From: Billi Goldberg <bigoldberg@igc.apc.org>
Subject: Cytotoxic T Lymphocytes
Note: Copyright 1992, Dan R. Greening. Non-commercial reproduction allowed.
Archive-Number: 6715
Organization: unspecified
Date: Tue, 5 Jan 93 08:09:50 PST

                      CYTOTOXIC T LYMPHOCYTES

Chapter 1 Principles of Tumor Immunity: Biology of Cellular Immune 
Responses by John R. Wunderlich and Richard J. Hodes. The book is 
"Biologic Therapy of Cancer" (1991) edited by Vincent T. DeVita, Jr., 
Samuel Hellman and Steven A. Rosenberg; published by J.B. Lippincott 
Company.

Pages 12-14

MHC-DEPENDENT, ANTIGEN-SPECIFIC CYTOTOXIC EFFECTOR CELLS

CYTOTOXIC T LYMPHOCYTES

Cytotoxic T lymphocytes, or CTLs, represent one of the most extensively 
studied and characterized functional T-cell populations. The generation 
of CTLs shows the classic hallmarks of Immune responses: (1) initiation 
of the response by exposure to antigen; (2) a complex MHC-dependent 
interaction of antigen-processing cells and T cells for generating and 
regulating the response; (3) MHC-dependent antigen specificity for the 
interaction between CTLs and target cells provided by the T-cell antigen 
receptor (TCR); and (4) memory responses following reexposure to the 
antigen, resulting in more rapid and higher-level responses. The 
requirements for the generation of CTLs were discussed above in the 
context of T-helper-cell function.

All CTLs express the TCR/CD3 complex described earlier, which is most 
readily detected by antibodies against the nonpolymorphic CD3 component. 
The vast majority of the CTLs that have been characterized express 
alpha-beta TCR. A small minority of peripheral blood T cells express 
gamma-delta TCR, and gamma-delta-expressing T cells can mediate 
antigen-specific cytotoxicity. Most CTLs do not express cell-surface 
receptors for immunoglobulin (FcR). Most mature CTL cells express either 
CD8 or CD4 molecules (primarily CD8), but not both. In mouse models, 
both CD8+ and CD4+ T cells mediate skin allograft rejection. As 
discussed earlier, MHC association of the target-cell antigen correlates 
with whether the CTL is CD4+ or CD8+; CD8+ CTLs primarily recognize MHC 
class I-associated antigens and CD4+ CTLs recognize primarily MHC Class 
II-associated antigens. Antigens associated with cell-surface MHC 
molecules appear to be proteins--usually fragments thereof, including 
proteins from cellular or extracellular sources, proteins from 
microorganisms including viruses, selected synthetic peptides, 
chemically modified autologous proteins, and some autologous proteins to 
which the host would normally be tolerant.

The distinction between T cells with cytotoxic function and those with 
helper function is not absolute. Fore example, CD4+ T-cell clones have 
been identified that release a relatively slow-acting cytotoxic product, 
lymphotoxin, They also release other lymphokines, such as IL-2, than can 
mediate help for CTL generation. Thus, these cells, which are referred 
to as TH1 or inflammatory CD4+ T cells, may be both helper and cytotoxic 
cells.

The cytotoxic activity of CTLs is normally initiated by CTL binding to 
target cells. The binding is facilitated by adhesion molecules expressed 
on the surface of T cells: LFA-1, CD2, CD4, and CD8. Adhesion complexes 
are formed by at least four receptor-ligand interactions than can 
enhance the binding of CTLs to target cells and thus increase the 
efficiency of cytotoxicity. LFA-1 (CD11a/CD18) on the CTL binds either 
to the intercellular adhesion molecule ([ICAM]-1 or CD54) or to other 
unidentified ligands on target cells, CD2 binds to LFA-3 (CD58), CD8 
binds to class I MHC, and CD4 binds to class II MHC. In addition to 
facilitating cell adhesion, these molecules also appear to be involved 
in T-cell triggering. The cellular distribution of receptors among 
leukocytes varies, and their level of expression also varies with cell 
function and activation state. For example, memory T cells express 
higher levels of LFA-1, CD2, and LFA3 than other T cells, which may 
relate to their enhanced response to restimulation with antigen.

Following CTL-target-cell binding, multiple TCR sites are cross-linked 
in an antigen-specific, MHC-dependent fashion, as a result of binding to 
MHC-associated antigens on the target cell. Cross-linking of TCR by 
target-cell antigens appears to be important for activating T-cell 
cytotoxicity. Although the initial cell-binding process is reversible, 
the cross-linking of TCR sites triggers the delivery of an irreversible 
lethal target-cell hit. The CTLs are not damaged as a result of 
attacking target cells, and they detach to recycle and attack other 
target cells.

The CTL-mediated cytotoxicity initiated by TCR recognition of target 
cells is normally specific to the target cells, both in vitro and in 
vivo. Thus, the release of cytotoxic mediators by CTLs is normal 
antigen-specific and MHC-restricted, because of the TCR-mediated 
triggering. Cytotoxic activity of the mediators, however, is not 
specific to target cells. Only to the extent that their activity is 
limited to the immediate vicinity of the triggered parent CTL is 
cytotoxicity specific for target cells bound by TCR to the CTL. Local 
activity is probably facilitated by concentration gradients, and/or a 
short burst of cytotoxic molecules from the parent CTL, and/or a short 
half-life of the mediator.

Two major types of cytotoxic mediators have been isolated from CTLs: 
cytolysin is related to the terminal components of complement. It is 
secreted with cytoplasmic granules from triggered CTLs into the junction 
between CTLs and target cells, and it forms pores in the target-cell 
membrane within minutes, leading to cell death without DNA 
fragmentation. Granule components, including cytolysin, are released 
from TCR-triggered CTLs previously exposed in vitro to high doses of 
IL-2, and are also released from triggered CTLs in vivo, albeit at lower 
levels. In addition, CTLs have been reported to lyse target cells by 
mechanisms not involving pore formation, which are independent of 
granules and cytolysin. Members of the lymphotoxin or TNF-alpha-like 
family, which are functionally and antigenically related, bind to 
cell-surface receptors on target cells but mediate cytotoxicity without 
pore formation. They cause cell death, including the DNA fragmentation 
that is characteristic of CTL-mediated lysis. Of note, 
lymphotoxin/TNF-alpha does not always cause target-cell lysis; the 
factors may cause cytostasis or may even stimulate growth and/or 
differentiation with some sources of cells. When lymphotoxin/TNF-alpha 
act alone, they work slowly--requiring as long as a day or more to kill 
target cells. Given rapid access to the target-cell cytoplasm, however, 
they can lyse target cells rapidly. Part of the mechanism of action of 
the lymphotoxin/TNF-alpha-related mediators of cytotoxicity may involved 
induction of target-cell factors that cause autodestruction in a form of 
"induced suicide." Additional mediators have been reported that may be 
unrelated to either cytolysin or lymphotoxin. The molecular mechanisms 
of target-cell lysis are controversial and appear to be numerous.

Three exceptions to the antigen specificity of CTL-mediated lysis of 
target cells have been described. In one case, nonspecificity appears to 
result from the cytotoxic mediator, lymphotoxin, or other members of the 
family. Release of this mediator from CTLs is antigen-specific and 
MHC-restricted, but given a lengthy in vitro incubation of triggered 
CTLs and target cells. Second, IL-2-activated T cells contribute to 
LAK-cell activity, as discussed below. Finally, the natural specificity 
of the TCR can be bypassed by artificially targeted CTLs to normally 
inappropriate target cells; for example, by using bispecific antibodies 
that bind, on the one hand, to a determinant in the TCR/CD3 complex and, 
on the other hand, to a determinant expressed on the target cell. With 
this technique, CTLs can be recruited to react with a specific target 
antigen in an MHC-independent fashion, irrespective of the original 
antigen specificity of the CTLs. Lectin-dependent cytotoxicity may 
involve the same principle.