Background Science
Cell death
Cell death is common, both in our bodies and in the laboratory. In each of these locations, cells die by two major mechanisms:
- On purpose – via the activation of a 'suicidal' cell-death programme, which is controlled by the cell itself. The best-known form of programmed cell death is apoptosis.
- By accident - following exposure to noxious agents or physical trauma. This type of cell death is commonly known as necrosis.
Apoptosis – purposeful cell death
In all our tissues, the size of individual cell populations is governed, on the one hand, by cell birth brought about by division of cells – mitosis – and, on the other hand, by a controlled cell deletion process known as apoptosis. The name apoptosis is derived from the ancient Greek meaning 'the falling of leaves from trees or petals from flowers'.
During embryonic development, we are fashioned not only through mitosis, cell differentiation and cell migration, but also through the 'useful cell death' that is apoptosis. Many of the tissues in our adult bodies are maintained by a fine balance between mitosis and apoptosis. For example, immune responses to infection are boosted by cell division and finely tuned and curtailed by apoptosis; cells of the gut and skin are renewed through cycles of mitosis and apoptosis; the lactating breast regresses post-lactation by apoptosis.
It is likely that all cells of all our tissues possess the ability to undergo apoptosis. The machinery that drives the process is genetically controlled and is known as the apoptosis programme. Often apoptosis is called 'programmed cell death'. The apoptosis programme can be activated by a wide diversity of triggers ranging from normal, physiological micro-environmental 'cues' to toxic stimuli, such as radiation-induced genomic damage. In many cases apoptosis occurs by default – that is apoptosis is activated when critical survival factors are lacking from the cell's environment. This indicates that the apoptosis programme is 'pre-formed' within cells in a dormant mode that awaits triggering. Amongst the dormant molecules that are key players in the apoptosis programme are the protein-cleaving enzymes, the caspases that reside in the cell in an inactive state, awaiting activation which allows them to cause the destruction of critical elements within the cell including the cytoskeleton and DNA. In other normal circumstances, apoptosis can be triggered by death receptors that engage with natural ligands known as death factors. In a given apoptosis pathway, activation of caspases may be initiated by death-receptor/death-factor interaction, or by the movement of molecules out of mitochondria, or both.
When apoptosis is triggered, cells undergo a series of stereotypical changes including shrinkage, loss of contact with their neighbours, changes in nuclear structure, controlled protein and DNA cleavage and alterations in molecules at the cell surface. Their altered surface architecture allows apoptotic cells to be recognised and engulfed (phagocytosed) by neighbouring cells. Apoptotic cells also release factors that allow professional scavenger cells of the immune system, the macrophages (from the Greek meaning "big eaters") , to sense the dying cells from a distance, seeking them out through directed locomotion called chemotaxis. The engulfment process is critical - it prevents dead cells from causing inflammation and tissue damage.
Necrosis – accidental and catastrophic cell death
Apoptotic cell death contrasts markedly with the accidental form of cell death, necrosis. Necrosis is abnormal, thermodynamically downhill cell death that is catastrophic. Necrotic cells rapidly lose plasma-membrane integrity and fall apart, causing direct or indirect (inflammatory) damage to their neighbours. Necrosis is not genetically regulated and serves no useful purpose.
Clearance of apoptotic cells
The natural habitat of the apoptotic cell is within a macrophage or neighbouring cell, since a variety of different cell types can engulf their apoptotic neighbours. The value of apoptosis lies in the capacity of the apoptotic cell to 'flag' itself for phagocytosis and in the capacity of the phagocyte to receive the apoptotic cell or its membrane-bound fragments (apoptotic bodies). Without phagocytosis, the apoptotic cell takes on the guise of a necrotic cell with its catastrophic tissue-damaging, pro-inflammatory properties. The key to the biological value of apoptosis is successful phagocytic clearance that is (1) rapid and (2) non-inflammatory. Apoptotic cells display on their surfaces "eat me" signals for rapid engulfment by phagocytes. The phagocytes display a variety of receptors that engage the "eat me" signals and mediate anti-inflammatory engulfment.
Thus, efficient clearance of dying cells in situ keeps tissues healthy. When cells are cultured in vitro, these clearance mechanisms are absent. This causes cell cultures to be sub-optimal, with dead cells exerting inhibitory effects on their viable neighbours.
Targeting cell death and the clearance process for therapeutic gain
Immunosolv is applying its core antibody technology directly to the cell-death program in vivo in order to develop novel anti-cancer therapeutic strategies. By targeting the features of apoptotic cells that are sensed by the host's immune system, our aim is to alter the response of the host to apoptosis for therapeutic benefit. For example, cell death is common in high-grade malignancies and we have found that macrophages, cells of the innate immune system that have anti-tumour potential, are recruited to tumours because of their ability to 'sense' apoptosis. However, rather than realising their anti-tumour potential, the recruited macrophages are subsequently conditioned by the apoptotic tumour cells to support net tumour growth. By changing the way apoptotic tumour cells are perceived by macrophages, Immunosolv aims to switch the conditioning process away from the supportive, pro-tumour mode to the destructive, anti-tumour response.
For example, by targeting the altered surface of apoptotic tumour cells (pink cells in figures below) using specific antibodies, we aim (1) to alter the receptors used by macrophages to respond to the apoptotic cells, and (2) cause persistence of apoptotic cells such that they acquire the pro-inflammatory features of necrotic cells. In each case, the tumour microenvironment is predicted to change from pro- to anti-tumour.
We have also recently discovered that apoptotic cells produce the anti-inflammatory iron-binding protein, lactoferrin which helps prevent granulocyte migration to sites of apoptosis [click here for link to publication]. Through targeting a range of molecules produced or displayed by apoptotic cells, our technology has the potential to modulate not only macrophage, but also granulocyte migration, with applications in cancer and inflammatory disorders.
Given the functions of lactoferrin (LTF, see below), we predict that in the tumour microenvironment LTF minimally (1) inhibits granulocyte migration, (2) activates macrophages to engulf apoptotic cells and support tumour-cell growth and (3) directly supports tumour-cell proliferation. Therefore, neutralising anti-LTF antibodies would be predicted to exert therapeutic anti-tumour effects through (1) releasing granulocytes from migration-inhibition allowing these cells to enter tumours and undertake antibody-dependent cellular cytotoxicity (ADCC) of tumour cells, (2) allow persistence of apoptotic cells and ADCC by macrophages and (3) block direct proliferative effects of LTF.
