Shaping of the alveolar landscape by respiratory infections and long-term consequences for lung immunity

Abstract

Respiratory infections and especially viral infections, along with other extrinsic environmental factors, have been shown to profoundly affect macrophage populations in the lung. In particular, alveolar macrophages (AMs) are important sentinels during respiratory infections and their disappearance opens a niche for recruited monocytes (MOs) to differentiate into resident macrophages. Although this topic is still the focus of intense debate, the phenotype and function of AMs that recolonize the niche after an inflammatory insult, such as an infection, appear to be dictated in part by their origin, but also by local and/or systemic changes that may be imprinted at the epigenetic level. Phenotypic alterations following respiratory infections have the potential to shape lung immunity for the long-term, leading to beneficial responses such as protection against allergic airway inflammation or against other infections, but also to detrimental responses when associated with the development of immunopathologies. This review reports the persistence of virus-induced functional alterations in lung macrophages, and discusses the importance of this imprinting in explaining inter-individual and lifetime immune variation.

Keywords: AM ontogeny; alveolar macrophages; lung immunity; niche imprinting; respiratory viruses; trained immunity.

Figure 1

Fluorescent reporter system and Cre-based mouse models for reporter labeling and cell ablation. (A) Reporter mice were developed by inserting a recombinant gene encoding a fluorescent protein (e.g. GFP, RFP) under the expression of a specific promoter. A general drawback of these reporter system is that fluorescence intensity is directly associated with the activity of the promoter. Therefore, one limitation is that expression level may fluctuate over time in lineage-tracing or during inflammation. Moreover, overlapping expression of different markers by EM-AMs and MO-AMS makes the distinction between these subsets quite challenging. (B) In the second generation of reporter mice, a Cre recombinase-encoding gene is inserted under the control of a specific promoter, while a fluorescent protein-encoding gene -preceded by a STOP codon flanked by loxP sites (flaxed) is inserted under the control of constitutively active promoter (e.g Rosa26). This system allows to create reporters with fluorescence of choice (e.g. GFP, RFP, mcherry, TdTomato,...), and labels cells in a on/off manner instead of depending on the fluctuating promoter activity. However, these fate-mapping models do not allow to discriminate the homeostatic contribution of MOs to the AM pool during life from the one specifically recruited in disease. (C) Inducible reporter mice, in wich the Cre-loxP system is activated by exogenous inducer (tamoxifen) that allows to assess the specific contribution of MOS to the AM pool at a given time. Indeed, upon exogenous administration, tamoxifen binds to estrogen-receptor (ER^T2 or MercreMer) leading to the dissociation of the complex that sequesters Cre recombinase in the cytoplasm and to the subsequent translocation into the nucleus, allowing reporter expression Interestingly, after tamoxifen treatment has been interrupted, short-lived cells rapidly loose the signal while long-lived cells remain labeled over time. (D) Cre-LoxP systems allowing for targeted cell ablation by combining cell-specific CreER^T2 expression with the induction of a cellular killing mechanism However, the results could be skewed by off-target effects or conversely, good Cre-driving promoter candidates may show only low activity, and present as poor Cre inducers with inadequate recombination efficiency. Figure adapted from Karen De Vlaminck PhD thesis (Bioengineering Sciences VUB).

Figure 2

Ontogeny and development of alveolar macrophages during life. During the pre- and post-natal period, alveolar macrophages (AMS) have an embryonic ontogeny as they derive from successive waves of precursors originating first from the yolk sac (YS-MACS) and then from the fetal liver (FL-MOs). These AMs of embryonic origin display a high proliferative capacity which allows them to occupy the expanding alveolar niche compartment and to self-sustain with little or no contribution from bone marrow-derived monocytes (BM-MOs) However, during life and the ageing process, BM-MOs will gradually infiltrate the lung, differentiate into AMs and progressively contribute to the AM population. The contribution of BM-MO to the pool of AMS increases following episodes of inflammation and/or respiratory infections inducing the depletion of the alveolar niche and the recruitment of MOs into the alveoli. The magnitude, persistence and functional consequences of this BM-MO contribution to the AM pool are context-dependent and the mechanisms dictating this heterogeneity are not fully elucidated. Figure adapted from Karen De Vlaminck PhD thesis (Bioengineering Sciences VUB).

Figure 3

Niche microenvironment is a major determinant governing AM development, proliferation and activation. The developmental dynamics of AMs and their plasticity are intimately linked to signals from the local environment. The alveolar niche, in addition to supplying the structural scaffold necessary for the development of AMs, harbours key cellular actors that provide the trophic factors determining the differentiation, maturation, proliferation or functional polarisation of these cells. Among the niche cells identified in the steady state, alveolar epithelial cells (AECs), innate lymphoid cells type 2 (ILC2s), and basophils imprint AM identity and function, both at birth and throughout life, notably via the secretion of GM-CSF. TGF-β. Upon lung infection, the EM-AM population is partially depleted making the alveolar niche more accessible to BM-MO infiltration and differentiation into AMs in a similar GM-CSF and TGF-B-depend manner. The contribution of MOS to the AM pool is highly dependent on the type of inflammation, the extent of depletion, the turnover rate of AMs and IMs in the lung, and the presence of other imprinted niche cells such as altered ILC2s and/or tissue-resident T cells. In particular, the combination of chemokines and cytokines such as IFN-γ released during inflammation, and the altered availability of trophic factors and nutrients may determine the differentiation of MOs into AMs, their polarisation as well as their renewal capacity.

Figure 4

Environmental factors imprint AM phenotype, metabolome and epigenome leading to diverses functional consequences in the context of health and disease. 'Immune imprinting' refers to an epigenetic, metabolic and functional long-term reprogramming of innate immune cells including EM-AMS or MO-AMS, that results in an altered response to subsequent unrelated triggers. This reprogramming can take place at the level of the inflammed niche (peripheral imprinting) and/or at the level of the progenitor stem cells in the bone marrow (central imprinting) explaining the possible very long term effects of an infectious event, despite the short half-life of immune cells such as MOS. The drivers of this immune imprinting are multiple Most of the currently known environmental factors are the BCG vaccine, microbial products such as LPS, β-glucans, and certain metabolites or pro-inflammatory cytokines. These stimuli can be transient and responsible for processes of trained immunity or long-term tolerance or they can be persistant and thus permanently/recurrently educate AMs. These central and/or peripheral stimuli trigger a cascade of intracellular signaling pathways that can lead to profound alterations in the metabolic and epigenetic profile of the cell The persistence of epigenetic marks and thus the long-term modification of chromatin accessibility at certain loci determines the phenotypic signature of imprinted cells and subsequently, their behaviour in heterologous inflammatory challenges. Interestingly, the imprinting of a given cell could lead to secondary alterations in the cells of the niche and thus modify the cross-talk established between the resident cents of the alveolar niche over the long term. Overall, the history of infection can shape the functional properties of AMs with consequences, depending on the context of imprinting and the context of challenge, being positive or detrimental for the host.