Elsevier

Biochemical Pharmacology

Volume 68, Issue 6, 15 September 2004, Pages 1033-1040
Biochemical Pharmacology

Cytotoxic lymphocytes; instigators of dramatic target cell death

https://doi.org/10.1016/j.bcp.2004.05.043Get rights and content

Abstract

Most mammalian cells are constantly threatened by viral infection and oncogenic transformation. To maintain healthy function of organs and tissues it is critical that afflicted cells are efficiently detected and removed. Cytotoxic lymphocytes (CL) are chiefly responsible for efficiently seeking out and eliminating damaged or infected cells. It is known that CLs must specifically recognize and bind to their targets, but the molecular events that occur within the target cell that lead to its death are still poorly understood. The two main processes initiated by CLs to induce target cell death are mediated by ligation of surface receptors or release of toxic proteins from secretory granules (granule exocytosis) of the CL. Here we review some of the key findings that have defined our knowledge of the granule exocytosis-mediated pathways to CL-mediated killing and discuss recent insights that challenge conventional views in the important area of CL effector function.

Introduction

Tissues are sculpted from many cells working co-operatively with their neighbours. Throughout life, developmental, homeostatic and immunological mechanisms rigorously regulate the number of cells in any tissue while protecting the body from disease. Each day, many billions of cells are created in the adult and as a consequence many billions of cells must also die. Each cell contains internal monitoring programs that determine whether it has become defective or reached the end of its useful life and these programs are capable of activating pathways that lead to cell death. If this mechanism fails and even one cell evades the death signal and continues to divide, cancer may result. Cell death is also used as a mechanism of host defence against viral pathogens. Viruses enter cells via surface receptors and hijack the cell’s metabolic machinery to proliferate, eventually escaping to infect neighbouring cells. Many viruses can spread rapidly and cause widespread tissue damage while some such as Human papilloma viruses have oncogenic potential. Therefore the efficient removal of infected cells is a very important process. As the majority of the viral life cycle occurs inside a cell, death of the infected cell is the most efficient, and sometimes the only, way to eliminate viral infections (Box 1).

In the initial stages, host defence against a viral infection is mediated by an ‘innate‘ immune response in which cytokines such as interferon γ or TNFα stem the ability of the virus to proliferate. Natural killer cells, a subset of cytotoxic lymphocytes (CL) (Box 2), participate in the innate response and act by detecting and disposing of infected cells. After several days, a more specific ‘adaptive’ immune response develops in which large numbers of virus-specific cytotoxic T lymphocytes are generated to target and kill the infected cells. These cells proliferate and persist in large numbers until the infection is resolved. Their number decline in response to reduction in antigen burden, however, a subset persist as a long-lived population of ‘memory’ cells that will provide lasting immunity to that virus.

The immune system can also recognize and destroy cells that have become transformed, a process often referred to as tumour immunosurveillance (supplementary movie 1). The physiological relevence of immunosurveillance has been the subject of a wide-ranging debate over several decades, but recent evidence strongly supports the concept. Organ transplants recipients who are treated with long-term immunosuppressive therapy have a higher relative risk of developing malignancy and this is presumed to be a consequence of the inhibitory effect of immunosuppressive drugs on T cell function [1], [2]. Animal models predict that the ability of CLs to kill tumour cells plays an important role in immunosurveillance as loss of key effectors of CL-mediated cell death, such as perforin, increases the incidence of chemically induced and spontaneous tumours [3], and spontaneously arising lymphoma [4].

Section snippets

The signature of a serial killer

Several CL subtypes have been defined, including cytotoxic T lymphocytes, natural killer cells, natural killer T cells and γδ T cells. These cells recognize their targets in different ways, however the mechanisms by which these cells kill is believed to be similar. Initially CL were believed to lyse their targets and the terms ‘cytolysis’ and ‘specific-lysis’ are still associated with CL killing. This terminology reflects that CL-mediated killing has largely been assessed by measuring the

The MO (modus operandi) of target cell death

The contents of cytotoxic granules have not been comprehensively studied, but include granule specific proteases (granzymes), other proteases and the pore forming protein perforin (Table 1). Studies of CLs from gene targeted animals have determined that the only protein absolutely required for granule-induced killing is perforin. Perforin-deficient mice are born and develop normally, however the CL from these mice are not able to kill via granule exocytosis [11]. As a consequence they have

Granzyme B, a silver bullet

Granules of CL contain several granule proteases (granzymes) that can induce death when administered to cells in combination with perforin [14]. Of these, granzyme B is the only agent that has been shown to induce caspase-dependent apoptosis [14], [19], [20]. Granzyme B is a 32 kDa serine protease with specificity for the C-terminus of certain specific aspartic acid residues. Granzyme B enters the cell by endocytosis, either by binding the mannose-6-phosphate receptor [21], or by constitutive

A “murder” of granzymes

An alternative explanation for the inability of Bcl-2 to protect cells from CL may be that granule components, other than granzyme B, induce death by MOMP independent mechanisms. This possibility is borne out by observations that CL from granzyme B deficient mice are still effective killers, but the molecular events within their target cells do not involve rapid DNA fragmentation suggesting that these cells die by a mechanism other than classic apoptosis [43]. The most likely candidates to

Epilogue

The mechanism of action of the granule components, their relative contribution to CL-mediated cell death and their importance to immune function are of critical interest. Although several important mediators of cell death and their mechanisms of action have been determined, there is much more to be learned about CL-mediated killing. CL induce apoptosis in target cells and the only granule protein known to induce death in this manner is granzyme B, suggesting that this enzyme must be a key

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    Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.bcp.2004.05.043

    1

    These authors contributed equally to this manuscript.

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