Learning from leprosy: insight into the human innate immune response.

Investigation into the innate immune response in leprosy has provided insight into host defense and immunopathology in human infectious disease. A key advance has been the delineation of pattern recognition receptors that detect pathogen-associated molecular patterns of the bacterium that causes leprosy, Mycobacterium leprae. From this knowledge, it has been possible to determine the cytokine responses as well as macrophage and dendritic cell differentiation programs that contribute to host defense and tissue injury in leprosy. These insights provide targets for therapeutic intervention to modulate innate immune responses against microbial infection in humans.

[Human monocyte-derived multinucleated giant cells].

Multinucleated giant cells (MGC) are characteristic cells in granulomatous disorders such as sarcoidosis and leprosy. There are two types of MGC; foreign body-type and Langhans-type cells. The exact mechanisms of the formation and the functional significance of MGC are not determined, although their morphological features are well understood. MGC are also formed in vitro from peripheral blood mononuclear cells by stimulation with cytokines and lectins. Particularly IFN-gamma is considered to play a pivotal role in monocyte fusion. IL-3, IL-4, IL-13, and GM-CSF are other reported cytokines involved in MGC formation. In addition to such inflammatory mediators, a factor derived from the pathogens of granulomatous disorders may be necessary for MGC formation. Muramyl dipeptide, a peptidoglycan portion of bacterial cell walls, is one of the candidates and can preferentially induce Langhans-type cells in in vitro MGC formation system. Although the exact mechanisms of in vitro MGC formation remains unknown, cell surface molecules such as P2X7 receptor, integrins, CD98, and macrophage fusion protein are considered to be involved in fusion process. Monocytes of sarcoidosis patients expressed higher levels of P2X7 and had a higher ability to induce MGC than those of healthy controls. Effective agents for sarcoidosis such as tranilast, alloprinol, and captopril inhibited in vitro MGC formation, suggesting their therapeutic effects through the direct effects on monocytes. Thus, an in vitro MGC formation model would be a useful tool to understand the relevance of MGC in granulomatous disorders.

Activation of signaling lymphocytic activation molecule triggers a signaling cascade that enhances Th1 responses in human intracellular infection.

T cell production of IFN-gamma contributes to host defense against infection by intracellular pathogens, including mycobacteria. Lepromatous leprosy, the disseminated form of infection caused by Mycobacterium leprae, is characterized by loss of cellular response against the pathogen and diminished Th1 cytokine production. Relieving bacterial burden in Ag-unresponsive patients might be achieved through alternative receptors that stimulate IFN-gamma production. We have previously shown that ligation of signaling lymphocytic activation molecule (SLAM) enhances IFN-gamma in mycobacterial infection; therefore, we investigated molecular pathways leading from SLAM activation to IFN-gamma production in human leprosy. The expression of the SLAM-associated protein (an inhibitory factor for IFN-gamma induction) on M. leprae-stimulated cells from leprosy patients was inversely correlated to IFN-gamma production. However, SLAM ligation or exposure of cells from lepromatous patients to a proinflammatory microenvironment down-regulated SLAM-associated protein expression. Moreover, SLAM activation induced a sequence of signaling proteins, including activation of the NF-kappaB complex, phosphorylation of Stat1, and induction of T-bet expression, resulting in the promotion of IFN-gamma production, a pathway that remains quiescent in response to Ag in lepromatous patients. Therefore, our findings reveal a cascade of molecular events during signaling through SLAM in leprosy that cooperate to induce IFN-gamma production and strongly suggest that SLAM might be a focal point for therapeutic modulation of T cell cytokine responses in diseases characterized by dysfunctional Th2 responses.

Activation and regulation of Toll-like receptors 2 and 1 in human leprosy.

The expression and activation of Toll-like receptors (TLRs) was investigated in leprosy, a spectral disease in which clinical manifestations correlate with the type of immune response mounted toward Mycobacterium leprae. TLR2-TLR1 heterodimers mediated cell activation by killed M. leprae, indicating the presence of triacylated lipoproteins. A genome-wide scan of M. leprae detected 31 putative lipoproteins. Synthetic lipopeptides representing the 19-kD and 33-kD lipoproteins activated both monocytes and dendritic cells. Activation was enhanced by type-1 cytokines and inhibited by type-2 cytokines. In addition, interferon (IFN)-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF) enhanced TLR1 expression in monocytes and dendritic cells, respectively, whereas IL-4 downregulated TLR2 expression. TLR2 and TLR1 were more strongly expressed in lesions from the localized tuberculoid form (T-lep) as compared with the disseminated lepromatous form (L-lep) of the disease. These data provide evidence that regulated expression and activation of TLRs at the site of disease contribute to the host defense against microbial pathogens.

Activation and regulation of toll-like receptors 2 and 1 in human leprosy

The expression and activation of Toll-like receptors (TLRs) was investigated in leprosy, a spectral disease in which clinical manifestations correlate with the type of immune response mounted toward Mycobacterium leprae. TLR2-TLR1 heterodimers mediated cell activation by killed M. leprae, indicating the presence of triacylated lipoproteins. A genome-wide scan of M. leprae detected 31 putative lipoproteins. Synthetic lipopeptides representing the 19-kD and 33-kD lipoproteins activated both monocytes and dendritic cells. Activation was enhanced by type-1 cytokines and inhibited by type-2 cytokines. In addition, interferon (IFN)-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF) enhanced TLR1 expression in monocytes and dendritic cells, respectively, whereas IL-4 downregulated TLR2 expression. TLR2 and TLR1 were more strongly expressed in lesions from the localized tuberculoid form (T-lep) as compared with the disseminated lepromatous form (L-lep) of the disease. These data provide evidence that regulated expression and activation of TLRs at the site of disease contribute to the host defense against microbial pathogens.

[Protective immunity against intracellular parasitic bacteria].

Facultative intracellular bacteria are resistant to the killing mechanism inside macrophages by virtue of various escape mechanisms. Activation of macrophages by cytokines is the key event to overcome of bacterial escape in macrophages of the infected host. Generation of TH1 type of antigen-specific T cell is the essentially required for the macrophage activation. This short review summarizes the escape mechanism of activated macrophages and the mechanisms involved in the generation of TH1 cells.

Dysregulation of IL-4 expression in lepromatous leprosy patients with and without erythema nodosum leprosum.

In order to increase our understanding of the immunological basis of erythema nodosum leprosum (ENL), we studied Th-like cytokine profiles in 130 leprosy patients, employing both the conventional and a novel, real-time, fluorogenic reverse transcriptase-based PCR (RT-PCR). The concomitant expression of both Th-like cytokines, interferon-gamma and IL-4, and the regulatory cytokines, IL-10 and IL-12, was studied in the peripheral blood cells of leprosy patients with and without ENL. In the conventional RT-PCR, varied cytokine profiles were observed in individual patients of all clinical types. Fifty-three percent of lepromatous patients without ENL and 59% of tuberculoid leprosy patients showed co-expression of IFN gamma and IL-4, indicating a non-polarized Th 0 pattern. Of the 36 patients with ENL, 58% demonstrated a polarized Th 1 pattern, with only 30% expressing both cytokines. Semiquantitative RT-PCR indicated a lower expression of IL-4 compared to that of IFN gamma in the lepromatous patients without ENL; the difference was even greater among those with ENL. The sensitive, real-time PCR confirmed the down-regulation of IL-4 and IL-10, with absence of IL-4 in half of the patients, resulting in skewing of the cytokine response toward a Th 1-like profile.

Spontaneous apoptosis in peripheral blood mononuclear cells of leprosy patients: role of cytokines.

Peripheral blood mononuclear cells from leprosy patients underwent spontaneous apoptosis upon culture for 24 h. The apoptosis was inhibited by anti-TNFalpha antibodies and to a certain extent by anti-IL-1alpha and IL-6, thus showing that T(H)2-type cytokines (mainly TNFalpha) are responsible for inducing apoptosis. This cytokine-mediated apoptosis could be inhibited by ionomycin and zinc, thereby suggesting that these metal ions can be used to decrease the levels of these inflammatory cytokines in various diseases.

Immunoregulation of cytokines in infectious diseases (leprosy), future strategies.

Leprosy is a dynamic disease model in which distinct Mycobacterium leprae-responsive T cell subsets play an important role in host defense and control clinical and immunological spectrum. Th1 cells are associated with tuberculoid leprosy patients that have strong M. leprae-specific CMI-DTH responses. Th2 cells are expressed in lepromatous leprosy patients that are characterized by strong humoral immune responses and lack of T cell responses. Recently cytokines are thought to play immunoregulatory role in both the protection and immunopathogenesis of the host. Recombinant cytokines for immunotherapy have been used for controlling mycobacterial infections including leprosy. The diversity of T-cell subsets contributing to Th1 and Th2 cell derived cytokines, other major cytokines of the immune system, their sources, modes of action and possible therapeutic potentials are discussed.

Expression and cytokine secretion in the states of immune reactivation in leprosy.

Leprosy is a chronic inflammatory disease caused by Mycobacterium leprae. The human response to this pathogen exhibits intriguing aspects which are up to now not well understood. The present study discusses the probable mechanisms involved in T cell-specific unresponsiveness observed in lepromatous patients. Analysis of the cytokine profile either in blood leukocytes or in skin specimens taken from leprosy lesions indicates that some parameters of Th1 immune response are present in lepromatous patients under reactional states.

Expression and cytokine secretion in the states of immune reactivation in leprosy

Leprosy is a chronic inflammatory disease caused by Mycobacterium leprae. The human response to this pathogen exhibits intriguing aspects which are up to now not well understood. The present study discusses the probable mechanisms involved in T cell-specific unresponsiveness observed in lepromatous patients. Analysis of the cytokine profile either in blood leukocytes or in skin specimens taken from leprosy lesions indicates that some parameters of Th1 immune response are present in lepromatous patients under reactional states.

[Attempts to elucidate reasons why mycobacterial infections are intractable, by using an experimental mouse infection model].

This paper reviews some recent studies which have been performed by us and other investigators, in order to clarify the reason why most mycobacterial infections such as due to Mycobacterium tuberculosis and M. avium complex infections are intractable, that is, why these organisms can escape from attack by microbicidal mechanisms of host macrophages and consequently persist for long time at sites of infection. This paper mainly dealt with the two major subjects, which were studied by using an experimental model for murine M. avium infection. The first subject is on the modes and mechanisms of mycobacterial killing in host macrophages and the mechanisms of bacterial escape from an onslaught by macrophages. The second is on the characteristics of immunosuppressive macrophages induced in M. avium complex infection and the role of the suppressor macrophages in the establishment of immune unresponsiveness of host mice in the progressed stage of infection.

Is there clinical, epidemiological and molecular evidence for two forms of Crohn's disease?

Crohn's disease is an idiopathic chronic panenteric intestinal inflammatory disease. Data concerning the pathogenesis of, and the immune responses occurring in, Crohn's disease are often conflicting. Current therapy is empirical and either non-specifically immunosuppressive or surgically ablative in nature. Although controversial, Crohn's disease may be thought of as having two different presentations, an aggressive fistulizing form and an indolent obstructive form. This is analogous to the tuberculoid and lepromatous manifestations of leprosy. If correct, this subclassification may provide key insights into the pathogenesis and differing host immune responses in Crohn's disease and also allow the development of more rational therapies.