A Functional Assessment of Fetal Liver and Monocyte-Derived Macrophages in the Lung Alveolar Environment

It is becoming clear that every organ is seeded by a population of fetal liver-derived macrophages that are replaced at different rates by monocyte-derived macrophages. Using the Ms4a3tdTomato reporter mouse that reports on monocyte-derived alveolar macrophages (Mo-AMs) and our ability to examine AM function using our multichannel intravital microscopy, we examined the fetal-liver derived alveolar macrophage (FL-AM) and Mo-AM populations within the same mouse under various environmental conditions. The experiments unveiled that AMs migrated from alveolus to alveolus and phagocytosed bacteria identically regardless of ontogenic origin. Using 50 PFU of influenza A virus (IAV) determined using the Madin-Darby canine kidney (MDCK) cell line, we noted that both populations were susceptible to IAV-induced immunoparalysis, which also led to impaired phagocytosis of secondary bacterial infections. Both FL-AMs and Mo-AMs were trained by β-glucan to resist IAV-induced paralysis. Over time (40 wk), Mo-AMs began to outperform FL-AMs, although both populations were still sensitive to IAV. Our data also show that clodronate depletion of AMs leads to replenishment, but by FL-AMs, and these macrophages do show some functional impairment for a limited time. Overall, the system is designed such that new macrophages rapidly assume the function of tissue-resident macrophages when both populations are examined in an identical environment. These data do differ from artificial depletion methods that compare Mo-AMs and FL-AMs.

Imaging reveals novel innate immune responses in lung, liver, and beyond

Highly dynamic immune responses are generated toward pathogens or injuries, in vivo. Multiple immune cell types participate in various facets of the response which leads to a concerted effort in the removal and clearance of pathogens or injured tissue and a return to homeostasis. Intravital microscopy (IVM) has been extensively utilized to unravel the dynamics of immune responses, visualizing immune cell behavior in intact living tissues, within a living organism. For instance, the phenomenon of leukocyte recruitment cascade. Importantly, IVM has led to a deep appreciation that immune cell behavior and responses in individual organs are distinct, but also can influence one another. In this review, we discuss how IVM as a tool has been used to study the innate immune responses in various tissues during homeostasis, injury, and infection.

Patrolling Alveolar Macrophages Conceal Bacteria from the Immune System to Maintain Homeostasis

During respiration, humans breathe in more than 10,000 liters of non-sterile air daily, allowing some pathogens access to alveoli. Interestingly, alveoli outnumber alveolar macrophages (AMs), which favors alveoli devoid of AMs. If AMs, like most tissue macrophages, are sessile, then this numerical advantage would be exploited by pathogens unless neutrophils from the blood stream intervened. However, this would translate to omnipresent persistent inflammation. Developing in vivo real-time intravital imaging of alveoli revealed AMs crawling in and between alveoli using the pores of Kohn. Importantly, these macrophages sensed, chemotaxed, and, with high efficiency, phagocytosed inhaled bacterial pathogens such as P. aeruginosa and S. aureus, cloaking the bacteria from neutrophils. Impairing AM chemotaxis toward bacteria induced superfluous neutrophil recruitment, leading to inappropriate inflammation and injury. In a disease context, influenza A virus infection impaired AM crawling via the type II interferon signaling pathway, and this greatly increased secondary bacterial co-infection.