Outcomes of extracorporeal membrane oxygenation in immunosuppressed vs. Immunocompetent patients.

INTRODUCTION: Immunosuppressed hosts represent a growing group of patients who suffer acute respiratory failure and may be considered for therapies such as extracorporeal membrane oxygenation (ECMO). OBJECTIVES: We conducted this retrospective study to determine whether acutely or chronically immunosuppressed patients placed on ECMO for cardiac and/or respiratory failure in our institution have different outcomes than immunocompetent patients placed on ECMO in our institution. METHODS: Adult patients placed on ECMO between June 31, 2010 and July 7, 2021 were identified within an IRB-approved database. Data was retrospectively extracted from the database and patients' medical records. Patients who survived ECMO decannulation were sub-grouped by the presence of acute or chronic immunosuppression, defined by the use of high-dose steroids or immunosuppressive agents for greater than four weeks prior to ECMO initiation. We analyzed and compared baseline characteristics and clinical outcomes using chi-squared tests for categorical variables and a one-way analysis of variance (ANOVA) for continuous variables. RESULTS: 385 patients were included in this study, with 39 identified as chronically immunosuppressed, 49 as acutely immunosuppressed, and 297 as immunocompetent. There was no statistical difference in ECMO survival (respectively 54%, 59%, 65% p = 0.359) or 30-day survival (33%, 51%, 48% p = 0.149) for chronically immunosuppressed, acutely immunosuppressed, and immunocompetent, respectively. There were significant differences in rates of pre-ECMO COVID infection (p<0.001), coronary artery disease (p<0.001), smoking (p = 0.003), and acute kidney injury (p = 0.032). Acutely immunosuppressed patients had the highest rates of new infections during ECMO (p = 0.006). CONCLUSION: When compared to immunocompetent patients, both acutely and chronically immunosuppressed patients had no significant difference in ECMO survival or 30-day survival. Acutely immunosuppressed patients had less comorbidities than chronically immunosuppressed patients, but they were more commonly infected during ECMO. ECMO may still be a valuable tool in appropriately selected patients with refractory respiratory or cardiac failure.

Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)-phosphate complex: a potential non-transferrin bound iron species.

In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe(3+) loading into apo-transferrin by forming Fe(III)-phosphate species. We report that phosphate competes with apo-transferrin for Fe(3+) by forming a soluble Fe(III)-phosphate complex. Once formed, the Fe(III)-phosphate complex is not a substrate for donating Fe(3+) to apo-transferrin. Phosphate (1-10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe(3+), such as iron nitrilotriacetic acid (Fe(3+)-NTA), and Fe(III)-citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe(3+) to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe(3+) and phosphate, the addition of Fe(2+) to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe(3+) into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.

The ferroxidase center is essential for ferritin iron loading in the presence of phosphate and minimizes side reactions that form Fe(III)-phosphate colloids.

Ferritin iron loading was studied in the presence of physiological serum phosphate concentrations (1 mM), elevated serum concentrations (2-5 mM), and intracellular phosphate concentrations (10 mM). Experiments compared iron loading into homopolymers of H and L ferritin with horse spleen ferritin. Prior to studying the reactions with ferritin, a series of control reactions were performed to study the solution chemistry of Fe(2+) and phosphate. In the absence of ferritin, phosphate catalyzed Fe(2+) oxidation and formed soluble polymeric Fe(III)-phosphate complexes. The Fe(III)-phosphate complexes were characterized by electron microscopy and atomic force microscopy, which revealed spherical nanoparticles with diameters of 10-20 nm. The soluble Fe(III)-phosphate complexes also formed as competing reactions during iron loading into ferritin. Elemental analysis on ferritin samples separated from the Fe(III)-phosphate complexes showed that as the phosphate concentration increased, the iron loading into horse ferritin decreased. The composition of the mineral that does form inside horse ferritin has a higher iron/phosphate ratio (~1:1) than ferritin purified from tissue (~10:1). Phosphate significantly inhibited iron loading into L ferritin, due to the lack of the ferroxidase center in this homopolymer. Spectrophotometric assays of iron loading into H ferritin showed identical iron loading curves in the presence of phosphate, indicating that the ferroxidase center of H ferritin efficiently competes with phosphate for the binding and oxidation of Fe(2+). Additional studies demonstrated that H ferritin ferroxidase activity could be used to oxidize Fe(2+) and facilitate the transfer of the Fe(3+) into apo transferrin in the presence of phosphate.

Ferritin iron mineralization proceeds by different mechanisms in MOPS and imidazole buffers.

The buffer used during horse spleen ferritin iron loading significantly influences the mineralization process and the quantity of iron deposited in ferritin. Ferritin iron loading in imidazole shows a rapid hyperbolic curve in contrast to iron loading in 3-(N-morpholino)propanesulfonic acid (MOPS), which displays a slower sigmoidal curve. Ferritin iron loading in an equimolar mixture of imidazole and MOPS produces an iron-loading curve that is intermediate between the imidazole and MOPS curves indicating that one buffer does not dominate the reaction mechanism. The UV-visible spectrum of the ferritin mineral has a higher absorbance from 250 to 450 nm when prepared in imidazole buffer than in MOPS buffer. These results suggest that different mineral phases form in ferritin by different loading mechanisms in imidazole and MOPS buffered reactions. Samples of 1500 Fe/ferritin were prepared in MOPS or imidazole buffer and were analyzed for crystallinity and using the electron diffraction capabilities of the electron microscope. The sample prepared in imidazole was significantly more crystalline than the sample prepared in MOPS. X-ray powder diffraction studies showed that small cores (~500 Fe/ferritin) prepared in MOPS or imidazole possess a 2-line ferrihydrite spectrum. As the core size increases the mineral phase begins to change from 2-line to 6-line ferrihydrite with the imidazole sample favoring the 6-line ferrihydrite phase. Taken together, these results suggest that the iron deposition mechanism in ferritin can be controlled by properties of the buffer with samples prepared in imidazole forming a larger, more ordered crystalline mineral than samples prepared in MOPS.

Non-reductive iron release from horse spleen ferritin using desferoxamine chelation.

The rate of Fe(3+) release from horse spleen ferritin (HoSF) was measured using the Fe(3+)-specific chelator desferoxamine (DES). The reaction consists of two kinetic phases. The first is a rapid non-linear reaction followed by a slower linear reaction. The overall two-phase reaction was resolved into three kinetic events: 1) a rapid first-order reaction in HoSF (k(1)); 2) a second slower first-order reaction in HoSF (k(2)); and 3) a zero-order slow reaction in HoSF (k(3)). The zero-order reaction was independent of DES concentration. The two first-order reactions had a near zero-order dependence on DES concentration and were independent of pH from 6.8 to 8.2. The two first-order reactions accounted for 6-9 rapidly reacting Fe(3+) ions. Activation energies of 10.5±0.8, 13.5±2.0 and 62.4±2.1kJ/mol were calculated for the kinetic events associated with k(1), k(2), and k(3), respectively. Iron release occurs by: 1) a slow zero-order rate-limiting reaction governed by k(3) and corresponding to the dissociation of Fe(3+) ions from the FeOOH core that bind to an Fe(3+) binding site designated as site 1 (proposed to be within the 3-fold channel); 2) transfer of Fe(3+) from site 1 to site 2 (a second binding site in the 3-fold channel) (k(2)); and 3) rapid iron loss from site 2 to DES (k(1)).