CAMs are required to control and kill mycobacteria and T. gondii
, which grow within the macrophages2
. Infected CAMs are activated by innate recognition pathways, including the TLR and interferon-γ pathways, to make NO. We found that macrophage Arg1 is involved in preventing this NO production, consistent with previous reports that arginases can compete with NO synthases for their common substrate, arginine28,37-39
. Our data suggest that in CAMs, where NO is thought to be involved in directly killing pathogens, successful chronic infections are associated with pathogen-induced Arg1 expression, which in turn keeps NO production in check. Indeed, the absence of Arg1 was associated with increased macrophage NO production and was linked to enhanced control of mycobacteria and T. gondii
. An implication of this finding is that transient interruption of macrophage Arg1 function by competitive inhibitors40
may augment the ability of the immune system to control or eliminate intracellular pathogens like toxoplasma and mycobacteria, which establish long-term latent infections.
The products of arginase catalysis are urea and ornithine. It remains to be determined whether obligate intracellular pathogens such as M. tuberculosis
hijack the Arg1 pathway not only to suppress NO production but also to supply substrates for growth and survival, as has been proposed for Leishmania
species, whose growth depends in part on host polyamines derived from ornithine41
. M. tuberculosis
strains made genetically deficient in urease are being tested for their ability to act as an attenuated live vaccine42
. The function of M. tuberculosis
urease is unknown, but the enzyme presumably uses urea as a substrate for production of compounds needed for survival within macrophages.
Our data provide a possible rationale for the previous finding that T. gondii
injects ROP kinases that activate the host's STAT proteins33
. It seems likely that T. gondii
–activated STAT6 induces Arg1 by bypassing the IL-4 or IL-13 receptors and directly activating the Arg1
enhancer. Other toxoplasma-induced mechanisms are probably involved in Arg1 expression, as we found rapid T. gondii
–induced Arg1 expression in macrophages derived from STAT6-deficient mice. T. gondii
therefore uses multiple strategies to induce macrophage Arg1, potentially as a means to regulate exposure to NO. T. gondii
induces a robust host inflammatory response that is considered essential to the parasite's ability to establish chronic infections. NO production from iNOS is crucial for host control of the chronic phase of toxoplasmosis31,32
. This is consistent with our finding that Arg1 is advantageous to the pathogen in the CAM-dependent response to T. gondii
infection. Although we attribute the elevated macrophage NO in the absence of Arg1 as a major mechanism for host survival in T. gondii
infection, it is possible that macrophage Arg1 has additional functions that the parasite could use as a survival strategy.
Unlike CAMs, AAMs are a distinct macrophage population that arise in polarized TH
2 responses and are not the cellular host for pathogens. AAMs are thought to be involved in immune responses associated with asthma, worm infections and pathological scenarios involving TH
. In AAMs, TH
2-driven immune responses, driven by IL-4 or IL-13, induce Arg1 expression and other markers of AAM activity, such as the mannose receptor, chitinases and metalloproteinases43
. AAM expression of Arg1 absolutely requires STAT6 (ref. 17
), and we and others have shown that IL-4- and IL-13- mediated expression of macrophage Arg1 requires direct binding of STAT6 to an upstream enhancer element in the −3 kb region of the Arg1
. However, AAMs are not known to require NO for antihelminth immunity, and AAMs do not express much iNOS. Thus, the functions of Arg1 in AAMs remain unknown.
We found that distinct mechanisms regulate Arg1 expression in different types of infections. AAMs require the functions of both STAT6 and C/EBPβ but are independent of MyD88. In contrast, expression of Arg1 induced by mycobacteria is independent of the STAT6 pathway but depends on C/EBPβ and MyD88. These data are consistent with studies documenting the induction of Arg1 expression by LPS35,44
. We speculate that the direct or indirect activation of C/EBPβ by LPS is most likely to be responsible for Arg1 induction in LPS-stimulated macrophages. As we found no obvious protective or pathogenic function for macrophage Arg1 in acute experimental LPS challenge, TLR-induced Arg1 has a more specific function for intracellular pathogens that require NO for their control.
Not all intracellular pathogens are killed by the NO pathway. NO has no obligate function in the clearance of chlamydia45
, which have evolved to parasitize various cell types in diverse anatomical niches including the eye, lungs, and genital tract. Notably, MyD88-dependent Arg1 expression was found in total lung homogenates in the early phase of Chlamydia pneumoniae
. The role of Arg1 in infections where NO is not essentially required for killing remains to be determined.
Our studies raise the issue of why an antipathogen response would include a component that favors intracellular pathogens. We speculate that the TLR-mediated induction of macrophage Arg1 has positive antimicrobial effector functions against other types pathogens detected by the TLR system.