Towards personalized medicine in sepsis: Quest for Shangri-La?

Commentary

In 1992, the American College of Chest Physicians and Society of Critical Care Medicine consensus conference defined sepsis as the systemic inflammatory response syndrome occurring as a result of infection [1]. Clinical and immunologic discoveries have challenged this view of a hyper-inflammatory state, described as a 'cytokine storm' [2,3], in light of the failure of anti-inflammatory mediators, such as high-dose corticosteroids, tumor necrosis factor (TNF) antagonists, and interleukin-1 (IL-1) pathway inhibitors, to improve survival [4-6]. Subsequent theories have proposed that some patients may be immunosuppressed and the compensatory anti-inflammatory response syndrome may predominate [7-9], leading to secondary infections and reactivation of dormant viruses [10,11]. These findings have been based largely on experimental murine sepsis models [12,13] and small human studies, in which most measurements were obtained in the circulation.

Boomer and colleagues [14] conducted a comprehensive assessment of immune dysfunction in human subjects who died of sepsis and multi-organ failure. Rather than focusing on circulating markers alone, the authors examined markers in tissues, including lungs since they are a common site for secondary infections. The authors identified multiple inhibitory mechanisms to explain the occurrence of immunosuppression, including dominance of inhibitory over activating receptors, expansion of suppressive cell types, and induction of inhibitory ligands on both antigen-presenting cells and tissue parenchymal cells.

Reduced expression of inflammatory cytokines

The authors confirmed that immunosuppression occurs within tissues, and expression of key inflammatory markers, such as TNF-α, interferon-gamma (IFN-γ), and IL-6 and IL-10, was significantly decreased when splenocytes and cells from the lung were stimulated with anti-CD3/CD28, lipopolysaccharide, and phorbol esters.

Expansion of suppressor cell type

Regulatory T (Treg) cells, which suppress immune response, were increased in the spleen of patients with sepsis, and increased expression of cells with suppressor phenotype (myeloid-derived suppressor cells) was noted in the lung tissue.

Dominance of inhibitory over activating receptors and induction of inhibitory ligands

There was reduced expression of CD28, a co-stimulatory signal that activates T cells and increases expression of PD-1 membrane protein. PD-1 negatively regulates T-helper and cytotoxic cell proliferation in both lung and spleen. CTLA-4, a protein receptor that downregulates the immune system, was increased, whereas IL-7Ra, which mediates cell survival, was reduced in T cells, indicating dominance of inhibitory over activating receptors.

Antigen-presenting cells

HLA-DR, a receptor that presents peptide antigens to the immune system for the purpose of eliciting T-cell response, was noted to be reduced. Immunohistochemistry showed intense airway epithelial staining for herpes virus entry mediator (HVEM) and the receptor ligand for BTLA (a B- and T-cell attenuator protein expressed on T cells mediating inhibitory function) on lung isolated from the majority of patients with sepsis. This is described mechanistically to mediate local immunosuppressive response in the lung in both pneumonia- and non-pneumonia-associated sepsis.

This is a novel study and provides clinical confirmation of previous experimental work showing profound immunosuppression in sepsis. Several findings of the study have important clinical implications. For instance, recovery of cytokine secretion at 22 hours was comparable for patients with sepsis and controls when removed from a septic milieu and suggests that immunosuppression may be reversible and indicates potential therapeutic possibilities.

This study has limitations. Despite an exhaustive assessment of immune markers, the sample size was small. There was significant heterogeneity in the micro-biologic etiology of patients with sepsis and control patients (transplant, trauma, and neurologic devastation). Whether results of this study can be generalized to most critically ill patients with sepsis is not clear. For example, sepsis patients who were alive were not included as a control group as it is difficult to obtain tissue specimens in these patients. Many subjects in this study had a long intensive care unit course (range of 1 to 195 days) and represent those with prolonged critical illness [15,16].

Finally, several questions regarding immunosuppression remain poorly understood. What is its time course, and is it a dynamic process? Increased inflammation and immunosuppression may occur concurrently, and some studies suggest that immunosuppression may occur as early as hospital presentation. Which factors affect immunosuppression? The authors found no difference across age, duration of sepsis, and nutritional status. Given the small sample size, the effect of these and other factors could not be assessed. Finally, whether measurements of immune markers in the tissues mirror those in the circulation is unclear. Many of the assays described in this study are not standardized and are difficult to deploy in large multi-center clinical trials.

What are the study's implications for the design of personalized strategies for patients with sepsis? First, recovery of cytokine secretion at 22 hours was comparable for patients with sepsis and controls when removed from a septic milieu. These findings and a proof-of-concept study by Meisel and colleagues [17] suggest that immunosuppression could be reversed. Second, using anti-inflammatory agents, such as glucocorticoids, for all patients with sepsis may not be appropriate, and targeted therapy based on the state of their immune function should be considered. Several therapies may improve immunosuppression. These therapies could work on multiple pathways, such as IFN-γ, or target specific pathways, such as PD-1 [18]. Instead of prior approaches in which these therapies are administered to broad populations, personalizing these therapies on the basis of immune profiles should be considered.

Conclusions

Sepsis may represent a spectrum of dynamic immunologic changes, which, if measurable, can direct therapies based on the preponderant mechanism. A subset of patients may still benefit from anti-inflammatory therapies, whereas others may respond to immunostimulatory therapies. The stage is set for making further strides toward personalizing 'immune therapy in sepsis', hitherto a Shangri-La for clinicians and sepsis investigators.

Abbreviations

IFN-γ: interferon-gamma; IL: interleukin; TNF: tumor necrosis factor.

Competing interests

The authors declare that they have no competing interests.

Note

University of Pittsburgh Department of Critical Care Medicine: Evidence-Based Medicine Journal Club, edited by Sachin Yende