The study matters because cochlear implants are the most successful neural prosthesis in medicine, restoring hearing to more than a million people worldwide. Yet every implant triggers a foreign body response, a chronic inflammatory reaction that leads to fibrotic tissue growth around the electrode array and can degrade implant performance over time. Until now, that response has been attributed almost entirely to macrophages, the generic immune cells long assumed to populate the cochlea. The Iowa team, led by Marlan R. Hansen, suspected the story was incomplete. Mononuclear phagocytes are a heterogeneous family that includes tissue-resident macrophages, circulating monocytes, dendritic cells, and microglia, the resident immune cells of the central nervous system. Each has a different embryonic origin, expresses different molecular markers, and performs divergent functions. Lumping them together as macrophages, the researchers argued, obscured the true cellular players in cochlear immunity.

Disentangling these cell types required overcoming a stubborn technical problem. Many commonly used immune markers, including IBA1, CD68, CD11b, and CD64, are expressed across multiple mononuclear phagocyte lineages and change with activation state, making them unreliable for classification. The team therefore turned to two of the most specific markers available. TMEM119 and the P2Y12 receptor are currently considered the gold-standard identifiers of microglia, the self-renewing resident macrophages of the brain and spinal cord. CD11c, meanwhile, is the most specific marker for dendritic cells, the stellate-shaped professional antigen-presenting cells that can activate naïve T cells and initiate adaptive immune responses. Rather than relying on antibodies alone, the researchers exploited two transgenic reporter mouse lines in which green fluorescent protein is expressed under the control of either the Tmem119 or the Itgax promoter, allowing microglia-like cells and dendritic cells to be visualized directly in intact tissue.

The experimental design was elegant in its precision. Young adult mice, ten to twelve weeks old, received a custom electrode array inserted into the scala tympani, one of the fluid-filled chambers of the cochlea, through a small cochleostomy in the basal turn, replicating the surgical approach used in human implantation. After twenty-eight days, a time point chosen to represent the chronic phase of the foreign body response, the cochleae were harvested alongside the contralateral, non-implanted ears of the same animals. The team then cut thirty-micrometer mid-modiolar sections, imaged them by confocal microscopy, and quantified cell densities in four anatomical compartments of the basal turn: the scala tympani, Rosenthal’s canal within the spiral ganglion, the lateral wall containing the spiral ligament and stria vascularis, and the modiolus, the central bony axis that houses the spiral ganglion neuron cell bodies.

The baseline map that emerged was remarkable for its regional structure. In healthy cochleae, CD11c-positive dendritic cells were concentrated in Rosenthal’s canal, interspersed individually among spiral ganglion neuron cell bodies, and in the spiral ligament and stria vascularis, connective tissue and vascular structures of the lateral wall. Density measurements revealed a clear hierarchy: the spiral ganglion held the highest mean density of dendritic cells, roughly 2.9 million cells per cubic micrometer, followed by the lateral wall at about 2.0 million and the modiolus at 1.3 million, while the scala tympani showed the lowest density at approximately 0.7 million. This restricted distribution contrasts sharply with the broader macrophage network previously described and suggests that dendritic cells occupy defined antigen-surveillance niches adjacent to neurons and vasculature, much as they do in the cornea and meninges of other sensory organs.

Microglia-like cells told a different story. TMEM119 reporter expression revealed a resident population distributed across the stria vascularis near the blood-labyrinth barrier, the modiolus, the spiral ligament, the spiral limbus, and the bony spiral lamina. The lateral wall showed the highest density, approximately 17.8 million cells per cubic micrometer, followed by the modiolus at 9.3 million and the spiral ganglion at 7.1 million, with minimal density in the scala tympani. Complementary immunolabeling for P2Y12 confirmed that these cells co-expressed the canonical microglial receptor, and the P2Y12 signal was restricted to the phagocyte population, never appearing on spiral ganglion neuron cell bodies. The presence of cells bearing both microglia-specific markers in the cochlea, an organ traditionally classified as part of the peripheral sensory nervous system, extends the known distribution of resident microglia-like phagocytes and identifies the spiral limbus, a structure adjacent to the organ of Corti involved in tectorial membrane attachment, as a previously unappreciated immune site warranting dedicated investigation.

Then came the implantation experiments, and with them the study’s most consequential finding. Twenty-eight days after electrode insertion, CD11c-positive dendritic cells had accumulated prominently at the implant-tissue interface in the scala tympani, clustering in close proximity to the electrode array surface. Paired statistical comparisons between implanted and contralateral ears showed that dendritic cell density trended higher in the implanted cochlea across all four regions examined, reaching statistical significance in the modiolus, where the mean difference was approximately 4.7 million cells per cubic micrometer. The modiolus houses the spiral ganglion neuron cell bodies and lies closest to the electrode trajectory, making it a plausible focal point for implant-induced tissue disruption and subsequent phagocyte recruitment. Crucially, these interface-localized dendritic cells strongly upregulated major histocompatibility complex class II, the molecular hallmark of antigen-presenting capability, indicating they had acquired the functional phenotype needed to sample biomaterial-associated proteins, cellular debris, or altered self-antigens and present them to T cells.

The microglia-like population responded very differently. TMEM119-positive cell density did not change significantly in any region following implantation, despite modest non-significant trends toward increased density in the scala tympani and spiral ganglion and decreased density in the lateral wall. TMEM119 expression persisted in cells adjacent to the implant site, suggesting either activation of resident microglia-like cells or phenotypic adaptation in response to injury, but the population remained comparatively undisturbed. This dissociation, in which implantation preferentially engages a recruited or locally activated dendritic cell population while leaving the tissue-resident microglia-like compartment largely intact, parallels findings in the central nervous system, where resident microglia maintain baseline surveillance while monocyte-derived cells drive acute inflammatory responses. The two populations also occupied distinct anatomical niches at baseline, with dendritic cells enriched in the spiral ganglion and microglia-like cells in the lateral wall, consistent with complementary rather than redundant surveillance functions shaped by local microenvironmental cues.

The implications reach well beyond basic immunology. If dendritic cells coordinate the adaptive immune arm of the cochlear foreign body response, then adaptive immune modulation becomes a plausible therapeutic strategy. Local delivery of immunomodulatory agents targeting dendritic cell activation, MHCII expression, or T cell recruitment could help preserve cochlear architecture and improve long-term implant performance, complementing existing approaches such as dexamethasone-eluting electrodes that dampen inflammation more broadly. Strategies targeting purinergic signaling pathways, which regulate microglial activation through receptors like P2Y12, could selectively suppress harmful neuroinflammation while preserving protective immune functions. The cochlea also offers a uniquely accessible model for studying neuroimmune interactions at the biomaterial-neuron interface, enabling precise spatial mapping of immune cells relative to implanted devices, neural structures, and vascular compartments, with relevance extending to brain implants and other neural technologies that face similar foreign body challenges.

The authors are careful to note the limits of their study. The work captured a single chronic time point, leaving the acute and subacute phases of immune recruitment unexplored. Reporter mice and immunostaining demonstrate the presence of distinct phagocyte populations but do not directly prove antigen presentation, cytokine production, or T cell activation within the cochlea. The team could not differentiate tissue macrophages from monocyte-derived macrophages, a distinction that markers like CCR2 could resolve, and the cochlear dendritic cells were not subclassified into the three major classes described in humans. The C57BL/6 background of the reporter strains, which develops early-onset age-related hearing loss, may itself influence immune activation, and modest sample sizes precluded testing for sex-specific differences. Future studies employing single-cell transcriptomics, fate mapping, and functional perturbation on more hearing-stable backgrounds will be needed to establish causal roles. Even so, the study fundamentally reframes the inner ear’s immunological identity. The cochlea is not an immune-privileged sanctuary but a compartmentally organized immune landscape, one in which dendritic cells stand poised as early antigen-presenting responders and microglia-like cells contribute to neuroimmune signaling within neural compartments. Understanding that landscape, the researchers argue, is the first step toward immune-informed strategies to make cochlear implants safer and more durable for the patients who depend on them.

Subject of Research: Tissue-resident and recruited mononuclear phagocyte organization in the cochlea under homeostasis and implant-induced inflammation

Article Title: Tissue-resident and recruited mononuclear phagocyte organization in the cochlea under homeostasis and implant-induced inflammation

Article References: Fatima, S. M., Rahman, M. T., Mostaert, B. J., Razu, I., Hunger, B., Afrin, S., Haque, S. N., Khan, N. A., Zhang, Z., Claussen, A. D., & Hansen, M. R. (2026). Tissue-resident and recruited mononuclear phagocyte organization in the cochlea under homeostasis and implant-induced inflammation. Frontiers in Cellular Neuroscience, 20, Article 1895870. https://doi.org/10.3389/fncel.2026.1895870

Image Credits: AI Generated

DOI: 10.3389/fncel.2026.1895870

Keywords: Tissue-resident, recruited, mononuclear, phagocyte, organization, cochlea, under, homeostasis, implant-induced, inflammation, scientific research