Early natural killer cell counts in blood predict mortality in severe sepsis

Severe sepsis (acute organ dysfunction secondary to infection) and septic shock (severe sepsis plus hypotension not reversed by fluid resuscitation) are major healthcare problems that affect millions of individuals around the world each year, killing one in four (and often more) and increasing in incidence [1‐3]. Similarly to polytrauma, acute myocardial infarction and stroke, the early initiation of therapy once severe sepsis is established is likely to influence the patient's prognosis. In consequence, early identification of individuals at risk for bad outcomes is dramatically important in this condition [4].

Because sepsis originates from a microbial infection, host immunity should play a principal role in determining both outcome and recovery. A protective role of naturally produced immunoglobulin G (IgG) in sepsis was described previously [5]. The participation of cellular immunity in this disease is poorly understood, and the available data are controversial [6‐8].

Identifying quantitative alterations in key humoral and cellular parameters could have a prognostic value in this condition. In an attempt to evaluate the quantitative changes in the status of immunocompetence in severe sepsis over time and its potential influence on clinical outcome, we monitored levels of blood of Immunoglobulins (IgG, IgA and IgM), complement factors (C3 and C4), lymphocyte subpopulations (T, B and natural killer (NK) cells) in 50 consecutive patients with a diagnosis of severe sepsis or septic shock at three moments during their hospitalization in the ICU. Our measurement of these parameters provide a first assessment of the status of both humoral and cellular arms of the immune response, and their evaluation is easily available in the vast majority of hospitals with critical care medicine units.

Our results provide evidence that differences in the systemic levels of a number of key host immunity elements in patients with severe sepsis influence their final outcome. Compared to those patients who survived, septic patients who died showed lower levels of IgG and C4, along with higher levels of NK cells, in the first 24 hours following admission to the ICU. Comparisons between fatal cases and survivors, as well as the results of our regression analysis, suggest that NK cell counts at day 1 are associated with increased risk of mortality in patients who present to the ICU with severe sepsis. The role of these cells in sepsis is controversial [14]. NK-cell depletion increases survival and decreases systemic levels of cytokines in experimental models of sepsis [7, 15‐21]. In humans, available data derived from patients in the ICU are scarce, and some of them diverge with the results derived from animal models. Gogos et al. [22] found increased absolute counts of NK cells in sepsis caused by community-acquired pneumonia. Giamarellos-Bourboulis et al. [8] reported improved survival in patients with severe Gram-negative sepsis and high NK counts. NK cells have sophisticated biological functions [23], participating with antigen presentation cells and T cells in the cellular response against pathogens. NK cells are key actors in innate immunity and as a consequence should play an important role in the very early moments of sepsis. In addition, NK cells could release high amounts of proinflammatory cytokines such as IFN-γ or immunosuppressive agents, such as IL-10, thus promoting tissue damage and interfering with the development of the adaptive immune response against the causative microbial agent [23].

When we performed a subanalysis considering only those patients with septic shock, NK cell counts at day 1 were still associated with increased risk of mortality. On the contrary, IgG levels at day 1 showed a protective trend close to statistical significance in the regression analysis. Taccone et al. [5] reported low concentrations of IgG as a common finding in patients with community-acquired septic shock. In their study, patients with hypo-IgG had greater vasopressor requirements, were more likely to develop acute lung injury and/or acute respiratory distress syndrome and had higher mortality. Studies with larger numbers of patients are necessary to confirm the protective trend of IgG in this disease.

The relatively lower levels of C4 observed in patients who died in our study could be a consequence of consumption of this complement in severe sepsis. In agreement with our results, Nakae et al. [24] found significantly lower levels of C3 and C4 in septic patients who did not survive than in those who did. Analysis of the variations over time in the levels of the immunological parameters evaluated showed that most survivors exhibited a progressive increase from day 1 to day 10 in most of the parameters. This result provides evidence for a compromised host response during the very first moments of severe sepsis (at least from the quantitative point of view) which improves at the latter stages of the disease in these patients. Interestingly, nonsurvivors failed to show significant increments in the levels of the immune parameters measured over time. These findings warrant further work with larger cohorts of patients to evaluate the inclusion of immunological parameters as part of the severity scores in sepsis. A limitation of our study is the definition employed for NK cells (CD3-CD16+CD56+ lymphocytes), since it did not include a subset of CD16- NK cells, which account for a minor percentage of total NK cells (5% to 10%). Nonetheless, the definition used in this work identified the largest population of these cells.

Our results demonstrate the prognostic role of NK cells (defined as CD3-CD16+CD56+ lymphocytes) in severe sepsis, proving a direct association of early blood counts of these cells with mortality. Further studies including functional analysis of NK cells are needed to confirm their exact role in the pathogenesis of this disease.