Alternative Endogenous Protein Processing via an Autophagy-Dependent Pathway Compensates forYersinia-Mediated Inhibition of Endosomal Major Histocompatibility Complex Class II Antigen Presentation

Abstract

Extracellular Yersinia pseudotuberculosis employs a type III secretion system (T3SS) for translocating virulence factors (Yersinia outer proteins [Yops]) directly into the cytosol of eukaryotic cells. Recently, we used YopE as a carrier molecule for T3SS-dependent secretion and translocation of listeriolysin O (LLO) fromListeria monocytogenes. We demonstrated that translocation of chimeric YopE/LLO into the cytosol of macrophages by Yersinia results in the induction of a codominant antigen-specific CD4 and CD8 T-cell response in orally immunized mice. In this study, we addressed the requirements for processing and major histocompatibility complex (MHC) class II presentation of chimeric YopE proteins translocated into the cytosol of macrophages by the Yersinia T3SS. Our data demonstrate the ability of Yersinia to counteract exogenous MHC class II antigen presentation of secreted hybrid YopE by the action of wild-type YopE and YopH. In the absence of exogenous MHC class II antigen presentation, an alternative pathway was identified for YopE fusion proteins originating in the cytosol. This endogenous antigen-processing pathway was sensitive to inhibitors of phagolysosomal acidification and macroautophagy, but it did not require the function either of the proteasome or of transporters associated with antigen processing. Thus, by an autophagy-dependent mechanism, macrophages are able to compensate for the YopE/YopH-mediated inhibition of the endosomal MHC class II antigen presentation pathway for exogenous antigens. This is the first report demonstrating that autophagy might enable the host to mount an MHC class II-restricted CD4 T-cell response against translocated bacterial virulence factors. We provide critical new insights into the interaction between the mammalian immune system and a human pathogen.Protein antigens are recognized by T cells as short peptide fragments bound either to major histocompatibility class (MHC) I or to MHC class II molecules on the surface of antigen-presenting cells (APCs). The location of antigens in distinct intracellular compartments of APCs influences their proteolytic processing as well as access to MHC molecules (16, 39). Classically, peptides generated in the cytosol (e.g., derived from viral proteins) by proteasomal degradation are bound to MHC class I molecules after transport across the endoplasmic reticulum membrane by the transporters associated with antigen processing (TAP) (90). Subsequently, MHC class I molecules present these endogenous antigenic peptides to CD8 T cells. In contrast, exogenous antigens (e.g., antigens derived from engulfed bacteria and soluble antigens) are directed into the endosomal/lysosomal pathway for degradation (63). In late endosomal compartments, degraded protein fragments interact with MHC class II molecules and are further trimmed into peptides for presentation to CD4 T cells.However, the synchrony of this system has been challenged by biochemical and functional studies of professional and nonprofessional APCs. Epitopes derived from a diverse ...

Autophagy in T‐cell development, activation and differentiation

“…Viral examples include the Epstein–Barr virus nuclear antigen 1 86 and the influenza matrix protein 1 52 . Bacterial peptides presented on MHC II via autophagy include Yersinia protein YopE, 87 Mycobacterium tuberculosis antigen Ag85B 88 and the bacterial transposon‐derived neomycin phosphotransferase II 89 . Lastly, tumor peptide presentation on MHC II is also promoted by autophagy 48 , 90 …”

Effector CD8+T Cells Are Generated in Response to an Immunodominant Epitope in Type III Effector YopE during Primary Yersinia pseudotuberculosis Infection

“…The Cb regulates the translocation of YopE through the binding of the chaperone SycE (27)(28)(29)(30)(31). When Y. pseudotuberculosis was tested as a vaccine carrier, epitopes from heterologous antigens (e.g., LLO 91-99 or ovalbumin amino acid residues 257 to 264 [OVA [257][258][259][260][261][262][263][264] ]) fused to the N-terminal 138 amino acids of YopE were presented by the MHC-I pathway after APCs were infected with Y. pseudotuberculosis carriers (32)(33)(34). Furthermore, the level of presentation of LLO 91-99 by APCs infected with a Y. pseudotuberculosis carrier was reduced in the presence of proteasome inhibitors (34), indicating that translocated YopE-LLO is processed by the conventional MHC-I pathway.…”

A Protective Epitope in Type III Effector YopE Is a Major CD8 T Cell Antigen during Primary Infection with Yersinia pseudotuberculosis

“…These carrier vaccine strains expressed YopE fused to model antigens such as listeriolysin O (LLO) or ovalbumin (OVA); after translocation into the host antigen-presenting cell (APC) cytosol, the antigens could be processed by the proteasome after polyubiquitination, then transported to the endoplasmic reticulum (ER) to be further trimmed, loaded onto major histocompatibility complex class I (MHC-I) molecules, and finally presented on the plasma membrane to CD8 T cells. The role of Yop effectors in modulating the CD8 T cell response to these model antigens in mice infected with Yersinia carrier vaccines has been examined (43,44,56,58). YopP inhibited the CD8 T cell response to YopE-LLO in mice orally infected with Y. enterocolitica, an activity that could be correlated with the killing of APCs by this effector (56).…”