Innate immunity

Cells of the innate immune system include neutrophils, monocytes (and their descendants, tissue macrophages), dendritic cells, and natural killer cells. These cells, to varying degrees, carry out functions including the recognition of pathogens, phagocytosis of these pathogens, intracellular killing, and presentation of digested antigens on cell surface molecules to facilitate activation of lymphocytes. In addition, innate immune cells elaborate cytokines and chemokines that modulate the local environment and/or recruit other immune cells to the area of infection or injury.

Innate immune cells are typically the first cellular elements to become activated in the face of an inflammatory stimulus. This occurs by virtue of their constitutive expression of cell surface receptors for pathogen-associated molecular patterns or endogenous damage-associated molecular patterns. These receptors can be activated by broad classes of ligands, an example of which is toll-like receptor 4, which is a key part of the pathogen-associated molecular patterns receptor for lipopolysaccharide (LPS). Thus, LPS should result in robust activation of innate immune cells upon their first exposure without requiring antigen presentation or immunologic memory. This vigorous and prompt response is responsible for the central role that innate immune cells are thought to play in the early and fulminant presentation of acute inflammatory conditions such as sepsis.

Adaptive immunity

Lymphocytes comprise the adaptive arm of the immune system. Most lymphocytes can be classified as T or B cells, with T cells being responsible for cytokine production (in the case of CD4 ⁺ cells) and cytotoxicity through the elaboration of lytic enzymes (in the case of CD8 ⁺ cells). B cells, once activated, differentiate into plasma cells, which are responsible for antibody production. Lymphocytes typically require the presentation of antigenic peptides by members of the innate immune system in order to become activated, with the notable exception of superantigens, which can directly activate lymphocytes. The T- and B-cell responses are generally highly antigen specific as the result of gene rearrangement within the T- and B-cell receptors. Accordingly, only a very small percentage of naïve lymphocytes are capable of responding to a given antigen. Last, once activated, a lymphocyte must clonally expand to mount a maximal immune response. Repeat exposure to an antigen, however, results in faster propagation of the immune response as the result of memory cells, which can persist for decades. While the lymphocyte may not be responsible for the initiation of the inflammatory response in many forms of critical illness, it can perpetuate and modulate that response in ways that can be beneficial or harmful to the host.

Systemic inflammatory response syndrome in critical illness

The elaboration of proinflammatory cytokines, including tumor necrosis factor (TNF)-α and interleukin (IL)-1β, and chemokines, such as IL-8, occurs rapidly following activation of most innate immune cells. These cytokines act to produce fever, vasodilation, and increased capillary permeability. Chemokines serve to promote recruitment of other immune cells to the area along a concentration gradient. These effects are beneficial when confined to a localized area of infection, allowing for increased delivery of leukocytes to these areas and enhanced killing of pathogens. When these effects become systemic, however, they become pathologic, leading to intravascular volume depletion, organ malperfusion, acidosis, organ failure, and death. Regardless of whether the insult prompting this activation is infectious (e.g., sepsis) or noninfectious (e.g., trauma, cardiopulmonary bypass, pancreatitis), the magnitude of the inflammatory response is frequently associated with adverse clinical outcomes.

Innate proinflammatory cytokines, such as TNF-α and IL-1β, peak early and have relatively short half-lives in the plasma. Often, by the time a patient comes to medical attention following an acute proinflammatory insult, the plasma levels of these cytokines are waning. IL-6 is made by multiple cell types, including immune and nonimmune cells in response to stress and/or exposure to proinflammatory cytokines. Therefore, plasma IL-6 levels remain elevated for far longer than those of TNF-α or IL-1β following the onset of a proinflammatory stimulus. Numerous investigators have shown that elevated plasma IL-6 levels predict mortality from sepsis and trauma in adults and children. However, it is important to understand that IL-6 is itself a pleiotropic cytokine. While it is a potent inducer of the hepatic acute-phase response, IL-6 also has antiinflammatory properties, including promoting production of endogenous glucocorticoids and antiinflammatory cytokines. IL-8 is a potent neutrophil chemokine whose levels in plasma can also be persistently elevated following an inflammatory insult. Low systemic IL-8 levels have been shown to predict favorable outcomes from pediatric septic shock while elevated IL-8 levels have been associated with increased mortality from pediatric trauma.

The use of plasma biomarkers such as IL-6 and IL-8 has the potential to identify high- or low-risk subjects for inclusion in clinical trials in the intensive care unit (ICU). For example, in one of the few positive phase III clinical trials of anticytokine therapy in adult patients with severe sepsis, anti-TNF antibody fragment therapy was associated with reduced mortality in subjects with plasma IL-6 levels greater than 1000 pg/mL.

Ferritin is another biomarker of the proinflammatory response that has been shown to be associated with mortality risk. While ferritin is an acute-phase reactant that is produced by the liver in response to IL-6 and other proinflammatory cytokines, it is also produced by activated macrophages and other immune cells. Marked hyperferritinemia is characteristic of macrophage activation syndrome and hemophagocytic lymphohistiocytosis, but it can also be seen in sepsis. Concurrent elevations in serum ferritin and C-reactive protein have been strongly associated with mortality in septic children.

Given the complexity and redundancy of the proinflammatory cascade, it is possible that a combinatorial approach may be superior to single biomarkers in predicting outcomes. Wong et al. have repeatedly demonstrated high sensitivity and specificity in predicting pediatric sepsis mortality using a panel of biomarkers, including IL-8, C-C chemokine ligand 3, heat shock protein 70, granzyme B, and matrix metalloproteinase-8. It remains unclear as to whether specific plasma biomarkers such as these represent potential direct mediators of harm versus indirect measures of the proinflammatory response. However, the relationship between the magnitude of this response and clinical outcomes from critical illness appears to be consistent.

Compensatory antiinflammatory response syndrome in critical illness

Within minutes of activation of the proinflammatory response, immune cells begin elaborating counter-regulatory antiinflammatory cytokines. These include molecules such as the soluble TNF receptor and IL-1 receptor antagonist, which impair the functionality of the proinflammatory cytokines. In addition, mediators such as IL-10 and transforming growth factor-β are produced that directly inhibit proinflammatory cells and/or repolarize them to an antiinflammatory/suppressed phenotype. Much as elevated plasma levels of proinflammatory cytokines are associated with adverse outcomes from critical illness, a similar relationship exists between antiinflammatory cytokine levels and clinical outcomes. High IL-10 levels have been associated with increased risks for secondary infection and/or mortality in the settings of adult and pediatric sepsis, ^{, ,} adult and pediatric trauma, ^, and pediatric cardiopulmonary bypass.

Immune cell death represents another important counter-regulatory response. Neutrophils typically die upon release of their cargo of lytic enzymes and free radicals. In addition, lymphocyte apoptosis has repeatedly been shown to occur in the setting of adult and pediatric sepsis. Phagocytosis of lymphocyte apoptotic bodies has also been shown to induce profound hyporesponsiveness of innate immune cells, demonstrating another example of innate and adaptive immune cell crosstalk.

While plasma cytokine levels reflect the systemic inflammatory milieu, they may not necessarily reflect leukocyte function, since cytokines are produced by parenchymal and endothelial cells in addition to white blood cells. Innate immune cell function has repeatedly been shown to be abnormal in many patients following the onset of critical illness. Human leukocyte antigen DR isotype (HLA-DR) is a class II major histocompatibility complex molecule present on the surface of antigen-presenting cells. These molecules display digested peptides on the innate immune cell’s surface for presentation of these antigens to adaptive immune cells. Monocyte HLA-DR expression is known to be reduced in the setting of sepsis, likely due to internalization of HLA-DR in subsurface vesicles. The phenomenon of reduced-monocyte HLA-DR expression has been demonstrated in adults and children in the aftermath of conditions as varied as trauma, cardiopulmonary bypass, multiple-organ failure, and pancreatitis.

The capacity of innate immune cells to respond to a new challenge has also been shown to be an important part of the compensatory antiinflammatory response syndrome (CARS) response. Specifically, the ability of blood samples to produce TNF-α when stimulated ex vivo with LPS has been shown to be reduced in many patients with critical illness. Ex vivo LPS-induced cytokine production capacity should be high in the healthy, immunocompetent state with innate immune cells (notably, monocytes) producing the majority of the measured TNF-α. Downregulation of innate immune cell responsiveness can be marked and persistent in critically ill patients. This may seem counterintuitive given the proinflammatory nature of many clinical phenotypes seen in the ICU. The simultaneous presence of high proinflammatory cytokine levels in the plasma and reduced cytokine production capacity in innate immune cells may be explained by the fact that cytokines can originate from injured tissues rather than from circulating leukocytes. Reductions in monocyte HLA-DR expression and/or ex vivo LPS-induced TNF-α production capacity that are severe and persistent following the onset of critical illness have been collectively termed immunoparalysis .

Last, specific cell types may play a role in the development and/or perpetuation of the immunoparalyzed phenotype. For example, work in critically ill adults has shown that the highly immunosuppressive regulatory T (T _reg ) cell, characterized by the CD4 ⁺ CD25 ⁺ FoxP3 ⁺ CD127 ^lo phenotype, can predominate in the subacute phase of sepsis. These T _reg cells are known to produce large quantities of IL-10 and transforming growth factor-β and are felt to be resistant to the wave of lymphocyte apoptosis that occurs in sepsis. T _reg cells have not been shown to be prevalent in pediatric sepsis, however. Recent evidence also points to the elaboration of myeloid-derived suppressor cells (MDSCs) from the bone marrow of critically ill adults. These MDSCs have the potential to produce high levels of antiinflammatory cytokines and facilitate T-cell suppression. They may also potentiate critical illness-induced immunosuppression, though their role in pediatric critical illness is unclear.

Temporal aspects of the systemic inflammatory response syndrome/compensatory antiinflammatory response syndrome response

The original paradigm of the SIRS/CARS response involved the presence of two temporally distinct phases of illness in which the proinflammatory phase of the SIRS was followed, over hours or days, by the antiinflammatory CARS phase. It is now appreciated that these two responses can and do coexist in the critically ill patient ( Fig. 105.1 ). This is evidenced by the frequent occurrence of “cytokine storm” in which both proinflammatory and antiinflammatory mediators are elevated in the plasma simultaneously. ^, Gene expression studies, using messenger RNA (mRNA) from circulating leukocytes, have demonstrated upregulation of innate immune signaling pathways and downregulation of adaptive immune pathways in the early phases of sepsis and trauma. Studies of functional measures of immunity (e.g., monocyte HLA-DR expression, ex vivo LPS-induced TNF-α production capacity, and phytohemagglutinin-induced cytokine production capacity) suggest that leukocyte function can be reduced in both arms of the immune system within hours of critical illness onset. ^, As noted earlier, the initial peak of the leukocyte inflammatory response may have occurred before the patient is admitted to the ICU. Similarly, the CARS response may be well established by the time of ICU admission despite persistently high levels of circulating proinflammatory mediators in the aftermath that follows the onset of critical illness.

Temporal aspects of the SIRS/CARS response. After the onset of critical illness, there is typically a surge in systemic inflammation associated with the SIRS response (pink) . A compensatory downregulation of immune cell function often occurs concomitant with this (blue) . Complicated outcomes (e.g., secondary infection, prolonged organ dysfunction, and death) are frequently associated with severe or persistent SIRS and CARS responses. Milder and more transient SIRS and CARS responses are associated with uncomplicated outcomes. CARS, Compensatory antiinflammatory response syndrome; SIRS, systemic inflammatory response syndrome.