## 3 Results  
**Ly6C\({}^{\text{Lo}}\) non-classical monocytes reside in the neonatal liver. ** We quantified the proportion of mature myeloid populations in the neonatal liver and bone marrow under homeostatic conditions. Using flow cytometry, we identified neutrophils (Ly6G+, MHCII\(-\)), macrophages (CD64+, CD11b+/\(-\)), and monocytes (CD64\(-\) CD11b+). We further separated monocytes into Ly6C\({}^{\text{Hi}}\) classical and Ly6C\({}^{\text{Lo}}\) non-classical monocytes (Figure 1a). Monocytes, particularly Ly6C\({}^{\text{Lo}}\) non-classical monocytes, were abundant in the liver but not in the bone marrow (Figure 1b). This finding was similar to that derived in our previous work, demonstrating the relative abundance of Ly6C\({}^{\text{Lo}}\) non-classical monocytes in the late-gestation fetus [12]. In P3 pups, monocytes comprised \(\sim\)8% of all CD45+ leukocytes in the liver, compared to \(\sim\)6% in the bone marrow (\(p=0.018\)). Ly6C\({}^{\text{Lo}}\) non-classical monocytes were the  
Figure 1: **Pro-reparative Ly6C\({}^{\text{Lo}}\) non-classical monocytes reside in the liver at the early neonatal timepoint. ****(a)** Myeloid populations including neutrophils, macrophages, Ly6C\({}^{\text{Hi}}\) classical monocytes and Ly6C\({}^{\text{Lo}}\) non-classical monocytes identified by flow cytometry. (**b)** Myeloid populations at an early neonatal timepoint (P3) as a proportion of all CD45% leukocytes. The neonatal liver (\(n=19\)) harvests significantly more monocytes, particularly Ly6C\({}^{\text{Lo}}\) non-classical monocytes, compared to the BM (\(n=18\)) (BM 3.76% vs liver 5.62%; \(p=0.0015\)). (**c)** At the juvenile timepoint (P14) the liver (\(n=12\)) no longer harvests monocytic populations. Ly6C\({}^{\text{Hi}}\) classical monocytes predominately reside in the BM (\(n=11\)) at this point (BM 2.11% vs liver 0.58%, \(p<0.0001\)). \({}^{*}\)\(p\leq 0.05\); \({}^{**}\)\(p\leq 0.01\); \({}^{***}\)\(p\leq 0.001\). predominant monocyte subset in the liver (5.62% vs. 3.76% for bone marrow, \(p=0.001\)) (Figure 1b). Higher proportions of neutrophils were seen in the bone marrow than in the liver (69.78% vs. 53.43%, \(p=0.002\)), whereas macrophages comprised \(\sim\)12% in both organs. In juvenile mice, monocytes no longer predominated in the liver at P14; instead, these were seen in higher proportions in the bone marrow (4.59% vs. liver 3.1%, \(p=0.01\)) (Figure 1c). Ly6C\({}^{\text{CH}}\) classical monocytes were seen in higher proportions in the bone marrow (2.11% vs. liver 0.58%, \(p<0.0001\)), whereas Ly6C\({}^{\text{Lo}}\) non-classical monocytes comprised \(\sim\)2.5% of all leukocytes in both organs at this timepoint (Figure 1c). Monocytes overall, and specifically Ly6C\({}^{\text{Lo}}\) non-classical monocytes, were present in higher proportions in the P3 liver than in the bone marrow, and made up a smaller proportion of total leukocytes in P14 mice than in neonates (Figure 1c). **Nr4a1 expression is highest in Ly6C\({}^{\text{Lo}}\) non-classical monocytes compared to other myeloid populations in the neonatal liver. ** We characterized Nr4a1 expression in mature myeloid populations under homeostatic conditions in the neonatal liver and bone marrow. To quantify the expression of Nr4a1, we used a reporter mouse in which the expression of enhanced green fluorescent protein (GFP) is under the control of the Nr4a1 regulatory region [20]. We observed that high proportions of all myeloid populations expressed Nr4a1 (%GFP-positive cells) in both the neonatal liver and the bone marrow (Figure 2a-d). In particular, significantly higher proportions of neutrophils, macrophages, and Ly6C\({}^{\text{CH}}\) classical monocytes in the neonatal bone marrow expressed Nr4a1 compared to the neonatal liver (Neutrophils: 80.03% vs. 66.52%; \(p=0.02\). Macrophages: 53.46% vs. 33.83%; \(p<0.001\). Ly6C\({}^{\text{CH}}\) classical monocytes: 66.13% vs. 56.51%; \(p=0.03\)) (Figure 2a-c).