Editorial: Control issues in sepsis: what modulates apoptosis?

Abstract

Vaki et al. propose that there may be factor(s) in the serum of patients with severe sepsis that modulate apoptosis of lymphocytes and monocytes.

The molecular pathophysiology of sepsis syndrome in humans remains poorly understood. As with many diseases, most of what is known is derived from studies in mice or from associative observations in human trials. Two problems further complicate the study of sepsis syndrome. The first problem is that there are substantial differences in the way species handle microbial challenge. Although experts debate the fine points of the varying animal models, there is general agreement that the syndrome induced by microbial challenge in rodents does not sufficiently mimic what we see clinically in humans in any of the models [1]. This lack of an appropriate rodent model to adequately reproduce the clinical syndrome seen in humans greatly complicates the discovery of underlying mechanisms, as well as the evaluation and development of strategies to alter these mechanisms. The second problem relates to the currently used definition of sepsis in humans. This definition, arrived at by consensus in 1992 [2] and readdressed in 2001 [3], uses broad clinical and/or laboratory criteria such as alterations in heart rate, temperature, respiration, white blood cells, or inflammatory biomarkers in the setting of infection to lump together patients with secondarily induced inflammation who differ in microbial causality, site of infection, and severity of disease. The heterogeneity of the patients captured by this definition makes it likely that numerous different immunopathological mechanisms are involved in the patients, making evaluation of any single mechanism highly complicated. Interpretation of studies of the underlying pathophysiology of sepsis must be taken in the context of these two problems.

One of the oldest hypotheses for the pathophysiology of sepsis relates to what might be called the “mediator theory of sepsis”, where the main concept is that secondary host molecules such as cytokines are released from immune cells in response to microbial challenge. These mediators are hypothesized to circulate in the bloodstream and lead to secondary distant organ inflammation and failure. The dominant cells in this process are believed to be macrophages. The importance of macrophages in sepsis is based at least in part on the experiments performed 25 years ago using D-galactosamine-sensitized mice. Adoptive transfer of macrophages from WT C3H/HeN mice into LPS hyporesponsive C3H/HeJ mice, whose macrophages do not recognize LPS (now known to be a result of a mutation in TLR4), restored LPS sensitivity [4]. This idea fit well with the prevailing concept at the time that sepsis was a result of release of host mediators from host immune cells. Initially, the proposed mediators were limited in number and consisted mainly of macrophage-derived cytokines such as TNF and IL-1, both of which have stood the test of time as proinflammatory cytokines of primary importance. Infusion of these recombinant cytokines reproduced some findings in sepsis, and specific blockade reduced these findings in animals challenged with microbial agonists [5, 6]. With time, the list of other potential upstream endogenous and exogenous proinflammatory mediators that activate macrophages has grown, and starting with the discovery of the TLRs, there has been intense study of mechanisms that modulate cellular signaling, with the hope that some of these processes could be therapeutic targets. Throughout this period of extensive research, the macrophage has retained its central role in the mediator theory of sepsis.

In the last decade, a fundamentally different hypothesis for the pathophysiology of sepsis has been popularized that might be called the “apoptosis theory of sepsis” [7]. This theory is based on the concept that there is massive apoptosis of lymphocytes in different tissues in patients with sepsis and that the apoptosis leads to loss of normal secondary immune responses. Apoptosis of lymphocytes has been described in both mouse models of sepsis and in human studies [8–10]. Most studies have focused on apoptosis of lymphocytes, although one study has reported that DCs but not macrophages are depleted in patients with sepsis [11]. As with the processes leading to activation of macrophages, the last decade has led to a tremendous increase in knowledge relating to signaling pathways of apoptosis. In models of sepsis using mice with specific gene deletions of a variety of cell receptors and signaling pathways, only deletion of MyD88 altered apoptosis of T and B lymphocytes, suggesting that at least in mice, there may be many redundant pathways [12]. Little is known about what modulates the apoptosis in humans who have sepsis syndrome.

In this issue of the Journal of Leukocyte Biology, Vaki et al. [13] report that sera from humans with severe sepsis dramatically alter the apoptosis of blood leukocytes prepared from normal volunteers cultured in vitro, as compared with control sera. Remarkably, these authors found opposite effects on monocytes and lymphocytes. Sera from patients with severe sepsis decreased apoptosis from CD14⁺ monocytes in their system, whereas the same sera increased apoptosis from CD4⁺ lymphocytes. Therefore, there are two novel findings in this article—one relevant to macrophages and the mediator hypothesis of sepsis and the other relevant to lymphocytes and the apoptosis hypothesis of sepsis. First, Vaki et al. [13] propose that molecule(s) in serum lead to decreased apoptosis of macrophages, a finding that by extension, might lead to prolonged release of inflammatory cytokines from these cells. The idea of decreased apoptosis and possibly inappropriate survival of monocyte/macrophages in this setting and that this may be caused by a serum factor is novel. The second finding is that there are circulating molecules(s) in serum that can cause the increased apoptosis of lymphocytes. Whereas apoptosis of lymphocytes in sepsis is well established, possible regulation by circulating factors in serum is novel.

There are many questions left unanswered. What is the nature of the factor(s) in serum? Is the factor of microbial- or host-origin? Is it a protein or some other type of molecule? Are the findings representative of a process that is actively regulated or simply some sort of secondary byproduct of the septic state. The kinetics seem unusual. It is described as an “early” factor, within 12 h after onset of organ failure. However, the processes leading to sepsis in these patients would likely have been present for some time. Do the factors modulate only circulating cells in blood or also macrophages in the reticuloendothelial system and lymphocytes in LNs and in the spleen? Are there separate factors responsible for the changes on monocytes and lymphocytes? Can the system be reproduced in an animal system, and if so, what species would be best given the resistance of mice to TLR agonists?

The findings in the article used a single method to study apoptosis (flow cytometry of Annexin-V), and they need to be repeated by other laboratories. However, this study could have substantial importance once confirmed and understood in more detail. It should be of interest to investigators interested in monocytes/macrophages as well as those studying lymphocytes. If indeed there are circulating factors in the blood of patients with severe sepsis that are causal in modulating apoptosis for each cell type, they might eventually be isolated and studied; the receptors might be identified, and ultimately, perhaps, the pathway(s) could be manipulated for therapeutic advantage.

Certainly, the field could use a breakthrough.