Toward a functional characterization of blood monocytes

Monocytes are bone marrow-derived circulating leukocytes that originate from the common monocyte, macrophage and dendritic cell (DC) precursor (MDP).1 They have been considered to be the circulating precursors for tissue macrophages and DCs; however, many DCs and macrophage cell types (for example, lymphoid organ DCs, plasmacytoid DCs (PDCs), skin Langerhans cells and brain microglia) originate from the MDP independently of monocytes,1, 2 and in some cases even develop independently from the bone marrow.3 Recent experimental evidence rather indicates that monocytes are innate effectors of the inflammatory response to microbes,4 killing pathogens via phagocytosis, the production of reactive oxygen species (ROS), nitric oxide (NO), myeloperoxidase and inflammatory cytokines. In some circumstances they can trigger and polarize T-cell responses4, 5 and may also contribute to tissue repair and neovascularization.6 In addition, monocytes can both stimulate and suppress T-cell responses in infectious and autoimmune diseases.4, 5, 7 In this issue of Immunology & Cell Biology, Harper and colleagues8 suggest that naïve monocytes can inhibit T-cell proliferation in vitro, but that the suppressive function of blood monocytes is impaired in mice immunized with MOG35−55 peptide to induce autoimmune encephalomyelitis (EAE). As adoptive transfer of 2 × 105 wild-type naïve monocytes did not confer protection to EAE, they suggest that environmental cues turn off the suppressive function of naïve monocytes. They observed that inhibition of T-cell proliferation is mediated in part by NO synthesis and cell–cell contact, and may be independent of Foxp3+ regulatory T cells.8 Monocytes are a heterogeneous cell population that consists of several functional subsets, the characterization of which is still in its infancy. Two main subsets of blood monocytes are known in the mouse.1 They do not express Ly6G, in contrast to granulocytes, but can be distinguished on the basis of Ly6C expression (also recognized by the Gr1 antibody) (Figure 1). They originate in the bone marrow from the MDP, but whether they arise directly from the MDP or from a common downstream immediate precursor has not yet been characterized.1 Inflammatory Gr1+ (Ly6C+) murine monocytes are recruited from the blood to tissues following infection, where they undergo activation.9 Their response appears to be pathogen dependent. In the spleen, following infection with Listeria monocytogenes, Gr1+ monocytes produce tumor necrosis factor α, NO and ROS, and they stimulate effector T-cell proliferation.4 In response to viruses, they produce type I interferon via a Toll-like receptor (TLR) 2-dependent pathway.10 When exposed to Toxoplasma gondii or Plasmodium chabaudi, Gr1+ monocytes secrete NO, which kills the invading parasite,11, 12 and in a model of spinal cord injury, recruitment of Gr1+ monocytes appears to be important for tissue repair.13 Gr1+ monocytes also contribute to a fraction of myeloid-derived suppressor cells that protect tumors from immune attack by suppressing T-cell functions.7 In a model of EAE, ‘suppressor’ Gr1+ monocytes can be found in the spleen of mice.14 It would be interesting to investigate whether the cells studied by Harper and colleagues8 belong to the Gr1+ subset, as suppression of T-cell proliferation was partially dependent on NO synthase. On the other hand, Gr1− (Ly6C−) monocytes, although less well characterized than the Gr1+ subset, are involved in the pathogenesis of autoimmune disease, such as lupus in mice, in particular lupus glomerulonephritis.15, 16 Gr1− monocytes exhibit long-range crawling over the endothelium of capillaries, and small veins and arteries, a process that may be involved in surveying the vasculature for tissue damage or infection.17 They respond better to TLR7 agonists than to lipopolysaccharide (LPS) and may be involved in the sensing of tissue damage such as dying and infected cells.18 Following cardiac ischemic injury, Gr1− monocytes are suggested to be involved in tissue repair by inducing myofibroblast accumulation, angiogenesis and collagen deposition.6 A role in regulating T-cell function is not yet described. To our knowledge, human ‘naïve’ blood monocytes have not been demonstrated to be able to suppress T-cell function, although monocyte-mediated T-cell suppression has been shown after human hematopoietic stem-cell transplantation.19 Phenotypes and functional heterogeneity of human blood monocytes were first described by a number of independent groups in the early 1980s.20 It was subsequently proposed that the expression of CD16 can distinguish two main subsets, namely CD14++CD16− (classical) and CD14+CD16++ (non-classical) monocytes.21 Recent data, however, indicate that this classification may be inappropriate and has led to confusion in functional studies, in part because CD16 expression is shared by many cell types and does not define a unique functional monocyte subset.18, 22 Human CD14+ monocytes, which consist of CD16+ and CD16− cells, resemble Gr1+ murine monocytes in gene expression analyses. They specialize in phagocytosis, production of ROS and the recognition of ligands for extracellular TLRs (such as the bacterial product LPS), to which they secrete inammatory cytokines. Among this subset, CD16 may be a marker of activation. In contrast, gene expression analyses indicate similarities between murine patrolling Gr1− monocytes and human monocytes that lack CD14 (CD14dim), which also express CD16.18 CD14dim monocytes patrol the endothelium of blood vessels after adoptive transfer, in an LFA-1-dependent manner, and respond to viruses and nucleic acid-containing immune complexes via a pro-inammatory TLR7–TLR8–MyD88–MEK pathway.18 CD14dim cells may be involved in the innate local surveillance of tissues with regard to cell death and viral infection, the scavenging of immune complexes and the pathogenesis of autoimmune diseases such as lupus.23 The work by Harper and colleagues,8 which suggests that naïve murine blood monocytes can suppress T-cell proliferation, is an incentive to further investigate this new putative function of these cells, to evaluate the extent to which monocytes can regulate T-cell functions, and to examine the respective roles of murine Gr1+ and Gr1− and human CD14+ and CD14dim monocytes in this phenomenon.