Proinflammatory cytokines such as IL-12 play critical roles in the induction of host resistance to M. tuberculosis
as well as other intracellular pathogens. These responses must be carefully regulated to avoid host tissue damage. The antiinflammatory cytokines IL-10 and TGF-β have been implicated as key protein mediators that prevent excess IL-12, TNF-α, and IFN-γ production in intracellular infections. Nevertheless, these downregulatory cytokines appear to have only limited effects in controlling M. tuberculosis
replication during infection in animal models (8
). In the present study, we report evidence for the role of a novel pathway involved in dampening M. tuberculosis
–driven proinflammatory immune responses and regulating bacterial growth that involves the 5-LO–dependent production of lipoxins.
Lipoxins such as LXA4
are biosynthesized by different cell types, including leukocytes, endothelial cells, and platelets by means of transcellular pathways (21
). Recently, LXA4
was shown to have downregulatory actions on several proinflammatory mechanisms including NK cell cytotoxicity (22
), leukocyte responses to proinflammatory cytokines (23
), and microbial stimulation (14
) as well as migration of both neutrophils (18
) and eosinophils (24
). Interestingly, stimulation of mucosal epithelial cells with lipoxin analogs induced the expression of a bactericidal/permeability-increasing protein, which exhibits antimicrobial activities and enables epithelial cells to engage in active microbial host defense (25
dramatically reduces T. gondii
–induced IL-12 production by DCs in vitro and by DCs as well as other cells in vivo (13
), which indicates a role for LXA4
in preventing uncontrolled proinflammatory responses. T. gondii
triggered high levels of LXA4
(~100 ng/ml) in sera of WT mice, while infected 5-LO–deficient mice produced elevated amounts of IL-12p40. Interestingly, it was recently shown that T. gondii
synthesizes its own LO that may play a role in increasing local concentrations of LXA4
). In the present report, using an aerosol model of infection with M. tuberculosis
, we also detected the induction of high levels of LXA4
as well as the leukotriene LTB4
in the sera of WT mice. However, LXA4
were induced with different kinetics, and only LXA4
persisted at high levels during chronic infection. Furthermore, we observed high levels of expression of 5-LO in lung endothelium and macrophages during infection. The latter results suggest that these cell populations participate in lipoxin generation in vivo and may be specifically involved in regulating local inflammatory responses during chronic experimental tuberculosis.
Whereas T. gondii–exposed 5-lo–/– mice succumbed rapidly to infection, the reduced lipoxin generation in 5-lo–/– mice infected with M. tuberculosis was associated with enhanced survival at high-dose aerosol challenge, although there were no apparent differences in mortality at the lower inoculum infections. These contrasting outcomes of the 2 infection models may stem from differences in the intrinsic virulence and immune-stimulatory properties of the pathogens in question. T. gondii is a highly virulent and fast-replicating microorganism that requires the induction of a potent immune response to protect the host and produce chronic persistent infections necessary for promoting its transmission. M. tuberculosis, while also inducing latent infections, replicates slowly and, at least in the mouse model, induces a weaker Th1 response than does T. gondii. Hence, in the absence of lipoxin-mediated counterregulation, the ensuing cellular responses are enhanced, triggering immunopathology and mortality in T. gondii infection, whereas in M. tuberculosis infection, this enhancement results in increased control of bacterial replication. The observed restriction in mycobacterial growth does not appear to be complete, however, since high-dose M. tuberculosis–infected 5-lo–/– mice eventually began to succumb at 150 days after infection, and this mortality is associated with an approximate log increase in mycobacterial load compared with earlier time points (e.g., day 42) (data not shown). Whether the late death of the infected 5-LO–deficient animals is due solely to increased bacterial burden remains unclear.
Since 5-LO is required for both leukotriene and lipoxin biosynthesis, reconstitution experiments were performed to more directly assess the role of the latter group of eicosanoids in the regulation of mycobacterial growth in vivo. Importantly, administration of the stable lipoxin analog ATLa2 to M. tuberculosis
mice, restored both pulmonary mycobacterial loads and IFN-γ production by purified protein derivative–stimulated splenocytes to levels comparable to those observed in infected WT animals. Although this observation does not rule out the possible participation of other 5-LO–dependent mediators, it demonstrates that a deficiency in lipoxins is sufficient to explain the effects on bacterial growth and host response seen in the infected 5-LO–deficient animals. ATLa2 treatment has previously been shown to reduce inflammatory cell infiltration in a number of different disease models (17
). Although this subject is not directly addressed in the present article, it is probable that the observed effects of ATLa2 reconstitution in our experiments result from decreased effector cell recruitment into infected lung.
In summary, our findings demonstrate the existence of a novel pathway involved in controlling proinflammatory and Th1 immune responses against M. tuberculosis
infection in vivo via the generation of 5-LO–dependent lipoxin formation. These observations suggest that the regulation of lipoxin biosynthesis merits further investigation as a potential immunopharmacologic intervention for enhancing the control of mycobacterial replication in tuberculosis patients. In this regard, it should be noted that 5-LO inhibitors are already in clinical trial for asthma, and therefore it may be possible to rapidly design and implement a study testing the efficacy of this strategy for intervention in tuberculosis (28