Coordinated chemokine expression defines macrophage subsets across tissues

Lung-resident macrophages, which include alveolar macrophages and interstitial macrophages (IMs), exhibit a high degree of diversity, generally attributed to different activation states, and often complicated by the influx of monocytes into the pool of tissue-resident macrophages. To gain a deeper insight into the functional diversity of IMs, here we perform comprehensive transcriptional profiling of resident IMs and reveal ten distinct chemokine-expressing IM subsets at steady state and during inflammation. Similar IM subsets that exhibited coordinated chemokine signatures and differentially expressed genes were observed across various tissues and species, indicating conserved specialized functional roles. Other macrophage types shared specific IM chemokine profiles, while also presenting their own unique chemokine signatures. Depletion of CD206hi IMs in Pf4creR26EYFP+DTR and Pf4creR26EYFPCx3cr1DTR mice led to diminished inflammatory cell recruitment, reduced tertiary lymphoid structure formation and fewer germinal center B cells in models of allergen- and infection-driven inflammation. These observations highlight the specialized roles of IMs, defined by their coordinated chemokine production, in regulating immune cell influx and organizing tertiary lymphoid tissue architecture.

A LTB4/CD11b self-amplifying loop drives pyogranuloma formation in chronic granulomatous disease

Sterile pyogranulomas and heightened cytokine production are hyperinflammatory hallmarks of Chronic Granulomatous Disease (CGD). Using peritoneal cells of zymosan-treated CGD (gp91phox-/-) versus wild-type (WT) mice, an ex vivo system of pyogranuloma formation was developed to determine factors involved in and consequences of recruitment of neutrophils and monocyte-derived macrophages (MoMacs). Whereas WT cells failed to aggregate, CGD cells formed aggregates containing neutrophils initially, and MoMacs recruited secondarily. LTB4 was key, as antagonizing BLT1 blocked neutrophil aggregation, but acted only indirectly on MoMac recruitment. LTB4 upregulated CD11b expression on CGD neutrophils, and the absence/blockade of CD11b inhibited LTB4 production and cell aggregation. Neutrophil-dependent MoMac recruitment was independent of MoMac Nox2 status, BLT1, CCR1, CCR2, CCR5, CXCR2, and CXCR6. As proof of concept, CD11b-deficient CGD mice developed disrupted pyogranulomas with poorly organized neutrophils and diminished recruitment of MoMacs. Importantly, the disruption of cell aggregation and pyogranuloma formation markedly reduced proinflammatory cytokine production.

TNFα: TNFR1 signaling inhibits maturation and maintains the pro-inflammatory programming of monocyte-derived macrophages in murine chronic granulomatous disease

Introduction

Loss of NADPH oxidase activity results in proinflammatory macrophages that contribute to hyperinflammation in Chronic Granulomatous Disease (CGD). Previously, it was shown in a zymosan-induced peritonitis model that gp91phox-/- (CGD) monocyte-derived macrophages (MoMacs) fail to phenotypically mature into pro-resolving MoMacs characteristic of wild type (WT) but retain the ability to do so when placed in the WT milieu. Accordingly, it was hypothesized that soluble factor(s) in the CGD milieu thwart appropriate programming.

Methods

We sought to identify key constituents using ex vivo culture of peritoneal inflammatory leukocytes and their conditioned media. MoMac phenotyping was performed via flow cytometry, measurement of efferocytic capacity and multiplex analysis of secreted cytokines. Addition of exogenous TNFα, TNFα neutralizing antibody and TNFR1-/- MoMacs were used to study the role of TNFα: TNFR1 signaling in MoMac maturation.

Results

More extensive phenotyping defined normal MoMac maturation and demonstrated failure of maturation of CGD MoMacs both ex vivo and in vivo. Protein components, and specifically TNFα, produced and released by CGD neutrophils and MoMacs into conditioned media was identified as critical to preventing maturation. Exogenous addition of TNFα inhibited WT MoMac maturation, and its neutralization allowed maturation of cultured CGD MoMacs. TNFα neutralization also reduced production of IL-1β, IL-6 and CXCL1 by CGD cells though these cytokines played no role in MoMac programming. MoMacs lacking TNFR1 matured more normally in the CGD milieu both ex vivo and following adoptive transfer in vivo.

Discussion

These data lend mechanistic insights into the utility of TNFα blockade in CGD and to other diseases where such therapy has been shown to be beneficial.

CCL5-producing migratory dendritic cells guide CCR5+ monocytes into the draining lymph nodes

Dendritic cells (DCs) and monocytes capture, transport, and present antigen to cognate T cells in the draining lymph nodes (LNs) in a CCR7-dependent manner. Since only migratory DCs express this chemokine receptor, it is unclear how monocytes reach the LN. In steady-state and following inhalation of several PAMPs, scRNA-seq identified LN mononuclear phagocytes as monocytes, resident, or migratory type 1 and type 2 conventional (c)DCs, despite the downregulation of Xcr1, Clec9a, H2-Ab1, Sirpa, and Clec10a transcripts on migratory cDCs. Migratory cDCs, however, upregulated Ccr7, Ccl17, Ccl22, and Ccl5. Migratory monocytes expressed Ccr5, a high-affinity receptor for Ccl5. Using two tracking methods, we observed that both CD88hiCD26lomonocytes and CD88-CD26hi cDCs captured inhaled antigens in the lung and migrated to LNs. Antigen exposure in mixed-chimeric Ccl5-, Ccr2-, Ccr5-, Ccr7-, and Batf3-deficient mice demonstrated that while antigen-bearing DCs use CCR7 to reach the LN, monocytes use CCR5 to follow CCL5-secreting migratory cDCs into the LN, where they regulate DC-mediated immunity.

Mapping Macrophage Polarization and Origin during the Progression of the Foreign Body Response to a Poly(ethylene glycol) Hydrogel Implant

Poly(ethylene glycol) (PEG) hydrogels hold promise for in vivo applications but induce a foreign body response (FBR). While macrophages are key in the FBR, many questions remain. This study investigates temporal changes in the transcriptome of implant-associated monocytes and macrophages. Proinflammatory pathways are upregulated in monocytes compared to control monocytes but subside by day 28. Macrophages are initially proinflammatory but shift to a profibrotic state by day 14, coinciding with fibrous capsule emergence. Next, this study assesses the origin of macrophages responsible for fibrous encapsulation using wildtype, C-C Motif Chemokine Receptor 2 (CCR2)^-/- mice that lack recruited macrophages, and Macrophage Fas-Induced Apoptosis (MaFIA) mice that enable macrophage ablation. Subpopulations of recruited and tissue-resident macrophages are identified. Fibrous encapsulation proceeds in CCR2^-/- mice similar to wildtype mice. However, studies in MaFIA mice indicate that macrophages are necessary for fibrous capsule formation. These findings suggest that macrophage origin impacts the FBR progression and provides evidence that tissue-resident macrophages and not the recruited macrophages may drive fibrosis in the FBR to PEG hydrogels. This study demonstrates that implant-associated monocytes and macrophages have temporally distinct transcriptomes in the FBR and that profibrotic pathways associated with macrophages may be enriched in tissue-resident macrophages.

LN Monocytes Limit DC-Poly I:C Induced Cytotoxic T Cell Response via IL-10 and Induction of Suppressor CD4 T Cells

Every immune response has accelerators and brakes. Depending on the pathogen or injury, monocytes can play either role, promoting or resolving immunity. Poly I:C, a potent TLR3 ligand, licenses cross-presenting dendritic cells (DC1) to accelerate a robust cytotoxic T cells response against a foreign antigen. Poly I:C thus has promise as an adjuvant in cancer immunotherapy and viral subunit vaccines. Like DC1s, monocytes are also abundant in the LNs. They may act as either immune accelerators or brakes, depending on the inflammatory mediator they encounter. However, little is known about their contribution to adaptive immunity in the context of antigen and Poly I:C. Using monocyte-deficient and chimeric mice, we demonstrate that LN monocytes indirectly dampen a Poly I:C induced antigen-specific cytotoxic T cell response, exerting a “braking” function. This effect is mediated by IL-10 production and induction of suppressor CD4⁺ T cells. In a metastatic melanoma model, we show that a triple-combination prophylactic treatment consisting of anti-IL-10, tumor peptides and Poly I:C works because removing IL-10 counteracts the monocytic brake, resulting in significantly fewer tumors compared to mice treated with tumor peptides and Poly I:C alone. Finally, in human LN tissue, we observed that monocytes (unlike DCs) express high levels of IL-10, suggesting that anti-IL-10 may be an important addition to treatments. Overall, our data demonstrates that LN monocytes regulate the induction of a robust DC1-mediated immune response. Neutralization of either IL-10 or monocytes can augment Poly I:C-based treatments and enhance T cell cytotoxicity.

Human and Mouse Transcriptome Profiling Identifies Cross-Species Homology in Pulmonary and Lymph Node Mononuclear Phagocytes

The mononuclear phagocyte (MP) system consists of macrophages, monocytes, and dendritic cells (DCs). MP subtypes play distinct functional roles in steady-state and inflammatory conditions. Although murine MPs are well characterized, their pulmonary and lymph node (LN) human homologs remain poorly understood. To address this gap, we have created a gene expression compendium across 24 distinct human and murine lung and LN MPs, along with human blood and murine spleen MPs, to serve as validation datasets. In-depth RNA sequencing identifies corresponding human-mouse MP subtypes and determines marker genes shared and divergent across species. Unexpectedly, only 13%-23% of the top 1,000 marker genes (i.e., genes not shared across species-specific MP subtypes) overlap in corresponding human-mouse MP counterparts. Lastly, CD88 in both species helps distinguish monocytes/macrophages from DCs. Our cross-species expression compendium serves as a resource for future translational studies to investigate beforehand whether pursuing specific MP subtypes or genes will prove fruitful.

Localization of Macrophages in the Human Lung via Design-based Stereology

Rationale: Interstitial macrophages (IMs) and airspace macrophages (AMs) play critical roles in lung homeostasis and host defense, and are central to the pathogenesis of a number of lung diseases. However, the absolute numbers of macrophages and the precise anatomic locations they occupy in the healthy human lung have not been quantified.Objectives: To determine the precise number and anatomic location of human pulmonary macrophages in nondiseased lungs and to quantify how this is altered in chronic cigarette smokers.Methods: Whole right upper lobes from 12 human donors without pulmonary disease (6 smokers and 6 nonsmokers) were evaluated using design-based stereology. CD206 (cluster of differentiation 206)-positive/CD43⁺ AMs and CD206⁺/CD43^- IMs were counted in five distinct anatomical locations using the optical disector probe.Measurements and Main Results: An average of 2.1 × 10⁹ IMs and 1.4 × 10⁹ AMs were estimated per right upper lobe. Of the AMs, 95% were contained in diffusing airspaces and 5% in airways. Of the IMs, 78% were located within the alveolar septa, 14% around small vessels, and 7% around the airways. The local density of IMs was greater in the alveolar septa than in the connective tissue surrounding the airways or vessels. The total number and density of IMs was 36% to 56% greater in the lungs of cigarette smokers versus nonsmokers.Conclusions: The precise locations occupied by pulmonary macrophages were defined in nondiseased human lungs from smokers and nonsmokers. IM density was greatest in the alveolar septa. Lungs from chronic smokers had increased IM numbers and overall density, supporting a role for IMs in smoking-related disease.

Immune Surveillance by Natural IgM Is Required for Early Neoantigen Recognition and Initiation of Adaptive Immunity

Early recognition of neoantigen-expressing cells is complex, involving multiple immune cell types. In this study, in vivo, we examined how antigen-presenting cell subtypes coordinate and induce an immunological response against neoantigen-expressing cells, particularly in the absence of a pathogen-associated molecular pattern, which is normally required to license antigen-presenting cells to present foreign or self-antigens as immunogens. Using two reductionist models of neoantigen-expressing cells and two cancer models, we demonstrated that natural IgM is essential for the recognition and initiation of adaptive immunity against neoantigen-expressing cells. Natural IgM antibodies form a cellular immune complex with the neoantigen-expressing cells. This immune complex licenses surveying monocytes to present neoantigens as immunogens to CD4⁺ T cells. CD4⁺ T helper cells, in turn, use CD40L to license cross-presenting CD40⁺ Batf3⁺ dendritic cells to elicit a cytotoxic T cell response against neoantigen-expressing cells. Any break along this immunological chain reaction results in the escape of neoantigen-expressing cells. This study demonstrates the surprising, essential role of natural IgM as the initiator of a sequential signaling cascade involving multiple immune cell subtypes. This sequence is required to coordinate an adaptive immune response against neoantigen-expressing cells.

Deletion of c-FLIP from CD11bhi Macrophages Prevents Development of Bleomycin-induced Lung Fibrosis

Idiopathic pulmonary fibrosis is a progressive lung disease with complex pathophysiology and fatal prognosis. Macrophages (MΦ) contribute to the development of lung fibrosis; however, the underlying mechanisms and specific MΦ subsets involved remain unclear. During lung injury, two subsets of lung MΦ coexist: Siglec-F^hi resident alveolar MΦ and a mixed population of CD11b^hi MΦ that primarily mature from immigrating monocytes. Using a novel inducible transgenic system driven by a fragment of the human CD68 promoter, we targeted deletion of the antiapoptotic protein cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP) to CD11b^hi MΦ. Upon loss of c-FLIP, CD11b^hi MΦ became susceptible to cell death. Using this system, we were able to show that eliminating CD11b^hi MΦ present 7-14 days after bleomycin injury was sufficient to protect mice from fibrosis. RNA-seq analysis of lung MΦ present during this time showed that CD11b^hi MΦ, but not Siglec-F^hi MΦ, expressed high levels of profibrotic chemokines and growth factors. Human MΦ from patients with idiopathic pulmonary fibrosis expressed many of the same profibrotic chemokines identified in murine CD11b^hi MΦ. Elimination of monocyte-derived MΦ may help in the treatment of fibrosis. We identify c-FLIP and the associated extrinsic cell death program as a potential pathway through which these profibrotic MΦ may be pharmacologically targeted.

A Consistent Method to Identify and Isolate Mononuclear Phagocytes from Human Lung and Lymph Nodes

Mononuclear phagocytes (MP) consist of macrophages, dendritic cells (DCs), and monocytes. In all organs, including the lung, there are multiple subtypes within these categories. The existence of all these cell types suggest that there is a clear division of labor and delicate balance between the MPs under steady state and inflammatory conditions. Although great strides have been made to understand MPs in the mouse lung, and human blood, little is known about the MPs that exist in the human lung and lung-draining lymph nodes (LNs), and even less is known about their functional roles, studies of which will require a large number of sorted cells. We have comprehensively examined cell surface markers previously used in a variety of organs to identify human pulmonary MPs. In the lung, we consistently identify five extravascular pulmonary MPs and three LN MPs. These MPs were present in over 100 lungs regardless of age or gender. Notably, the human blood CD141⁺ DCs, as described in the literature, were not observed in non-diseased lungs or their draining LNs. In the lung and draining LNs, expression of CD141 was only observed on HLADR⁺ CD11c⁺ CD14⁺ extravascular monocytes (often confused in the LN as resident DCs based on the level of HLADR expression and mouse LN data). In the human lung and LNs there are at least two DC subtypes expressing HLADR, DEC205 and CD1c, along with circulating monocytes that behave as either antigen-presenting cells or macrophages. Furthermore, we demonstrate how to distinguish between alveolar macrophages and interstitial macrophage subtypes. It still remains unclear how the human pulmonary MPs identified here align with mouse MPs. Clearly, we are now past the stage of cell surface marker characterization, and future studies will need to move toward understanding what these cell types are and how they function. Our hope is that the strategy described here can help the pulmonary community take this next step.

Isolation and Characterization of Mononuclear Phagocytes in the Mouse Lung and Lymph Nodes

There is a diverse population of mononuclear phagocytes (MPs) in the lungs, comprised of macrophages, dendritic cells (DCs), and monocytes. The existence of these various cell types suggests that there is a clear division of labor and delicate balance between the MPs under steady-state and inflammatory conditions. Here we describe how to identify pulmonary MPs using flow cytometry and how to isolate them via cell sorting. In steady-state conditions, murine lungs contain a uniform population of alveolar macrophages (AMs), three distinct interstitial macrophage (IM) populations, three DC subtypes, and a small number of tissue-trafficking monocytes. During an inflammatory response, the monocyte population is more abundant and complex since it acquires either macrophage-like or DC-like features. All in all, studying how these cell types interact with each other, structural cells, and other leukocytes within the environment will be important to understanding their role in maintaining homeostasis and during the development of disease.

Isolation and Identification of Interstitial Macrophages from the Lungs Using Different Digestion Enzymes and Staining Strategies

Interstitial macrophages (IMs) are present in multiple organs. Although there is limited knowledge of the unique functional role IM subtypes play, macrophages, in general, are known for their contribution in homeostatic tissue maintenance and inflammation such as clearing pathogens and debris and secreting inflammatory mediators and growth factors. IM subtypes have been identified in the heart, skin, and gut, and more recently we identified three distinct IMs in the lung. IMs express on their surface high levels of MerTK, CD64, and CD11b, with differences in CD11c, CD206, and MHC II expression, and referred to the three pulmonary IM subtypes as IM1 (CD11c^loCD206⁺MHCII^lo), IM2 (CD11c^loCD206⁺MHCII^hi), and IM3 (CD11c^hiCD206^loMHCII^hi). In this chapter, we highlight how to extract IMs from the lung using three different digestion enzymes: elastase, collagenase D, and Liberase TM. Of these three commonly used enzymes, Liberase TM was the most effective at IM extraction, particularly IM3. Furthermore, alternative staining strategies to identify IMs were examined, which included CD64, MerTK, F4/80, and Tim4. Thus, future studies highlighting the functional role of IM subtypes will help further our understanding of how tissue homeostasis is maintained and inflammatory conditions are induced and resolved.

Three Unique Interstitial Macrophages in the Murine Lung at Steady State

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin, such as microglia in the brain, whereas other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs, such as the skin, there are both embryonic and postnatal-derived macrophages. In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages, dendritic cells, and few extravascular monocytes. We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover, and lack of survival dependency on fractalkine receptor, CX₃CR1. Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature and phenotype. All IMs expressed MER proto-oncogene, tyrosine kinase, CD64, CD11b, and CX₃CR1, and were further distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2, and MHC class II, along with the absence of Ly6C, Ly6G, and Siglec F. Most intriguingly, in addition to the lung, similar phenotypic populations of IMs were observed in other nonlymphoid organs, perhaps highlighting conserved functions throughout the body. These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

Three unique interstitial macrophages in the murine lung at steady state

Rationale

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin such as microglia in the brain, while other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs such as the skin, there are both embryonic and postnatal-derived macrophages.

Objectives

In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages (AMs), dendritic cells and few extravascular monocytes.

Methods

We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover and lack of survival dependency on CX3CR1.

Measurements and Main Results

Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature. All IMs expressed MerTK+CD64+ CD11b+ CX CR1+ and were furthermore distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2 and MHCII, along with the absence of Ly6C, Ly6G, and Siglec F. Ex vivo analysis revealed that all three IMs were highly phagocytic compared to AMs. Finally, similar phenotypic populations of IMs were present in other non-lymphoid organs, suggesting that these IMs may not be unique to the lung.

Conclusions

These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

Coordinated immune activation by distinct mononuclear phagocytes subtypes against the mismatch of minor antigens

Recently, we identified that Batf3-dependent DCs were solely responsible for the rejection of minor antigen-mismatched grafts, and demonstrated in Batf3−/− female mice, lacking Batf3-dependent DCs, the complete acceptance of minor antigen-mismatched male cells, which are normally rejected in WT female mice. Intriguingly, this rejection occurs in the absence of PAMPs, leading to considerable uncertainty of how endogenous APC subtypes become activated and licensed to induce an adaptive immune response against male cells in female mice. Investigating this gap in knowledge is the main goal of this study. Rejection of minor antigen-mismatched grafts involves all branches of the immune system: adaptive immunity (T cells), humoral immunity (B cells) and innate immunity (DCs and antigen-presenting monocytes). Although we showed that depletion of one cell type dampens or omits the inflammatory outcome of minor antigen-mismatched graft rejection, here we show how lymphoid and myeloid cell subtypes act sequentially and in concert for the induction of minor antigen-mismatched graft rejection without the presence of PAMPs. In conclusion, we believe that understanding the mechanisms of action against minor antigen-mismatched graft rejection will also apply more broadly to other diseases such as autoimmunity and cancer, in which an immune response is elicited against self-associated minor antigens.

Ly6C(+) monocyte efferocytosis and cross-presentation of cell-associated antigens

Recently it was shown that circulating Ly6C⁺ monocytes traffic from tissue to the draining lymph nodes (LNs) with minimal alteration in their overall phenotype. Furthermore, in the steady state, Ly6C⁺ monocytes are as abundant as classical dendritic cells (DCs) within the draining LNs, and even more abundant during inflammation. However, little is known about the functional roles of constitutively trafficking Ly6C⁺ monocytes. In this study we investigated whether Ly6C⁺ monocytes can efferocytose (acquire dying cells) and cross-present cell-associated antigen, a functional property particularly attributed to Batf3⁺ DCs. We demonstrated that Ly6C⁺ monocytes intrinsically efferocytose and cross-present cell-associated antigen to CD8⁺ T cells. In addition, efferocytosis was enhanced upon direct activation of the Ly6C⁺ monocytes through its corresponding TLRs, TLR4 and TLR7. However, only ligation of TLR7, and not TLR4, enhanced cross-presentation by Ly6C⁺ monocytes. Overall, this study outlines two functional roles, among others, that Ly6C⁺ monocytes have during an adaptive immune response.

Flow Cytometric Analysis of Mononuclear Phagocytes in Nondiseased Human Lung and Lung-Draining Lymph Nodes

RATIONALE

The pulmonary mononuclear phagocyte system is a critical host defense mechanism composed of macrophages, monocytes, monocyte-derived cells, and dendritic cells. However, our current characterization of these cells is limited because it is derived largely from animal studies and analysis of human mononuclear phagocytes from blood and small tissue resections around tumors.

OBJECTIVES

Phenotypic and morphologic characterization of mononuclear phagocytes that potentially access inhaled antigens in human lungs.

METHODS

We acquired and analyzed pulmonary mononuclear phagocytes from fully intact nondiseased human lungs (including the major blood vessels and draining lymph nodes) obtained en bloc from 72 individual donors. Differential labeling of hematopoietic cells via intrabronchial and intravenous administration of antibodies within the same lobe was used to identify extravascular tissue-resident mononuclear phagocytes and exclude cells within the vascular lumen. Multiparameter flow cytometry was used to identify mononuclear phagocyte populations among cells labeled by each route of antibody delivery.

MEASUREMENTS AND MAIN RESULTS

We performed a phenotypic analysis of pulmonary mononuclear phagocytes isolated from whole nondiseased human lungs and lung-draining lymph nodes. Five pulmonary mononuclear phagocytes were observed, including macrophages, monocyte-derived cells, and dendritic cells that were phenotypically distinct from cell populations found in blood.

CONCLUSIONS

Different mononuclear phagocytes, particularly dendritic cells, were labeled by intravascular and intrabronchial antibody delivery, countering the notion that tissue and blood mononuclear phagocytes are equivalent systems. Phenotypic descriptions of the mononuclear phagocytes in nondiseased lungs provide a precedent for comparative studies in diseased lungs and potential targets for therapeutics.

Cutting Edge: Roles for Batf3-Dependent APCs in the Rejection of Minor Histocompatibility Antigen-Mismatched Grafts

In transplantation, a major obstacle for graft acceptance in MHC-matched individuals is the mismatch of minor histocompatibility Ags. Minor histocompatibility Ags are peptides derived from polymorphic proteins that can be presented by APCs on MHC molecules. The APC subtype uniquely responsible for the rejection of minor Ag-mismatched grafts has not yet been identified. In this study, we examined graft rejection in three mouse models: 1) mismatch of male-specific minor Ags, 2) mismatch of minor Ags distinct from male-specific minor Ags, and 3) skin transplant. This study demonstrates that in the absence of pathogen-associated molecular patterns, Batf3-dependent dendritic cells elicit the rejection of cells and grafts expressing mismatched minor Ags. The implication of our findings in clinical transplantation may be significant, as minor Ag reactivity has been implicated in the pathogenesis of multiple allograft tissues.