Potential role of B7-1 and CD28 molecules in immunosuppression in leprosy

Abstract

In order to understand the mechanism of unresponsiveness towards Mycobacterium leprae antigens in leprosy, we evaluated the role of M. leprae sonicate antigens in regulating the expression of the costimulatory molecules B7-1, CD28, intercellular adhesion molecule-1 (ICAM-1), LFA-1alpha, LFA-1beta and Mac-1 on the lymphocytes of both leprosy patients and healthy subjects. It was observed that the expression of B7-1 and CD28 was significantly decreased but the levels of ICAM-1 and LFA-1alpha were increased in patients with untreated borderline leprosy (BL)/lepromatous leprosy (LL) disease. No remarkable change was noticed in the case of borderline tuberculoid (BT) leprosy or treated BL/LL patients. Further, a striking finding was that lymphocytes from healthy subjects cultured with a particularly high dose of M. leprae sonicate antigens down-regulated the expression of B7-1 and CD28 molecules, but up-regulated the display of ICAM-1 and LFA-1alpha. Furthermore, proliferation induced by M. leprae sonicate was inhibited only by anti-B7-1 antibody. Mycobacterium leprae antigen-induced suppression of the proliferation of lymphocytes of healthy volunteers and LL patients was reversed by culturing the lymphocytes with purified protein derivative (PPD). It may be concluded from the findings in this study that down regulation of B7-1 and CD28 in BL/LL leprosy patients may be responsible for a defective T cell signalling by the B7-1/CD28 pathway caused by M. leprae antigens. This may lead to clonal inactivation of M. leprae-reactive T cells, consequently the bacilli grow without restriction in macrophages.

Fig. 1

Effect of anti-B7-1, anti-CD28, anti-intercellular adhesion molecule-1 (ICAM-1), anti-LFA-1α, anti-LFA-1β and Mac-1 MoAbs in the inhibition of proliferation of lymphocytes of healthy subjects induced by Mycobacterium leprae sonicate (Mls) and purified protein derivative (PPD). Peripheral blood mononuclear cells (PBMC) isolated from healthy volunteers were cultured either with Mls (10 μg/ml) (a), PPD (100 U/ml) (b), Mls (50 μg/ml) +PPD (100 U/ml) (c). MoAbs against the accessory molecules were added in the cultures to monitor the inhibition of proliferation. ³H-TdR was added 72 h later and the cells were harvested after 16 h. The radioactivity incorporated was monitored by liquid scintillation counting. The lymphocytes cultured with 50 μg/ml Mls showed 6915 ± 1651 ct/min. The control cultures containing isotype-matched MoAbs could not inhibit the Mls or PPD-induced T cell proliferation. The blocking of the proliferation of CD4⁺ T cells was achieved either by incorporation of anti-CD4 MoAb in the cultures containing PBMC and Mls (3924 ± 1822 ct/min) and PPD (2764 ± 352 ct/min), or treating the PBMC with anti-CD4 MoAb and complement (1049 ± 211 ct/min). Results are expressed as mean ± s.d.; n = 7.

Fig. 2

Effect of anti-B7-1, anti-CD28, anti-intercellular adhesion molecule-1 (ICAM-1), anti-LFA-1α, anti-LFA-1β and Mac-1 MoAbs in the inhibition of proliferation of lymphocytes of borderline leprosy (BL)/lepromatous leprosy (LL) and borderline tuberculoid (BT) leprosy patients. Peripheral blood mononuclear cells (PBMC) were isolated from untreated BL/LL (a) and BT (b) patients. The culture conditions were the same as for Fig. 1. The results expressed are mean ± s.d.; n = 3.

Fig. 3

Expression of B7-1, CD28, intercellular adhesion molecule-1 (ICAM-1), LFA-1α, LFA-1β and Mac-1 on freshly isolated lymphocytes of leprosy patients. The lymphocytes were stained using MoAbs against the costimulatory molecules (5 ng/ml) and secondary FITC-labelled antibodies. Data shown are represented as percent change in the mean fluorescence intensity (MFI) of the costimulatory molecules expressed on the surface of the lymphocytes obtained from the leprosy patients (mean ± s.d.; borderline leprosy (BL)/lepromatous leprosy (LL) treated (T), n = 3; BL/LL untreated (UT), n = 6; borderline tuberculoid (BT) leprosy treated (T), n = 7; BT untreated (UT), n = 3), compared with the expression of costimulatory molecules on the lymphocytes of healthy subjects (mean ± s.d.; n = 7). No staining was viewed in the controls containing isotype-matched MoAbs.

Fig. 4

Mycobacterium leprae sonicate (Mls) alters the expression of B7-1, CD28, intercellular adhesion molecule-1 (ICAM-1) and LFA-1α on the lymphocytes of healthy volunteers. The lymphocytes were cultured with Mls or purified protein derivative (PPD) for 48 h and labelled with the MoAbs against the costimulatory molecules for FACScan. Staining was done as described in the legend to Fig. 3. Data are presented as percentage difference in mean fluorescence intensity (MFI) of the expression of costimulatory molecules compared with the lymphocytes cultured in the absence of Mls or PPD (mean ± s.d.; n = 7).