Nosocomial infections and immunity: lesson from brain-injured patients

Of brain-injured patients admitted to intensive care units, a significant number acquires nosocomial infections. Increased susceptibility to infectious agents could, at least partly, be due to transient immunodepression triggered by brain damage. Immune deficiency in patients with severe brain injury primarily involves T cell dysfunction. However, humoral and phagocytic deficiencies are also detectable. Activation of the hypothalamo–pituitary–adrenal axis and the sympathetic nervous system plays a crucial role in brain-mediated immunodepression. In this review we discuss the role of immunodepression in the development of nosocomial infections and clinical trials on immunomodulation in brain-injured patients with hospital-acquired infections.


Introduction
An infection that is not present or incubating when a patient is admitted to hospital but is detected 48-72 hours after admission is considered to a nosocomial rather than community-acquired infection [1]. Nosocomial infections affect about 30% of patients in intensive care units (ICUs; incidence rates range between 9% and 37%, depending on the population studied and the definition used) [2]. Patients with severe brain injury appear to be at greater risk for nosocomial infections than other ICU patients. In one study [3], 41 out of 82 (50%) patients with severe head injury experienced at least one infectious complication during their hospitalization. Piek and coworkers [4] examined 734 patients with severe head trauma and identified pulmonary infections in 41% and septicaemia in 10% of patients. Fassbender and colleagues found that 1 week after admission to hospital 27% of 52 patients with ischaemic stroke fulfilled criteria for hospital-acquired infection [5]. Hilker and coworkers [6] prospectively evaluated 124 patients with acute stroke who were treated at a neurological ICU. In that study the incidence of stroke-associated pneumonia was 21%. Berrouane and colleagues [7] found higher incidence rates of early-onset pneumonia in patients with neurotrauma than in patients without neurotrauma hospitalized in a neurosurgical ICU (20.1/1000 versus 15.7/1000 patient days and 34.2/1000 versus 27.9/100 ventilation days). Ventilator-associated pneumonia is among the most important subtypes of nosocomial infections, and the incidence of this type of pneumonia in brain-injured patients ranges from 28% to 40% [8].
Development of nosocomial infection depends on two key factors: decreased host defences and colonization by pathogenic micro-organisms. Here, we review the significance of immune status in development of nosocomial infections in brain-injured patients. Mechanical causes of immunodepression (e.g. intubation and invasive procedures) are not discussed here. Data for the review were identified by searches of the Medline database, the Cochrane Library and references from relevant articles (January 1980 to June 2003). Search terms included the following: 'head trauma', 'brain injury', 'infection', 'immunity' and 'intensive care'.

Why are infections so frequent in brain-injured patients -is 'immunoparalysis' involved?
Immune deterioration has been reported in patients after trauma, surgery, or blood loss (for review [9,10]). Alterations in host defence mechanisms after trauma that are potentially important to development of infectious complications include the following: paralysis of monocyte function (transient nonreactivity of monocytes toward stimulation with endotoxin, depression of antigen presentation capacity, and enhanced secretion of the anti-inflammatory cytokine IL-10); suppression of T cell functions (decreased response to mitogenic activation and decreased IL-2 production); and impairment of B-cell function (decreased capacity to produce antibodies).
The high frequency of infection in brain-injured patients suggests that host defences may be compromised after severe brain trauma, even in the absence of clinically important systemic injury. In the early 1990s several groups showed independently that severe brain injury precipitates significant deficiencies in the immune system, and this finding was confirmed and later extended by other researchers. Immunological abnormalities that are found in patients with brain injury are summarized in Table 1 [11][12][13][14][15][16][17][18][19][20][21].
The cellular arm of immunity is the most affected, although phagocytic and humoral deficiencies are also detectable. Defects in cellular immunity include reduced number of circulating T cells, increased proportion of suppressor cells, depressed mitogen-induced proliferative response, and depressed delayed-type hypersensitivity reaction [11][12][13][14][15][16]. Moreover, it has been postulated that monocytes could be target cells in brain-mediated immunosuppression; monocyte deactivation, with decreased capacity for antigen presentation and depressed secretion of proinflammatory cytokines, increased the risk for infectious complications [21].
Immunodeficiencies are noted soon after brain injury; for example, T-cell anergy is seen within several hours after brain damage [12]. These deficiencies are most prominent during the first few days after brain injury [14,17] and precede the infectious complications, which reach a peak incidence at 5-11 days after brain trauma [4]. The immunodepression in brain-injured patients appears to be a transient phenomenon. Recovery of T-cell function was observed 3 months after head injury [14]. In patients with a vegetative state, all neutrophil functions (superoxide release, migration and chemotactic capability) were found to be normal when measured several months after the brain damage [18]. Further studies are needed to determine how the immune system recovers after brain damage and to compare the recovery process between brain-injured and other ICU patients.
The mechanisms that lead to immunological defects in patients with head trauma or severe stroke are still poorly understood. In many cases immune deficiencies (defect in cellular immunity, monocyte deactivation) closely resemble those observed in patients after 'non-neurological' trauma, surgical injury, or blood loss [10]. Generally, local infection or sterile trauma induces a local inflammatory response, with release of proinflammatory mediators (tumour necrosis factor [TNF]-α, IL-1β, IL-6). Overwhelming immune activation can result in systemic inflammatory response syndrome and septic shock. To control the potentially harmful proinflammatory response, the immune system releases several antiinflammatory mediators (IL-10, IL-1 receptor antagonist, soluble TNF-α receptor), causing compensatory antiinflammatory response syndrome (CARS). Monocyte deactivation with decreased capacity for antigen presentation and depressed secretion of proinflammatory cytokines appear to be critical events in CARS. Table 1 Immunological defects in brain-injured patients

Can we effectively prevent and treat nosocomial infections in brain-injured patients -is there a role for immunomodulatory therapy?
Various immunomodulatory agents, including IFN-γ, granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor and immunoglobulins, have been used in ICU patients to prevent or treat nosocomial infections by activating the immune system. Here we discuss the results of studies focused on brain-injured patients.
G-CSF promotes the differentiation and proliferation of neutrophil precursor cell, prolongs the survival of neutrophils, and acts as a chemoattractant for granulocytes. In a randomized, placebo-controlled, double-blind, multicentre phase II study [29], 40 patients with head trauma were given one or two daily doses of recombinant human G-CSF (filgrastim) for up to 10 days after hospitalization. The primary study end-points were an increase in absolute neutrophil count; frequencies of nosocomial pneumonia, urinary tract infection and primary bacteraemia; and safety of G-CSF. Secondary end-points were serum G-CSF level; duration of hospitalization, antibiotic use and ICU stay; and 28-day survival. Filgrastim caused a dose-dependent increase in absolute neutrophil count. There was no difference in pneumonia and urinary tract infections between groups. The incidence of hospital-acquired bacteraemia was significantly reduced in patients treated with high-dose G-CSF (300 µg/day) compared with those treated with placebo (0/19 [0%] versus 5/17 [29%]). There was no difference between groups in any of the secondary end-points. That study did not address the issue of the possible deleterious effects of G-CSF on the injured brain [30], although this drug appears to be safe for extracerebral complications [31].
In another study [32], administration of recombinant human G-CSF ameliorated life-threatening infections without causing lung injury or brain swelling in patients with severe head injuries who were treated with a combination of high-dose barbiturates and mild hypothermia. In that study eight patients with head trauma received recombinant human G-CSF for 5 days, and the results of treatment were compared with those in 22 patients who were not administered recombinant human G-CSF. In patients treated with recombinant human G-CSF, total leucocyte count, nucleated cell count and neutrophil function increased significantly, whereas levels of C-reactive protein and IL-6 decreased. Seven out of eight patients treated with recombinant human G-CSF recovered from life-threatening infections, and none of the eight patients died. In contrast, in patients who did not receive recombinant human G-CSF, infections continued after 5 days in 17 out of 22 patients, seven of whom died from severe infections during hospitalization.
Intravenous immunoglobulin (IVIG) can modulate the immune response in several ways, including by Fcγ receptor mediated immunomodulation, by its impact on the idiotype/anti-idiotype network, and by elimination of immunostimulating microbial products (e.g. toxins, superantigens) [33]. An analysis of randomized trials conducted by the Cochran Infectious Diseases Group showed that polyclonal IVIG significantly reduced mortality in sepsis and septic shock [34]. Gooding and coworkers [35] conducted a randomized, double-blind trial to determine whether IVIG decreases the incidence of secondary infections in head-injured children. Eighteen children with severe head injury received IVIG (400 mg/kg) and 14 received albumin placebo within 48 hours after admission. Unfortunately, no significant differences in the incidence of pneumonia or in any other type of infection were noted. In addition, there were no differences between groups in the number of days on mechanical ventilation and in the number of hospital days. In another study [38], nine patients with severe closed-head injury were prospectively randomized either to early parenteral nutrition (four patients) at day 1 or to delayed parenteral nutrition (five patients) at day 5. Significant increases in total CD4 + cell counts, a rise in the CD4 + /CD8 + ratio and improved lymphocyte responses after mitogen stimulation were noted in patients receiving early nutrition as compared with those receiving delayed parenteral nutrition.
The results of a systematic review assessing the significance of nutritional support for head-injured patients suggested that early feeding may be associated with a trend toward better outcome in terms of survival and disability, but further studies are needed [39].
Rixen and coworkers [28] demonstrated an immunomodulatory effect of the histamine-2 receptor antagonist ranitidine, both at cellular and mediator levels, in patients after severe head injury. In that randomized, prospective, doubleblind study, nine patients received continuous infusion of ranitidine (6.25 mg/hour) for up to 5 days and 11 patients received placebo. Treatment with ranitidine, but not with placebo, was associated with a significant increase in CD4 + lymphocytes, increased IFN-γ production after mitogen stimulation, and significant decrease in CD8 + lymphocytes. The mortality rate was similar between groups; one patient died in placebo group, and among those treated with ranitidine no patients died.

Conclusion
Reports published to date on modulation of immune function in brain-injured patients have several flaws. The number of included patients was too small to draw firm conclusions. Examined groups were heterogeneous with respect to aetiology of brain injury (trauma, haemorrhage) and severity of disease. Although T lymphocytes appear to be the most affected in patients with brain injury, there is a lack of studies attempting to modulate cell-mediated immunity in braininjured patients.
Several important issues should be addressed in future studies. First, the mechanisms responsible the immunodepression in brain-injured patients (e.g. endocrinological and stress-related mechanisms) require further exploration. Second, future studies should be conducted in large groups of carefully selected patients at high risk for infection. It is important to select appropriate patients for immunotherapy.
Patients with severe brain injury are not good candidates for immunotherapy because death in this group is usually not directly related to infectious complications but rather is caused by brain-stem damage. Therefore, in this group potentially beneficial effects of immunotherapy can be overwhelmed by the primary brain damage. Finally, the specific cytokines or growth factors that have the greatest therapeutic impact, and which are the patient populations that will derive the greatest benefit remain to be defined.