The Relationship Between Sepsis-induced Immunosuppression and Serum Toll-like Receptor 9 Level

Discussion

Today, despite all research, the usage of modern antibiotics and advanced resuscitation methods in intensive-care units, sepsis is still seen at a high frequency and is one of the most important causes of death in hospitals and intensive care units (9-11). The pathogenesis of the sepsis process is tremendously complex and involves a dynamic interplay between the pathogen and the host immune system. In order to reduce sepsis-induced morbidity and mortality, it is essential to understand the molecular mechanisms associated with development of sepsis and sepsis-linked organ injury (7, 8).

The immune system consists of two parts: natural and acquired. The innate immune system is an essential form of host defense against infections. TLRs are recognized as a component of the natural immune system. TLRs are transmembrane proteins. They have a considerable role in host immune system by both creating a host natural immune response against many pathogens and activating the gained immune response (12, 13). The stimulation of TLR receptors by pathogenic microbial substances leads to the activation of many inflammatory cytokines and downstream inflammatory pathways (14, 15). This is the host immune inflammatory response.

Additionally, TLR9 is required for optimal activation of bactericidal activity. It carries the pattern of recognition for microbial DNA. TLR9 is essential for the uptake and intracellular killing of the bacteria during infection with K. pneumoniae (16). Some studies defined the role of TLR9 as recruiting and activating leukocytes, including dendritic cells and macrophages (17-18). Similar results for Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis have been published in other studies, where TLR9 has critical functions in the generation of the IL12/interferon-γ response (19-21).

There is a hyperinflammatory response during the initial phase of sepsis. During this period, the complement system is activated and TLRs are highly expressed on monocytes and macrophages (22). Termination of this hyperinflammatory response and restoration of the host immune system are mediated by compensatory anti-inflammatory cytokines after control of infection. This compensatory anti-inflammatory response syndrome appears in the majority of patients with sepsis (14). Indeed, the host immune system produces two types of responses against the sepsis. These two immunological responses are consecutive and overlapping phases. If the compensatory anti-inflammatory response is ineffective, the inflammatory reaction can result in destruction of the host cells and became injurious to the body overall. This may influence sepsis-related organ injury or dysfunction and eventual mortality (23-25). On the other hand, deterioration in the regulation of control mechanisms during sepsis may lead to loss of control of inflammation due to excessive activation or extreme suppression of the inflammatory response. For instance, as a result of the long duration of infection or the inability for it to be controlled by antibiotics and other treatment methods, it is possible to switch to the anti-inflammatory state. This is the period of immuno-paralysis or immuno-suppression and is a hallmark of sepsis. These two extreme immune responses in the fight against infection, inflammatory and anti-inflammatory processes, can ultimately harm the patient (26).

It is known that sepsis affects immune responses leading to immunosuppression. Studies demonstrated that sepsis impairs host immune system homeostasis (27, 28). This is an important pathological mechanism in sepsis and is one of the important breakpoints in sepsis-related death (29, 30). In patients, secondary infection or herpes virus reactivation may develop during the sepsis-associated immunosuppressive stage (31). In our study, infection due to multiple pathogenic microorganisms was detected at a high rate in patients with sepsis.

In a septic-shock study, investigators found that 35% of patients had a down-regulation of TLR gene expression compared to their baseline values (32). We are of the opinion that the low levels of serum TLR9 detected in our study were because of down-regulation in these sepsis cases. The decrease in the serum level of TLR9 in the sepsis group in our study can be explained by the fact that the patients had severe sepsis. Again, to stress the point, TLR9 levels in our study were measured only in patients with serious sepsis and septic shock. TLR9 levels may be reduced due to compensatory anti-inflammatory condition in an advanced stage in severe cases of sepsis. Additionally, there was also a statistically significant relationship between these low values and mortality in our study.

Myocardial functions are also affected by various mechanisms in sepsis. Cardiovascular dysfunction is a serious complication in about 40% of patients with sepsis and increases mortality by 20-70%. TLR2 and TLR4 particularly contribute to cardiac dysfunction (33, 34). Nevertheless there is little information on the role of TLR9 in myocardium, but TLR9 expression in the heart has been shown (35). It is also known that TLR9 stimulation leads to nuclear factor ĸB activation in different tissues (36). In one study, it was found that bacterial DNA increased myocardial cytokine production via TLR9 and caused depletion of cardiomyocyte contractile ability (37).

In addition to hyperinflammation, severe sepsis complicated by hypoperfusion and organ dysfunction due to this cardiovascular dysfunction develops. One of the best indicators in diagnosis of severe sepsis is the level of lactate (38). Our study shows that there is a relationship between serious sepsis progressing to septic shock and increased serum TLR9 level, particularly in those cases where the lactate level increased to >5 mmol/l due to tissue hypo-perfusion. Our conclusion is that there is a decrease in serum TLR9 level in serious sepsis through its down-regulation, while there is an increase in serum TLR9 level, perhaps in connection with TLR9 re-expression, in the cases in which tissue perfusion deteriorates, leading to high values of lactate. Recurrent cycles of hyper- and hypoinflammatory condition may occur in sepsis (39). It is not clear which clinical conditions trigger or cause this change. An increase in serum lactate level may play a crucial role as an immunostimulant or an agent that triggers this change in the TLR9 pathway.

In fact, both inflammatory and anti-inflammatory processes reflect not only uncontrolled systemic response but also survival efforts of the patients with sepsis. Current inflammatory status determination and appropriate management are important points for sepsis treatment. Targeted therapies for inflammatory cytokines in a patient during the hyperinflammatory phase may be performed. However, in clinical trials, these therapies have shown no clinical benefit or, in some cases, have worsened survival (40, 41). Furthermore, compensatory anti-inflammatory response to counter-regulate the host immune response in sepsis may result in immunosuppression, whereas immunostimulants may be more beneficial in these patients (42-44). The immunological state of each patient may differ.

For this reason, while one patient may benefit from targeted therapy, another may suffer as a result of the same therapy. Targeted immune therapies designed according to the patient's current inflammatory condition should be planned. Therefore the immunological status of the patient should be well determined for sepsis-appropriate management and treatment. Ideally, specific biomarkers should be used to help management of sepsis (45). The importance of TLRs in this topic will be determined by future studies.

This study has some limitations; the most important of them is that the current clinic study had a relatively small sample size, it was single-institutional and serum TLR9 levels were analyzed only once. Repetitive segmental determinations would make the evaluations and progress of patients more understandable. We believe that our results are encouraging, but they need to be supported by study in a larger sample and perhaps in different phases of sepsis.

Sepsis is characterized by a hyperinflammatory response in the early stage, but it may be followed by an immunosuppressive phase. This immunosuppression is believed to be the potent factor which is tightly associated with high mortality in sepsis. Modulation of this immunosuppression may help to develop new therapeutic strategies. TLRs have an important role in sepsis pathophysiology. However, the role of TLRs in the development of sepsis and in the creation of tissue damage and sepsis-related organ injury is not clear. Targeting TLRs is an exciting and promising area for inflammation control and sepsis management (46).

Actually, developing sepsis-specific TLR-targeted therapies might not be easy. Blocking a single mediator or activating a single channel will not be enough to intervene in sepsis (47, 48). Further experimental studies are required to understand the underlying mechanism of infection pathogenesis. Animal sepsis models can provide valuable information about the role of TLRs and the underlying mechanism of infection pathogenesis.

A better understanding of TLR biology and the change of the TLR9 level in sepsis may unveil novel therapeutic approaches for sepsis. It may be possible to detect those patients who are on the cusp of septic shock, which would infer higher mortality, and specific alterations in treatment might be considered. Studies have shown that not only microbial products but also some endogenous molecules can stimulate TLRs (49). The host immune system can be activated by means of TLR9- related systems. Hypoperfusion and hyperlactatemia may play an effective stimulatory role in reactivation of the immune system.