Mononuclear Phagocyte System

Monocytes

Monocytes are the most abundant mononuclear phagocyte and third most abundant leukocyte subset in the complete blood count, after neutrophils and lymphocytes. They represent approximately 10% of all nucleated blood cells. Their concentration in neonatal blood ranges from 300 to 3300 (mean 1400) cells/μL at 40 weeks, approximately twice as abundant as counts in healthy adults. One to 10 million new monocytes are produced each day from the bone marrow in a healthy adult in a cyclic (∼5 days) manner. Of these, approximately 20% are free flowing in the circulation, and the rest are “marginalized” (i.e., adherent and rolling on the endothelium surface of the blood vessels). Monocytes generally reside only for a short period in blood (<48 hours) after being produced in the bone marrow, following which they rapidly transmigrate to peripheral tissues (e.g., lung, kidney, peritoneum, and gastrointestinal or reproductive tracts), where a majority of them differentiate into macrophages or DCs. ^, In tissues, a minority of monocytes conserve their original phenotype (so-called tissue monocytes ) and may play an important and specific role in vascular remodeling. ^,

Monocytes have been studied for more than a century and were originally identified based on morphologic criteria. However, with the use of flow cytometry, three main, functionally distinct subsets have been defined in human blood, based on their expression of cell surface CD14, a co-receptor for the gram-negative cell wall component lipopolysaccharide (LPS), and CD16, the immunoglobulin (Ig)G Fc receptor FcγR-III ( Fig. 118.2 ). ^, The subset composition and function of human monocytes have been comprehensively reviewed and vary considerably across development, suggesting distinct physiologic roles during fetal ontogeny. ^,

Classical monocytes express high levels of the CD14 surface receptor but lack expression of the receptor CD16; thus they are commonly referred to as CD14 ^high /CD16 ^– monocytes. They are efficient at phagocytosis and produce high amounts of IL-10 upon stimulation with LPS. They represent the main monocyte subset and make up the majority (>80%) of monocytes in adult blood. However, they are less predominant in neonatal blood (<40% to 60%). ^,

Nonclassical monocytes express CD16 but only low levels of CD14 (CD14 ^low /CD16 ⁺ ). They are capable of efficient antigen presentation and express high levels of the human leukocyte antigen (HLA)-DR molecule. They are particularly responsive to intracellular nucleic acids such as those from single-stranded viruses, due to high expression of certain pattern recognition receptors (PRRs), Toll-like receptor (TLR) 7 and TLR8 (see later). They are smaller in size and represent 5% to 10% of the monocytes in adult blood. In neonatal blood, they are relatively more abundant earlier in gestation, representing up to 40% of monocytes less than 32 weeks of gestation. ^, Nonclassical monocytes are also potent producers of IL-1β and tumor necrosis factor (TNF)-α. ^, Because of their specific role patrolling the vascular endothelial surface, nonclassical monocytes may also play an important role as immediate responders in neonatal sepsis.

A third subset of intermediate monocytes express high levels of CD14 and CD16 (CD14 ^high /CD16 ⁺ ). This subset represents a minority of monocytes in adult blood (<5% of all monocytes) compared with neonatal (cord) blood where they are more abundant (∼10% to 20%, up to 30% of monocytes based on another study ). Intermediate monocytes play an important role in the resolution of inflammation. ^, Although little is known regarding their specific in vivo role during fetal life, they appear to differentiate directly from fetal liver embryonic stem cells rather than from classical monocytes as reported in mature, adult individuals. Their high production of proangiogenic factors such as endoglin and vascular endothelial growth factor (VEGF) receptor 2 and their unique ability to induce the generation of capillary networks suggest that these cells may play a distinct, important role in angiogenesis and vascular remodeling. ^,

Macrophages

Macrophages are phenotypically heterogeneous, existing in several different specialized forms depending on the tissue of residence. Specialized tissue-specific macrophages include osteoclasts in bone, alveolar macrophages in the lung, Kupffer cells in the liver, Langerhans cells in the epidermis, and microglia in the CNS (see Fig. 118.2 ). In the placenta, decidual macrophages play an important role ensuring immunologic tolerance between the fetus and its mother.

As discussed earlier, macrophages originate from EMPs during the primitive hematopoiesis wave from the yolk sac or during the definitive hematopoietic wave from liver- or bone marrow–derived monocytes. Therefore the embryonic wave of tissue-resident macrophage is independent from the pool of monocyte-derived macrophage originating from the bone marrow. In addition, tissue-resident macrophages can also be produced from the differentiation of a distinct type of EMPs that express the transcription factor c-MYB ^a

a MYC was discovered as viral gene that transforms cells into a myeloblast phenotype. The c in c-MYC stands for “conserved” and denotes the host homolog of the gene as opposed to the viral homology v-MYC.

and that seed the liver from the yolk sac during embryogenesis to give rise to fetal monocytes that then will become macrophages in tissues. Based on fate-mapping studies where the traveling of labeled cells can be followed over periods in mice, the proportion of macrophages directly originating from embryonic versus embryonic liver-derived versus definitive liver- or bone marrow derived–monocytes vary considerably among tissues. Monocyte-derived macrophages are most abundant in tissues such as the spleen, skin, gut, lungs, and heart, whereas in the CNS, the vast majority of macrophages and DCs may originate from the self-renewal of tissue-resident embryonic macrophages that were produced during embryonic life. ^{, ,}

The proinflammatory or antiinflammatory function of macrophages can be polarized by cytokine milieu, referred to also as M1 and M2 macrophages, respectively. GM-CSF–derived macrophages are able to carry out highly efficient antimicrobial functions and promote adaptive cellular T-cell responses through the production of proinflammatory cytokines, high expression of costimulatory molecules (for antigen presentation), and production of enzymes such as the inducible nitric oxide synthase (iNOS) (at least in mice) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. M-CSF–differentiated macrophages are relatively anergic to inflammatory stimuli and are well adapted for the clearance of debris through phagocytosis in the promotion of wound healing and resolution of inflammation. This function is mediated by the production of antiinflammatory cytokines, immunosuppressive arginase (in mice), and scavenger receptors.

Dendritic Cells

DCs specialize in presentation of antigens to T lymphocytes and therefore express high levels of major histocompatibility complex (MHC) class II molecules such as HLA-DR. At least three main subsets of DCs are recognized, according to the expression of surface markers, functional characteristics, ontogeny, and tissue localization. ^, Two subsets of conventional (or myeloid) DCs (cDCs) can be distinguished based on the high expression of the surface markers CD141 and CD1c, whereas a third subset of plasmacytoid dendritic cells (pDCs) more distinctively expresses the cell surface marker CD123. These two main DC subsets are present in high proportions from early in the third trimester of gestation throughout adult life. ^, Immature DCs are also found in blood and can give rise to tissue DCs. The ontogeny of DCs in human fetuses and neonates has been much less studied than monocytes, in part because of their low abundance in blood makes them difficult to obtain in large enough numbers. pDCs are the main producer of interferon (IFN)-α, a key cytokine in antiviral defense, and neonatal pDCs generally produce relatively low amounts of this cytokine in response to most stimuli. During gestation, DCs that highly express arginase 2 promote tolerance to maternal and fetal antigens. ^,

Pattern Recognition Receptors

A main function of phagocytes is surveillance, which consists in patrolling organs to provide a rapid, early detection and clearance of microbial pathogens by the innate immune system. This is particularly relevant in neonates, whose adaptive immune system is largely naïve and thus unable to respond quickly. ^, This surveillance is achieved through a series of microorganism-recognizing receptors, termed PRRs . PRRs are highly conserved among species. Each PRR targets a different structurally conserved motif uniquely present on bacteria, fungi, or viruses, termed pathogen-associated molecular patterns (PAMPs) . PRRs can be intracellular or extracellular, and include TLRs, nucleotide-binding oligomerization domain-containing protein (NOD), and NOD-like receptors (NLRs), C-type lectin receptors (CLRs) such as the dectin-1 receptor (see later), and the retinoic acid-inducible gene I (RIG-I) and RIG-I–like receptors (RLRs). Stimulation of PRRs results in a cascade of cellular events leading to the activation of phagocytes and production of cytokines and chemokines ( Fig. 118.3 ).

Before the third trimester of gestation, the function of most PRRs is profoundly diminished. At birth, most PRRs are already functional, although some adopt an adult phenotype only later in postnatal life. In between, PRRs do not become responsive all at once, but rather, mature sequentially during the early third trimester of gestation, following an hierarchy, with intracellular PRRs maturing earlier than extracellular PRRs. Authors have hypothesized that this hierarchical maturation of PRRs follows corresponding selective advantages and requirements during the establishment of self-tolerance to maternal and fetal self-antigens as the adaptive immune system is deployed. For example, in the skin, the strength of inflammatory response determines whether the body considers microorganisms to be commensal versus pathogenic during the neonatal period. Suppression of PRR activity in early fetal life may also serve to limit potentially harmful inflammatory responses prior to the fetus reaching a viable stage. Intriguingly, PRR functions are naturally dampened in adult bats, making them more tolerant to disease rather than able to fight infection.