Gamma interferon in experimental leprosy.

We have shown that preactivated MACs are able to cope with newly acquired leprosy bacilli, and a high intracellular burden of live M. leprae induces a refractory response of the host MAC to activation by IFN-gamma. Our studies underscore the fact that MAC function in LL is dependent on localized conditions, influenced by the high intracellular burden of leprosy bacilli and, in part, involving the production of prostanoids. These findings preclude inferences about the function of granuloma MAC s in leprosy based on the responses of MACs from other easily accessible anatomical compartments such as the peritoneal cavity or peripheral blood. We feel that the clearance of bacilli from the LL lesion as a consequence of local immunotherapeutic measures or chemotherapy likely depends on the influx of new competent MAC rather than the activation of resident lepromatous MACs.

Inhibition of interferon-gamma-mediated activation in mouse macrophages treated with lipoarabinomannan.

Lipoarabinomannan (LAM), purified from the cell walls of Mycobacterium leprae and M. tuberculosis, is a potent inhibitor of interferon-gamma (IFN-gamma) mediated activation of macrophages. The capability of LAM to inhibit IFN-gamma activation of macrophages in vitro was dose dependent and required a 24-h pre-exposure. Defective activation was evident as a block in IFN-gamma-induced cytocidal activity for tumour cell targets and microbicidal capacity for intracellular Toxoplasma gondii. Additionally, LAM treatment blocked the induction of surface Ia antigens on peritoneal macrophages by IFN-gamma. The requirement for pretreatment with LAM was further substantiated by the finding that peritoneal macrophages that were activated in vivo were not affected by LAM treatments and retained full microbicidal function. However, once inhibited by LAM treatment in vitro, macrophages remained fully refractory to IFN-gamma activation for up to 5 days in culture. Inhibition of IFN-gamma activation in macrophages treated with LAM was not overcome by 100-fold increases in the dose of IFN-gamma used or by a constant dose of IFN-gamma in combination with 100-fold increases in the level of endotoxin used to trigger cytotoxic activity. The defect in IFN-gamma unresponsiveness was not due to altered receptor function, as control and LAM-treated macrophages showed similar capacity to bind, internalize, and digest radiolabelled IFN-gamma. Based on the in vitro findings reported here, the inhibition of IFN-gamma-mediated macrophage activation by exposure to LAM may contribute to defective macrophage function observed in lepromatous granulomas and thus constitutes an important aspect of pathogenesis in mycobacterioses.

Induction of unresponsiveness to gamma interferon in macrophages infected with Mycobacterium leprae.

We have previously demonstrated that Mycobacterium leprae-burdened granuloma macrophages isolated from infected nude mice are refractory to activation by gamma interferon (IFN-gamma). To explore further both the afferent and efferent functional capacity of M. leprae-infected macrophages, we examined the IFN-gamma-mediated activation of resident mouse peritoneal macrophages infected in vitro with live or dead M. leprae. When IFN-gamma was administered within 24 h of M. leprae infection, macrophages were fully activated. However, defective activation was evident at 3 to 5 days postinfection in macrophages that were heavily burdened with viable M. leprae. This defect was evident by four parameters of activation in which IFN-gamma failed to stimulate the enhancement of microbicidal activity, cytotoxicity for tumor target cells, O2- production, and surface Ia antigen expression. The development of defective activation closely followed an increase in macrophage production of prostaglandin E2. Defective activation of M. leprae-burdened macrophages was reversible by indomethacin, and a similar block in IFN-gamma activation was observed in three of these four parameters in normal macrophages treated with exogenous prostaglandin E2. Thus, infection of mouse macrophages with M. leprae appears to restrict IFN-gamma-mediated activation at least in part by induction of inhibitory levels of prostaglandin E2.

Mycobacterial lipoarabinomannan inhibits gamma interferon-mediated activation of macrophages.

The principal efferent role of the macrophage in acquired resistance to intracellular pathogens depends on activation by T-cell lymphokines, primarily gamma interferon (IFN-gamma). However, mouse macrophages that are heavily burdened with Mycobacterium leprae are refractory to activation by IFN-gamma and are thus severely compromised in their capacity for both enhanced microbicidal and tumoricidal activities. We report here that lipoarabinomannan (LAM), a highly immunogenic lipopolysaccharide that is a prominent component of the cell walls of M. leprae and M. tuberculosis, was a potent inhibitor of IFN-gamma-mediated activation of mouse macrophages in vitro. Inhibition of macrophage activation by LAM required preincubation for approximately 24 h, resulting in uptake of LAM into cytoplasmic vacuoles of macrophages. Intact LAM was necessary to inhibit IFN-gamma-mediated activation, as this property was lost when the acyl side chains were removed from LAM by mild alkaline hydrolysis. In addition, LAM was an abundant constituent of macrophages isolated from lepromatous granulomas of M. leprae-infected nude mice and likely contributed to the defective activation of granuloma macrophages by IFN-gamma.

Defective activation of granuloma macrophages from Mycobacterium leprae-infected nude mice.

We have previously demonstrated that granuloma macrophages from the foot pads of Mycobacterium leprae-infected nude mice are functionally normal despite heavy intracellular burdens of bacilli. However, unlike peritoneal macrophages, these macrophages fail to restrict the intracellular growth of Toxoplasma gondii when stimulated with recombinant murine gamma interferon (Mu IFN-gamma) and thus appear defective in their response to macrophage-activating factor(s). In further characterizing this defect we have examined tumoristatic capacity, superoxide radical formation, and expression of la antigens on granuloma macrophages before and after treatment with Mu IFN-gamma. By all three criteria, M. leprae-burdened granuloma macrophages failed to become activated by doses of Mu IFN-gamma that readily activate peritoneal macrophages from M. leprae-infected nude mice or normal Balb/c mice. M. leprae-infected granuloma macrophages produced elevated levels of prostaglandin E2 (PGE2) in vitro, which was suppressed by indomethacin. However, the inhibition of PGE2 production for 48 hr in vitro did not restore normal responses to Mu IFN-gamma.

Intracellular fate of Mycobacterium leprae in normal and activated mouse macrophages.

Mycobacterium leprae replicates within mononuclear phagocytes, reaching enormous numbers in the macrophage-rich granulomas of lepromatous leprosy. To examine the capability of macrophages to digest M. leprae, we studied the intracellular fate of M. leprae organisms in normal and activated mouse macrophages by using the electron-dense secondary lysosome tracer Thoria Sol. Intracellular M. leprae organisms, surrounded by a characteristic electron-transparent zone, were contained within phagosomal vacuoles of macrophages cultured in vitro for 1 to 6 days. In normal macrophages, a majority of phagosomes containing freshly isolated live M. leprae cells resisted fusion with Thoria Sol-labeled lysosomes. The extent of fusion was not significantly affected by pretreatment of M. leprae with human patient serum high in specific immunoglobulin G and M antibodies. In contrast, a majority of phagosomes containing gamma-irradiated M. leprae cells underwent lysosome fusion in normal macrophages. In addition, increased phagolysosome fusion was observed with live M. leprae-containing phagosomes in macrophages activated with gamma interferon. Increased fusion was associated with an increase in the number of fragmented and damaged bacilli, suggesting that increased digestion followed fusion. This study indicates that activated macrophages may have an increased capacity for clearance of normally resistant M. leprae.

Mycobacterium leprae-burdened macrophages are refractory to activation by gamma interferon.

Mycobacterium leprae grows to enormous numbers in the nu/nu mouse footpad, producing granulomas resembling those of lepromatous leprosy in humans. Footpad granuloma cells gorged with M. leprae were established in primary cell culture to examine their functional capabilities. These cells were classified as macrophages by the following criteria: positive staining for nonspecific esterase, reduction of Nitro Blue Tetrazolium during phagocytosis of Candida albicans, possession of Fc receptors, and possession of Mac-1 antigen. Footpad macrophages also phagocytized and supported the intracellular growth of Toxoplasma gondii. However, unlike peritoneal macrophages, footpad macrophages could not be activated to kill or inhibit T. gondii by macrophage-activating factor produced by mitogen-stimulated spleen cells or by recombinant gamma interferon. Thus, although the lepromatous macrophages appeared to be normal in many of their functions, they were defective in response to macrophage-activating signals.