The Role of Human Breast Milk-Derived Exosomes in Regulating Microglial Inflammation

Premature birth, accounting for ~11% of births worldwide, is the most significant risk factor of infant mortality and long-term neurological disabilities, caused mainly by ischemic and infectious injury, with hypoxic-ischemic encephalopathy and gram-negative bacterial infections reaping the most disastrous outcomes, respectively. Ischemic/infectious causes can have a bidirectional relationship. Excessive neonatal neuroinflammation can promote debilitating neurological diseases (i.e., cerebral palsy, neurocognitive delay, epilepsy) that burden families and the healthcare system. While external pathologies initiate brain damage, the immaturity of the neonatal nervous and immune systems causes an exaggerated inflammatory response stemming from reactive gliosis, which has been linked to several neurological diseases. Microglia, the resident immune cells of the CNS, are at the center of this response. While they are integral in neurodevelopment, maintenance, and immune surveillance, conversely, they exacerbate hyperinflammatory conditions, directly causing significant brain injury. Neonatal microglia have a particularly robust immune response to the gram-negative bacterial endotoxin, lipopolysaccharide (LPS). Thus, our work modeled the in vitro microglial response to pathogenic invasion of the CNS. Proposed treatment strategies, though effective experimentally, have not shown clinical efficacy and may have long-term adverse effects. Exosomes are endogenous, nanosized (30 – 200 nm), membrane-bound extracellular vesicles that transport proteins, microRNAs, and other metabolites intercellularly. Their low immunogenicity makes them an ideal therapeutic option. Hence, we utilized human breast milk-derived exosomes (hBMEs), which exert immunomodulatory effects, including suppression of the inflammatory response. Firstly, we isolated and purified hBMEs utilizing differential centrifugation, then characterized them with nanoparticle tracking analysis, transmission electron microscopy, small-molecule localized microscopy, and western blot analysis. hBMEs attenuated LPS-induced microglial inflammation by inhibiting the TLR4/NF-κB signaling cascade, resulting in the suppression of Iba1, CD40, NLRP3, and IL-1β; and an increase in IL-10. Next, we found that CD9, a membrane-spanning tetraspanin abundantly expressed on hBMEs, facilitates microglial internalization of hBMEs. After CD9 blockade, hBMEs transfected with GFP-tagged plasmids had decreased uptake into resting and LPS-induced HMC3 microglia. Congruently, CD9 blockade inhibited hBME-mediated upregulation of anti-inflammatory cytokine IL-10 secretion from LPS-induced BV2 microglia. In summary, this work suggests that hBMEs attenuate microglial inflammation through CD9 function and may be a biologically safe, easily accessible, and reproducible therapeutic option for attenuating the inflammatory response in preterm neonates.