While the role of different cytokines in AP has been extensively studied, the role of cellular immunity is poorly evaluated. Innate immune cells are the major leukocyte population in the inflamed pancreas.
Natural killer cells
Natural killer (NK) cells are predominantly studied in response to infection and immunosurveillance against tumours. They are part of the innate immune system, giving them the ability to respond without prior sensitisation. They also carry certain abilities of adaptive immunity, as they are primed during development, their receptors can exhibit antigen specificity, they undergo clonal expansion during infection and generate long-lived memory cells. Natural killer cells can undergo clonal-like expansion through specific and non-specific immune responses. While the specific response occurs via interaction of their activating receptors with viral antigens, the non-specific response is driven by the production of cytokines and proliferation following exposure to proinflammatory cytokines in the absence of TCR signals and co-stimulation[39,40]. Natural killer cells have immunological memory, which enables them to react faster and more aggressively in familiar surroundings. The most important cytokines produced by NK cells after activation are TNF-α and IFN-γ. It is thought that NK cells that produce proinflammatory cytokines can contribute to dysregulation of the immune response as seen in sepsis. The cytokine IL-15 pays a role in the maintenance of NK cells. The half-life of mature NK cells is about 1 wk, but in the absence of IL-15 they disappear in 48 h. These cells can also serve as an immunotherapeutic target.
Dabrowski et al reported significant depletion of the NK cell population on the first day of severe AP, while there was no significant change in NK cell number in mild AP. These findings are consistent with the idea that severe forms of AP are related to immune suppression. Profound inhibition of innate cell immunity can be explained by the migration of NK cells and natural killer T (NKT) cells to the site of inflammation.
The most important anti-inflammatory cytokine is interleukin (IL)-10. It down-regulates the production of proinflammatory cytokines and the expression of HLA-DR on monocytes. If the compensatory anti-inflammatory response is too intense, however, it may lead to immunosuppression and complications including infection. The concentration of IL-10 is highest in the early phase of severe AP. As infection is considered to be one of the prognostic factors related to disease severity, IL-10 may be a promising predictive marker of organ failure. There are conflicting reports for the use of IL-10 in the prevention of post-ERCP AP. In a randomised double-blind study, Deviere et al showed a reduced incidence of post-ERCP AP after IL-10 usage, although this was not supported by a study by Dumot et al.
As a key proinflammatory mediator, IL-6 regulates the synthesis of acute-phase proteins in the liver as well as macrophage-conditioned tissue damage. It reaches its peak value 24-48 h after clinical expression. In necrotising pancreatitis, the peak levels of IL-6 occur after 24 h. Minkov et al concluded that IL-6 represents an independent factor for predicting severity in acute non-ERCP pancreatitis.
The highest values of C-reactive protein (CRP) are recorded after 48–72 h, which is later than that of IL-6. Although CRP has been identified as a late marker in laboratory monitoring, Messman et al found that both IL-6 and CRP peak earlier in patients with ANP.
IL-1β-mediated signalling is required for full pancreatic and distal organ injury and inflammation, and is the pivotal inflammatory mediator in cell death associated with sterile inflammation. Serum levels of IL-1β do not correlate with AP severity in humans, although it has been found that the values peak after 24 h and are greater in patients with severe AP compared to mild AP. In animal models, peak serum IL-1β precede peak serum IL-6 values[50,53]. It is possible that IL-1β is required for the induction of IL-6 production, which is strongly correlated with disease severity in humans. IL-1β and TNF-α are considered the primary cytokines that initiate and propagate most of the consequences of the SIRS in AP[55,56]. IL-6 prevents the synthesis of IL-1β and TNF-α.
Kilciner et al compared early changes (within 24 h) in the serum levels of IL-2, IL-4, TNF-α and IL-6 in the development of post-ERCP pancreatitis. They used patients who underwent ERCP as well as a control group consisting of patients with non-ERCP AP caused by gallstones, drugs or alcohol. They found that IL-4, an anti-inflammatory cytokine, was significantly lower in post-ERCP and non-ERCP AP patients compared to patients who did not develop pancreatitis. The TNF-α level was not significantly different after 24 h in patients who developed PEP compared to those who did not develop pancreatitis after ERCP. After 24 h, the IL-6 levels did not differ from the control group, but they were significantly higher compared to patients who did not go on to develop pancreatitis after ERCP.
The role of IL-18 may depend on the presence of other cytokines. It plays an important role in the local immune response to pancreatic injury, and can also be found in serum. It has been described to prime NK cells, and NK cells that were unable to receive IL-18 signals were found to have defective cytotoxicity and cytokine secretion after stimulation.
AP is the most frequent complication after the ERCP procedure. Although the incidence of AP after ERCP is low, it is reported to occur in 0.5% of patients, PEP has a greater severity index compared to non-ERCP AP. As the mild form of PEP is not a clinically relevant condition, it would be useful to identify early markers to predict whether a patient will develop the severe form of PEP.
The serial changes in amylase and lipase levels in patients without PEP suggest the existence of subclinical pancreatic damage. Messmann found that amylase and lipase levels increased equally among all patients after ERCP. Amylase and lipase are released into the systemic circulation due to disturbed transport and increased ductal permeability; however, they are not thought to be responsible for inducing further inflammation. Based to these findings, we conclude that serum amylase values can’t serve as an adequate future therapeutic goal.
The role of cytokines, especially IL-10, IL-6 and TNF-α, have been extensively studied for the prediction of disease severity[45,48,55,56]. These cytokines can be used to predict the severity of PEP after 12-24 h; however, measurements taken 4 h after the procedure showed no significant difference between patients who developed PEP and those who did not develop PEP[51,58].
Further research on the initial inflammatory response is necessary, particularly as organ failure has been reported to occur earlier in severe forms of AP, either at admission or 14 h later. Furthermore, in PEP, organ failure occurs twice as fast than in non-ERCP AP. Direct comparison of the initial inflammatory response between PEP and non-ERCP AP would be of significant importance to clarify these statements. Found difference in clinical response to initial injury might be explained by different initial immune response.
Infection is considered to be the most important prognostic factor for disease severity. Similarities between cytokines and inflammatory mediators in sepsis and AP are often compared. Kjaergaard et al reported that the expression of NKG2D receptors on NK cells and CD14 on monocytes can be valuable prognostic markers of an unbalanced immune response, and may predict a worse outcome for critically ill patients. Also, Guo et al presented natural killer cells as critical to eliminate pathogens during the early phase of sepsis and prevent patients from developing secondary infection. We suggest that similar components should be used in PEP and non ERCP AP.
In addition to searching for adequate biomarkers to assess disease severity, it is our opinion that novel therapeutic strategies for both of these conditions lie in uncovering the immune pathways.