Transfusable neutrophil progenitors as cellular therapy for the prevention of invasive fungal infections.

The use of mature neutrophil (granulocyte) transfusions for the treatment of neutropenic patients with invasive fungal infections (IFIs) has been the focus of multiple clinical trials. Despite these efforts, the transfusion of mature neutrophils has resulted in limited clinical benefit, likely owing to problems of insufficient numbers and the very short lifespan of these donor cells. In this report, we employed a system of conditionally immortalized murine neutrophil progenitors that are capable of continuous expansion, allowing for the generation of unlimited numbers of homogenous granulocyte-macrophage progenitors (GMPs). These GMPs were assayed in vivo to demonstrate their effect on survival in 2 models of IFI: candidemia and pulmonary aspergillosis. Mature neutrophils derived from GMPs executed all cardinal functions of neutrophils. Transfused GMPs homed to the bone marrow and spleen, where they completed normal differentiation to mature neutrophils. These neutrophils were capable of homing and extravasation in response to inflammatory stimuli using a sterile peritoneal challenge model. Furthermore, conditionally immortalized GMP transfusions significantly improved survival in models of candidemia and pulmonary aspergillosis. These data confirm the therapeutic benefit of prophylactic GMP transfusions in the setting of neutropenia and encourage development of progenitor cellular therapies for the management of fungal disease in high-risk patients.

Allogeneic blood or marrow transplantation with haploidentical donor and post-transplantation cyclophosphamide in patients with myelofibrosis: a multicenter study.

We report the results from a multicenter retrospective study of 69 adult patients who underwent haploidentical blood or marrow transplantation (haplo-BMT) with post-transplantation cyclophosphamide (PTCy) for chronic phase myelofibrosis. The median age at BMT was 63 years (range, 41-74). Conditioning regimens were reduced intensity in 54% and nonmyeloablative in 39%. Peripheral blood grafts were used in 86%. The median follow-up was 23.1 months (range, 1.6-75.7). At 3 years, the overall survival, relapse-free survival (RFS), and graft-versus-host-disease (GVHD)-free-RFS were 72% (95% CI 59-81), 44% (95% CI 29-59), and 30% (95% CI 17-43). Cumulative incidences of non-relapse mortality and relapse were 23% (95% CI 14-34) and 31% (95% CI 17-47) at 3 years. Spleen size ≥22 cm or prior splenectomy (HR 6.37, 95% CI 2.02-20.1, P = 0.002), and bone marrow grafts (HR 4.92, 95% CI 1.68-14.4, P = 0.004) were associated with increased incidence of relapse. Cumulative incidence of acute GVHD grade 3-4 was 10% at 3 months and extensive chronic GVHD was 8%. Neutrophil engraftment was reported in 94% patients, at a median of 20 days (range, 14-70). In conclusion, haplo-BMT with PTCy is feasible in patients with myelofibrosis. Splenomegaly ≥22 cm and bone marrow grafts were associated with a higher incidence of relapse in this study.

In Vivo Efficacy of Neutrophil-Mediated Bone Regeneration Using a Rabbit Calvarial Defect Model.

Reconstruction of bone due to surgical removal or disease-related bony defects is a clinical challenge. It is known that the immune system exerts positive immunomodulatory effects on tissue repair and regeneration. In this study, we evaluated the in vivo efficacy of autologous neutrophils on bone regeneration using a rabbit calvarial defect model. Methods: Twelve rabbits, each with two surgically created calvarial bone defects (10 mm diameter), were randomly divided into two groups; (i) single application of neutrophils (SA-NP) vs. SA-NP control, and (ii) repetitive application of neutrophils (RA-NP) vs. RA-NP control. The animals were euthanized at 4 and 8 weeks post-operatively and the treatment outcomes were evaluated by micro-computed tomography, histology, and histomorphometric analyses. Results: The micro-CT analysis showed a significantly higher bone volume fraction (bone volume/total volume) in the neutrophil-treated groups, i.e., median interquartile range (IQR) SA-NP (18) and RA-NP (24), compared with the untreated controls, i.e., SA-NP (7) and RA-NP (14) at 4 weeks (p < 0.05). Similarly, new bone area fraction (bone area/total area) was significantly higher in neutrophil-treated groups at 4 weeks (p < 0.05). Both SA-NP and RA-NP had a considerably higher bone volume and bone area at 8 weeks, although the difference was not statistically significant. In addition, immunohistochemical analysis at 8 weeks revealed a higher expression of osteocalcin in both SA-NP and RA-NP groups. Conclusions: The present study provides first hand evidence that autologous neutrophils may have a positive effect on promoting new bone formation. Future studies should be performed with a larger sample size in non-human primate models. If proven feasible, this new promising strategy could bring clinical benefits for bone defects to the field of oral and maxillofacial surgery.

Efficient production of human neutrophils from iPSCs that prevent murine lethal infection with immune cell recruitment.

Neutrophils play an essential role in innate immune responses to bacterial and fungal infections, and loss of neutrophil function can increase the risk of acquiring lethal infections in clinical settings. Here, we show that engineered neutrophil-primed progenitors derived from human induced pluripotent stem cells can produce functional neutrophil-like cells at a clinically applicable scale that can act rapidly in vivo against lethal bacterial infections. Using 5 different mouse models, we systematically demonstrated that these neutrophil-like cells migrate to sites of inflammation and infection and increase survival against bacterial infection. In addition, we found that these human neutrophil-like cells can recruit murine immune cells. This system potentially provides a straight-forward solution for patients with neutrophil deficiency: an off-the-shelf neutrophil transfusion. This platform should facilitate the administration of human neutrophils for a broad spectrum of physiological and pathological conditions.

The AML-associated K313 mutation enhances C/EBPα activity by leading to C/EBPα overexpression.

Mutations in the transcription factor C/EBPα are found in ~10% of all acute myeloid leukaemia (AML) cases but the contribution of these mutations to leukemogenesis is incompletely understood. We here use a mouse model of granulocyte progenitors expressing conditionally active HoxB8 to assess the cell biological and molecular activity of C/EBPα-mutations associated with human AML. Both N-terminal truncation and C-terminal AML-associated mutations of C/EBPα substantially altered differentiation of progenitors into mature neutrophils in cell culture. Closer analysis of the C/EBPα-K313-duplication showed expansion and prolonged survival of mutant C/EBPα-expressing granulocytes following adoptive transfer into mice. C/EBPα-protein containing the K313-mutation further showed strongly enhanced transcriptional activity compared with the wild-type protein at certain promoters. Analysis of differentially regulated genes in cells overexpressing C/EBPα-K313 indicates a strong correlation with genes regulated by C/EBPα. Analysis of transcription factor enrichment in the differentially regulated genes indicated a strong reliance of SPI1/PU.1, suggesting that despite reduced DNA binding, C/EBPα-K313 is active in regulating target gene expression and acts largely through a network of other transcription factors. Strikingly, the K313 mutation caused strongly elevated expression of C/EBPα-protein, which could also be seen in primary K313 mutated AML blasts, explaining the enhanced C/EBPα activity in K313-expressing cells.

Successful Cord Blood Transplantation for Idiopathic CD4+ Lymphocytopenia.

Idiopathic CD4+ lymphocytopenia (ICL) is the depletion of CD4+ lymphocytes to <300 cells/mm3 without human immunodeficiency virus infection or other causes of lymphocytopenia. ICL causes fatal infections; its etiology remains unclear and it lacks consensus regarding therapeutic options. We report the first patient with ICL who had a successful clinical course following a cord blood transplant (CBT). A 45-year-old woman was diagnosed with ICL and underwent partial hepatectomy for an abscess caused by the Mycobacterium avium complex. No specific gene alterations were detected through next generation sequencing-based evaluation. Following a reduced-intensity conditioning (RIC) regimen consisting of fludarabine, busulfan, and 4 Gy total body irradiation, a single-unit CBT was performed. Neutrophils were engrafted on day +14. CD4+ lymphocyte counts increased to over 300 cells/mm3 on day +436. After 75 months, she was alive without any sequelae. CBT with an RIC regimen could be a curable treatment option for ICL.

Murine Myeloid Progenitors Attenuate Immune Dysfunction Induced by Hemorrhagic Shock.

Hemorrhagic shock induces an aberrant immune response characterized by simultaneous induction of a proinflammatory state and impaired host defenses. The objective of this study was to evaluate the impact of conditionally immortalized neutrophil progenitors (NPs) on this aberrant immune response. We employed a mouse model of hemorrhagic shock, followed by the adoptive transfer of NPs and subsequent inoculation of Staphylococcus aureus to induce pneumonia. We observed that transplant of NPs decreases the proportion of host neutrophils that express programmed death ligand 1 and intercellular adhesion molecule 1 in the context of prior hemorrhage. Following hemorrhage, NP transplant decreased proinflammatory cytokines in the lungs, increased neutrophil migration into the airspaces, and enhanced bacterial clearance. Further, hemorrhagic shock improved NP engraftment in the bone marrow. These results suggest that NPs hold the potential for use as a cellular therapy in the treatment and prevention of secondary infection following hemorrhagic shock.

Extracellular adenosine signaling reverses the age-driven decline in the ability of neutrophils to kill Streptococcus pneumoniae.

The elderly are susceptible to serious infections by Streptococcus pneumoniae (pneumococcus), which calls for a better understanding of the pathways driving the decline in host defense in aging. We previously found that extracellular adenosine (EAD) shaped polymorphonuclear cell (PMN) responses, which are crucial for controlling infection. EAD is produced by CD39 and CD73, and signals via A1, A2A, A2B, and A3 receptors. The objective of this study was to explore the age-driven changes in the EAD pathway and its impact on PMN function. We found in comparison to young mice, PMNs from old mice expressed significantly less CD73, but similar levels of CD39 and adenosine receptors. PMNs from old mice failed to efficiently kill pneumococci ex vivo; however, supplementation with adenosine rescued this defect. Importantly, transfer of PMNs expressing CD73 from young mice reversed the susceptibility of old mice to pneumococcal infection. To identify which adenosine receptor(s) is involved, we used specific agonists and inhibitors. We found that A1 receptor signaling was crucial for PMN function as inhibition or genetic ablation of A1 impaired the ability of PMNs from young mice to kill pneumococci. Importantly, activation of A1 receptors rescued the age-associated defect in PMN function. In exploring mechanisms, we found that PMNs from old mice failed to efficiently kill engulfed pneumococci and that A1 receptor controlled intracellular killing. In summary, targeting the EAD pathway reverses the age-driven decline in PMN antimicrobial function, which has serious implications in combating infections.

Bone marrow haploidentical transplant with post-transplantation cyclophosphamide: does graft cell content have an impact on main clinical outcomes?

We analyzed data relative to cell content in 88 consecutive patients receiving HLA haploidentical bone marrow (BM) transplants with post-transplantation cyclophosphamide (PT-CY). Median age was 54.5 (range, 17-72); diagnoses were acute leukemia (n = 46), lymphoproliferative disorders (n = 24), myelofibrosis (n = 11) and myelodysplastic syndromes (n = 5). Total nucleated cell (TNC) and CD34+, CD3+, CD4+ and CD8+ cell doses were stratified as higher than first, second and third quartile and the dose effect on various clinical outcomes was assessed. Median time to engraftment was 17 days for neutrophils and 24 days for platelets. To receive a dose of TNC ≥3.2 x 106/kg or CD34+ cells ≥2.7 x 106/kg significantly shortened the time to neutrophil and platelet engraftment and reduced the blood product requirements in the 30-day period after transplantation. Overall, TNC and CD34+ cell doses had no effect on acute graft-versus-host disease (GVHD) incidence, whereas patients receiving higher CD3+ and CD8+ cell doses seemed to have less chronic GVHD. No effect on non-relapse mortality, progression-free survival and overall survival was observed at different cell dose thresholds. These data suggest that in HLA haploidentical BM transplant with PT-CY, appropriate cell doses are relevant to the engraftment. The association between low CD3+/CD8+ cells and chronic GVHD deserves further investigation.

Neutrophils promote the development of reparative macrophages mediated by ROS to orchestrate liver repair.

Phagocytes, including neutrophils and macrophages, have been suggested to function in a cooperative way in the initial phase of inflammatory responses, but their interaction and integration in the resolution of inflammation and tissue repair remain unclear. Here we show that neutrophils have crucial functions in liver repair by promoting the phenotypic conversion of pro-inflammatory Ly6ChiCX3CR1lo monocytes/macrophages to pro-resolving Ly6CloCX3CR1hi macrophages. Intriguingly, reactive oxygen species (ROS), expressed predominantly by neutrophils, are important mediators that trigger this phenotypic conversion to promote liver repair. Moreover, this conversion is prevented by the depletion of neutrophils via anti-Ly6G antibody, genetic deficiency of granulocyte colony-stimulating factor, or genetic deficiency of NADPH oxidase 2 (Nox2). By contrast, adoptive transfer of WT rather than Nox2-/- neutrophils rescues the impaired phenotypic conversion of macrophages in neutrophil-depleted mice. Our findings thus identify an intricate cooperation between neutrophils and macrophages that orchestrate resolution of inflammation and tissue repair.

Neutrophils Derived from Genetically Modified Human Induced Pluripotent Stem Cells Circulate and Phagocytose Bacteria In Vivo.

Bacterial and fungal infections are a major cause of morbidity and mortality in neutropenic patients. Donor-derived neutrophil transfusions have been used for prophylaxis or treatment for infection in neutropenic patients. However, the short half-life and the limited availability of large numbers of donor-derived neutrophils for transfusion remain a significant hurdle in the implementation of neutrophil transfusion therapy. Here, we investigate the in vitro and in vivo activity of neutrophils generated from human induced pluripotent stem cells (iPSC), a potentially unlimited resource to produce neutrophils for transfusion. Phenotypic analysis of iPSC-derived neutrophils reveal reactive oxygen species production at similar or slightly higher than normal peripheral blood neutrophils, but have an ∼50%-70% reduced Escherichia coli phagocytosis and phorbol 12-myristate 13-acetate induced formation of neutrophil extracellular traps (NET). Signaling of granulocytic precursors identified impaired AKT activation, but not ERK or STAT3, in agonist-stimulated iPSC-derived neutrophils. Expression of a constitutively activated AKT in iPSC-derived neutrophils restores most phagocytic activity and NET formation. In a model of bacterial induced peritonitis in immunodeficient mice, iPSC-derived neutrophils, with or without corrected AKT activation, migrate similarly to the peritoneal fluid as peripheral blood neutrophils, whereas the expression of activated AKT significantly improves their phagocytic activity in vivo. Stem Cells Translational Medicine 2019;8:557-567.

NAMPT signaling is critical for the proangiogenic activity of tumor-associated neutrophils.

Tumor-associated neutrophils (TANs) regulate many processes associated with tumor progression, and depending on the microenvironment, they can exhibit pro- or antitumor functions. However, the molecular mechanisms regulating their tumorigenicity are not clear. Using transplantable tumor models, we showed here that nicotinamide phosphoribosyltransferase (NAMPT), a molecule involved in CSF3R downstream signaling, is essential for tumorigenic conversion of TANs and their pro-angiogenic switch. As a result tumor vascularization and growth are strongly supported by these cells. Inhibition of NAMPT in TANs leads to their antitumor conversion. Adoptive transfer of such TANs into B16F10-tumor bearing mice attenuates tumor angiogenesis and growth. Of note, we observe that the regulation of NAMPT signaling in TANs, and its effect on the neutrophil tumorigenicity, are analogous in mice and human. NAMPT is up-regulated in TANs from melanoma and head-and-neck tumor patients, and its expression positively correlates with tumor stage. Mechanistically, we found that targeting of NAMPT suppresses neutrophil tumorigenicity by inhibiting SIRT1 signaling, thereby blocking transcription of pro-angiogenic genes. Based on these results, we propose that NAMPT regulatory axis is important for neutrophils to activate angiogenic switch during early stages of tumorigenesis. Thus, identification of NAMPT as the critical molecule priming protumor functions of neutrophils provides not only mechanistic insight into the regulation of neutrophil tumorigenicity, but also identifies a potential pathway that may be targeted therapeutically in neutrophils. This, in turn, may be utilized as a novel mode of cancer immunotherapy.

Allogeneic Hematopoietic Cell Transplantation for Chronic Granulomatous Disease: Controversies and State of the Art.

Chronic granulomatous disease (CGD) is a congenital disorder characterized by recurrent life-threatening bacterial and fungal infections and development of severe inflammation secondary to a congenital defect in 1 of the 5 phagocyte oxidase (phox) subunits of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex. Hematopoietic cell transplant (HCT) is a curative treatment for patients with CGD that provides donor neutrophils with functional NADPH and superoxide anion production. Many characteristics of CGD, including preexisting infection and inflammation and the potential for cure with mixed-donor chimerism, influence the transplant approach and patient outcome. Because of the dangers of short-term death, graft-versus-host disease, and late effects from chemotherapy, HCT historically has been reserved for patients with high-risk disease and a matched donor. However, as advances in CGD and HCT treatments have evolved, recommendations on transplant eligibility also must be amended, but the development of modern guidelines has proven difficult. In this review, we provide an overview of HCT in patients with CGD, including the debate over HCT indications in them, the unique aspects of CGD that can complicate HCT, and a summary of transplant outcomes.

Novel Role of IL (Interleukin)-1ß in Neutrophil Extracellular Trap Formation and Abdominal Aortic Aneurysms.

OBJECTIVE: Neutrophils promote experimental abdominal aortic aneurysm (AAA) formation via a mechanism that is independent from MMPs (matrix metalloproteinases). Recently, we reported a dominant role of IL (interleukin)-1ß in the formation of murine experimental AAAs. Here, the hypothesis that IL-1ß-induced neutrophil extracellular trap formation (NETosis) promotes AAA was tested. APPROACH AND RESULTS: NETs were identified through colocalized staining of neutrophil, Cit-H3 (citrullinated histone H3), and DNA, using immunohistochemistry. NETs were detected in human AAAs and were colocalized with IL-1ß. In vitro, IL-1RA attenuated IL-1ß-induced NETosis in human neutrophils. Mechanistically, IL-1ß treatment of isolated neutrophils induced nuclear localization of ceramide synthase 6 and synthesis of C16-ceramide, which was inhibited by IL-1RA or fumonisin B1, an inhibitor of ceramide synthesis. Furthermore, IL-1RA or fumonisin B1 attenuated IL1-ß-induced NETosis. In an experimental model of murine AAA, NETs were detected at a very early stage-day 3 of aneurysm induction. IL-1ß-knockout mice demonstrated significantly lower infiltration of neutrophils to aorta and were protected from AAA. Adoptive transfer of wild-type neutrophils promoted AAA formation in IL-1ß-knockout mice. Moreover, treatment of wild-type mice with Cl-amidine, an inhibitor NETosis, significantly attenuated AAA formation, whereas, treatment with deoxyribonuclease, a DNA digesting enzyme, had no effect on AAA formation. CONCLUSIONS: Altogether, the results suggest a dominant role of IL-1ß-induced NETosis in AAA formation.

ICAM-1null C57BL/6 Mice Are Not Protected from Experimental Ischemic Stroke.

Accumulation of neutrophils in the brain is a hallmark of cerebral ischemia and considered central in exacerbating tissue injury. Intercellular adhesion molecule (ICAM)-1 is upregulated on brain endothelial cells after ischemic stroke and considered pivotal in neutrophil recruitment as ICAM-1-deficient mouse lines were found protected from experimental stroke. Translation of therapeutic inhibition of ICAM-1 into the clinic however failed. This prompted us to investigate stroke pathogenesis in Icam1tm1Alb C57BL/6 mutants, a true ICAM-1null mouse line. Performing transient middle cerebral artery occlusion, we found that absence of ICAM-1 did not ameliorate stroke pathology at acute time points after reperfusion. Near-infrared imaging showed comparable accumulation of neutrophils in the ischemic hemispheres of ICAM-1null and wild type C57BL/6 mice. We also isolated equal numbers of neutrophils from the ischemic brains of ICAM-1null and wild type C57BL/6 mice. Immunostaining of the brains showed neutrophils to equally accumulate in the leptomeninges and brain parenchymal vessels of ICAM-1null and wild type C57BL/6 mice. In addition, the lesion size was comparable in ICAM-1null and wild type mice. Our study demonstrates that absence of ICAM-1 neither inhibits cerebral ischemia-induced accumulation of neutrophils in the brain nor provides protection from ischemic stroke.

Plasminogen Activator Inhibitor-1 Promotes Neutrophil Infiltration and Tissue Injury on Ischemia-Reperfusion.

OBJECTIVE: Ischemia-reperfusion (I/R) injury significantly contributes to organ dysfunction and failure after myocardial infarction, stroke, and transplantation. In addition to its established role in the fibrinolytic system, plasminogen activator inhibitor-1 has recently been implicated in the pathogenesis of I/R injury. The underlying mechanisms remain largely obscure. APPROACH AND RESULTS: Using different in vivo microscopy techniques as well as ex vivo analyses and in vitro assays, we identified that plasminogen activator inhibitor-1 rapidly accumulates on microvascular endothelial cells on I/R enabling this protease inhibitor to exhibit previously unrecognized functional properties by inducing an increase in the affinity of ß2 integrins in intravascularly rolling neutrophils. These events are mediated through low-density lipoprotein receptor-related protein-1 and mitogen-activated protein kinase-dependent signaling pathways that initiate intravascular adherence of these immune cells to the microvascular endothelium. Subsequent to this process, extravasating neutrophils disrupt endothelial junctions and promote the postischemic microvascular leakage. Conversely, deficiency of plasminogen activator inhibitor-1 effectively reversed leukocyte infiltration, microvascular dysfunction, and tissue injury on experimental I/R without exhibiting side effects on microvascular hemostasis. CONCLUSIONS: Our experimental data provide novel insights into the nonfibrinolytic properties of the fibrinolytic system and emphasize plasminogen activator inhibitor-1 as a promising target for the prevention and treatment of I/R injury.

Neutrophil peptidyl arginine deiminase-4 has a pivotal role in ischemia/reperfusion-induced acute kidney injury.

Ischemia/reperfusion is a common cause of acute kidney injury (AKI). However, mechanisms underlying the sudden loss in kidney function and tissue injury remain to be fully elucidated. Here, we investigated the role of peptidyl arginine deiminase-4 (PAD4), which converts arginine to citrulline and plays a role in epigenetic regulation and inflammation, in renal ischemia/reperfusion injury. PAD4 expression was highly induced in infiltrating leukocytes 24 hours following renal ischemia and reperfusion. This induction was accompanied by citrullination of histone H3 and formation of neutrophil extracellular traps in kidneys of wild-type mice. By contrast, PAD4-deficient mice did not form neutrophil extracellular traps, expressed lower levels of pro-inflammatory cytokines and were partially protected from renal ischemia/reperfusion-induced AKI. Furthermore, PAD4-deficient mice recovered kidney function 48 hours after ischemia/reperfusion, whereas kidney function in the wild-type mice progressively worsened. Administration of DNase I, which degrades neutrophil extracellular traps or the PAD-specific inhibitor YW3-56 before ischemia, partially prevented renal ischemia/reperfusion-induced AKI. Notably, transfer of neutrophils from wild-type, but not from PAD4-deficient mice, was sufficient to restore renal neutrophil extracellular trap formation and impair kidney function following renal ischemia/reperfusion. Thus, neutrophil PAD4 plays a pivotal role in renal ischemia/reperfusion-induced AKI.

Neutrophil-mediated Suppression of Influenza-induced Pathology Requires CD11b/CD18 (MAC-1).

Severe influenza virus infection can lead to life-threatening pathology through immune-mediated tissue damage. In various experimental models, this damage is dependent on T cells. There is conflicting evidence regarding the role of neutrophils in influenza-mediated pathology. Neutrophils are often regarded as cells causing tissue damage, but, in recent years, it has become clear that a subset of human neutrophils is capable of suppressing T cells, which is dependent on macrophage-1 antigen (CD11b/CD18). Therefore, we tested the hypothesis that immune suppression by neutrophils can reduce T cell-mediated pathology after influenza infection. Wild-type (WT) and CD11b-/- mice were infected with A/HK/2/68 (H3N2) influenza virus. Disease severity was monitored by weight loss, leukocyte infiltration, and immunohistochemistry. We demonstrated that CD11b-/- mice suffered increased weight loss compared with WT animals upon infection with influenza virus. This was accompanied by increased pulmonary leukocyte infiltration and lung damage. The exaggerated pathology in CD11b-/- mice was dependent on T cells, as it was reduced by T cell depletion. In addition, pathology in CD11b-/- mice was accompanied by higher numbers of T cells in the lungs early during infection compared with WT mice. Importantly, these differences in pathology were not associated with an increased viral load, suggesting that pathology was immune-mediated rather than caused by virus-induced damage. In contrast to adoptive transfer of CD11b-/- neutrophils, a single adoptive transfer of WT neutrophils partly restored protection against influenza-induced pathology, demonstrating the importance of neutrophil CD11b/CD18. Our data show that neutrophil CD11b/CD18 limits pathology in influenza-induced, T cell-mediated disease.