Abstract

Background

Osteocytes are the most abundant cells in bone and are increasingly recognised as active regulators of skeletal remodelling and inflammatory signalling. While osteocytes are known to possess innate immune responsiveness, their capacity to participate in adaptive immune pathways remains poorly defined⁽¹⁾. This study investigated the expression and functionality of major histocompatibility complex (MHC) Class II-associated pathways in human primary osteocyte-like cells under basal and infectious conditions.

Methods

Human cancellous bone and donor-matched peripheral blood were collected from participants undergoing joint replacement surgery. Bone explant-derived osteoblastic cells were tested undifferentiated, differentiated to an osteocyte stage, and analysed using flow cytometry, immunofluorescence confocal imaging, and Western blotting. Expression and ability to stimulate autologous CD4⁺ T cell proliferation were tested using an osteocyte Staphylococcus aureus intracellular infection model. ⁽²⁾ Human periprosthetic joint infection (PJI) bone was examined to assess clinical relevance.

Results

Immunofluorescence and confocal microscopy identified intracellular localisation of MHC Class II and the peptide-loading chaperone HLA-DM within acidic vesicular compartments consistent with canonical antigen-processing machinery. Staphylococcus aureus exposure induced marked upregulation of MHC Class II protein levels. Infected osteocytes stimulated autologous CD4⁺ T cell proliferation, demonstrating functional MHC Class II-restricted antigen presentation. Osteocyte MHC Class II protein expression was detected in human PJI bone tissue, supporting the clinical relevance of these findings.

Conclusion

This study identifies for the first time osteocytes as capable of expressing functional MHC Class II and participating directly in the adaptive immune response, suggesting they play a previously unrecognised immune-protective role in bone.