cells. Other highly pathogenic PAMPs include porins, mannose-containing glycoproteins, lipoteichoic acid, flagellin, double-strain RNA and quorum sensing molecules. All these elements are able to alter kidney microcirculation, induce apoptosis and functional alterations of tubular cells and concomitantly modulate the immune response in septic patients [3, 11, 12].
Damage-Associated Molecular Patterns (DAMPs). DAMPs are endogenous molecules released by injured or necrotic cells: RNA, single/double strain DNA, ATP, histones and high-mobility group box 1 (HMGB-1). Also, these molecules activate specific receptors located on the surface of both immune and renal cells (i.e., P2Xr for ATP or TLR-2 for HMGB-1) and have physiological roles in spreading “the alert signal,” inducing the recruitment of activated immune cells: indeed, the over-activation of DAMP pathways is a further source of renal damage trough direct and indirect (immune-mediated) effects [13, 14].
Fig. 1. Modulation of sepsis-associated acute kidney injury by extracorporeal therapies. ADMA, asymmetric dimethylarginine; ATP, adenosine tri-phosphate; DAMPs, damage-associated molecular pathways; HMGB-1, high-mobility group box 1; LPS, lipopolysaccharide; NO, nitric oxide; RAD, renal assist device.
Inflammatory Cytokines and Chemokines. Cytokines/chemokines are actively produced by injured/activated cells with the aim of modulating the inflammatory response. The immune system is the main source of cytokines, but several other tissues are able to release them. During inflammatory processes, podocytes, TEC and renal endothelium can massively produce IL-18, IL-6 and chemokines such as IL-8 [9, 15]. Several studies have demonstrated the impact of these molecules in S-AKI prognosis; in particular, IL-6 and TNF correlated with AKI severity and with a worse patient survival [16].
Vasoactive Agents and Other Hormones. The release of all the above mentioned molecules coupled with organ dysfunction activates hormones and growth factors involved in homeostasis maintenance. These mediators could only partially counteract tissue injury and promote regeneration processes. However, the excess of signaling of specific stimuli can lead to the development of maladaptive responses. Examples include the massive production of nitric oxide (NO) that induces harmful NO-derived reactive oxygen species, the hyper-activation of the renin-angiotensin-aldosterone system (RAAS) and the sepsis-related catecholamine release that strongly contributes to microvascular vasoconstriction, thrombosis and/or hemorrhage [7, 17].
Immune Products, Activators of Complement and Coagulation Cascades. Cell injury exposes matrix proteins to bloodstream and activates the coagulation cascade. In parallel, complement system is activated by pathogens (mannose pathway), immune complexes (direct pathway) and by downregulation of complement-inhibiting proteins within injured cells (indirect-pathway). Moreover, the loss of glomerular filtration barrier causes proteinuria and exposes complement products to tubular brush border enzymes, thus inducing intra-luminal complement activation [18]. Additionally, activated immune cells release pathogen-killing factors (perforin, granzyme-B, etc.) able to worsen and to perpetuate cell injury [19].
Metabolites and Uremic Toxins. Uremic toxin is an omni-comprehensive term that includes all factors accumulating/upregulated during renal failure that cause any kind of tissue injury. Based on this definition, several molecules included in the previous categories can be defined as uremic toxins (i.e., some cytokines and RAAS). However, the largest part of uremic toxins is constituted by detrimental metabolic products, normally excreted by the kidney. p-Cresol sulfate and indoxyl sulfate are protein-bound metabolites not filtered by glomeruli and secreted by TEC; their accumulation in AKI and CKD is associated with several harmful effects such as endothelial injury and immune dysfunction [20].
Extracellular Vesicles (EV), Apoptotic Bodies and Other Cell Fragments. EV are membrane fragments actively produced to shuttle proteins, nucleic acids, lipids and other metabolites from an origin to a target cell [21]. EV play a key role in cell-to-cell communication processes and are classified as exosomes (30–120 nm in size and released by multi-vesicular bodies) or microvesicles (>120 nm and released by a membrane-sorting process), and they are involved in tissue repair and homeostasis. In the course of sepsis, a significant increase of plasma EV concentration is observed [22]. EV can be released by different cell types including monocytes, platelets and injured endothelial cells. The biological effects of EV may change in relation to the state of activation of the origin cell: this is of particular relevance in S-AKI patients in which plasma EV may represent not only a new biomarker for the early detection of disease, but also a key element in the pathogenic mechanisms of renal damage [22, 23]. Plasma EV are able to modulate NO and prostacyclin endothelial release, activate the coagulation cascade and cytokine production. EV isolated from septic animals and injected in healthy ones were able to induce the same functional and biological alterations observed in the course of the systemic inflammatory response, suggesting that EV can somehow transfer the septic disease to a healthy animal [24]. Moreover, sepsis-induced tissue injury promotes the passive release of other cell fragments such as apoptotic and necrotic bodies that have high DAMP concentrations. Several pathogens may also exploit EV to spread the infection or to transport PAMPs/toxins. Of interest, preliminary data from our research group demonstrated that despite their small size, EV are electrically charged and are not easily removed by standard diffusive and/or convective RRT.
Fluid Overload
Recent studies demonstrated the relevant role of fluid overload as AKI determinant. Indeed, a number of retrospective analysis investigating critically ill patients correlated central venous pressure and fluid overload with mortality and worse renal outcomes [25]. All these findings have been recently confirmed by a large multicenter, prospective, observational trial: authors found that the severity and speed of fluid accumulation are independent risk factors for ICU mortality [26].
New Biomarkers and Phenotypic Analysis of Cells in the Peripheral Blood
Diverse studies showed the association between serum levels of specific mediators and a worse outcome in septic patients. Soluble CD40-ligand, Fas-ligand and angiopoietin-2 are middle molecules involved in inflammation, coagulation and apoptotic cell damage that were found to be significantly increased in septic critically ill patients with a worse outcome [27]. In recent years, sepsis research also focused on the alterations of peripheral blood cells: it has been shown that a decreased expression of HLA-DR on monocytes is an indicator of immune paralysis and increased death risk [28].
