**4. Discussion**

Primary nonfunction, poor early graft function, and decreased patient and allograft survival have all been associated with the use of steatotic liver allografts for transplantation [1–3,5]. This, combined with inaccurate and inconsistent reporting of liver allograft biopsies, has led to a high discard rate of grafts with moderate (>30%) steatosis [11]. While the risk of adverse events with severely steatotic liver allografts (>60%) remains well recognized, the use of moderately steatotic grafts (30%–60%) has been increasing [7].

It has been our experience that steatotic grafts almost universally exhibit early allograft dysfunction and require additional resource utilization postoperatively specific to the development of post-LT AKI and the need to initiate new dialysis [7,12–15]. In this study, the occurrence of AKI post-LT was noted to be markedly increased at 52.5% compared to 19.6% observed in the general liver transplant recipient pool. Although early allograft dysfunction, best demonstrated by significantly elevated transaminases, was common to all steatotic grafts, not all recipients developed post-LT AKI (Figure 2). We have clinically observed this divergent pattern; however, it remains difficult to quantify why some steatotic grafts behave in this manner while others do not. In this study, patients receiving steatotic grafts that developed post-LT AKI were noted to have higher inotrope requirements post-reperfusion. Despite having higher inotrope requirements, no differences were observed in hemodynamic parameters (systolic blood pressure and MAP) between the patients with and without post-LT AKI, suggesting that variables other than hemodynamics influence the development of AKI. Not surprisingly, a higher MELD score was associated with an increased risk of post-LT AKI. This association was particularly strong when steatotic grafts with lipopeliosis on biopsy were transplanted to patients with MELD scores of 20 or above (Tables 2 and 5) (Figure 4).

Proper classification of graft steatosis remains challenging even within the transplant community [9]. Historically, steatosis was classified as microvesicular or macrovesicular, based on hepatocyte fat droplet size and nucleus displacement. Although often reported, true microvesicular steatosis is rare and manifests histologically as diffuse deposition of small lipid droplets in the hepatocyte cytoplasm with a resulting foamy appearance (Figure 1D). Two types of fat droplets are seen in the setting of macrovesicular steatosis: small droplets and large droplets (Figure 1C). Fat droplets in small droplet steatosis are not large enough to displace the nucleus; this finding is often inaccurately reported as being microvesicular steatosis on biopsy. Although both small and large droplet steatosis contribute to overall macrovesicular steatosis, historically small droplet macrovesicular steatosis and microvesicular steatosis were used synonymously, resulting in ongoing confusion [3]. We hypothesize that some of the observed di fferences in post-LT AKI between otherwise similarly-appearing steatotic grafts might be related to this histological variation.

The term lipopeliosis was first identified in the early days of hepatic transplantation and describes the coalescence of fat droplets from ruptured hepatocytes into larger droplets of fat in the sinusoidal space [16]. Due to the universal finding of preservation-related injury in these biopsies, lipopeliosis was presumed to not be of clinical significance [17]. In our experience, however, the histologic finding of lipopeliosis is, by far, more common in patients who develop post-LT AKI and, in our experience, has been associated with inferior post-transplant outcomes [18].

Although the current study was not designed to elucidate the mechanisms of AKI post-LT, we have previously shown that fat droplets, through the process of lipopeliosis, embolize to the pulmonary vasculature following reperfusion with resulting respiratory failure [18]. This mechanism is likely similar to that seen after long bone traumatic injuries, where fat droplets are released into the venous system and migrate to the pulmonary capillary beds [19,20]. Microvascular lodging results in ischemia, inflammation, and release of inflammatory mediators. The breakdown of fat emboli by pneumocytes can results in release of free fatty acids that, in turn, enter systemic circulation and result in multisystem dysfunction [21]. The finding of fat droplets in the urine under these circumstances correlates with the development of AKI and a systemic process triggered by fat embolization [22]. Whether lipopeliosis in steatotic liver graft biopsies can be used to predict clinical instability and the development of post-LT AKI will need to be validated in a larger prospective study. Limitations to this study include its overall small cohort size. The results also represent the experience of only two centers. Patients with pre-LT AKI were also excluded in this study to better assess post-LT outcomes specific to AKI development. The impact of steatosis on renal function in patients with preexisting AKI remains uncertain.

In conclusion, utilization of moderately steatotic grafts is associated with a significantly higher risk for developing post-LT AKI. This risk appears to be independent of pre-reperfusion operative hemodynamics. In utilizing these grafts, laboratory abnormalities persist 2 to 3 months post-LT, but there does not appear to be an impact on long-term renal function, patient, or graft survival. The risk for AKI in this setting appears increased when the MELD score is greater than 20 and lipopeliosis is histologically present on biopsy. These outcomes are more favorable as compared to older studies, and sugges<sup>t</sup> that, with lower MELD recipients, satisfactory outcomes can be achieved with the use of these grafts [5].

**Author Contributions:** For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used "Conceptualization, C.J. and T.T.; methodology, C.J. and T.T.; software, C.J. and M.N.; validation, C.J., M.N. and T.T.; formal analysis, C.J., M.S., M.N., S.M., D.A., K.R., A.M., B.A., T.T.; investigation, C.J., M.S., T.T.; resources, C.J.; data curation, C.J., M.S., M.N., S.M, D.A. and T.T.; writing—original draft preparation, C.J., M.S., M.N., S.M., D.A., K.R., A.M., B.A., T.T.; writing—review and editing, C.J., M.S., M.N., S.M., D.A., K.R., A.M., B.A., T.T.; visualization, C.J. and T.T.; supervision, T.T.; project administration, C.J.; funding acquisition, Not applicable. All authors have read and agreed to the published version of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
