Hematopoiesis is the process of mature blood and immune cell production and maintenance from hematopoietic stem and progenitor cells (HSPCs). The majority of "traditional" blood and immune cells, including eosinophils, have short half-lives and are continually replaced by HSPCs 1 . In contrast, fetally-derived tissue resident macrophage (trMacs) self-maintain in their respective tissues for life, without contribution from adult HSCs, thereby leading us to consider them "nontraditional" mature immune cells [2] [3] [4] [5] . Recently, our lab uncovered a novel role for interleukin 7 receptor (IL7R) in trMac specification during development 6 . Though IL7R had originally been reported to be essential exclusively for lymphocyte development and survival, a few recent studies, including ours, have found that it also plays roles in myeloid cell development [6] [7] [8] . Interestingly, in addition to our finding that IL7R regulates trMac specification during mouse fetal development 4 , studies in humans have reported that other myeloid cell types, including monocytes and eosinophils, may express IL7R 7, 9 . Furthermore, these cells can upregulate IL7R mRNA and surface protein upon stimulation with lipopolysaccharide (LPS), a known activator of eosinophils.
Here, we used germline knockout mice, lineage tracing, and transplantation assays to determine whether IL7R plays a role in adult-derived "traditional" myeloid cell types in the lung, peripheral blood (PB), and bone marrow (BM).
During our previous studies where we found that IL7R unexpectedly regulates trMac development, we also noted alterations in other cells types in the lungs of IL7Ra -/mice. We therefore employed Flk2 -/and IL7Ra -/mice and crossed the two strains to also create homozygous double mutants (referred to as FIDKO mice here) to investigate lung cellularity in three mutant cohorts. Flk2 is a tyrosine kinase receptor that regulates hematopoietic development 10 , including robust numbers of common myeloid progenitor cells (CMPs) 10, 11 , the presumed precursors of eosinophils. Previously, our lab established that all adult-derived "traditional" mature blood and immune cells, but not trMacs 6, 12 , develop through a Flk2+ pathway 6, 11, 13, 14 . As expected based on previous reports of circulating lymphocytes 15 , B lymphocytes in the lungs were decreased in all three mutant strains relative to the WT controls, with drastic reductions in the IL7Ra -/and FIDKO mice (Fig.1A) , while neutrophil numbers were normal in all three genetic models (Fig. 1B) .
Surprisingly, deletion of IL7Ra also led to a significant reduction in numbers of eosinophils in the lungs, with a further significant reduction in the FIDKO mice (Fig. 1C) . To determine whether this was a tissue-specific phenotype, we also analyzed the PB and BM of WT and IL7R -/mice. We observed that eosinophils were similarly affected in these tissues ( Fig. S1B and D) . Interestingly, there was no significant difference in neutrophils numbers in the BM of these mice, but there were significantly more circulating neutrophils in the IL7Ra -/mice ( Fig. S1A and C) . These data indicate that IL7R is differentially required for eosinophil and neutrophil homeostasis across tissues.
Recently, we established that IL7R-Cre, but not Flk2-Cre, robustly labels trMacs across several tissues due to transient, developmental expression of IL7Ra in the monocyte-macrophage lineage 6 . To interrogate whether eosinophils have a history of IL7R expression, we crossed mice expressing IL7R-Cre 16 to mTmG mice containing a dual color fluorescent reporter 17 , thereby creating the "IL7RαSwitch" model analogous to the previously described FlkSwitch mouse ( Fig.   2A -2B) 6, 13, 14, 18 . In both models, all cells express Tomato (Tom) unless Cre-mediated recombination cause an irreversible switch to GFP expression by Cre-expressing cells and all of their progeny ( Fig. 2A-2B ). Labeling of lung B lymphocytes was high (>95%) in both models ( Fig. 2C ), as has previously been reported for circulatory lymphocytes 6, 16 . Similar to myeloid cells in the peripheral blood (PB), labeling of lung neutrophils and eosinophils was high (>95%) in the FlkSwitch mice ( Fig. 2D-2E ). In contrast, in the IL7RaSwitch mice labeling of lung neutrophils was nominal (<3%) (Fig. 2D ), similar to previously reported PB neutrophils 6, 16 . Despite the significant reduction in eosinophil numbers in IL7Ra -/mice (Fig. 1C) , labeling of eosinophils was also nominal (<3%) in the IL7RαSwitch mice (Fig. 2E) . These data show that most lung eosinophils do not develop through an IL7Ra+ pathway yet depend on IL7R for maintenance of normal cell numbers.
To determine whether IL7R-deficient progenitors lack the intrinsic potential to make eosinophils, we transplanted IL7Rα -/or wild type (WT) HSCs into sublethally irradiated WT recipients and analyzed immune cell reconstitution in the blood and lung >4 months post-transplantation (Fig.  3A ). As expected, the IL7Rα -/-HSCs showed diminished ability to produce circulating lymphoid, but not "traditional" myeloid, cells in the blood of the same mice (Fig. S2A,B) . In contrast, lung neutrophils and eosinophils were generated with equal efficiency by WT and IL7Rα -/-HSCs (Fig.   3B ). The strong reconstitution potential of eosinophils and neutrophils of the lung, comparable to the GM donor chimerism in the periphery, further supports that these cells are "traditional", adult HSC-derived myeloid cell types and that they do not intrinsically require IL7R for their development.
To determine whether the requirement for IL7R was eosinophil-extrinsic, WT HSCs were transplanted into either WT or IL7Rα -/recipients ( Fig. 3C ). WT HSCs efficiently contributed to circulating GMs, Bs, and Ts ( Fig. S2C,D) , but, intriguingly, displayed significantly impaired reconstitution of neutrophils and eosinophils in the lungs of IL7Rα -/recipients ( Fig. 3D ). Taken together, these data suggest that extrinsic IL7R is required for homeostasis of neutrophils and eosinophils in the lung, but not for other "traditional" myeloid cells which circulate in the periphery.
Previous literature has reported several factors implicated in eosinophil development, homing, and survival, including Eotaxin 19,20 , IL-5 21-24 , and others 7, 8, 25 . To determine which factors may be influencing eosinophil development and survival in the IL7R mutant mice, we collected serum from WT and mutant mice and compared the relative concentration of several cytokines. Because IL-7 has previously been reported to be elevated in IL7Rα -/mice 7 , we first tested IL-7 levels as our positive control. Consistent with previous data, we observed that IL-7 was upregulated several fold in IL7Rα -/mice, although borderline statistically significant here (~10-fold, p<0.1; Fig. 3E ).
Importantly, we also observed that eosinophil-promoting Eotaxin was significantly downregulated in the IL7Rα -/mice (Fig. 3E) . Additionally, IL-5 was decreased 3-fold in the mutant mice, near statistical significance (p<0.1; Fig. 3E ). Taken together, these data indicate that downregulation of Eotaxin and IL-5, and possibly additional factors, may play important roles in eosinophil development and survival in the IL7R mutant mice.
Our data presented here reveal that IL7R is required for specific myeloid cell homeostasis in the lungs, PB, and BM of adult mice. Other groups have reported that human eosinophils have detectable levels of IL7Ra mRNA and surface protein 7, 8 , and that IL7R mRNA can be induced in human monocytes with LPS stimulation 9 . Similarly, we previously found that trMacs in the lung and other tissues transiently express IL7Rα during development 6 . However, our IL7R-Cre lineage tracing data of traditional lung myeloid cells reported here show that only a small proportion of eosinophils and neutrophils are labeled at steady state. This strongly argues against a requirement for expression by eosinophils or their precursors. Instead, we favor a model of cell-extrinsic requirement for IL7Rα in eosinophil development. This notion is supported by the reciprocal transplantation assays, where we found that WT and IL7Rα -/-HSCs are equally capable of contributing to eosinophils and neutrophils in WT hosts (Fig. 3B) . Conversely, when we transplanted WT HSCs into an IL7R null background, we observed that the donor chimerism of eosinophils and neutrophils in the lungs were significantly impaired (Fig. 3D ), but that circulating "traditional" myeloid cells were efficiently generated (Fig. S2B) . Interestingly, it has been reported that eosinophil homeostasis relies on lymphocyte-and stromal-secreted survival factors 21,22,25-28 .
We found that two known eosinophil regulators, Eotaxin and IL-5, were reduced in the IL7Rα -/mice. Both of these cytokines are known to be secreted by lymphoid cells to promote eosinophil development, homing, and survival 19, 20, [22] [23] [24] 29 . Additionally, as previously reported 7 , we observed that IL-7 was upregulated in the IL7R -/mice, likely due to unbound excess IL-7 in the absence of lymphoid cells. These data could potentially explain the mechanisms behind the eosinophil decrease in IL7Rα -/mice (Fig. 1C , S1B,D, 3D), which have drastically reduced numbers of lymphocytes 16 (Fig. 1A, S2B,D) . The lack of lymphoid cells results in less secretion of Eotaxin and IL-5, resulting in poor eosinophil development and survival. Taken together, this suggests an extrinsic requirement for IL7R for eosinophil and neutrophil homeostasis. The data reported here represent a novel role of IL7R in "traditional" myeloid cell homeostasis, in a cell type-specific manner. We interpret our findings to suggest a cell-intrinsic role for IL7R in lymphoid cell development and survival, which then, in turn, support traditional myeloid cells in the lungs of adult mice. The results presented here add to the body of knowledge of how complex and dynamic Flk2 and IL7R expression and function cooperate in the regulation of a variety of immune cells, including traditional myeloid cells in the adult lung (Fig. 3F) . These findings are significant for lung health, because understanding hematopoietic homeostasis in the lung provides insight on susceptibility to respiratory disease.
All animals were housed and bred in the AALAC accredited vivarium at UC Santa Cruz and group housed in ventilated cages on a standard 12:12 light cycle. All procedures were approved by the 
Mice were sacrificed by CO2 inhalation. Adult peripheral blood was collected by femoral artery knick. Lungs were dissected, manually dissociated, and incubated in 1x PBS(+/+) with 2% serum, 1-2mg/ml collagenase IV (Gibco) with 100U/ml Dnase1 for 1-2 hours. Following incubation, all tissues were passed through a 16g needle ~10X followed by a 19g needle ~10X to make a single cell suspension, and then filtered through a 70 µM filter to obtain a single cell suspension. Cells were pelleted by centrifugation (1200g/ 4 degrees C /5 minutes). Numbers neutrophils, eosinophils, and B lymphocytes were analyzed and compared from the same tissue preparations from the same mice.
Cell labeling was performed on ice in 1X PBS with 5 mM EDTA and 2% serum 6 
Transplantation assays were performed as previously described 11, 18, [30] [31] [32] [33] [34] . Briefly, sorted cells were isolated from wild type (WT) or IL7Ra knockout mice (IL7Ra -/-) donor bone marrow. WT recipient mice aged 8-12 weeks were sublethally irradiated (750 rad, single dose) using a Faxitron CP-160 (Faxitron). Under isofluorane-induced general anesthesia, sorted cells were transplanted retro-orbitally.
Serum was harvested from wild type or IL7R -/mice via cardiac puncture. Several hundred microliters of blood was collected and transferred to a 1.5ml anti-coagulant free tube and allowed to clot at room temp for 30 min. Blood was centrifuged at 3,000 rpm for 10 min at 4C. Serum was carefully transferred to a fresh tube and stored at -80C until shipment to Eve Technologies (Calgary, Canada) for analysis..
Number of experiments, n, and what n represents can be found in the legend for each figure.
Statistical significance was determined by two-tailed unpaired student's T-test. All data are shown as mean ± standard error of the mean (SEM) representing at least two to three independent experiments. Power calculations for cytokine analysis indicated that 3 mice was sufficient to 