TGF-beta-Mediated Sustained ERK1/2 Activity Promotes the Inhibition of Intracellular Growth of Mycobacterium avium in Epithelioid Cells Surrogates

Abstract

Transforming growth factor beta (TGF-beta) 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-beta 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-beta and inhibit the intracellular growth of M. avium. Here we asked whether the levels of TGF-beta produced by ECs are sufficient to induce a self-sustaining autocrine TGF-beta signaling controlling mycobacterial replication in infected-cells. We showed that while exogenous addition of increased concentration of TGF-beta to infected-macrophages counteracted M. avium replication, pharmacological blockage of TGF-beta receptor kinase activity with SB-431542 augmented bacterial load in infected-ECs. Moreover, the levels of TGF-beta 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-beta 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-beta production. in summary, our findings indicate that the amplitude of TGF-beta signaling coordinates the strength and duration of ERK1/2 activity that is determinant for the control of intracellular mycobacterial growth.