From mouth to macrophage: mechanisms of innate immune subversion by Mycobacterium avium subsp. paratuberculosis

Abstract

Johne's disease (JD) is a chronic enteric infection of cattle caused by Mycobacterium avium subsp. paratuberculosis (MAP). The high economic cost and potential zoonotic threat of JD have driven efforts to develop tools and approaches to effectively manage this disease within livestock herds. Efforts to control JD through traditional animal management practices are complicated by MAP's ability to cause long-term environmental contamination as well as difficulties associated with diagnosis of JD in the pre-clinical stages. As such, there is particular emphasis on the development of an effective vaccine. This is a daunting challenge, in large part due to MAP's ability to subvert protective host immune responses. Accordingly, there is a priority to understand MAP's interaction with the bovine host: this may inform rational targets and approaches for therapeutic intervention. Here we review the early host defenses encountered by MAP and the strategies employed by the pathogen to avert or subvert these responses, during the critical period between ingestion and the establishment of persistent infection in macrophages.

Figure 1

Uptake of orally ingested MAP. MAP is ingested (1) and travels through the GI tract. It may also be taken up in tonsillar crypts and transported to the ileum. In the rumen (2) the bacterium’s FAP is activated and is opsonized by fibronectin upon entering the lower digestive tract (3). After reaching the ileum (4) MAP is phagocytized by M cells of Peyer’s patches following recognition of the bacterium through the fibronectin receptor (5) and travels across the epithelium to intra-epithelial macrophages, which take up complement-coated MAP via complement receptors (6). Infected macrophages form granulomas (7), which harbor latent MAP infections. During active JD (8), MAP may be transmitted to an unborn calf (5), to neonates following priming in mammary glands and in milk causing increased virulence (9), or through fecal matter contaminating the environment (10). (a) Mouth (b) Salivary Glands (c) Esophagus (d) Rumen (e) Reticulum (f) Omasum (g) Abomasum (h) Gallbladder (i) Pancreas (j) Duodenum (k) Jejunum (l) Ileum (m) Cecum (n) Large Intestine (o) Anus (p) Uterus.

Figure 2

Inhibition of phagolysosomal maturation by MAP. MAP sulpholipds inhibit the formation of the phagolysosome by hindering the merging of the phagosome with the lysosome. SapM dephosphorylates phosphotidylinositol phosphates, disrupting membrane-trafficking regulation. V-ATPase is involved in phagosome-lysosome fusion. It is bound by mycobacterium protein PtpA and excluded from the phagosome thus inhibiting fusion. Rab5 stimulates fusion of early endosomes. Through retention of Rab5, as well as inhibition of recruitment of early endosomal autoantigen 1 (EEA1) to mycobacterial phagosomes, MAP is able to avert the maturation of endosomes into functional mycobacteriocidal compartments. Normally TACO is released from the phagosome allowing the lysosome to fuse. This release is inhibited by MAP. Mycobacteria are known to influence MAPK-p38 through LAM activation of TLR2. This ultimately leads to the inhibition of EEA1. TLR2 also induces production of IL-10 inhibiting a number of other innate immune signaling pathways.

Figure 3

Inhibition of toll-like receptor signaling by MAP. Signaling pathways based on known phosphorylation events of TLR9. A coloring scheme is used to illustrate phosphorylation events that were detected by peptide array kinome analysis when analyzing lysates from bovine monocytes stimulated with CpG ODNs in the presence and absence of MAP infection. Differential levels of phosphorylation relative to the media treated control (p < 0.1) are presented. Green for increased phosphorylation, red for decreased phosphorylation and blue for insignificant phosphorylation or peptide not present on array. Adapted from Arsenault et al. [89]. (Copyright © American Society for Microbiology).

Figure 4

Inhibition of IFNg signaling by MAP. MAP infection inhibits JAK-STAT signaling via increased expression of negative regulators of the IFNγ receptor, SOCS1 and SOCS3, as well as decreased expression of IFNγ receptor chains 1 and 2. These modifications of cellular responsiveness occurred in temporal fashion with early induction of SOCS1 and 3 with subsequent repression of the IFNγ receptor. MAP inhibition of IFNγ responsiveness may be mediated by MAP effector molecule PtpB. PtpB may disrupt the normal IFNγ receptor immune signaling pathway by dephosphorylating a key signaling intermediate, or perhaps the receptor itself.