HLA-DR expression on monocytes is decreased in polytraumatized patients

Sepsis, Systemic Inflammatory Response Syndrome (SIRS) and Multiple Organ Dysfunction Syndrome (MODS) remain the most frequent causes of complications and death in severely injured patients. Authors could detect a malfunction of the immune system after major trauma as one reason for that [1]. The efficient function of the immune system is essential for an organism to fight pathogens. The innate immune system represents the first line of host defence. It performs phagocytosis as well as releases cytokines, thus activating the adaptive immune system. Surgical stress and trauma also lead to an activation of the innate immune system [2, 3]. In this case, so-called danger-associated molecular patterns have been identified as danger signals that mediate the early post-traumatic inflammatory response [2, 3]. Both pathogen- and danger-associated molecular patterns are recognized by immunologically competent cells [4]. HLA-DR molecules play a central role in the specific immune response, as they are required for antigen presentation and activation of helper T lymphocytes. These molecules are expressed on the surface of professional antigen-presenting cells such as macrophages or dendritic cells. Several studies have shown that HLA-DR expression on monocytes decreases after major injury, surgery and organ transplantation [5, 6]. Reduced HLA-DR expression on monocytes was considered to correlate with infectious complications and the development of sepsis.

One critical aspect following trauma is the ability of the organism to present MHC II antigens (HLA-DR) on monocytes, because the T helper cells will only react to foreign antigens that are presented on the macrophage surface using HLA-DR. Studies could show that in patients with an uneventful recovery from severe trauma or surgery, the level of monocyte HLA-DR expression fell within hours of trauma or surgery, but returned to normal within 1 week [6, 7]. In contrast, 3 weeks were required for HLA-DR expression to return to base level in those who developed infection, but finally recovered. Interestingly, in those who developed infection and sepsis and died as a result, HLA-DR expression decreased and never regained base levels.

Numerous markers of immune failure have been described in patients. Among these, the decreased expression on circulating monocytes of HLA-DR measured by flow cytometry has repeatedly been described as a robust marker of immune dysfunctions in septic shock patients [5]. So far, not many data exist about the association between HLA-DR expression on monocytes of severely injured patients and the development of a post traumatic SIRS, or MODS. Several studies are published regarding the HLA-DR expression and development of sepsis in patients after abdominal surgery or during long ICU stay; only one publication exists concerning the HLA-DR expression in polytraumatized patients with sepsis [8].

Up to now, the post traumatic kinetics of HLA-DR expression levels on monocytes after polytrauma has sparsely been shown. Moreover, it is not completely clear whether the HLA-DR expression on monocytes of polytraumatized patients is different on those who develop sepsis, SIRS or MODS during the post-traumatic course. As the early detection of the patients at risk for development one of these highly feared complications is most important for early treatment, this could be of great importance for the treatment of those patients during ICU stay. Therefore, the aim of this study was to analyse the HLA-DR expression on monocytes from polytraumatized patients during a 14-day post-traumatic course. Furthermore, it was the aim to investigate whether these kinetic changes were associated with the development of sepsis, SIRS or MODS.

In the present study, the post -traumatic kinetic of HLA-DR expression on monocytes after polytrauma compared to surgery and healthy volunteers has been analysed. Moreover, the use of HLA-DR expression on monocytes as a prognostic marker for development of MODS, SIRS and sepsis in polytraumatized patients has extensively been researched and analysed. A significant decrease in HLA-DR expression on monocytes during the first 14 days after severe trauma could be detected. This result comes along with the findings of several other authors analysing HLA-DR levels after surgery or organ transplantation [12‐15]. Persisting low HLA-DR values were found in patients who subsequently developed secondary infections, whereas HLA-DR levels increased generally within 1 week in individuals who recovered uneventfully [6 , 8, 16, 17]. Previously, HLA-DR expression was usually assessed as a predictor of septic complications after trauma, surgery or pancreatitis [6, 8, 16, 17], but no statements concerning MODS or SIRS were made so far. Only one study deals with sepsis development in polytraumatized patients and HLA-DR expression on monocytes but in this study neither MODS nor SIRS development was analysed. Moreover, neither the healthy HLA-DR expression in volunteers nor the HLA-DR expression after surgery had been analysed as control groups. In contrast, Ditschkowski et al. separated the polytrauma collective in three groups depending on the severity of injury [8]. In contrast to Ditschkowski et al. no significant differences between the HLA-DR expression of polytrauma patients with sepsis and without sepsis could be found in this study. One reason for this might be the patient number. While Ditschkowski et al. enrolled 77 patients of whom 20 developed a sepsis, we could include 24 patients of whom eight developed a sepsis. Although we have enrolled less patients, the sepsis rate of both collectives was more or less 30% . The HLA-DR expression curves in our study looked similar to the ones from Ditschkowski et al. and they followed the same trend [8].

Other authors could also show a correlation between a decreased HLA-DR expression on monocytes and the development of a sepsis during the post-operative course [18‐20]. These data are not comparable with the presented results, as they have analysed different patient groups like burn patients or patients suffering from abdominal or vascular surgery. Nevertheless, a trend towards lower HLA-DR expression could be observed in our sepsis patients.

We believe that polytrauma patients cannot be compared with patients suffering from sepsis due to nosocomial infections, burns or abdominal surgery complications because the immunological processes after a polytrauma are different. Due to polytrauma, the patient has to face the first immunological hit (e.g. hypoxia, hypotension, organ and soft tissue injuries, fractures), followed by a second hit (e.g. ischaemia/reperfusion injuries, operative interventions, infections). This induces a host defence response, which is characterized by local and systemic release of pro-inflammatory cytokines, complement factors and acute phase proteins, as well as hormonal mediators. In parallel, anti-inflammatory mediators are produced (compensatory anti-inflammatory response syndrome, CARS). An imbalance here mayo be responsible for organ dysfunction and increased susceptibility to infections.

Therefore, the HLA-DR expression may be dependent on the pro- or anti-inflammatory phase the individual is passing through at the moment of blood collection. SIRS is defined as a hyper-inflammatory phase and, therefore, associated with normal or raised HLA-DR expression [21].

Later during the anti-inflammatory phase, CARS, the production of anti-inflammatory mediators is increased and monocytes downregulate the HLA-DR expression [21]. Especially, IL-10 causes an internalization of HLA-DR thus decreasing the HLA-DR expression on monocytes. This can be prevented by anti-IL-10 treatment. Thus, in individual patients, profound CARS is associated with low numbers of HLA-DR expressing monocytes. However, in other patients the blood sample might have been taken while the individual was in the SIRS phase, in which case monocyte HLA-DR may be normal or raised whatever the prognosis of the patient. Measurements of monocyte HLA-DR expression may be worth pursuing as one possible indicator as to whether the patient is in an inflammatory or anti-inflammatory phase.

Some studies exist analysing the different impact of ageing on monocytic HLA-DR expression. Seidler et al., for example, could show a decrease of monocyte HLA-DR expression in aged healthy volunteers (>50 years). The difference was significant compared to the very young control group (<30 years) but it was not significant compared tot he middle aged (>30 years) [22]. Others did not find any correlation with HLA-DR expression and age [23]. Therefore, we do not think that the age difference between our study group (ca. 30 years) and the surgery group (ca. 69 years) is crucial for our results.

20 % of our severely injured patients developed MODS during the post traumatic course, which is comparable with results from other studies, analysing patients with blunt abdominal trauma, with MODS rates between 7.9 and 26.1 % [24].

Decreased expression of HLA-DR on monocytes is a hallmark of altered immune status in patients with SIRS. West et al. could show that HLA-DR expression on monocytes correlates with injury severity [25]. They could also show decreased monocytic HLA-DR levels 48 h after trauma as others have shown before [25, 26]. This comes along with our results. Nevertheless, we could not detect significant differences of HLA-DR expression on monocytes of patients suffering from SIRS and those who did not after multiple trauma.

Kim et al. could detect that HLA-DR expression is differently modulation on CD14^HIGH (classical) and CD14^LOW (inflammatory) monocytes after systemic inflammation.This shows that also the HLA-DR expression on monocyte subpopulations is involved in the development of a SIRS [27].

There are several limitations to this study. First, the group size of 24 polytraumatized patients is very small. This is due to the fact that the analysed patient population, traumatized patients with an ISS >16 points, is a rare entity. Nevertheless, more studies with a bigger collective are needed. Moreover, HLA-DR expression on monocytes was analysed only at some time points (directly after trauma, 3, 7 and 14 days post trauma). Therefore, significant changes in between might be missed. However, the time curve after severe trauma and the important time points have been extensively studied during the past decades and most immune responses are detected during the first 14 days with a peak directly after trauma up to 3 days postop [28].

In conclusion, post-traumatic HLA-DR expression on monocytes is significantly reduced after multiple trauma. HLA-DR levels are at 14 days normal. No significant changes in HLA-DR expression on monocytes from severely injured patients suffering from SIRS, MODS or sepsis compared to on those who did not have complications could be detected. Nevertheless, HLA-DR expression on monocytes may be used to identify the immunological pro- or anti-inflammatory phase the patient is going through in combination with other immunological markers. More studies with a larger polytrauma collective are needed to confirm and widen our knowledge about post-traumatic immune system dysfunctions.