Monocyte Cell Overview

Monocytes were first described in 1882 by Ilya Metchnjkoff and have since been demonstrated to be critical in human health and disease [1]. Monocytes are a leukocyte subset that play key roles in homeostasis, pathogen challenge and clearance, and inflammation. Monocytes represent approximately 4% of total leukocytes in mice and 10% in humans [2,3]. After production and development from progenitors in the bone marrow, monocytes circulate in the vasculature, bone marrow, and spleen; they do not proliferate in a steady state. Whether in the normal course of development or due to pathogen challenge, once monocytes migrate to and into peripheral tissues, they differentiate into dendritic cells or macrophages. Plasticity and heterogeneity are monocyte hallmarks, as they can rapidly adjust their functional phenotype in response to changing immunological cues.

Monocytes are members of the mononuclear phagocyte system (MPS), which is a comprehensive classification of all highly phagocytic mononuclear cells and their precursors [3,4,5]. The MPS comprises all myeloid immune cells other than polymorphonuclear granulocytes. The origin and lineage of the cells in the MPS remained poorly understood until the availability of multicolor fluorescence-activated cell sorting (FACS), which allows identification of progenitors and differentiated cell populations based on the expression of specific cellular markers [4,5]. Current models of monocyte differentiation propose that circulating monocytes originate from hematopoietic stem cell (HSC)-derived progenitors with myeloid restricted potential [4,5]. Subsequent commitment steps during monocyte differentiation in the bone marrow involve common myeloid progenitors (CMPs), granulocyte-macrophage precursors (GMPs), and macrophage and DC precursors (MDPs) (Figure 1) [5,6].

Human monocyte markers

Expression of CD14, CD16 (Fcγ RIII), CD64 (Fcγ RI), and the chemokine receptors CD192 and CX3CR1 define human peripheral blood monocyte populations into three different subsets: classical, intermediate, and nonclassical (Table 1). These subpopulations can be further characterized by different levels of HLA-DR and CD195, as well as of the receptors TNFR1 (CD120a) and TNFR2 (CD120b). TNFR1 expression is highest on intermediate monocytes, whereas TNFR2 expression is highest on nonclassical monocytes [5,6,7,8].

Table 1. Markers for human monocytes.

Mouse monocyte markers

Mouse monocytes also have three distinct subpopulations (Table 2), defined by their cell surface expression of Ly6C, CD11b, and chemokine receptors CD192 and CX3CR1. In addition, a high level of CD115 expression discriminates between peripheral blood monocytes and granulocytes, as well as lymphocytes, which also express CD11b (Mac-1) [5,6,8].

Table 2. Markers for mouse monocytes.

Chemokines and monocyte trafficking

CCR2

CCL2 (also known as MCP1) and CCL7 (also known as MCP3) are CC chemokines that bind to CCR2 and mediate Ly6C^Bright monocyte recruitment [9,10,12]. Although expression of CCR2 is restricted to only a few cell types, most (if not all) nucleated cells express CCL2 in response to activation by pro-inflammatory cytokines or stimulation of innate immune receptors by a range of microbial molecules. Many infections induce the expression of CCL2 and result in high circulating levels of CCL2 in serum and within inflamed tissues. CCL2 dimerizes and associates with tissue glycosaminoglycans, establishing gradients that guide monocytes towards sites of infection or inflammation. In support of this model, monocyte recruitment is impaired by amino acid substitutions in CCL2 that prevent dimerization or association with glycosaminoglycans.

CCR1 and CCR5

Monocytes also express CCR1 and CCR5, and these receptors bind to a variety of chemokines, including the shared ligands CCL3 (also known as MIP1α) and CCL5 [11,12]. In vitro transmigration assays have shown: (1) CCR1 predominantly mediates the arrest of monocytes in the presence of shear flow; (2) CCR5 contributes to monocyte “spreading”; (3) both CCR1 and CCR5 support transendothelial chemotaxis towards CCL5. These observations suggest that the two receptors have specialized roles in monocyte recruitment. Table 3 contains a summary of chemokine molecules involved in monocyte recruitment.

Table 3. Chemokines receptors involved in monocyte trafficking.

Adhesion molecules and monocyte trafficking

Monocyte recruitment follows the general paradigm of leukocyte adhesion and trafficking, which involves rolling, adhesion, and transmigration. The migration of leukocytes, including monocytes, depends on integrins and other adhesion molecules. LY6C^Hi monocytes in mice express L-selectin, P-selectin glycoprotein ligand 1 (PSGL1), lymphocyte function-associated antigen 1 (LFA1, also known as α_Lβ₂ integrin), macrophage receptor 1 (MAC1, also known as integrin α_Mβ₂), platelet endothelial cell adhesion molecule (PECAM1), and very late antigen 4 (VLA4, also known as integrin α₄β₁), which all contribute to leukocyte adhesion and migration [13].

Patrolling by LY6C^Low monocytes along endothelial cells in resting dermal blood vessels is mediated by the integrin LFA1. In contrast, the early recruitment of LY6C^Hi monocytes is not affected by LFA1 deficiency. These results suggest that the adhesion mechanisms used by distinct monocyte subsets may differ depending on the type of tissue and the inflammatory state [13]. Table 4 contains a summary of adhesion molecules involved in monocyte recruitment.

Table 4. Adhesion molecules involved in monocyte trafficking.

Isolation

Primary human monocytes can be isolated from peripheral blood mononuclear cells (PBMCs). PBMCs can be isolated from human blood or peripheral leukopacs by density gradient centrifugation. Monocytes can be isolated from PBMCs using Invitrogen Dynabeads Untouched Human Monocytes Kit, which isolates pure and viable monocytes from PBMC by negative isolation. The kit depletes T cells, B cells, NK cells, dendritic cells, granulocytes, and erythrocytes, while the negatively isolated human monocytes are left in the sample. If cell sorting instrumentation is available, then CD14, CD64, and CD192 can be used as specific markers for human monocytes.

Culture

Primary human monocytes can be cultured in vitro; however, care must be taken because within 5 days in the presence of serum or M-CSF, cultured monocytes will start to differentiate into macrophages. Monocytes grow as a mixture of adherent and nonadherent cells, with the proportion of adherent cells dependent on the culture medium and activation stimuli (if added). If autologous serum is used, RPMI 1640 culture medium is recommend such as Gibco Advanced RPMI 1640 Medium supplemented with 5% FBS.
 

IL-34 and CSF-1 promote the differentiation and survival of monocytes under homeostatic conditions while they are regulated by GM-CSF during inflammation. Monocytes respond to chemokine (CCL2, CCL3, CCL5) gradients that cause them to migrate from peripheral blood cells into tissues where they differentiate into macrophages, myeloid dendritic cells, or osteoclasts depending on the stimulus. Monocytes are a source of pro-inflammatory cytokines such as TNF-α, IL-1β, IL-6, and IL-8 which are essential for the innate response to pathogens. Multiplexed immunoassays provide an effective way to measure monocyte associated cytokine production in plasma, serum, or cell cultures. Invitrogen multiplex assays provide an easy to use online selection tool to custom configure multiplex panels in addition to pre-configured panels.

Table 5: Key cytokines involved in monocyte differentiation, recruitment, and secretion.

Multiplex immunoassays