Inflammatory responses encompass a series of host defense and repair mechanisms. Inflammatory processes fight off microbial and viral invaders. They aid the repair of injured tissues. Thus, the operation of inflammatory pathways proves essential to survival of the individual and of the species. Yet, these very processes that can protect from infection or injury, when unleashed inappropriately, or if the inflammatory response fails to resolve, can cause disease. The aging of the population favors the development of chronic diseases. The control of communicable diseases by sanitation, vaccination, antibiotic treatment, and other societal measures permits more individuals to survive to the point of developing chronic diseases of many organ systems. Inflammatory pathways contribute to many of the chronic conditions that currently challenge successful aging. Inflammation contributes to the pathogenesis of diseases as diverse as forms of dementia, the arthritides, chronic lung and kidney diseases, and of course cardiovascular conditions. Thus, understanding and combatting inflammation holds a key for the prevention and treatment of many chronic conditions associated with aging. Moreover, recognition has increased that oncogenesis, and the growth, invasion, and metastasis of many cancers hijack inflammatory pathways.
Given the critical role of inflammation in host defenses and tissue homeostasis, interventions that modulate inflammatory processes require careful targeting and modulation, lest adverse consequences outweigh potential benefits. While antiinflammatory strategies have long contributed to the management of forms of arthritis and airways disease, the advent of strategies that modulate inflammation have just begun to show promise in cardiovascular conditions. Given the delicate balance required to intervene successfully to modulate therapeutically inflammation, a background in the basics of inflammation biology provides a framework for this undertaking.
Basic Concepts of Inflammation Biology
Inflammatory pathways encompass two major sets of responses that have evolved in an interdependent manner ( Fig. 7.1 ). The innate immune response arose early in evolution. Marine invertebrates such as the starfish and limulus crab have innate immune responses. The innate pathways mobilize rapidly. Their triggers include products of infectious agents known as pathogen-associated molecular patterns (PAMPs). Damaged or dead cells can release triggers for innate immunity known as damage-associated molecular patterns (DAMPs). These signals interact with a series of pattern recognition receptors, prominently the class known as Toll-like receptors (TLRs). These receptors recognize hundreds or just a few thousand different DAMPs or PAMPs. Thus, although innate immune responses mobilize rapidly, they are relatively blunt, as they recognize a fairly restricted set of patterns.
In contrast, the adaptive immune response arose much later in evolution. Most arms of adaptive immunity require “education.” Specific structures can serve as antigens for instigating cellular or humoral immune responses. Defined structural features of proteins, carbohydrates, and some lipids can engender adaptive immune responses. Rather than hundreds or thousands of structures as triggers, adaptive immunity can recognize millions or even billions of structures. The “education” of the adaptive immune response helps to limit recognition of self structures, to minimize autoimmune responses. Also, the affinity of antibodies and specificity of cellular responses to antigens can increase with time. Thus, in contrast to the innate immune response, adaptive immunity shows exquisite specificity and considerable power, given the fine-tuning of adaptive immune responses over time after encountering an initial antigenic stimulus.
While the mononuclear phagocyte exemplifies the main cellular mediator of innate immunity, lymphocytes give rise to the adaptive immune response. T lymphocytes comprise the afferent limb of cellular immunity. Several important subclasses of T lymphocytes subserve specific functions. T helper 1 (Th1) cells, characterized by secretion of a signature cytokine immune interferon or interferon-gamma (IFN-γ), promote adaptive immune responses. Th2 lymphocytes tend to mediate allergic responses. Th2 cells secret interleukin (IL) -4 and -10 as signature cytokines. Regulatory T cells (T reg ) secrete transforming growth factor-beta (TGF-β) and can calm immune responses and promote tissue repair through fibrosis. B lymphocytes mediate humoral immunity and give rise to plasma cells that produce large quantities of antibody. In an exception to the rule that adaptive immune responses require “education,” B1 lymphocytes secrete “natural” antibodies, some of which can mitigate experimental atherosclerosis. B2 lymphocytes tend to aggravate adaptive immune responses.
Although innate immunity evolved before the adaptive response, adaptive immune factors regulate innate immunity. Innate immune cells such as the dendritic cell (a relative of the mononuclear phagocytes) serve to present antigens to the T cell, initiating the adaptive immune response. IFN-γ elaborated by Th1 cells can activate macrophages strongly. Cytokines derived from Th2 cells such as IL-10 can dampen innate immune responses mediated by macrophages. These examples provide an illustration of the complex crosstalk underway in any moment in complex organisms ( Fig. 7.1 ). The inflammatory status of an individual can vary immensely and depends on an intricate balance of proinflammatory, antiinflammatory, and proresolving pathways.
In acute bacterial infections the rapidly mobilized innate immune system responds rapidly, causing fever and a series of host defense mechanisms that help to fight off the invaders including mobilization of polymorphonuclear leukocytes. The extreme example of an acute inflammatory response, Gram-negative bacterial sepsis, familiar to all clinicians, illustrates the rapidity and devastating nature of an undampened acute inflammatory response. Fulminant myocarditis illustrates another acute inflammatory response that affects the cardiovascular system.
On the other end of the spectrum, the chronic conditions that plague the cardiovascular system often involve much more muted inflammatory responses that play out over months and years rather than hours and days as in the case of acute inflammation. An example of a chronic immune response mediated by macrophages, tuberculosis, illustrates the long-term and indolent nature of the chronic immune response. The intersection of innate and adaptive immune responses that give rise to chronic diseases of many organ systems, including atherosclerosis, exemplifies the ravages of a chronic immune response in the cardiovascular system.
Stimuli for the Inflammatory Response
A number of stimuli can initiate inflammatory responses in the context of cardiovascular disease ( Fig. 7.2 ). Atherosclerosis represents the best-studied chronic inflammatory condition of the cardiovascular system. Although low-density lipoprotein (LDL) doubtless plays a pivotal permissive role in atherosclerosis, it does not appear to exert most of its proatherogenic actions primarily by instigating inflammation. While macrophages of the innate immune system accumulate lipid and become foam cells when exposed to excessive concentrations of atherogenic lipoproteins, unmodified LDL itself does not lead to lipid overload. The LDL receptor responds exquisitely to intercellular cholesterol concentrations. Hence, loading cells with cholesterol requires uptake of cholesterol-containing lipoproteins via receptors besides the classical LDL receptor. Such scavenger receptors tend to recognize modified LDL particles that have undergone oxidation or glycation. Foam cell formation may thus depend less on native LDL than on cholesterol derived from other atherogenic lipoproteins.
Although initial evidence pointed to oxidized LDL as a possible antigen triggering the adaptive immune response in the context of atherosclerosis, more recent data suggest that the T cells recognize native LDL more readily than modified LDL. Finally, an intervention that lowers LDL by augmenting activity of the LDL receptor, inhibitors of proprotein convertase subtilisin/kexin type 9 (PCSK9), can mitigate atherosclerotic events without decreasing biomarkers of inflammation. This observation underscores the contention that LDL itself has modest proinflammatory properties at best.
Hypertension and mediators of high blood pressure can involve both adaptive and innate immunity. High concentrations of angiotensin II, a prototypical vasoconstrictor hormone implicated in the pathogenesis of many forms of hypertension, can elicit the production of the proinflammatory cytokine IL-6 from various cell types involved in atherosclerosis, including smooth muscle cells. Considerable experimental evidence supports the involvement of adaptive immunity in hypertension. Yet, antiinflammatory drugs tend not to improve blood pressure in hypertensive individuals. Indeed, nonsteroidal antiinflammatory agents tend to increase blood pressure modestly. These findings argue against inflammation as a major contributor to chronic forms of hypertension.
Many have invoked oxidative stress, and oxidatively modified lipoproteins in particular as instigators of both innate and adaptive immune responses in the context of atherosclerosis. Yet, all antioxidant vitamins tested, and a number of inhibitors of oxidative pathways including the production of oxidized LDL, have failed to reduce cardiovascular events in rigorously conducted clinical trials. Once again, despite considerable preclinical evidence and the results of observational studies, oxidative stress and lipoprotein oxidation have not shown promise as therapeutic targets. These observations demote the clinical relevance of oxidative pathways as instigators of immune and inflammatory responses.
A large body of experimental and observational epidemiologic literature support associations between various infectious agents and atherosclerosis. While viral myocarditis indubitably represents an example of an inflammatory trigger for cardiovascular disease, targeting infectious agents has not proven actionable in general in atherosclerosis. While bacteria and viruses can stimulate both innate and adaptive immunity, and microbial products can serve as PAMPs, rigorously conducted and appropriately sized antibiotic intervention studies have failed to show reduced cardiovascular events. These studies have used several classes of antibiotic agents that target microorganisms implicated by experimental and seroepidemiologic studies in atherosclerosis, e.g., Chlamydia pneumoniae. The types of agent used in the larger randomized clinical trials include macrolides (e.g., azithromycin) and fluoroquinolones (e.g., gatifloxacin). While various viruses, notably Herpesviridae, can inhabit many human tissues including atheromata, rigorous evidence implicating viral agents as triggers to innate and adaptive immunity in usual forms of human atherosclerosis has not emerged. Experimentally, a herpes virus can cause an atherosclerotic-like disease in avian species (Marek disease), and cytomegalovirus can also enhance arterial disease in rodents.
Considerable recent interest has highlighted the potential of the microbiome as a contributor to cardiovascular disease. The clinical extrapolation of intriguing experimental results showing the ability of intestinal microbiota to produce metabolites that putatively potentiate atherosclerosis, such as trimethylamine N-oxide (TMAO), to humans still requires reinforcement.
Tissue damage can instigate innate immune responses through the production of DAMPs ( Fig. 7.1 ). Experimental evidence substantiates the possibility that DAMPs elaborated from the infarcted myocardium can augment innate immune responses. For example, dying cardiac myocytes can release DNA that can initiate an inflammatory response through a pathway mediated by interferon regulatory factor 3 (IRF3). Myocardial infarction can mobilize systemic inflammatory responses that can augment remote inflammatory responses, including in preexisting atherosclerotic lesions. These pathways involve mobilization of macrophages and their activation in response to tissue injury ( Fig. 7.3 ). These examples show how ischemic damage to myocardium produced by preexisting atherosclerotic plaques can elicit and amplify immune responses. Yet, these pathways likely participate in the potentiation of inflammatory responses to preexisting disease, rather than proving pathogenic in initiation of primary atherosclerotic plaques.
A good deal of recent work has firmly established that adipose tissue can contribute to inflammatory states. In particular, ectopic fat deposition around the viscera, associated with the android fat distribution (male or “apple” pattern), associates with markers of innate immune activation such as C-reactive protein (CRP). Such ectopic depots of adipose tissue team with inflammatory cells including macrophages and T lymphocytes. Ectopic adipose tissue elaborates proinflammatory mediators such as tumor necrosis factor that can mediate insulin resistance. Perivascular adipose tissue may participate in local “outside in” inflammatory signaling that can potentiate vascular disease. Thus, weight loss either pharmacologic or through bariatric surgery might mitigate inflammation in cardiovascular diseases. The use of pharmacologic agents in this regard has proved quite challenging due to adverse or off-target effects. For example, certain classes of weight loss drugs can produce pulmonary hypertension or valvular heart disease. The thiazolidinediones that activate peroxisome proliferation activation receptor gamma (PPAR-γ) may cause a redistribution of adipose tissue away from the visceral depot, but also can cause fluid retention and exacerbate heart failure. These limitations have frustrated pharmacologic management of adiposity and as antiinflammatory strategies in cardiovascular disease.
Exposure to foreign tissues, such as transplanted organs, can also lead to activation of the immune response ( Fig. 7.4 ). Such adaptive immune reactions to foreign tissues known as the allogeneic immune response, clearly contribute to rejection of solid organ transplants. This is one arena where therapies that mitigate immune and inflammatory responses have proven of daily applicability in the practice of cardiovascular medicine. Allograft rejection can be hyperacute, acute, or chronic. Preformed antibodies against donor determinants mediate hyperacute rejection, a process that occurs within minutes to hours after transplantation. This form of allograft rejection has become relatively rare due to the practice of prospective cross-matching in sensitized recipients. Acute rejection can be cellular or humoral. In acute cellular rejection, T cells directed against the donor myocardium trigger an inflammatory response that leads to myocyte necrosis and graft failure. Antibodies directed against the graft vasculature mediate acute humoral rejection. This type of rejection can lead to local complement activation, vessel damage, and graft failure. Allograft vasculopathy also called “chronic rejection” results from immune and nonimmune responses against the graft vasculature and leads to diffuse and concentric narrowing of donor coronary arteries including smaller intramyocardial branches ( Fig. 7.3 ). We prefer the term allograft vasculopathy to chronic rejection, as the major immunological mechanisms differ substantially. CD8 + T-cell–mediated myocardiocytolysis characterizes acute cellular parenchymal rejection. In contrast, CD4 + T cells likely dominate in the pathogenesis of proliferative lesions in the arterial intima of graft vasculopathy. Acute cellular and humoral rejection contribute to early transplant death, while allograft vasculopathy typically causes later transplant mortality.
Transplant immunosuppression can be divided into induction and maintenance therapies ( Table 7.1 ). Induction therapies, including antithymocyte globulin or basiliximab, may be given for a limited duration, shortly after transplant, to allow earlier reduction of steroid dosage or to delay introduction of calcineurin inhibitors (CNIs) in patients at risk for nephrotoxicity. Antithymocyte globulin or basiliximab may reduce the risk of acute rejection without substantially altering posttransplant survival or complications. Maintenance immunosuppression consists of lifelong therapy with some combination of corticosteroids, CNIs (cyclosporine or tacrolimus), and antimetabolites (azathioprine or mycophenolate mofetil). Mammalian target of rapamycin (mTOR) inhibitors (everolimus and sirolimus) can be used in combination with low-dose CNI, or after withdrawal of CNI, to reduce progression of allograft vasculopathy or CNI-induced nephrotoxicity. While these potent inhibitors of adaptive immune responses have greatly attenuated acute allograft rejection, they have not allayed the chronic inflammatory response that begets the allograft vascular disease that remains a major challenge to the longevity of transplanted hearts.
| Drug | Mechanism of action | Indications | Potential side effects |
|---|---|---|---|
| Antithymocyte globulin (rabbit or horse) | Polyclonal antibodies that deplete T cells, modulate adhesion and cell-signaling molecules, interfere with dendritic cell function, induce B-cell apoptosis and regulatory and natural killer T-cell expansion |
| Neutropenia, thrombocytopenia, anaphylaxis, severe cytokine release syndrome, hyperkalemia, infection |
| Basiliximab | Anti-IL-2 receptor monoclonal antibody that prevents IL-2 mediated T-cell proliferation | Prophylaxis of acute rejection | Anaphylaxis |
| Corticosteroids | Inhibit crucial transcriptional regulators of inflammatory genes, including NF-κB and AP-1. |
| Volume retention, hypertension, hyperglycemia, obesity, mood and behavioral changes, infection, osteopenia, avascular necrosis, gastritis/perforation, myopathy, cataracts |
| Calcineurin Inhibitors (cyclosporine and tacrolimus) | Inhibit calcineurin leading to reduced IL-2 production and decreased T-cell proliferation | Prophylaxis of acute rejection | Cyclosporine : Renal dysfunction, hypertension, tremor, hirsutism, gingival hyperplasia, infection. Tacrolimus: Renal dysfunction, hypertension, tremor, hyperlipidemia, diabetes, infection |
| Anti-metabolites (azathioprine, mycophenolate mofetil) | Inhibit de novo purine synthesis and thus limit T- and B-cell proliferation | Prophylaxis of acute rejection | Azathioprine: Leukopenia, infection, malignancy, teratogenic Mycophenolate mofetil: Neutropenia, pure red cell aplasia, infection, malignancy, teratogenic, progressive multifocal leukoencephalopathy |
| mTOR inhibitors (everolimus, sirolimus) | Inhibit mTOR, a key regulatory protein required for cytokine driven T-cell proliferation. Also inhibits antibody production. | Prophylaxis of acute rejection | Stomatitis, hypertriglyceridemia, proteinuria, renal dysfunction, diarrhea, rash, infection, noninfectious pneumonitis |
For reasons that remain unclear, self-antigens can occasionally trigger the immune response leading to myocarditis. Cardiac infiltration by inflammatory cells resulting in myocardial necrosis characterizes the myocardidites. Myocarditis can have many etiologies including viral (Coksackievirus B, parvovirus, and adenovirus being among the most common), pharmacologic (e.g. anthracyclines), hematologic (e.g. eosinophilic myocarditis), and autoimmune (e.g., giant cell myocarditis).
Knowledge regarding the immunobiology of myocarditis emerged from two types of animal experiments: (1) viral myocarditis caused by infection of mice with Coxsackievirus B3 and (2) experimental autoimmune myocarditis caused by immunization of mice with cardiac myosin or a myocarditogenic peptide derived from cardiac α-myosin heavy chain, or forced expression of ovalbumin in the myocardium in mice with ovalbumin responsive T cells.
In viral myocarditis, myocardial injury results from a direct viral cytopathic effect as well as activation of the host cellular immune response. During acute infection, NK cells infiltrate the myocardium and play a critical role in the early host response by preventing viral replication. A second wave of infiltrating leukocytes, composed primarily of CD3 + T cells that colocalize with CD68 + macrophages, peaks 7–14 days after viral infection. They destroy infected myocytes to promote viral clearance, and the ensuing damage exposes cryptogenic intracellular antigens, such as myosin-derived peptides, that generate an autoimmune cardiac- specific response leading to chronic inflammation, fibrosis and progression to dilated cardiomyopathy. Similarly in other forms of autoimmune myocarditis, unknown triggers, combined with specific host factors, activate the innate immune response leading to myocyte damage and exposure of cryptic antigens that are recognized by autoimmune T-cell clones leading to sustained activation of the adaptive immune response and progressive myocardial damage.
The therapeutic approach to myocarditis has relied on immunosuppressive therapy as a way to target proinflammatory mediators of disease. The Myocarditis Treatment Trial failed to show a benefit of corticosteroids, in combination with cyclosporine or azathioprine, in patients with histologically proven lymphocytic myocarditis and left ventricular dysfunction compared to placebo. The failure of immunosuppressive therapy in this study may have resulted from suppression of the beneficial effects of the innate immune response on viral replication. Another study evaluated the effects of corticosteroids with azathioprine in patients with histologically proven myocarditis and chronic heart failure with no evidence of myocardial viral genomes. In this study, immunosuppression resulted in a significant improvement in left ventricular function and volumes compared to placebo. Clinically, treatment of myocarditis does not routinely use immunosuppression, save for patients with giant cell myocarditis where registry data indicate that the use of immunosuppression improved mean survival from 3 to 12.3 months. However, only 11% of patients survived without transplantation. Recently, the rising use of immune checkpoint inhibitors for the treatment of solid tumors has also given rise to T-cell–mediated myocarditis in < 1% of patients. Despite treatment with high-dose immunosuppressive therapy, almost half of these patients suffer major adverse cardiovascular events including cardiovascular death, cardiogenic shock, cardiac arrest, and hemodynamically significant complete heart block.
Another arm of innate immunity, the complement system, also participates in certain vasculitides. A large body of experimental work has implicated complement activation in ischemia-reperfusion injury. Yet, anticomplement strategies have not proven productive in the clinical management of either vasculitis or reperfusion injury in humans.
A novel aspect of inflammation has emerged from studies of clonal hematopoiesis ( Fig. 7.5 ). With age, humans often host clones of myeloid cells, classical participants in innate immunity, that bear mutations in genes implicated in driving acute leukemia. Over 10% of individuals over 70 will harbor such clones of mutant leukocytes. The development of acute leukemia generally requires successive accumulation in the same clone of two or three mutations in leukemia driver genes. Thus, most individuals with clonal hematopoiesis will never develop leukemia. Yet, these individuals have a striking enrichment in cardiovascular risk. This condition is called clonal hematopoiesis of indeterminate potential (CHIP). As in the case of monoclonal gammopathy of unknown significance (MGUS), most individuals with CHIP will never develop a hematologic malignancy.
