Mechanisms of Antigen Presentation 
protein — the major outer membrane protein 
(MOMP) . Dr. Chakrabarti has succeeded in estab- 
lishing stable cell lines expressing the MOMP 
gene. These cell lines are being used for the de- 
tection and characterization of cytotoxic re- 
sponses in infected mice. 
Juerg Baenziger, a Howard Hughes Associate, 
has been studying another aspect of class I- 
mediated processes. This study is related to the 
behavior of the peptide-class I complexes in T 
lymphocytes after they reach the surface. Several 
years ago it was observed that class I complexes 
are endocytosed, or internalized, by T lympho- 
cytes, but only when the lymphocytes are acti- 
vated. To explore further the molecular basis as 
well as the function of T cell-specific endocyto- 
sis of class I complexes, he has constructed a se- 
ries of site-directed mutants of the human class I 
MHC molecule A2.1. These constructs include 
various truncations, deletions, and substitutions 
within the cytoplasmic domain of this mem- 
brane-spanning protein. They also include chi- 
meras in which the extracellular domain is de- 
rived from A2.1 and the transmembrane and 
cytoplasmic domains are derived from either the 
T cell surface marker, CD4, or the receptor for 
low-density lipoprotein (LDL) . 
Several of the constructs have been introduced 
into Jurkat cells, a human T cell lymphoma line. 
The properties of the chimeras in these cells with 
regard to endocytosis exactly parallel those of the 
donors of the cytoplasmic tails. Specifically, CD4 
chimeras endocytose only when phosphorylated, 
whereas LDL receptor chimeras are endocytosed 
and recycled constitutively. 
In addition to providing insights into structure- 
function relationships in the trafficking of the na- 
tive molecules, studies in these cell lines have 
led Dr. Baenziger to conclude that the class I mol- 
ecules that are endocytosed in activated T lym- 
phocytes are structurally distinct from normal 
class 1 molecules. By comparing messenger RNA 
from resting and activated human T cells, he 
found that activated cells produce a single new 
species of class I heavy chains. These heavy 
chains, which lack the domain that normally an- 
chors them in the membrane, are secreted from 
the cell. Current studies are directed toward 
determining whether these soluble class I 
molecules account in whole or in part for the 
endocytosis observed in activated T lymphocytes. 
Such a mechanism would have novel implica- 
tions for regulation of T cell-mediated immune 
responses. 
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