Immune regulatory effect of pHSP65 DNA therapy in pulmonary tuberculosis: activation of CD8+ cells, interferon-gamma recovery and reduction of lung injury.

A DNA vaccine based on the heat-shock protein 65 Mycobacterium leprae gene (pHSP65) presented a prophylactic and therapeutic effect in an experimental model of tuberculosis. In this paper, we addressed the question of which protective mechanisms are activated in Mycobacterium tuberculosis-infected mice after immune therapy with pHSP65. We evaluated activation of the cellular immune response in the lungs of infected mice 30 days after infection (initiation of immune therapy) and in those of uninfected mice. After 70 days (end of immune therapy), the immune responses of infected untreated mice, infected pHSP65-treated mice and infected pCDNA3-treated mice were also evaluated. Our results show that the most significant effect of pHSP65 was the stimulation of CD8+ lung cell activation, interferon-gamma recovery and reduction of lung injury. There was also partial restoration of the production of tumour necrosis factor-alpha. Treatment with pcDNA3 vector also induced an immune stimulatory effect. However, only infected pHSP65-treated mice were able to produce significant levels of interferon-gamma and to restrict the growth of bacilli.

Increased expression of fas ligand in human tuberculosis and leprosy lesions: a potential novel mechanism of immune evasion in mycobacterial infection.

To study the location and mechanism of apoptosis within the human tuberculosis (TB) and leprosy lesions, parallel sections were analyzed for mycobacterial antigens (M.Ag), Fas ligand (FasL), Fas, CD68 and Mac387 by immunohistochemistry, and apoptotic cells by the terminal deoxynucleotidyl-transferase-mediated dUTP-digoxigenin nick end labelling method. Cutaneous leishmaniasis and foreign body granulomas were analyzed for comparison. The heavily infected macrophages in multibacillary TB and leprosy granulomas very strongly expressed FasL, indicating that a mycobacterial infection can induce an increased expression of FasL in a population of infected macrophages, which may protect them from the attack of Fas-expressing lymphocytes. However, macrophages with high levels of leishmania amastigotes did not selectively express FasL, suggesting that this phenomenon is specific for the mycobacteria. Interestingly, in the well-formed TB granulomas, 84% of the multinucleated giant cells strongly expressed FasL. The expression of Fas was weak (34%) or absent. A higher number (33%) of epithelioid cells expressed FasL than Fas (23%). Lymphocytes were scanty among the epithelioid cells. The frequency of apoptotic cells was higher in the epithelioid cells (0.25%) than the mononuclear cells in the mantle zone (0.14%). Thus, the epithelioid cells and the multinucleated giant cells by virtue of the increased expression of FasL may make these granulomas an immune privileged site for mycobacteria.

Differential ability of T cell subsets to undergo activation-induced cell death.

Human T cell clones were analyzed for their susceptibility to activation-induced cell death (AICD) in response to CD3/T cell receptor ligation. AICD was observed only in Th1 clones and was Fas-mediated, whereas Th2 clones resisted AICD. Analysis of a panel of Th0 clones, characterized by their ability to secrete both Th1 and Th2 cytokines, revealed that this subset included both AICD-sensitive (type A) and -resistant (type B) clones. Resistance to AICD by Th2 and Th0-type B clones was not due to lack of expression of either Fas receptor or its ligand. Paradoxically, the AICD-resistant clones were susceptible to apoptosis when Fas receptor was directly ligated by anti-Fas antibodies. However, prior activation of the resistant clones by monoclonal antibodies to CD3/TCR complex induced resistance against Fas-mediated apoptosis. Thus, the Fas-FasL pathway is critical for the induction of AICD in T cells, and moreover this pathway can be negatively regulated in the AICD-resistant clones by signals that are generated from ligation of the CD3/TCR complex.