CRISPR-MI and scRNA-Seq Reveal TREM2's Function in Monocyte Infiltration and Macrophage Apoptosis During Abdominal...

Abdominal Aortic Aneurysm (AAA) represents a life-threatening vascular pathology characterized by the progressive dilation and potential rupture of the aortic wall, yet effective pharmacological interventions remain entirely unavailable for clinical use. Prior research has shown that the infiltration of circulating monocytes into the arterial tunica media and adventitia is a fundamental driver that initiates the inflammatory cascade and subsequent tissue remodeling. These recruited myeloid cells eventually differentiate into mature macrophages, which release proteolytic enzymes and pro-inflammatory mediators that weaken the structural integrity of the vessel. Despite the recognized importance of this cellular influx, the precise genetic regulators and molecular signaling pathways that dictate the magnitude and timing of monocyte recruitment remain largely uncharacterized. The complexity of the vascular microenvironment has historically limited the ability of researchers to identify these regulatory elements using traditional in vitro models or low-throughput genetic studies. This absence of evidence motivated the current investigation into the systemic and local factors that govern monocyte trafficking during the early and late stages of aneurysm induction.

This investigation seeks to identify the specific genetic determinants that modulate the recruitment of monocytes into the aortic wall during the initiation of Abdominal Aortic Aneurysm (AAA). The researchers aimed to develop and validate a novel, high-throughput screening platform capable of evaluating gene function in vivo within the context of vascular inflammation. A central objective involved determining how the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) influences the migratory capacity and adhesive properties of these immune cells. The study evaluates the hypothesis that specific myeloid receptors act as checkpoints to prevent excessive cellular infiltration and subsequent tissue damage. Investigators also examined the temporal consequences of TREM2 deficiency on the survival and persistence of macrophages within the established aneurysmal lesion. By mapping these interactions, the team intended to clarify the paradoxical relationship between early immune cell recruitment and the ultimate progression of the disease.

The authors developed an innovative technique termed in vivo genome-wide CRISPR/Cas9 screening of monocyte infiltration (CRISPR-MI) to systematically probe genetic functions within a living organism. This methodology integrates Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology with Single-Cell Ribonucleic Acid Sequencing (scRNA-Seq) to provide high-resolution transcriptomic and functional data. Myeloid progenitor cells were subjected to large-scale genetic modification to observe the impact of specific knockouts on their ability to home to the inflamed vasculature. In vitro assays were subsequently employed to measure the adhesive interactions and chemotactic responses of monocytes lacking the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) protein. The researchers utilized a standardized murine model of Abdominal Aortic Aneurysm (AAA) induction, involving the application of elastase or angiotensin II, to track cell movement in a complex biological environment. Histological examinations and flow cytometry were performed to quantify the density of infiltrating immune populations and assess the extent of vascular remodeling at multiple time points.

Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) functions as a potent negative regulator of monocyte infiltration during the initial induction phase of Abdominal Aortic Aneurysm (AAA). Deletion of the Trem2 gene leads to a significant upregulation of various adhesion molecules, chemotactic receptors, and pro-inflammatory cytokines within the circulating monocyte population. Experimental data demonstrate that Trem2 knockout (KO) cells exhibit significantly enhanced migration toward chemoattractants and increased binding to endothelial layers compared to wild-type controls. In vivo observations confirmed that the absence of this receptor leads to a marked increase in the total number of monocytes entering the aortic wall during the early disease stages. Paradoxically, the Trem2 KO mice displayed a notable reduction in the overall severity and diameter of the aneurysm during the late stages of the study. This unexpected attenuation of disease progression is attributed to a prominent increase in macrophage apoptosis, which limits the chronic inflammatory response within the vessel wall.

The study establishes CRISPR-MI as a powerful and versatile platform for identifying the genetic drivers of immune cell trafficking in various disease conditions in vivo. These findings highlight a complex, dichotomous role for Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) in the pathogenesis and progression of Abdominal Aortic Aneurysm (AAA). While the receptor serves to limit the initial recruitment of monocytes, its presence is essential for maintaining the survival of macrophages within the chronic inflammatory environment. Targeting the TREM2 signaling pathway may therefore offer a nuanced therapeutic approach that varies depending on the clinical stage of the vascular pathology. Future research should investigate whether the modulation of this receptor can be optimized to prevent early inflammation without compromising the clearance of apoptotic cells. This work provides a critical foundation for exploring other myeloid-specific regulators that could serve as novel pharmacological targets for treating life-threatening aortic diseases.