T2 is much less affected than that of HLA molecules. 
In this case, transport occurs despite the lack of 
associated peptides. The precise cellular mecha- 
nism responsible for the discrimination between 
"empty" H-2 class 1-^2^ complexes and "empty" 
HLA class I-/?2m complexes is unclear, but may re- 
late to differential afi&nity of these molecules for 
intracellular retention factors important in class I- 
restricted antigen processing. 
In collaboration with Dr. Thomas Spies (Dana 
Farber Cancer Center) , an antiserum to a synthetic 
peptide corresponding to the carboxyl terminus of 
the TAP.l protein was used to show that the TAP.l 
and TAP. 2 proteins coassociate. This may reflect 
dimer formation and suggests that dimers of TAP. 1 
and TAP. 2 form the structure critical for transport of 
cytosolic peptides into the ER. Reconstitution of 
mutant cell lines lacking TAP.l or TAP. 2 with the 
gene encoding the missing protein clearly restores 
the class I-restricted antigen-processing function. 
Function of MHC Class II Molecules 
The a and 13 subunits of class II MHC molecules 
assemble in the ER with the invariant chain, a type II 
transmembrane protein that is proteolytically re- 
moved from the class II-a/3 dimer in a post-Golgi 
endosomal compartment. Dr. Cresswell and his col- 
leagues have shown that the class II invariant-chain 
complex is a nine-subunit structure, containing 
three a subunits, three /3 subunits, and three invari- 
ant-chain molecules. In class Il-negative, invariant- 
chain-positive mutant cells, the invariant chain 
forms a trimer. In wild-type cells the invariant chain 
also rapidly forms trimers upon synthesis in the ER 
and subsequently binds a and 0 subunits, accumu- 
lating them to form the nine-chain structure. After 
transport through the Golgi, the invariant chain is 
degraded to release a/3 dimers associated with 
peptides. 
Dr. John Newcomb, an HHMI associate in Dr. 
Cresswell 's laboratory, has isolated and sequenced a 
number of the peptides associated with the HLA- 
DRl 1 molecule. They vary in length from 1 3 to 16 
amino acids. Eight of nine peptides sequenced were 
from known sources, either from secretory proteins, 
the extracytoplasmic regions of transmembrane 
proteins, or heat-shock proteins. 
The first two types (secretory and transmembrane 
protein-derived peptides) are consistent with the 
notion that class II molecules acquire endosomally 
generated peptides. Peptides derived from heat- 
shock proteins are more surprising. They may result 
from transport of these proteins into lysosomes from 
the cytosol, a process known to occur when cells 
undergo serum deprivation. No peptides were 
found associated with ajS-invariant-chain com- 
plexes, consistent with previous observations from 
the laboratory suggesting that one function of the 
invariant chain is to prevent class II molecules from 
binding inappropriate peptides in the early stages of 
transport. 
Dr. Cresswell and his colleagues have shown that 
the T2 cell line, defective in class I-restricted anti- 
gen processing, is also defective in class II- 
restricted antigen processing. T2 transfectants ex- 
pressing HLA-DR or I-A*' molecules assemble class 
Il-invariant-chain complexes normally. These com- 
plexes are transported through the Golgi, the invari- 
ant chain is degraded, and a/3 dimers are expressed 
on the cell surface. However, these dimers do not 
contain normally processed peptides and are unable 
to stimulate antigen-specific class Il-restricted T 
cells. They also lack the stability of association in 
SDS (sodium dodecyl sulfate) characteristic of class 
II-a/3 dimers. When peptides were isolated from 
HLA-DR3 molecules purified from T2 transfectants, 
they all proved to be derived from a small region of 
the invariant chain (residues 81-104). This may de- 
fine the region responsible for blocking access of 
peptides to invariant-chain-associated class II mole- 
cules, a region that is replaced in wild-type cells by 
peptides generated from endocytosed proteins. 
The nature of the genetic lesion in T2 responsible 
for the class II antigen-processing defect is under 
investigation. The affected gene (or genes) is clearly 
in the class II region of the MHC, since T2 has a large 
deletion in this region and the parental cell line 
with an intact MHC has normal antigen-processing 
functions. However, the precise nature of the defect 
remains unknown. 
Dr. Cresswell is also Professor of Immunobiol- 
ogy at Yale University School of Medicine. 
Articles 
Cresswell, P. 1992. Chemistry and functional role 
of the invariant chain. Curr Opin Immunol 
4:87-92. 
Crumpacker, D.B., Alexander, J., Cresswell, P., and 
Engelhard, V.H. 1992. Role of endogenous pep- 
tides in murine allogeneic cytotoxic T cell 
responses assessed using transfectants of the 
antigen-processing mutant 174xCEM.T2. / Im- 
munol 148:3004-3011. 
Lamb, C.A., and Cresswell, P. 1992. Assembly and 
transport properties of invariant chain trimers and 
HIA-DR-invariant chain complexes. / Immunol 
148:3478-3482. 
Riberdy, J.M., and Cresswell, P. 1992. The antigen- 
IMMUNOLOGY 321 
