Page 115 - Clinical Immunology_ Principles and Practice ( PDFDrive )

CHaPter 6 Overview of T-Cell Recognition 99

Class II Peptide Loading HLA-DM and Peptide Exchange
The precise intracellular compartment where class II peptide HLA-DM (DM) is the critical mediator in the release of CLIP
21
loading occurs has been a subject of much investigation. from class II and its replacement with peptide. 22-26 HLA-DM α
Endosomal vesicles “mature” over time, becoming more acidic and β heterodimers are encoded within the HLA gene complex.
as they progress toward an end-stage lysosome. MHC class II Instead of associating with invariant chain, HLA-DM gains access
molecules change conformation with acidic pH, becoming more to endosomal compartments via a tyrosine-based motif in its
“peptide receptive.” Also, as pH decreases, endosomal proteolytic cytosolic tail that allows rapid internalization into endosomal
enzymes generally become more active. Some resident endosomal compartments after a brief cell-surface intermediate. The release
proteases are synthesized as inactive enzymes, termed zymogens, of CLIP from MHC class II molecules (the dissociation reaction)
which become activated either directly by pH-dependent con- and the acquisition of antigenic peptide (the binding reaction)
formational changes or by cleavage of other endosomal pH are greatly enhanced by the presence of DM. Both dissociation
dependent proteases. In most APCs, late endosomal compartments and binding are greatly affected by acidic pH. At acidic pH,
are enriched in serine proteases (cathepsins A and G), aspartic interactions between DM and class II are initiated along their
proteases (cathepsins D and E), and cysteine cathepsins (cathepsins lateral faces on the side of class II that bears the amino terminal
S and L). The low pH in late endosomes promotes antigen segment of peptide.
unfolding and access to reductases, such as the IFN-γ–inducible DM prefers class II molecules that are open and have
thiolreductase (GILT) that cleaves disulfide bonds. The combina- unstable interactions with peptide. DM is thought to bind and
tion of low pH and reduction of disulfide bonds thus facilitates stabilize this open intermediate of class II–peptide complexes
protein unfolding, proteolytic digestion, and generation of that is promoted by low pH. This accelerates peptide release.
antigenic peptide fragments. This same open conformation is apparently also readily able
Although MHC class II molecules traffic through many to capture peptide, promoting a rapid exchange of CLIP for
endosomal compartments, they are most highly enriched in antigenic peptides. When a stable interaction between class II and
late endosomes that have a pH of approximately 4–5. These antigenic peptide forms and the antigenic peptide is fully docked
heterogeneous intracellular compartments are termed MIICs within the MHC class II molecule, DM is released from class II
(MHC class II compartments) or MVBs (multivesicular bodies) molecules. MHC class II–peptide complexes can then escape to
and can have either multilamellar or multivesicular organization, the cell surface for recruitment of CD4 T cells. Accordingly, DM
consisting of both a limiting membrane and internal membranes. acts as a catalyst of peptide exchange for class II, binding to a
In these compartments, class II molecules localize with the transition state until the final product (MHC-class II–peptide) is
other critical components of the antigen presentation pathway. formed.
The precise organization of the class II–containing compart-
ment likely varies with the APC studied (B cells, macrophages, Selection of Immunodominant Peptides
or DCs) and can change upon signaling or antigen receptor Of the many potential peptides that can bind to class II molecules
engagement. and recruit CD4 T cells when introduced into the host as single
The exchange reaction of the invariant chain fragment for peptides, only a subset of these epitopes recruit CD4 T cells
antigenic peptides represents an intriguing biochemical event when the host encounters an intact antigen or pathogen. For
that derives from many factors of MHC class II structure and example, in a 50-kDa protein that has more than 80 potential
biology. The first is the polymorphic nature of the peptide-binding 15–18mer peptides, the CD4 T cells may focus on just 3–5 peptide
pocket of the class II heterodimer. Much of the genetic variability epitopes. These peptides are termed immunodominant. Another
in both α and β chains lies within the peptide-binding cleft of subset of peptides is capable of recruiting CD4 T cells in the
class II molecules. This highly localized genetic polymorphism host when introduced as single peptides. These are termed cryptic
is thought to allow alternate MHC molecules expressed in different because they are sequestered from the immune responses to
individuals to capture distinct subsets of peptides from pathogenic complex protein antigens. Other peptides can neither bind to
organisms. the host class II nor recruit CD4 T cells.
All allelic forms of class II thus must be able to accom- Early models to explain the selectivity of CD4 T-cell responses
modate the CLIP segment while they are associated with suggested that intracellular antigen proteolysis would play a
an intact invariant chain. This ensures conformational integrity prominent role in selection of epitopes because intact antigens,
of the class II molecule early in biosynthesis. Once invariant but not peptides, require internalization and degradation. By
chain is proteolytically cleaved, the various allelic forms of this model, the position of the peptide within the three-
class II are differentially able to sustain interactions with dimensional structure of antigenic proteins might limit access
CLIP and acquire peptides. Class II molecules with high affin- to proteolytic enzymes and thus availability to bind to class II
ity for CLIP require an efficient mechanism to release CLIP. molecules. Conversely excessive degradation, leading to destruc-
Class II molecules devoid of peptide have a tendency to tion of the peptide epitopes by endosomal proteases, could lead
aggregate and become degraded, particularly at the low pH of to diminished yield of some peptides for class II binding and
the endosome. Peptide binding to class II molecules, even at low presentation. These differential proteolytic events could thus
pH where peptide acquisition is facilitated, is quite slow. These restrict the potential presented repertoire to a limited number
combined characteristics of class II structure and biochemistry of peptides of the appropriate size (typically 12–25 amino acids).
led to the early speculation that there must be a mechanism Although it is likely that in some cases such differential antigen
to ensure the release of CLIP from all alleles of class II and processing might impact the yield of available peptide, accumu-
rapid subsequent binding of antigenic peptide. The protein that lated data now suggest that selective presentation of potential
facilitates this process is termed HLA-DM (in humans) or H-2M antigenic peptides in association with MHC class II molecules
(in mice). is primarily as a result of intracellular DM editing.