## 4 Discussion  
In this study, we hypothesized that Nr4a1 is important for Ly6C\({}^{\text{Lo}}\) non-classical monocyte differentiation, and is therefore a key regulator of perinatal liver inflammation. Our results demonstrate that in the liver and bone marrow, Nr4a1 expression was highest among hepatic neonatal Ly6C\({}^{\text{Lo}}\) non-classical monocytes compared to neutrophils, macrophages, and Ly6C\({}^{\text{Hi}}\) classical monocytes. In a murine model of BA, although the levels of hepatic Ly6C\({}^{\text{Lo}}\) non-classical monocytes decreased, their expression of Nr4a1 increased significantly. However, in the bone marrow, Nr4a1 expression decreased significantly during inflammation, but increased in Ly6C\({}^{\text{Hi}}\) classical monocytes. Collectively, these results support the idea that Nr4a1 is an important transcription factor for the pro-reparative response mediated by hepatic Ly6C\({}^{\text{Lo}}\) non-classical monocytes during inflammation in murine BA. Ly6C\({}^{\text{Lo}}\) non-classical monocytes have been shown to promote a pro-reparative response in the setting of inflammation, and specifically promote the resolution of periportal liver inflammation to confer protection to liver disease in murine neonates [9, 10, 11, 12]. In our  
Figure 4: Ly6C\({}^{\text{Lo}}\) non-classical monocytes increase Nr4a1 expression in the setting of inflammation. (**a,b**) Myeloid populations and monocyte subsets at (P3) as a proportion of all CD45% leukocytes under homeostatic (\(n=19\)) and inflammatory (\(n=13\)) conditions in the neonatal liver. (**c**) Reduction of Ly6C\({}^{\text{Lo}}\) non-classical monocyte population in the liver by flow cytometry. (**d–g**) Intensity of Nr4a1 expression (MFI) is increased on macrophages and Ly6C\({}^{\text{Lo}}\) non-classical monocytes in the setting of inflammation (WT \(n=14\), RRV \(n=12\)). * \(p\leq 0.05\); ** \(p\leq 0.01\); ** \(p\leq 0.001\). **study, we demonstrate that the liver acts as a reservoir for Ly6C\({}^{\text{Lo}}\) non-classical monocytes in neonatal mice before the physiologic homing process of hematopoietic cells to the bone marrow. While the homing process is a well-described phenomenon, the normal physiology and development of the murine hematopoietic system shortly before and after birth are not yet fully elucidated [25; 26; 27]. Hematopoietic cells have been examined extensively in the murine fetus [28; 29; 30]; however, mature cells at later stages of development, particularly in perinatal pups, have not been examined as thoroughly [31; 32]. Hematopoietic cells are believed to migrate to the fetal liver at approximately day 10 post-coitus, and at or near birth, cells then migrate from the liver to the bone marrow, where they remain throughout the animal's adult life [25; 26; 27]. Our results demonstrate that by postnatal day 14, the myeloid populations predominately reside in the bone marrow, and the liver no longer maintains a significant proportion of monocytes. Not only does this finding clarify the timing of the homing process from the liver to the bone marrow, but it also highlights that the neonatal liver initially maintains a high proportion of the pro-reparative Ly6C\({}^{\text{Lo}}\) non-classical monocytes, raising the possibility that these cells may confer resistance to perinatal injury. Previous studies have demonstrated the importance of Nr4a1 in the differentiation and survival of the pro-reparative Ly6C\({}^{\text{Lo}}\) non-classical monocytes [13; 33]. Monocyte subsets arise from a common monocyte progenitor (cMoP) in the adult murine bone marrow, but the intricacies of this development remain poorly understood due to the challenges of lineage tracing [33]. The most accepted hypothesis currently supports the idea that cMoPs give rise to Ly6C\({}^{\text{Hi}}\) classical monocytes, which can then differentiate into Ly6C\({}^{\text{Lo}}\) non-classical monocytes [34; 35; 36; 37; 38].