Mechanisms of Antigen Processing 
Peter Cresswell, Ph.D. — Investigator 
Dr. Cresswell is also Professor of Immunobiology at Yale University School of Medicine. He was born 
and educated in the United Kingdom. He received undergraduate degrees in chemistry and microbiology 
from the University of Newcastle upon Tyne and his Ph.D. degree in biochemistry and immunology 
from London University. Dr. Cresswell took postdoctoral training at Harvard University with 
Jack Strominger. Before assuming his position at Yale, he was Chief of the Division of Immunology 
at Duke University Medical Center. 
FOREIGN protein antigens must be proteo- 
lyzed into peptides and must associate with 
specialized membrane glycoproteins before they 
can be recognized by T lymphocytes. These 
membrane glycoproteins are known as major his- 
tocompatibility complex (MHC) molecules, be- 
cause of their original definition as the critical 
antigens responsible for organ graft rejection be- 
tween members of the same species. Two types of 
MHC molecules have evolved, apparently to deal 
with two different categories of antigens. Class I 
MHC molecules bind peptides derived from cyto- 
solic proteins synthesized by the cell that bears 
them, and are critical for T cell recognition of 
virally infected cells. Class H MHC molecules 
bind peptides derived from proteins internalized 
by a number of class Il-positive cell types, such 
as B cells or macrophages, collectively known as 
antigen-presenting cells. The major current inter- 
est of my laboratory is in the molecular mecha- 
nisms involved in generating these MHC-peptide 
complexes. 
Class I MHC-Peptide Association 
The association of cytosolic protein-derived 
peptides with class I MHC molecules presents a 
topological problem, in that the peptides or their 
precursor proteins must cross an intracellular 
membrane for binding to occur. Evidence from 
the literature suggests that peptide binding oc- 
curs early in the intracellular transport of class I 
MHC molecules, perhaps in the endoplasmic re- 
ticulum (ER). 
A mutant cell line, T2, derived in our labora- 
tory from a similar cell line, .174 (produced by 
Robert DeMars at the University of Wisconsin- 
Madison) , is defective in one or more of the steps 
involved in generating class I MHC-peptide com- 
plexes. Somatic cell genetic evidence suggested 
that the gene (or genes) involved is localized in a 
region of the MHC that is deleted in T2. Two of 
these genes are homologous to a group of pro- 
teins with multiple membrane-spanning domains 
known as the ATP-binding cassette (ABC) family 
of membrane transporters. 
These molecules are generally involved in the 
active transport of small molecules, or occasion- 
ally larger proteins, across membranes. It has 
been proposed that the members of the family 
encoded in the MHC are responsible for the trans- 
location of cytosolic peptides into the ER, where 
they bind to class I MHC molecules. Data from my 
laboratory have shown that most class I alleles 
expressed in T2 indeed lack peptides when they 
are affinity purified and associated peptides are 
analyzed chromatographically. 
Human class I MHC (HLA) molecules generally 
fail to be transported to the cell surface when 
expressed in T2, a probable consequence of the 
lack of associated peptides. An exceptional al- 
lele, HLA-A2, is significantly surface-expressed at 
20-50 percent of wild-type levels. HLA-A2 is as- 
sociated with a limited set of peptides in T2, 
three of which have been isolated and se- 
quenced. Two proved to be derived from the sig- 
nal sequence of an interferon-inducible protein 
known as IP- 30, and the third corresponded to no 
known protein. 
The two identified peptides, a nonamer and 
1 1 -mer respectively, are presumably transpor- 
ted into the ER by the conventional signal- 
recognition mechanism involved in the translo- 
cation of secretory and transmembrane proteins. 
This would explain their association with HIA-A2 
in a cell line defective in the normal mechanisms 
of peptide transport. In wild-rype cells, class I- 
associated peptides are generally nonameric. The 
existence of the 1 1-mer peptide, and the fact that 
the third peptide is a 1 3-mer, argues that the gen- 
eration of nonamers in wild-type cells may be a 
normal facet of peptide generation and transport. 
Curiously, mouse class I MHC (H-2) molecules 
in general are well surface-expressed in the T2 
cell line, even though they are devoid of asso- 
ciated peptides. This has led us to hypothesize 
that the human class I molecules are subject to a 
specific retention mechanism that prevents them 
from leaving the ER unless they are associated 
with a peptide. Understanding the difi'erential 
transport properties of class I H-2 and HLA mole- 
cules in both normal and mutant cells is a major 
area of emphasis in our laboratory and should un- 
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