Sepsis and acute respiratory distress syndrome (ARDS) constitute devastating conditions with high morbidity and mortality. Sepsis results from abnormal host immune response, with evidence for both pro- and anti-inflammatory activation present from the earliest phases. The “proinflammatory” response predominates initially causing host injury, with later-phase sepsis characterized by immune cell hypofunction and opportunistic superinfection. ARDS is characterized by inflammation and disruption of the alveolar-capillary membrane leading to injury and lung dysfunction. Sepsis is the most common cause of ARDS. Approximately 20% of deaths worldwide in 2017 were due to sepsis, while ARDS occurs in over 10% of all intensive care unit patients and results in a mortality of 30 to 45%. Given the fact that sepsis and ARDS share some—but not all—underlying pathophysiologic injury mechanisms, the lack of specific therapies, and their frequent coexistence in the critically ill, it makes sense to consider therapies for both conditions together. In this article, we will focus on the therapeutic potential of mesenchymal stem/stromal cells (MSCs). MSCs are available from several tissues, including bone marrow, umbilical cord, and adipose tissue. Allogeneic administration is feasible, an important advantage for acute conditions like sepsis or ARDS. They possess diverse mechanisms of action of relevance to sepsis and ARDS, including direct and indirect antibacterial actions, potent effects on the innate and adaptive response, and pro-reparative effects. MSCs can be preactivated thereby potentiating their effects, while the use of their extracellular vesicles can avoid whole cell administration. While early-phase clinical trials suggest safety, considerable challenges exist in moving forward to phase III efficacy studies, and to implementation as a therapy should they prove effective.

Cell therapy, particularly mesenchymal stem/stromal (MSC) therapy, has been investigated for a wide variety of disease indications, particularly those with inflammatory pathologies. However, recently it has become evident that the MSC is far from a panacea. In this review we will look at current and future strategies that might overcome limitations in efficacy. Many of these take their inspiration from stem cell niche and the mechanism of MSC action in response to the injury microenvironment, or from previous gene therapy work which can now benefit from the added longevity and targeting ability of a live cell vector. We will also explore the nascent field of extracellular vesicle therapy and how we are already seeing enhancement protocols for this exciting new drug. These enhanced MSCs will lead the way in more difficult to treat diseases and restore potency where donors or manufacturing practicalities lead to diminished MSC effect.

Carbon dioxide (CO 2 ) has long been considered, at best, a waste by-product of metabolism, and at worst, a toxic molecule with serious health consequences if physiological concentration is dysregulated. However, clinical observations have revealed that ‘permissive’ hypercapnia, the deliberate allowance of respiratory produced CO 2 to remain in the patient, can have anti-inflammatory effects that may be beneficial in certain circumstances. In parallel, studies at the cell level have demonstrated the profound effect of CO 2 on multiple diverse signalling pathways, be it the effect from CO 2 itself specifically or from the associated acidosis it generates. At the whole organism level, it now appears likely that there are many biological sensing systems designed to respond to CO 2 concentration and tailor respiratory and other responses to atmospheric or local levels. Animal models have been widely employed to study the changes in CO 2 levels in various disease states and also to what extent permissive or even directly delivered CO 2 can affect patient outcome. These findings have been advanced to the bedside at the same time that further clinical observations have been elucidated at the cell and animal level. Here we present a synopsis of the current understanding of how CO 2 affects mammalian biological systems, with a particular emphasis on inflammatory pathways and diseases such as lung specific or systemic sepsis. We also explore some future directions and possibilities, such as direct control of blood CO 2 levels, that could lead to improved clinical care in the future.

Background Mesenchymal stem cell (MSC) derived extracellular vesicles (EVs) have been proposed as an alternative to cell therapy, creating new possible delivery modalities such as nebulisation. We wished to investigate the therapeutic potential of directly nebulised MSC-EVs in the mitigation of Escherichiacoli-induced pneumonia. Methods EV size, surface markers and miRNA content were assessed pre- and post-nebulisation. BEAS2B and A459 lung cells were exposed to lipopolysaccharide (LPS) and treated with nebulised bone marrow (BM) or umbilical cord (UC) MSC-EVs. Viability assays (MTT) and inflammatory cytokine assays were performed. THP-1 monocytes were stimulated with LPS and nebulised BM- or UC-EVs and phagocytosis activity was measured. For in vivo experiments, mice received LPS intratracheally (IT) followed by BM- or UC-EVs intravenously (IV) and injury markers assessed at 24 h. Rats were instilled with E. coli bacteria IT and BM- or UC-EVs delivered IV or by direct nebulisation. At 48 h, lung damage was assessed by physiological parameters, histology and inflammatory marker presence. Results MSC-EVs retained their immunomodulatory and wound healing capacity after nebulisation in vitro. EV integrity and content were also preserved. Therapy with IV or nebulised MSC-EVs reduced the severity of LPS-induced lung injury and E. coli-induced pneumonia by reducing bacterial load and oedema, increasing blood oxygenation and improving lung histological scores. MSC-EV treated animals also showed lower levels of inflammatory cytokines and inflammatory-related markers. Conclusions MSC-EVs given IV attenuated LPS-induced lung injury, and nebulisation of MSC-EVs did not affect their capacity to attenuate lung injury caused by E. coli pneumonia, as evidenced by reduction in bacterial load and improved lung physiology.

IntroductionMesenchymal stromal cells (MSC) are a promising therapeutic for pneumonia-induced sepsis. Here we sought to determine the efficacy of delayed administration of naïve and activated bone marrow (BM), adipose (AD), and umbilical cord (UC) derived MSCs in organized antibiotic resistant Klebsiella pneumosepsis.MethodsHuman BM-, AD-, and UC-MSCs were isolated and expanded and used either in the naïve state or following cytokine pre-activation. The effect of MSC tissue source and activation status was assessed first in vitro. Subsequent experiments assessed therapeutic potential as a delayed therapy at 48 h post infection of rodents with Klebsiella pneumoniae, with efficacy assessed at 120 h.ResultsBM-, AD-, and UC-MSCs accelerated epithelial healing, increased phagocytosis, and reduced ROS-induced epithelial injury in vitro, with AD-MSCs less effective, and naïve MSCs more effective than pre-activated MSCs. Delayed MSC administration in pre-clinical organized Klebsiella pneumosepsis had no effect on physiologic indices, but enhanced resolution of structural lung injury. Delayed therapy with pre-activated MSCs reduced mRNA concentrations of fibrotic factors. Naïve MSC treatment reduced key circulating cell proportions and increased bacterial killing capacity in the lungs whereas pre-activated MSCs enhanced the phagocytic index of pulmonary white cells.DiscussionDelayed MSC therapy enhanced resolution of lung injury induced by antibiotic resistant Klebsiella infection and favorably modulated immune cell profile. Overall, AD-MSCs were less effective than either UC- or BM-MSCs, while naïve MSCs had a more favorable effect profile compared to pre-activated MSCs.