and Antigen Presentation
A. Rudensky, C. Beers
5.1 Introduction . . . 82 5.2 The MHC Class II Pathway . . . 82 5.3 Lysosomal Cysteine Proteases . . . 85 5.4 MHC Class II Invariant Chain Degradation
by Lysosomal Cysteine Proteases . . . 87 5.5 Role of Cathepsin S and L in Generation
of MHC Class II-Bound Peptides . . . 89 5.6 Regulation of Cathepsin S and L Activity
and Their Role in Ii Degradation in Macrophages . . . 90 5.7 Role of Cathepsin S and L in MHC Class II Presentation
by Nonprofessional Antigen-Presenting Cells . . . 91 References . . . 92
Abstract. Lysosomal proteinases are involved in two critical stages of MHC
class II-mediated antigen presentation, i.e., degradation of invariant chain,
a chaperone molecule critical for MHC class II assembly and transport, and
generation of class II-binding peptides in the endocytic compartment. We found
that two lysosomal cysteine proteinases, cathepsins S and L, were found to be
differentially expressed in different types of “professional” and “nonprofession-
al” antigen presenting cells, including B cells, macrophages, specialized thymic
epithelium, intestinal epithelium, and dendritic cells. In this chapter, our recent
studies on the role of cathepsin S and L in MHC class II-mediated antigen
processing and presentation in these cells types are highlighted.
5.1 Introduction
T lymphocytes recognize antigens in the form of short peptides bound to major histocompatibility complex (MHC) molecules displayed on the surface of antigen-presenting cells (APCs). MHC molecules sample products of proteolysis in two major sites of constitutive protein break- down within the cells: MHC class I molecules present peptides generated in the cytosol and MHC class II molecules present peptides generated in the endosomal/lysosomal compartment. Peptides presented by MHC class I are recognized by CD8
+T cells, while peptides generated in the endosomal/lysosomal compartment are presented in the context of MHC class II to CD4
+T cells (Cresswell 1994).
CD4
+T cell development is dependent upon recognition of self- MHC class II-peptide complexes in the thymus by TCR receptors ex- pressed on immature thymocytes. Thymocytes capable of recognition of relatively low avidity MHC class II-self-peptide ligands expressed on cortical TECs proceed further with their differentiation program. Thy- mocytes displaying TCR with a high avidity for MHC class II-peptide complexes, expressed primarily by thymic bone marrow-derived cells, are subjected to deletion via apoptosis in the process dubbed negative se- lection. Elimination of these precursors of autoreactive T cells serves as a critical mechanism of autoimmunity prevention. Dendritic cells repre- sent the most efficient type of bone marrow-derived antigen-presenting cells mediating negative selection in the thymus and presenting foreign and self-antigens in the periphery.
In this review, we discuss results of our recent studies of proteolytic mechanisms involved in MHC class II-mediated pathway of antigen presentation. Specifically, we focus on the role of lysosomal cysteine proteinases cathepsin S and L in maturation of MHC class II molecules and antigen processing in different types of antigen-presenting cells (APCs), including thymic cortical epithelial cells, dendritic cells, B cells, macrophages, and intestinal epithelial cells.
5.2 The MHC Class II Pathway
MHC class II molecules are constitutively expressed on the surface of
professional antigen-presenting cells (APCs) of the immune system, i.e.,
B cells, dendritic cells (DCs), macrophages (M φs), and cortical thymic epithelial cells (TECs), which present proteolytic fragments of self and foreign protein antigens to the CD4
+T helper cells (Cresswell 1994;
Watts 1997; Steimle et al. 1994). Expression of MHC class II can also be transiently induced by inflammatory signals in other cell types such as endothelial cells, fibroblasts, and different types of epithelial cells (Hershberg and Mayer 2000; Sartor 1994).
MHC class II molecules assemble in the endoplasmic reticulum with the help of the chaperone molecule invariant chain (Ii), a type II gly- coprotein that promotes the proper folding and assembly of the MHC α/β heterodimer (Anderson and Miller 1992; Cresswell 1996) (Fig. 1).
Alpha-beta heterodimers associate with the Ii binding to a particular region of Ii, termed CLIP (class II-associated Ii peptide), within the peptide-binding groove. The roles of Ii include stabilization of the nascent MHC class II heterodimers and prevention of the premature binding of polypeptides or partially folded proteins within the ER. Ii also facilitates intracellular trafficking of newly synthesized MHC class II molecules to the endosomes. The latter function of Ii is mediated by its cytoplasmic tail, which contains an endosomal/lysosomal tar- geting motif (Bakke and Dobberstein 1990; Lotteau et al. 1990; see for review Cresswell 1996). Upon arrival at the endosomal/lysosomal compartment, the invariant chain is degraded by lysosomal proteases in a step-wise fashion. Initial cleavage results in generation of the p22 and p18 Ii degradation intermediates followed by generation of the p12 fragment, all of which are still associated with the α/β heterodimer. At the late stages of Ii degradation, the p12 fragment is removed, leaving only CLIP associated with the MHC class II peptide binding groove (Maric et al. 1994; Neefjes and Ploegh 1992; Riese et al. 1996) (Fig. 2).
The MHC class II-like chaperone molecule HLA-DM (H-2M in mice) mediates removal of CLIP in exchange for the diverse array of peptides derived from self or foreign protein antigens generated in the endosomes and lysosomes. Upon peptide binding, MHC class II-peptide complexes traffic to the plasma membrane via a poorly understood mechanism or retrograde transport from the late aspects of the endosomal/lysosomal compartment (Denzin and Cresswell 1995; Martin et al. 1996) (Fig. 1).
Thus, lysosomal proteases are involved in two critical processes within
Antigen Presenting Cell
Antigen
H- 2M
Endoplasmic Reticulum Golgi
Plasma Membrane
Lysosome/Endosome CD4+
T cell
MHC class II a/b heterodimer
Invariant chain Proteases
CD4+
T cell CD4+
T cell
Fig. 1. The MHC class II presentation pathway. MHC class II α/β heterodimers
assemble in the endoplasmic reticulum (ER) with the assistance of invariant
chain (Ii). The cytoplasmic tail of Ii contains a motif that targets the MHC
class II:Ii complex to the endosomal/lysosomal pathway. Maturation of the
early endosomes leads to activation of lysosomal enzymes, including cysteinal
proteases, which degrade endogenous and exogenous internalized proteins and
Ii. Following Ii cleavage, the MHC class II peptide-binding groove remains
occupied by the class II-associated invariant chain peptide (CLIP), which
prevents peptide loading. Removal of CLIP and loading of peptides is mediated
by the MHC-like molecule H-2M (DM). These newly assembled peptide:MHC
class II complexes then traffic to the plasma membrane
p12 H2-M p18
p22 p31
CLIP Antigenic
Peptide
Endoplasmic
Reticulum Endosomes/Lysosomes Invariant Chain
Cysteine Proteases (leupeptin sensitive)
Aspartyl or Cysteine Protease (leupeptin insensitive)
Presentation to CD4+T cells Internalized Antigen
MHC class IIa/b Heterodimer