Modulation of Donor Cardiac Macrophages is Sufficient to Suppress Rejection and Extend Allograft Survival

Purpose Cellular rejection confers significant morbidity and mortality after heart transplant. Current immunosuppression targets recipient immune cells and increases the risk of life-threatening complications. An alternative approach is to target donor immune cells prior to or at the time of transplant. This approach may be advantageous as it avoids consequences of systemic immunosuppression. Macrophages represent the major immune cell in the donor heart. The heart contains 2 distinct macrophage populations defined by their expression of C-C Chemokine Receptor 2 (CCR2+ and CCR2-) with divergent functions. Their roles in the pathophysiology of heart transplant rejection are yet to be explored. Methods We used a murine allogenic heart transplant model (CD45.2 Bl6 donor, CD45.1 Balb/c recipient) with CTLA4-Ig immunosuppression (200µg days 0,2,4,6). Inclusion of congenic and macrophage reporter strains allowed high fidelity separation of donor CCR2- and CCR2+ macrophages from recipient immune cells. Macrophage depletion, cellular imaging, single cell RNA sequencing and conditional knockout studies were employed to investigate the functions and pathways responsible for donor macrophage activation during rejection. Results High resolution imaging demonstrated that donor macrophages are activated following transplant based on increased size, number of projections, and clustering behavior. Selective depletion of donor CCR2- macrophages resulted in accelerated rejection (16d vs 28 d, p Conclusion Donor CCR2- and CCR2+ macrophage have opposing roles in the pathogenesis of allograft rejection. Prevention of donor CCR2+ macrophage activation through inhibition of MyD88 signaling represents a new therapeutic target to prevent or suppress heart transplant rejection.