77
generate C3a and C5a (anaphylatoxins). These complement fragments stimulate the release of vasoactive amines (including histamine and 5 hydroxytryptamine) and chemotactic factors from mast cells and basophils. C5a is also chemotactic for basophils, cosinophils and neutrophils.
Recent work with knockout mice indicates that complement has a less pro-inflammatory role than previously thought, wheras cell bearing Fc receptors for IgG and IgE appear to be critical for developing inflammation, with complement having a protective effect. The vasoactive amines released by platelets, basophils and mast cells cause endothelial cell retraction and thus increase vascular permeability, allowing the deposition of immune complexes on the blood vessel wall. The deposited complexes continue to generate C3a and C5a. Platelets also aggregate on the exposed collagen of the vessel basement membrane, assisted by interactions with the Fc regions of deposited immune complexes, to form microthrombi. The aggregate platelets continue to produce vasoactive amines and to stimulate the production of C3a and C5a. (Platelets are also a rich source of growth factors – these may be involved in the cellular proliferation seen in immune-complex diseases such as glomerulo-nephritis and rheumatoid arthritis).
Polymorphs are chemotactically attracted to the site by C5a. They attempt to engulf the deposited immune complexes, but are unable to do so because the complexes are bound to the vessel wall. Thus, they exocytose their lysosomal enzymes onto the site of deposition. If simply released into the blood or tissue fluids these lysosomal enzymes are unlikely to cause much inflammation, because they are rapidly neutralized by serum enzyme inhibitors. But if the phagocyte applies itself closely to the tissue-trapped complexes through Fc binding, then serum inhibitors are excluded and the enzymes may damage the underlying tissue.
78 Figure 2.9
Immune Complex lattice formation at different molar ratios of antigen and antibody: When antigen or antibody is in great excess, small soluble complexes form. When antigen and antibody are in molar equivalence, large, insoluble complexes form. As antigen/antibody ratios approach molar equivalence, ICs are larger but remain soluble. IC = immune complex (Rojko et al., 2014)
79
Figure 2.10: Immune Complexes and vascular system (Rojko et al., 2014)
80
2.14.2.1. Endogenous (Assaults) Sources of Immune Complexes
Tissue damage caused by trauma, infection or inflammation is associated with the release of endogenous proteins that signal impending danger to the host. The terms ―damage-associated molecular patterns (DAMPs)‖ or ―alarmins‖ have been used to collectively describe endogenous proteins that signal tissue and cell damage which may be present in the absence of microbial pathogens. These molecules help to explain the initiation of an inflammatory response in the absence of infection such as in trauma or the classical example of acute pancreatitis (Coveney et al., 2015). Inflammatory mediators play important roles in the development and progression of cancer. Cellular stress, damage, inflammation, and necrotic cell death cause release of endogenous damage-associated molecular pattern (DAMP) molecules or alarmins, which alert the host of danger by triggering immune responses and activating repair mechanisms through their interaction with pattern recognition receptors. Recent studies show that abnormal persistence of these molecules in chronic inflammation and in tumour microenvironments underlies carcinogenesis and tumour progression, indicating that DAMP molecules and their receptors could provide novel targets for therapy (Srikrishna and Freeze, 2009). Multicellular animals detect pathogens via a set of receptors that recognize pathogen-associated molecular patterns (PAMPs). However, pathogens are not the only causative agents of tissue and cell damage: trauma is another one. Evidence is accumulating that trauma and its associated tissue damage are recognized at the cell level via receptor-mediated detection of intracellular proteins released by the dead cells. The term ―alarmin‖ is proposed to categorize such endogenous molecules that signal tissue and cell damage. Intriguingly, effector cells of innate and adaptive immunity can secrete alarmins via nonclassical pathways and often do so when they are activated by PAMPs or other alarmins (Coveney et al., 2015). Endogenous alarmins and exogenous PAMPs therefore convey a similar message and elicit similar responses (forming circulating immune complexes); they can be considered subgroups of a larger set, the damage-associated molecular patterns (DAMPs) (Bianchi, 2006, Coveney et al., 2015). Multicellular animals must distinguish whether their cells are alive or dead and detect when microorganisms intrude, and have evolved surveillance/defense/repair mechanisms to this end (Bianchi, 2006, Coveney et al., 2015). Tissues can be ripped, squashed, or wounded by mechanical forces, like falling rocks or simply the impact of one‘s own body hitting the ground. Animals can be wounded by predators. In addition, tissues can be damaged by excessive heat (burns), cold, chemical insults (strong acids or bases, or a number of different cytotoxic poisons), radiation, or the withdrawal of oxygen and/or nutrients. Finally, humans can also be damaged by specially designed drugs, such as chemotherapeutics, that are meant to kill their tumor cells with
81
preference over their healthy cells. Very likely, we would not be here to discuss these issues if evolution had not incorporated in our genetic program ways to deal with these damages, which are not caused by pathogens but are nonetheless real and common enough. Inflammation is also activated by these types of insults. A frequently quoted reason for the similarity of the responses evoked by pathogens and trauma is that pathogens can easily breach wounds, and infection often follows trauma; thus, it is generally effective to respond to trauma as if pathogens were present.
In my opinion, an additional reason is that pathogens and trauma both cause tissue and cell damage and thus trigger similar responses (Bianchi, 2007).
The best known DAMPs are high mobility group box-1 (HMGB1), S100A8 (MRP8, calgranulin A) HSP70, uric acid and S100A9 (MRP14, calgranulin B), and Serum amyloid A (SAA).
Increased serum levels of these DAMPs have been associated with many inflammatory diseases, including sepsis, arthritis, atherosclerosis, lupus, Crohn‘s disease and cancer. Therapeutic strategies are being developed to modulate the expression of these DAMPs for the treatment of these diseases. These DAMDs have all been found to induce Toll-like receptor (TLR)-dependent inflammatory response (Cavassani et al., 2008) Significantly, some of these molecules, including HMGB1 and HSP70 are not released during apoptosis, which is in keeping with the idea that programmed cell death does not result in an inflammatory response. Some DAMPs can engage TLRs to induce and amplify the inflammatory response. TLR2 and TLR4 signaling have been shown to mediate NF-κB activation initiated by HMGB1, S100A8 and SAA. Different signaling pathways are involved that may cross-talk at several levels, but all culminate in the activation of NF-κB (Conevey et al., 2015). The sources of endogenous CICs formation involve physiological processes such as inadequate removal of apoptosed cells. Apoptosis produces cell fragments called apoptotic bodies that under go efferocytosis (phagocytic cells are able to engulf and quickly remove apoptotic bodies before the contents of the cell can spill out onto surrounding cells and cause damage) (Poon et al., 2014). The possible means of endogenous antigens that form immune complexes include the continual response of the body's immune system, which overloads the ability of the body to remove the immune complexes that formed, expression of mutated gene products (Poon et al., 2014) and proliferation of cancer cells, breakdown of tissue structure due to injuries, resulting from persistent inflammatory response.
These have lead to release into circulation sequestered antigens (endogenous and/or auto-antigen)(Poon et al., 2014). It is important to note that continual progression of cancer cells and persistence of tissue injury, would continually induce the activation of the apoptotic pathway and activation of cytotoxic T cell activities, thereby generating more cell debris (apoptotic bodies
82
and/or cell fragments). The increase in these cell fragments may overwhelm the phagocytic cells thereby frustrating efferocytosis. Antibodies are formed against this debris leading to increase in immune complex formation, as well as fuelling the persistent circulation of immune complexes.
2.14.2.2. Exogenous Sources of Immune Complexes
Sources of exogenous CICs include microbial infections, Toxins and Chemicals Irritants.
Chronic infections with persistent pathogens such as helminths, mycobacteria, Plasmodium, bacteria and hepatitis viruses affect more than a third of the human population and are associated with increased susceptibility to other pathogens as well as reduced vaccine efficacy. Although these observations suggest an impact of chronic infections in modulating immunity to unrelated antigens, little is known regarding the underlying mechanisms. Diseases from persistent infections impact a large portion of humanity (Stelekati and Wherry, 2012). In developing countries, in particular, infection with at least one persistent pathogen is common. Although major efforts are focused on the control of persistent pathogens, current vaccines and treatments for many of these infections are lacking, ineffective or unavailable (Stelekati and Wherry, 2012).
Mounting evidence suggests that persistent infections can alter immunity to unrelated pathogens and vaccines. In some cases co-infections may provide a benefit to the host (Stelekati and Wherry, 2012). The high incidence of co-infection with multiple chronic pathogens suggests an increased susceptibility to secondary infections. Moreover, responses to many vaccines are reduced in chronically infected patients, rendering those individuals more susceptible to subsequent infections. Epidemiological studies suggest that chronic infections can pre-dispose patients to secondary infections (Stelekati and Wherry, 2012). Since many pathogens causing chronic pathology are co-endemic, one could argue that the high rate of co-infection is simply due to enhanced co-exposure. Although the geographical overlap of pathogen spread cannot be excluded as a potentially contributing factor, mathematical models suggest that chronic infections, such as malaria and human immunodeficiency virus (HIV), actively contribute to the increased rate of infection with unrelated pathogens (Abu-Raddad et al., 2006, Stelekati et al., 2014). The combined effects of a low-grade persistent infection (such as occur with a parasite such as Plasmodium species or in viral hepatitis) together with a weak antibody response, form chronic immune complexes (ICs) with the eventual deposition of the complexes in body tissues (Basile et al., 2012). Continuous exposure to these exogenous substances would be a source to retain pathological level of immune complexes in circulation. In this study, it is hypothesize that due to constant exposure to many infectious agents or foreign pathogens, with continuous infection and re-infection as may occur in some developing countries, IC accumulation may
83
reach a plateau, deposit on organs and constitute a great immunological risk factor to carcinogenesis. Hence it becomes imperative to measure the rate of formation. Supportively, Mart (1982), reported that concentration of immune complex at any giving time in circulation depends on the rate of immune complex formation and rate of removal (Mart 1982, Nydegger, 2007). The rate of immune complex formation in turn depends on the rate of antibody synthesis and rate of availability of specific antigen. The rate of immune complex removal in turn depends on the rate of removal by mononuclear phagocyte system (MPS), and on the deposition of immune complex on tissues. In cases with inefficient clearance by the mononuclear phagocytes system (MPS) only, pathological consequences will be expected, in particular by immune complexes formed with moderate excess of antigen (Nydegger, 2007).