CYTOTOXIC T LYMPHOCYTES 
Michael J. Bevan, Ph.D., Investigator 
Cytotoxic T lymphocytes (CTLs) recognize for- 
eign antigens as peptide fragments bound to the cell 
membrane class I molecules that are encoded in the 
major histocompatibility complex (MHC). These 
peptides bind the class I molecule in the endoplas- 
mic reticulum and are an essential stabilizing fea- 
ture of the final structure. The peptides can come 
from any protein present within the cell — normal 
self proteins or an antigenic viral protein, for exam- 
ple. The so-called classical class I MHC molecules 
— HLA-A, -B, and -C in humans and H-2K, -D, and -L 
in mice — are extremely polymorphic, and each al- 
lelic form binds a different range of peptides. 
In addition to these well-known class I MHC mole- 
cules that act as peptide presenters, there are a large 
number of so-called nonclassical, or class lb, MHC 
molecules with much more limited polymorphism. 
When an activated CTL recognizes a cell expressing 
a foreign peptide, it can rapidly lyse the cell and 
release cytokines. In this way CTLs provide a sur- 
veillance mechanism against virally and bacterially 
infected cells and possibly tumor cells expressing 
aberrant genes. 
Responses to Intracellular Bacteria 
A small number of bacteria, such as Listeria, Shi- 
gella, and Rickettsia, gain access to the cytoplasm 
of cells, where they are able to replicate and hide 
from the antibody-mediated and phagocytic arms of 
the immune system. Listeria monocytogenes is a 
ubiquitous food-borne pathogen. Dr. Bevan and his 
colleagues have been studying the class I MHC- 
restricted response to peptides derived from Lis- 
teria and presented on the surface with class I MHC 
molecules. A year ago they reported the classical 
presentation of a Listeria peptide to CTLs by a con- 
ventional class I molecule. During the course of 
these studies, they became aware that some of the 
peptides present on the surface of Listeria-'mtected 
cells could be recognized in a non-MHC-restricted 
way. This implied that the peptides were not being 
presented by the classical MHC molecules but per- 
haps by the class lb molecules. In collaboration 
with Dr. Kirsten Fischer Lindahl (HHMI, University 
of Texas Southwestern Medical Center at Dallas), 
they were able to show that the relatively nonpoly- 
morphic class lb gene H-2M3 was indeed responsi- 
ble for presenting this Listeria peptide to CTL. 
The peptide itself is resistant to aminopeptidase 
digestion, implying that it may have a blocked 
amino terminus, and its interaction with the class I 
molecule is competed by peptides that begin with 
A'-formylmethionine. All bacterial proteins are initi- 
ated with A'-formylmethionine, whereas nuclear eu- 
karyotic proteins initiate with methionine. Within 
eukaryotes only 1 3 genes, those encoded by mito- 
chondria, initiate with A'-formylmethionine, reflect- 
ing their bacterial origin. It seems likely, therefore, 
that the nonclassical class I molecule H-2M3 has 
evolved to bind A'-formylmethionine peptides de- 
rived from intracellular bacteria and to present 
them to specific CTLs. This is the first time that the 
nonclassical class I molecules have been shown to 
be recognized during the course of an infection. It is 
not yet known whether the other nonclassical class I 
molecules also have a similarly unique role in anti- 
gen presentation to T lymphocytes. 
A grant from the National Institute of Allergy and 
Infectious Diseases, National Institutes of Health, 
provided support for the project described above. 
How Do CTLs Protect 
Against Intracellular Bacteria? 
Most bacteria live extracellularly, and 99% of 
them are removed by phagocytosis, aided in some 
cases by opsonizing antibody. Most researchers 
would have predicted that the protective T cell re- 
sponse to intracellular bacteria would be heavily 
dependent on the interferon-7 (IFN-7) release by 
CTLs serving to activate macrophages for phagocyto- 
sis. It came as a surprise, therefore, to discover that 
CTLs specific for a single nonameric bacterial pep- 
tide plus a class I MHC molecule could protect ani- 
mals in an IFN-7-independent fashion. 
These experiments were done by loading the ani- 
mals with a neutralizing anti-IFN-7 antibody. In 
such animals CD4 class Il-restricted T cells lose 
their ability to protect against Listeria monocyto- 
genes infection, implying that the IFN-7-mediated 
activation of macrophages is very important in this 
case. However, with class I-restricted CTLs, neutral- 
ization of the IFN-7 had no effect. It seems likely 
that the CTLs work by rapidly lysing infected cells 
and thus limiting their rate of multiplication. CTLs 
probably cannot kill bacteria directly, and it may be 
that neutrophils kill them in the extracellular space 
following their release from infected cells. 
Positive Selection 
CTLs and helper T cells are selected in the thymus 
on the basis of their antigen/MHC-specific recep- 
tors. It is a common assumption that cortical epithe- 
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