C-Reactive Protein Aggravates Acute Allograft Rejection via Subset-Specific Monocyte Activation Visualized in a VCA Model

Introduction: The impact of the innate immune response on acute and chronic allograft rejection is poorly understood. We have recently identified a molecular mechanism that leads to a conformational change within the structure of C-reactive protein (CRP), leading to activation of monocyte subsets and aggravation of the ischemia/reperfusion injury. Here, we visualized monocyte subsets in the early phase of allogeneic immune response and their differential regulation by CRP using intravital imaging in a hind limb VCA rat model. Materials and Methods: We performed allogeneic hind limb transplantations in the rat with and without the additional administration of human CRP to assess subset-specific infiltration and tissue distribution of recipient-derived monocytes at different time points postoperatively. We performed adoptive cell transfer with FACS-isolated GFP-labelled monocyte subsets from CD68-GFP Wistar rats for intravital visualization of cell adhesion and transmigration. Results: During acute allograft rejection, we identified a differential transmigration pattern of monocyte subsets by intravital imaging. We found non-classical monocytes to be pro-inflammatory and infiltrate the allograft primarily. CRP significantly increased activation and cytokine expression of this monocyte subset, thus accelerating clinical allograft rejection. Conclusion: Non-classical monocytes primarily infiltrate allogeneic tissue and contribute to acute rejection after allogeneic transplantation. Differential activation of these cells by CRP aggravates innate immune response and accelerates clinical allograft rejection, thus representing potential targets for immunomodulatory therapies.