Engineered Macrophages Deliver Anti-Aging Antibodies Directly Into Bone

Age-related skeletal disorders—osteoporosis, impaired fracture healing, and osteoarthritis—affect hundreds of millions globally, yet current treatments remain largely palliative. A major barrier to progress has been identifying the molecular drivers shared across these conditions and building delivery platforms capable of reaching bone tissue specifically. This study addresses both challenges simultaneously.

Using RNA sequencing of serum from young (<40 years) and elderly (>65 years) patients, as well as bone biopsies from elderly patients undergoing total hip arthroplasty, the team identified MMP9 as the most consistently and significantly upregulated gene. STRING network analysis placed MMP9 at the hub of differentially expressed genes linked to Wnt signaling, stem cell differentiation, osteoblast activity, and extracellular matrix remodeling—pathways central to skeletal homeostasis. ELISA and PCR validation confirmed elevated MMP9 at both protein and transcript levels in aged serum and osteoporotic bone versus controls, establishing MMP9 as a SASP (senescence-associated secretory phenotype) effector and therapeutic target.

To neutralize MMP9 in situ, the researchers engineered apoptosis-mimicking lipid nanoparticles (aMMP9-LNPs) by incorporating phosphatidylserine (PtdSer) into the lipid shell and reducing DSPE-PEG2000 content to 0.5%. PtdSer mimics the 'eat-me' signal of apoptotic cells, triggering recognition by macrophage surface receptors (PVEERs such as TIM-4 and MerTK) and enhancing efferocytosis-mediated uptake. The optimal formulation (0.5% PEG, 1% PtdSer) achieved the highest macrophage transfection efficiency among tested variants. Encapsulated within these LNPs was a circular RNA (circRNA) encoding a full-length anti-MMP9 IgG antibody—designed with an IgK signal peptide, self-cleaving 2A peptide, and an IRES element for cap-independent translation—producing a ~3.2 kb circular construct with high purity and structural integrity confirmed by RNase R digestion.

In vitro, aMMP9-LNP-treated macrophages showed sustained anti-MMP9 antibody secretion, significantly reducing extracellular MMP9 concentrations over time without cytotoxicity. In aged mouse models, systemic intravenous administration led macrophages to traffic to bone marrow, where they secreted anti-MMP9 antibodies locally. This resulted in reduced skeletal stem cell senescence, restored osteoblast-osteoclast balance, decreased p21 and MMP3 expression, dampened SASP signaling, accelerated fracture repair, and attenuated cartilage degradation in osteoarthritis models. Biodistribution studies confirmed bone-targeted delivery with preserved tissue homeostasis in major organs.

Mechanistically, MMP9 blockade interrupted a feed-forward senescence loop: by reducing SASP components, it restored stem cell function and osteogenic differentiation capacity while limiting osteoclast-driven bone resorption. The circRNA platform offers sustained but non-integrating antibody expression, circumventing the safety concerns of viral gene therapy while outperforming conventional mRNA in expression duration. The study establishes a proof-of-concept for macrophage-directed in vivo antibody engineering as a versatile platform for degenerative diseases beyond bone.