Microfilariae of Brugia malayi Inhibit the mTOR Pathway and Induce Autophagy in Human Dendritic Cells

Abstract

Immune modulation is a hallmark of patent filarial infection, including suppression of antigen-presenting cell function and downmodulation of filarial antigen-specific T cell responses. The mammalian target of rapamycin (mTOR) signaling pathway has been implicated in immune regulation, not only by suppressing T cell responses but also by regulating autophagy (through mTOR sensing amino acid availability). Global proteomic analysis (liquid chromatography-tandem mass spectrometry) of microfilaria (mf)-exposed monocyte-derived dendritic cells (DC) indicated that multiple components of the mTOR signaling pathway, including mTOR, eIF4A, and eIF4E, are downregulated by mf, suggesting that mf target this pathway for immune modulation in DC. Utilizing Western blot analysis, we demonstrate that similar to rapamycin (a known mTOR inhibitor), mf downregulate the phosphorylation of mTOR and its regulatory proteins, p70S6K1 and 4E-BP1, a process essential for DC protein synthesis. As active mTOR signaling regulates autophagy, we examined whether mf exposure alters autophagy-associated processes. mf-induced autophagy was reflected in marked upregulation of phosphorylated Beclin 1, known to play an important role in both autophagosome formation and autolysosome fusion, in induction of LC3II, a marker of autophagosome formation, and in induced degradation of p62, a ubiquitin-binding protein that aggregates protein in autophagosomes and is degraded upon autophagy that was reduced significantly by mf exposure and by rapamycin. Together, these results suggest that Brugia malayi mf employ mechanisms of metabolic modulation in DC to influence the regulation of the host immune response by downregulating mTOR signaling, resulting in increased autophagy. Whether this is a result of the parasite-secreted rapamycin homolog is currently under study.

Can Parasitic Worms Cure the Modern World’s Ills?

“…For example, recent studies suggest mTORC2 signalling appears essential and specific to AAM-M2 differentiation and its deficiency in macrophages renders mice incapable of clearing pulmonary infection with N. brasiliensis and prevents their ability to regulate metabolic control of thermogenesis[82]. Moreover, as mTOR and autophagy are counter-regulatory[60], this provides another point of potential intervention by helminths: consistent with its induction of autophagy to limit TLR-mediated inflammation, ES-62 suppresses activation of PI3K/AKT, upstream regulators of mTOR[59,83], whilst Brugia malayi microfilariae inhibit the mTOR pathway and induce autophagy in human DCs[84]. However, the ability of helminths to induce autophagy is not always good news, as the chronic oxidative stress underpinning transformation in hepatocellular carcinoma by the fluke Dicrocoelium dendriticum is associated with induction of autophagic vesicles by its somatic antigens[85].…”

Human innate lymphoid cells (ILCs) in filarial infections

“…Interestingly, these cKit+ILCs produced primarily GM‐CSF (and not the prototypical cytokines IL‐5, IL‐17A and IFN‐γ) following TLR agonist‐induced stimulation . Therefore, cytokine production and TLR signalling represent two pathways by which mf may attenuate ILC function; similar to the modulatory properties demonstrated for mf on monocytes and DC …”

“…TLR signalling represent two pathways by which mf may attenuate ILC function; similar to the modulatory properties demonstrated for mf on monocytes[92][93][94][95][96][97] and DC [98][99][100][101]. …”

Modulation of the mTOR pathway plays a central role in dendritic cell functions after Echinococcus granulosus antigen recognition

“…Different pathogens, including helminth parasites, modulate the mTOR pathway for controlling and shaping the effector responses of immune cells, through promoting or inhibiting host translation 33,42 .…”

“…In helminths, Brugia malayi inhibits the phosphorylation of mTOR and its downstream proteins promoting autophagy 33 The hookworm Nippostrongylus brasiliensis modulates mTOR pathway in macrophages to induce differentiation and functional abilities of M2 profile and Schistosoma mansoni-soluble egg antigens via mTOR-dependent and -independent pathway condition human DCs to skew immune responses towards Th2 response 34,35 .…”