As essential components of the central nervous system, glial cells and their intercellular communication within the glial ecosystem play a pivotal role in maintaining neuronal functional integrity and brain homeostasis. In the pathological progression of Alzheimer's disease (AD), neurodegenerative changes caused by β-amyloid (Aβ) deposition and abnormal tau phosphorylation are closely linked to glial dysfunction. However, the precise mechanisms by which glial cells regulate the accumulation and clearance of these pathological proteins remain key scientific questions to be addressed in neuroscience.
A recent study by Swirski and Tanzi's team, published in Nature under the title "Astrocytic interleukin-3 programs microglia and limits Alzheimer’s disease," systematically revealed that interleukin-3 (IL-3), as a critical molecule mediating cross-cell communication between astrocytes and microglia, plays a central role in regulating microglial function and inhibiting AD pathological progression through multi-level experimental design. This provides a novel theoretical basis and potential intervention targets for AD-targeted therapy.
Previous clinical studies have found that IL-3 levels are associated with the risk and severity of AD, and in vitro experiments have suggested that IL-3 may be involved in the regulation of neurodegeneration. However, the specific role and mechanism of IL-3 in the pathological process of AD remain unclear. To verify the in vivo function of IL-3, the research team constructed a hybrid model (Il3⁻/⁻5xFAD) of IL-3 gene knockout (Il3⁻/⁻) and 5xFAD mice. The 5xFAD mice, which express five familial AD-related mutant genes, can stably reproduce typical AD pathological features such as Aβ deposition and cognitive decline.
Pathological analysis showed that compared with wild-type 5xFAD mice, the number of Aβ aggregates in the cortical area of Il3⁻/⁻5xFAD mice increased significantly, the volume of individual Aβ plaques increased obviously, and the overall levels of soluble and insoluble Aβ showed a statistically significant increase. Behavioral evaluation further confirmed that IL-3 deficiency exacerbated cognitive impairment in AD model mice: in the Y-maze test, Il3⁻/⁻5xFAD mice showed reduced spontaneous alternation behavior, indicating short-term memory impairment; in the Morris water maze test, their positioning navigation latency was prolonged, and the residence time in the target quadrant was shortened, indicating that spatial learning and memory retention abilities were significantly decreased. These results for the first time confirmed that IL-3 has a clear protective effect on AD pathology in vivo, laying an important foundation for subsequent mechanism research.
To clarify the tissue source of IL-3, the research team first detected the levels of IL-3 in plasma and cerebrospinal fluid of wild-type and 5xFAD mice, and found that there was no significant difference in plasma IL-3 concentration between the two groups, but the level of IL-3 in cerebrospinal fluid was 4 times that in plasma, suggesting that IL-3 may be synthesized locally in the brain. Using CRISPR-Cas9 technology to construct IL-3 green fluorescent protein-labeled mice (Il3 GFPfl/fl), immunofluorescence co-localization experiments showed that IL-3 co-localized with the astrocyte-specific marker GFAP, but not with other cell markers such as neurons and microglia. Further verification using astrocyte reporter mice (Aldh1l1 GFP) found that IL-3 was only expressed in ALDH1L1⁺ astrocytes, and the activation state of astrocytes or IL-3 deficiency did not affect the morphology and distribution of astrocytes, indicating that there is a subpopulation of astrocytes that constitutively secrete IL-3 in the brain.
As the main immune cells in the central nervous system, the functional state of microglia is regulated by a variety of cytokines. The study found that the expression of IL-3 receptor α subunit (IL-3Rα) in microglia of 5xFAD mice showed an age-dependent increase, and this upregulation was cell-specific—only occurring in microglia, while other nerve cells did not show this change. More importantly, IL-3Rα⁺ microglia highly overlapped with the Aβ plaque area, suggesting that IL-3Rα may mediate the response of microglia to Aβ deposition. By comparing the microglial transcriptomes of wild-type, Trem2⁻/⁻, 5xFAD, and Trem2⁻/⁻5xFAD mice, it was found that the expression of IL-3Rα depends on TREM2 (a key receptor regulating microglial phagocytic function), and the expression levels of TREM2 and its adapter protein DAP12 (Tyrobp) in IL-3Rα high-expression (IL-3Rαhi) microglia were significantly higher than those in IL-3Rα low-expression (IL-3Rαlo) cells, confirming that IL-3Rα⁺ microglia are a TREM2-dependent activated subpopulation, which may enhance immune response function through IL-3 signals.
To explore the clinical relevance of IL-3 signaling in human AD, the research team analyzed frontal cortex samples from AD patients and age-matched controls, and found that IL-3 co-localized with astrocytes in AD patient brain tissue, and the expression of IL-3Rα in microglia was significantly higher than that in the control group, suggesting that the IL-3/IL-3Rα signaling axis also plays a role in human AD pathology.
Further mechanism studies showed that IL-3 deficiency did not affect the number and proliferation ability of microglia in 5xFAD mice, but significantly changed their transcriptome characteristics: RNA sequencing analysis found that the expression of genes related to immune response, cell migration, and morphological remodeling in microglia of Il3⁻/⁻5xFAD mice was significantly downregulated, among which the expression levels of key immune regulatory genes such as Trem2 and Tyrobp were significantly reduced, confirming that IL-3 signaling may regulate microglial function through the TREM2 downstream pathway. In vivo imaging experiments further revealed that IL-3 deficiency significantly inhibited the motility of microglia and their chemotaxis to Aβ plaques—in Il3⁻/⁻5xFAD mice, the diffusion rate of microglia decreased, and the aggregation density around Aβ plaques decreased, resulting in a significant reduction in the contact efficiency between microglia and Aβ. These results indicate that IL-3 promotes the recruitment of microglia to pathological areas and enhances immune response by remodeling the transcriptome and functional phenotype of microglia, thus participating in the regulation of Aβ clearance.
To simulate the regulatory effect of IL-3 on microglial movement in the human AD microenvironment, the research team established a 3D microfluidic culture system containing astrocytes, neurons, and microglia. The results showed that adding IL-3 to the ternary culture system of the AD model could significantly promote the migration of microglia to the central cavity containing Aβ and p-tau, confirming that IL-3 has a direct chemotactic effect on human microglia.
In in vivo intervention experiments, using a 5xFAD mouse model with astrocyte-specific knockout of IL-3, it was found that the deficiency of IL-3 derived from astrocytes led to a 75% decrease in cerebrospinal fluid IL-3 level, accompanied by increased Aβ deposition, decreased expression of phagocytosis-related factors such as microglial Apoe, and reduced aggregation around plaques, further verifying the key role of astrocyte-derived IL-3 in AD protection. More importantly, stereotactic injection of recombinant IL-3 (rIL-3) into the cortex of 5xFAD mice could rapidly mobilize microglia to aggregate towards Aβ plaques, increase the binding efficiency of microglia to Aβ, and significantly improve the cognitive function of mice, providing direct experimental evidence for the therapeutic application of IL-3.
This study systematically clarified the molecular mechanism by which IL-3, as a key mediator of astrocyte-microglia crosstalk, plays a protective role in the pathological process of AD by regulating the activation, migration, and functional phenotype of microglia. The study confirmed that IL-3 constitutively secreted by astrocytes binds to IL-3Rα on the surface of microglia, activates TREM2-dependent signaling pathways, promotes the recruitment of microglia to Aβ plaques and enhances their clearance ability, ultimately reducing pathological damage and cognitive impairment in AD model mice.
This finding provides a new perspective for the study of AD pathological mechanisms—the cytokine communication between glial cells may be a key link in regulating the progression of AD pathology. In view of the repeated failures of traditional therapeutic strategies such as anti-Aβ and anti-tau, therapeutic methods targeting the IL-3 signaling axis are expected to become a new direction for AD intervention. Future studies need to further explore the role differences of IL-3 in different AD stages, the cross-regulation mechanism with other cytokines, and the safety and long-term effects of IL-3 therapy, so as to lay a foundation for promoting the transformation of IL-3 from basic research to clinical application. At the same time, the development of drugs that specifically enhance IL-3 secretion from astrocytes or IL-3Rα signaling in microglia may provide a new breakthrough for the precise treatment of AD.
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