Both AKI and ALI significantly reduce survival, particularly when they occur together. Nonetheless, only limited and at times even controversial information is available regarding kidney-lung cross-talk under pathological conditions. Moreover, there are no data regarding the mechanisms of kidney-lung interactions during bacterial ALI and AKI. A better understanding is of utmost importance, as pneumonia represents the most frequent cause of sepsis which in turn is the leading cause of AKI in critically-ill patients.24,25
Using a murine two-hit model of AKI and bacterial pneumonia, we show that FA-induced AKI itself has no clinically overt effect on healthy lungs, as indicated by unchanged oxygenation 48h after induction of AKI. Similarly, lung histology is largely unaffected by FA-induced AKI, although there is a slight trend toward increased lung MPO-activity, indicating mild neutrophil influx. Preexisting AKI, however, attenuates pulmonary neutrophil recruitment during pneumonia but does not affect plasma IL-6 levels during pneumonia. AKI appears to increase bacterial load during pneumonia, to further impair oxygenation, and to ultimately worsen pneumonia. We were able to reproduce these findings in a clinically relevant model of myohemoglobinuric AKI and to thereby further substantiate the general concept of AKI-impaired neutrophil recruitment into the inflamed lung.
Our findings confirm recent studies showing that neutrophils and their recruitment control development and severity of bacterial ALI (P. aeruginosa
As outlined before, there is experimental evidence to substantiate the concept of AKI-induced inflammation in the healthy lung.3,9,27–30
We have also observed a non-significant, slight increase in pulmonary neutrophil content in mice with isolated AKI (group 2). We observed similar changes for plasma IL-6 level after induction of AKI. We observed similar non-significant changes for plasma IL-6 level after induction of AKI. This is in contrast to other mouse models of AKI, such as ischemia-reperfusion and bilateral nephrectomy. However, these models require major abdominal or retroperitoneal surgery which by themselves could explain the elevated levels of IL-6.3,4
In models of bilateral nephrectomy and renal ischemia-reperfusion, we could not find signs of impaired oxygenation or significant pulmonary edema.10
To the contrary, both our previous study10
and our current study suggest that AKI exerts a strong anti-inflammatory and clinically relevant effect on the injured lung. We have previously demonstrated in two different murine models of AKI combined with aseptic ALI that uremic neutrophils fail to recruit into the inflamed lung and thereby protect from ALI. 10
These effects appeared to depend solely on uremic neutrophils and occurred in both normal and uremic milieus. 10
In our current study, we provide further evidence to support the idea of AKI-induced inhibition of pulmonary neutrophil recruitment. Pre-existing AKI attenuates neutrophil recruitment into the lung after inhalation of P. aeruginosa
. Similar to neutrophil-depleted mice, mice with pre-existing AKI display worse oxygenation and higher bacterial load after inhalation of P. aeruginosa
than mice without AKI. Moreover, neutrophils from mice with FA-induced AKI also demonstrated significantly impaired transmigration and F-actin polymerization in vitro
. Combining our previous results and these findings, we postulate that AKI-induced inhibition of neutrophil recruitment gives rise to clinically relevant aggravation of bacterial ALI with worse oxygenation and higher bacterial load.
Although our previous findings suggest that the observed effects depend directly on neutrophils rather than soluble factors, we could not find changes in the expression of neutrophil surface molecules, e.g. L-selectin (data not shown). We therefore focused on intracellular events controlling neutrophil recruitment. Directed neutrophil migration depends on the coordinated rearrangement of cytoskeleton structures that provide both protrusion and contraction. Actin filaments polymerize at the leading edge, generating a protrusive leading edge that pushes the cell forward. Contraction occurs through the assembly of actomyosin filaments along the main body and at the tail, suppressing lateral protrusions but enabling protrusions at the leading edge.31–33
Our data show that neutrophils from mice with FA-induced AKI have a substantial transmigration defect. Moreover, these neutrophils cannot limit F-actin formation. As a single and restricted lamellipodia is essential for directed migration,32,34
, the defective migration of neutrophils from mice with FA-induced AKI seen in our transwell assays may be attributed to abnormal cytoskeleton regulation.
Available data together allow us to hypothesize that the effect of AKI on pulmonary neutrophil recruitment depends on the condition of the lung. Under non-inflammatory conditions, mechanisms causing AKI or AKI itself appear to have remote, pro-inflammatory effects on the lung that can enhance neutrophil recruitment. AKI, nonetheless, does not promote neutrophil recruitment during pulmonary inflammation, which inherently triggers neutrophil recruitment into the lung. Here, AKI exerts anti-inflammatory effects toward neutrophils and ultimately inhibits neutrophil recruitment. Histological signs of less pulmonary inflammation in mice with pneumonia plus AKI compared to those with only pneumonia further add to the notion of AKI-induced anti-inflammatory effects. Neutrophil recruitment itself can trigger excessive inflammatory responses in the affected tissue. Thus, a lack of neutrophil recruitment can lead to attenuated tissue inflammation.35
Moreover, these findings also allow us to hypothesize that pulmonary damage through P. aeruginosa
itself, e.g. via exotoxins,36
plays a greater role under theses conditions than damage caused by neutrophil-dependent pulmonary inflammation per se.
Findings from a recent study about kidney-lung interaction during AKI and ventilation-induced lung injury also support a differential concept regarding pulmonary neutrophil recruitment during AKI.37
In the setting of high tidal volume ventilation, which in itself causes lung inflammation, AKI impaired pulmonary neutrophil recruitment into the lung. Under less inflammatory conditions, e.g., low tidal volume ventilation or spontaneous breathing, these effects were not seen.
Finally, as neutrophils and their recruitment represent the body’s the first-line defense against infections35
, our experimental findings also have the potential to explain several clinical observations. For example, patients with AKI more frequently develop bacteremia and poor outcome in the setting of peritonitis, hematologic malignancies, and after cardiac surgery than patients without AKI.38–41
Various mechanisms have been attributed to ALI-induced AKI. Mostly, mechanisms of injury have been considered a consequence of mechanical ventilation, including a reduction in cardiac output, redistribution of renal blood flow, and stimulation of hormonal and sympathetic pathways.13
Recent animal data suggest that maintenance of stable hemodynamics and alveolar ventilation are crucial in preserving renal function in the setting of ALI.42
Mechanical ventilation also induces apoptosis in tubular epithelial cells17
as well as a so-called biotrauma.12,16
We here provide further evidence that bacterial ALI (pneumonia) itself, without underlying mechanical ventilation, can cause remote organ injury. Here, pneumonia leads to AKI within 24h, as evidenced by corresponding changes in plasma creatinine and cystatin C concentrations. Pneumonia-induced AKI appears to be due to direct tubular damage rather than pre-renal factors, as evidenced by the significantly increased NGAL concentrations and stable pulse distension. Moreover, direct bacterial damage to the kidney is also unlikely, as only a small number of mice had blood cultures positive for P. aeruginosa, and we could not find histological evidence of bacterial invasion into the kidneys.
Previously unrecognized as mediators of remote organ failure, here, platelets have emerged as crucial for the development of pneumonia-induced AKI. Despite worsening pneumonia, platelet-depleted mice showed nearly normal plasma creatinine concentrations. Platelet and platelet-derived microparticles represent well-known prognostic factors for the severity of septic organ failure.43,44
Nonetheless, whether platelets themselves or platelet-derived microparticles or both are responsible for this effect, requires further studies,
Our study reveals clinically relevant kidney-lung interactions in a murine model of bacterial pneumonia combined with AKI. Our data further substantiate the notion of AKI-induced inhibition of neutrophil recruitment, which translates into higher bacterial load and worse oxygenation in the setting of bacterial pneumonia. Impaired neutrophil recruitment during AKI seems, at least partially, to be due to altered cytoskeleton arrangement. On the other side, bacterial pneumonia, causes AKI in a platelet-dependent manner.