T cell--macrophage interactions in tuberculosis

Acquired resistance against tuberculosis paradigmatically depends on specific T lymphocytes and mononuclear phagocytes. The etiological agent, Mycobacterium tuberculosis is capable of replicating in mononuclear phagocytes which act both as habitat and as effectors of protection. Upon interaction with antigen-specific T lymphocytes infected mononuclear phagocytes acquire tuberculostatic activities. Here, data from experimental tuberculosis studies in mice are summarized which show that: interleukins produced by cloned T cells and recombinant interferon-gamma are capable of activating tuberculostatic capacities in macrophages; both CD4 and CD8 T cells, after adequate stimulation, produce interferon-gamma; CD8 T cells lyse macrophages in an antigen-specific way; not only CD8 but also CD4 T cells possess an antigen-specific cytolytic potential; lysis of infected macrophages results in mycobacterial growth inhibition. Evidence is also presented that tuberculostatic activities of activated macrophages depend on phagosome-lysosome fusion and are independent of reactive oxygen metabolites and that some strains of M. tuberculosis are resistant against interferon-gamma activities macrophages. These findings suggest that both helper and cytolytic T cells participate in the immune response to tuberculosis and that similar T cell mechanisms contribute to resistance as well as pathogenesis. Protection against tuberculosis, therefore, depends on subtle coordination of the immune response.

Mycobacteria-reactive Lyt-2+ T cell lines

The biological activities of mycobacteria-reactive Lyt-2+ T cells were characterized in vitro. T cells from mice immunized with killed Mycobacterium tuberculosis or viable M. bovis were restimulated in vitro and cloned under limiting dilution conditions. Several L3T4-Lyt-2+ T cell lines, some of them KJ16+, were established. These T cell lines were capable of lysing mycobacteria-primed macrophages in an antigen-specific way. The cytolytic activity of some T cell lines was found to be class I restricted, whereas others showed antigen-specific killing in the absence of apparent H-2 restriction. Several T cell lines produced interferon-gamma after appropriate stimulation. Furthermore, these T cell lines could induce tuberculostatic macrophage capacities by apparently two different mechanisms, namely by secretion of lymphokines (most probably interferon-gamma) and by direct cell contact. We conclude that CD8 T cells with antigen-specific cytolytic potential are generated during tuberculosis and that these T cells are involved in the immune response to tubercle bacilli.