Purpose: To study distinct aspects of human monocyte-derived macrophage (MDM) activation by human corneal tissue as a possible initial stage in human corneal allograft rejection.
Methods: Human monocytes were isolated from peripheral blood mononuclear cells (PBMC) and differentiated into MDM. Human corneas with or without endothelium were fragmented using a standardized protocol. MDM were stimulated with human corneal fragments, corneal fragment supernatant, lipopolysaccharide (LPS) or interferon-gamma (IFNγ), and expression profiles for 34 cytokines were determined in MDM-conditioned media using a Luminex bead-based multiplex assay. Data from clinical aqueous humour samples served for comparison and validation. To assess cell recruitment, immunogenicity of corneal endothelial cells (CEC), monocyte survival and differentiation, we applied transwell migration assays, cell viability assays and fluorescence-activated cell sorting, respectively.
Results: Corneal fragments induced MDM to release distinct cytokines into the medium. Media thus conditioned in vitro by stimulated MDM shared cytokine patterns, namely MCP-1, MIP-1α and MIP-1β, with human aqueous humor samples obtained in human corneal allograft rejection. The presence of CEC in tissue fragments used for MDM stimulation attenuated the upregulation of distinct pro-inflammatory chemokines, like MCP-3 and IL-8, reduced the monocyte survival time, and diminished monocyte-to-macrophage differentiation induced by conditioned media. Distinct anti-inflammatory cytokines, like IL-4 and IL-13, were upregulated in the presence of corneal endothelium. Cornea fragment-stimulated MDMs induced recruitment of monocytes from a PBMC pool in a transwell migration model, modulated immune cell viability and promoted further immune cell recruitment and differentiation.
Conclusions: Human macrophages respond to allogenic corneal tissue and generate an inflammatory milieu. This can drive further recruitment of immunocompetent cells and modulate cell survival and differentiation of the cells recruited. These observations are consistent with the hypothesis that macrophages play a significant role in the initiation of corneal transplant rejection. Our data also indicate that distinct aspects of early human corneal transplant rejection can be modelled in vitro.