Abstract
Transforming growth factor beta (TGF-β) has been implicated in the pathogenesis of several diseases including infection with intracellular pathogens such as the Mycobacterium avium complex. Infection of macrophages with M. avium induces TGF-β production and neutralization of this cytokine has been associated with decreased intracellular bacterial growth. We have previously demonstrated that epithelioid cell surrogates (ECs) derived from primary murine peritoneal macrophages through a process of differentiation induced by IL-4 overlap several features of epithelioid cells found in granulomas. In contrast to undifferentiated macrophages, ECs produce larger amounts of TGF-β and inhibit the intracellular growth of M. avium. Here we asked whether the levels of TGF-β produced by ECs are sufficient to induce a self-sustaining autocrine TGF-β signaling controlling mycobacterial replication in infected-cells. We showed that while exogenous addition of increased concentration of TGF-β to infected-macrophages counteracted M. avium replication, pharmacological blockage of TGF-β receptor kinase activity with SB-431542 augmented bacterial load in infected-ECs. Moreover, the levels of TGF-β produced by ECs correlated with high and sustained levels of ERK1/2 activity. Inhibition of ERK1/2 activity with U0126 increased M. avium replication in infected-cells, suggesting that modulation of intracellular bacterial growth is dependent on the activation of ERK1/2. Interestingly, blockage of TGF-β receptor kinase activity with SB-431542 in infected-ECs inhibited ERK1/2 activity, enhanced intracellular M. avium burden and these effects were followed by a severe decrease in TGF-β production. In summary, our findings indicate that the amplitude of TGF-β signaling coordinates the strength and duration of ERK1/2 activity that is determinant for the control of intracellular mycobacterial growth.
Highlights
Bacteria of the Mycobacterium avium complex are facultative intracellular microorganisms that mainly infect mononuclear phagocytes and cause disseminated infection in immunocompromised patients [1,2]
We have previously shown that epithelioid cell surrogates (ECs) derived from primary murine peritoneal macrophages through a process of differentiation induced by recombinant IL-4, overlap several morphological and functional features of epithelioid cells found in granulomas [24]
We have previously described that primary murine peritoneal macrophages pulsed with recombinant IL-4 acquire morphological and functional characteristics of epithelioid cells after 7 days in culture [24], suggesting that signaling through IL-4 receptor is involved in the differentiation of macrophages into epithelioid cells surrogates (ECs)
Summary
Bacteria of the Mycobacterium avium complex are facultative intracellular microorganisms that mainly infect mononuclear phagocytes and cause disseminated infection in immunocompromised patients [1,2]. Once the bacteria are transported into the deeper tissues by macrophages and perhaps other phagocytic cells, additional macrophages gather at individual infected foci to form granulomas. Granulomas likely begin as aggregates of mononuclear phagocytes that surround individual infected macrophages [7,8]. These macrophages become activated, a transformation reflected by an increase in their size and subcellular organelles, ruffled cell membranes, and enhanced phagocytic and microbicidal capabilities [9,10]. A common feature of all Mycobacterium granulomas is the differentiation of macrophages into epithelioid cells that have tightly interdigitated cell membranes in zipper-like arrays linking adjacent cells [7,10]. It is speculated that mycobacteria enter into a latent phase within mature granulomas and remain contained within these structures for prolonged periods of time [11]
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