Synthesis of cytokines during tumour development in mice immunized with the mycobacterial antigen complex A60

Tuberculosis II: the failure of the BCG vaccine

BCG (bacille Calmette-Guerin) is an attenuated pathogen characterized by its capacity to induce cellular and humoral immune responses primarily against a nonpeptidic antigen, lipoarabinomannan. Immune responses against this substance contribute to the immunoprotection of the patient if the production of IL-2 and INF-gamma is not impaired. The most adequate production of INF-gamma and IL-2 is obtained by immunoreactivity against proteinic antigens. The formation of IgG-type antibodies and of cellular immunity against mycobacterial peptidic and proteinic antigens is an additional immunological response essential for a good protection. This is achieved by the BCG vaccine in only a small proportion of the vaccinees. A vaccine adjuvant that also finds application as an immunotherapeutic agent is composed of proteinic antigens such as sonicates of Mycobacterium vaccae and antigen 60 of Mycobacterium bovis. These enhance the beneficial Th1-pole of the immune response. In addition, A60 induces the formation of antibodies against species-specific proteinic antigens. Despite the questioning of its innocuousness and efficacy, the BCG vaccine was imposed worldwide in 1950 by medical and political organizations that showed no concern for these questions. The contemporary structures of research administration in this area make it unlikely that the efficacious means recently developed to complement the action of the vaccine and of chemotherapies to face the surge of tuberculosis (TB) will be readily adopted.

Tuberculosis I: a conceptual frame for the immunopathology of the disease

An analysis of the cellular and humoral immune responses after bacille Calmette-Guerin (BCG) vaccination and during tuberculosis treatment favors the hypothesis of an immune defence developed in four overlapping successive stages. The initial immune response is innate. The following two intermingle innate and specific responses against low molecular weight oligopeptidic and nonpeptidic antigens, as muramyldipeptide and trehalose dimycolate, and large molecular weight nonpeptidic antigens such as lipoarabinomannan. The ultimate specific response is directed against protein antigens as Antigen 60. BCG and primary tuberculosis (TB) infections induce cellular and humoral immune responses essentially against oligopeptidic and small and large molecular weight nonpeptidic antigens. Immune responses against non-peptidic substances contribute to the immunoprotection of the infected person who develops a primary infection. Some infected people allow the expression of the immunosuppressive activity of the pathogen. This results in the synthesis of interleukin-10 (IL-10), which suppresses the formation of interferon-gamma (INF-gamma) and IL-2, and of IL-6, which suppresses T-cell responses. These patients have a skewed immune response against non-peptidic antigens and present with symptoms. They will not recover unless responses directed against proteinic antigens occur, which restore INF-gamma and IL-2 production. The formation of immumoglobulin-G (IgG)-type antibodies and of a cellular immunity against mycobacterial peptidic antigens is essential for a good protection against a post-primary infection.

Immunological relatedness of the protective mechanisms against tuberculosis and cancer

BACKGROUND

Bacille Calmette-Guérin (BCG), an attenuated strain of tuberculous bacillus, is the source of vaccines providing unclear and variable protection against tuberculosis (TB) and cancer. Thermostable macromolecular antigens (TMAs) are major mycobacterial complexes immunodominant in disease. A60 (TMA complex of BCG) protects mice against TB development, via T lymphocyte (TL)-mediated macrophage (Mphi) activation, halting intracellular mycobacterial replication. In most A60-primed mice, cytolytic TLs and Mphi infiltrate cancer tissue, resulting in 80-100% rejection. Adoptive TL transfer is indispensable for Mphi-dependent tumour cell inactivation via oxygen and nitrogen radicals. Neoplasm development induces immune anergy with depletion ofA60-specific TL and activated Mphi. A60 protects mice against TB and cancer by inducing the synthesis of three lymphokines: interleukin 2 (IL-2), interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha). Tumour cells prevent A60-dependent synthesis of these lymphokines in vivo and in vitro.

CONCLUSION

These data provide some clues to immune surveillance and tumour escape mechanisms, as well as to the antituberculous and antineoplastic BCG action.

In vitro analysis of cancer prevention by a mycobacterial antigen complex and of cancer-promoted inhibition of immune reactions

The antigen complex A60 of Mycobacterium bovis bacillus Calmette-Guérin protected mice against experimental tuberculous infection, and prevented cancer development after challenge with EMT 6 cells. Although humoral and cellular immune reactions elicited by A60 in vivo remained unaffected in cases of tumor rejection, they were suppressed in the case of neoplastic growth. In the present work, these in vivo observations were analyzed by in vitro techniques. Activated macrophages played a major role, and cytolytic T lymphocytes a minor role, in A60-promoted cancer cell cytolysis leading to tumor rejection. In vitro, EMT 6 cells weakly inhibited the proliferation of A60-specific B lymphocytes and strongly inhibited the functions of activated macrophages. However, the collapse of both humoral and cellular immune reactions during the course of cancer development was also accompanied by an inhibitory action of EMT 6 cells on the multiplication and functions of A60-specific T lymphocytes. Tumor-dependent repression of macrophage activation was therefore due to both a direct action of tumor cells on macrophages and an indirect one via inhibition of macrophage-activating T cell functions. On the other hand, tumor-induced collapse of the anti-A60 Ig synthesis was mainly due to inhibition of B-cell-activating T cells, with a weaker direct effect of tumor cells on B lymphocytes. Consequently, A60 and tumor cells exert opposite effects on the immune system at several levels.