Self-renewing tissue-resident macrophages are thought to be exclusively derived from embryonic progenitors. However, whether circulating monocytes can also give rise to such macrophages has not been formally investigated. Here we use a new model of diphtheria toxin-mediated depletion of liver-resident Kupffer cells to generate niche availability and show that circulating monocytes engraft in the liver, gradually adopt the transcriptional profile of their depleted counterparts and become long-lived self-renewing cells. Underlining the physiological relevance of our findings, circulating monocytes also contribute to the expanding pool of macrophages in the liver shortly after birth, when macrophage niches become available during normal organ growth. Thus, like embryonic precursors, monocytes can and do give rise to self-renewing tissue-resident macrophages if the niche is available to them.

Cells of the monocyte–macrophage lineage play a central role in the orchestration and resolution of inflammation. Plasticity is a hallmark of mononuclear phagocytes, and in response to environmental signals these cells undergo different forms of polarized activation, the extremes of which are called classic or M1 and alternative or M2. NF-κB is a key regulator of inflammation and resolution, and its activation is subject to multiple levels of regulation, including inhibitory, which finely tune macrophage functions. Here we identify the p50 subunit of NF-κB as a key regulator of M2-driven inflammatory reactions in vitro and in vivo. p50 NF-κB inhibits NF-κB–driven, M1-polarizing, IFN-β production. Accordingly, p50-deficient mice show exacerbated M1-driven inflammation and defective capacity to mount allergy and helminth-driven M2-polarized inflammatory reactions. Thus, NF-κB p50 is a key component in the orchestration of M2-driven inflammatory reactions.

Alternatively activated macrophages (aaMφ) display molecular and biological characteristics that differ from those of classically activated macrophages (caMφ). Recently, we described an experimental model of murine trypanosomosis in which the early stage of infection of mice with a Trypanosoma brucei brucei variant is characterized by the development of caMφ, whereas in the late and chronic stages of infection, aaMφ develop. In the present study, we used suppression subtractive hybridization (SSH) to identify genes that are expressed differentially in aaMφ versus caMφ elicited during infection with this T. b. brucei variant. We show that FIZZ1 and Ym1 are induced strongly in in vivo- and in vitro-elicited aaMφ as compared with caMφ. Furthermore, we demonstrate that the in vivo induction of FIZZ1 and Ym1 in macrophages depends on IL-4 and that in vitro, IFN-γ antagonizes the effect of IL-4 on the expression of FIZZ1 and Ym1. Collectively, these results open perspectives for new insights into the functional properties of aaMφ and establish FIZZ1 and Ym1 as markers for aaMφ.

Tumor-associated macrophages (TAM) are an important component of the tumor stroma and exert several tumor-promoting activities. Strongly pro-angiogenic TAMs that reside in hypoxic tumor areas highly express macrophage mannose receptor (MMR, CD206). In this study, we targeted MMR⁺ TAMs using nanobodies, which are single-domain antigen-binding fragments derived from Camelidae heavy-chain antibodies. MMR-specific nanobodies stained TAMs in lung and breast tumor single-cell suspensions in vitro, and intravenous injection of ^99mTc-labeled anti-MMR nanobodies successfully targeted tumor in vivo. Retention of the nanobody was receptor-specific and absent in MMR-deficient mice. Importantly, co-injection of excess unlabeled, bivalent anti-MMR nanobodies reduced nanobody accumulation in extratumoral organs to background levels, without compromising tumor uptake. Within tumors, the ^99mTc-labeled nanobodies specifically labeled MMR⁺ TAMs, as CCR2-deficient mice that contain fewer TAMs showed significantly reduced tumor uptake. Further, anti-MMR nanobodies accumulated in hypoxic regions, thus targeting pro-angiogenic MMR⁺ TAMs. Taken together, our findings provide preclinical proof of concept that anti-MMR nanobodies can be used to selectively target and image TAM subpopulations in vivo. Cancer Res; 72(16); 4165–77. ©2012 AACR.