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. Correspondence and requests for reprints should be addressed to Claudia V. Jakubzick, Ph.D., Department of Pediatrics and Immunology, National Jewish Health, 1400 Jackson Street, Denver, CO 80206. E-mail: jakubzickc@njhealth.org This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org The human respiratory tract has a branching structure that terminates in millions of alveoli, whose luminal surface covers an approximate area of 50 to 100 m 2 . In comparison to other barrier surfaces, such as the skin (2 m 2 ) and the gut (10 m 2 ), this surface area is massive, and therefore, comprises the body's largest interface with the ambient environment. Because of normal respiratory function, the average human exchanges 7,000 to 9,000 L of gas each day and inhales billions of particles, allergens, and microbes. Accordingly, the human lung constitutes a major site for the innate and adaptive immune responses. In this context, cells in the mononuclear phagocyte system (MPS), which consists of macrophages, monocytes, monocytederived cells, and dendritic cells (DCs), play critical roles. T...

• Of the 30 000 genes, there are ;0.1% genes whose expression is linked to the origin of the cell rather than the environment.• Marco was most conserved by embryonic origin and not altered by the environment, whereas C1qb and Plbd1 were most conserved by adult origin.Alveolar macrophages (AMs) reside on the luminal surfaces of the airways and alveoli where they maintain host defense and promote alveolar homeostasis by ingesting inhaled particulates and regulating inflammatory responses. Recent studies have demonstrated that AMs populate the lungs during embryogenesis and self-renew throughout life with minimal replacement by circulating monocytes, except under extreme conditions of depletion or radiation injury. Here we demonstrate that on a global scale, environment appears to dictate AM development and function. Indeed, transcriptome analysis of embryonic host-derived and postnatal donor-derived AMs coexisting within the same mouse demonstrated >98% correlation and overall functional analyses were similar. However, we also identified several genes whose expression was dictated by origin rather than environment. The most differentially expressed gene not altered by environment was Marco, a gene recently demonstrated to have enhancer activity in embryonic-derived but not postnatal-derived tissue macrophages. Overall, we show that under homeostatic conditions, the environment largely dictates the programming and function of AMs, whereas the expression of a small number of genes remains linked to the origin of the cell. (Blood. 2015;126(11):1357-1366 Introduction Alveolar macrophages (AMs) reside on the luminal surfaces of the airways and airspaces where they serve critical roles in host defense and alveolar homeostasis, ingesting particulates and microbes that are constantly encountered in the lungs. Importantly, under most circumstances the phagocytosis of inhaled foreign agents is silent, such that inflammatory responses are activated only under circumstances when host defenses become overwhelmed.1 Indeed, compared with macrophages from other sites, AMs are relatively ineffective at initiating immune responses.2,3 Furthermore, compared with other tissue macrophages, they display a unique repertoire of cell surface molecules and have a distinct transcriptome profile. [4][5][6] AMs are now known to derive primarily from fetal liver monocytes and self-renew throughout life with minimal replenishment from circulating monocytes. [7][8][9][10][11][12][13][14][15] This self-renewal is not only maintained under steady-state conditions, but also during acute and chronic inflammation.16 These concepts were illustrated in lung-protected bone marrow (BM) chimera studies in which lead shields were used to protect AMs during radiation. Eight weeks after BM transplantation, the lungs of these chimeras contained AMs of host origin, whereas circulating monocytes were donor-derived. 16 In these chimeric mice, we showed that during inflammation (lipopolysaccharide or influenza A infection), BM donor-derived monocytes were rapidly...

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.

Complements, such as C1q and C3, and macrophages in the splenic marginal zone (MZMs) play pivotal roles in the efficient uptake and processing of circulating apoptotic cells. SIGN-R1, a C-type lectin that is highly expressed in a subpopulation of MZMs, regulates the complement fixation pathway by interacting with C1q, to fight blood-borne Streptococcus pneumoniae. Therefore, we examined whether the SIGN-R1-mediated classical complement pathway plays a role in apoptotic cell clearance and immune tolerance. SIGN-R1 first-bound apoptotic cells and this binding was significantly enhanced in the presence of C1q. SIGN-R1-C1q complex then immediately mediated C3 deposition on circulating apoptotic cells in the MZ, leading to the efficient clearance of them. SIGN-R1-mediated C3 deposition was completely abolished in the spleen of SIGN-R1 knockout (KO) mice. Given that SIGN-R1 is not expressed in the liver, we were struck by the finding that C3-deposited apoptotic cells were still found in the liver of wild-type mice, and dramatically reduced in the SIGN-R1 KO liver. In particular, SIGN-R1 deficiency caused delayed clearance of apoptotic cells and aberrant secretion of cytokines, such as TNF-a, IL-6, and TGF-b in the spleen as well as in the liver. In addition, anti-double-and single-stranded DNA antibody level was significantly increased in SIGN-R1-depleted mice compared with control mice. These findings suggest a novel mechanism of apoptotic cell clearance which is initiated by SIGN-R1 in the MZ and identify an integrated role of SIGN-R1 in the systemic clearance of apoptotic cells, linking the recognition of apoptotic cells, the opsonization of complements, and the induction of immune tolerance.

Immunoglobulins are glycoproteins produced by the cells of the immune system. Their primary function is to protect the body from pathogenic infection. Moreover, a concentrated polyclonal mixture of immunoglobulin G (IgG), the so-called intravenous IgG (IVIG), has been used to treat various chronic and systemic disorders of the immune system. Studies on the effects of IVIG in autoimmune disease models have revealed that IgG Fc fragments confer protection against various autoimmune diseases. The identification of this IgG Fc immunomodulatory component is important for the development of IVIG substitutes. The focus of this review is to introduce one of the Fc regulatory entities and to provide a summary of the current knowledge of the putative general mechanisms underlying IVIG activity in vivo on the basis of these Fc fragments. We also address the recent insights into several approaches for the development of IVIG substitutes.