The role of monocytes and macrophages in delayed xenograft rejection

Despite the development of successful strategies for averting hyperacute rejection (HAR) in both small and large animal xenograft models, a delayed xenograft rejection (DXR) ultimately occurs. This process is characterized by endothelial cell activation and graft infiltration with activated monocytes and natural killer (NK) cells. We evaluated the role of monocytes and macrophages in a guinea pig-to-rat model of DXR. Our results suggest that specific interactions between these cells and the xenograft occur that result in their activation, since adoptive transfer of xenoactivated splenocytes significantly accelerated both DXR and allograft rejection, while adoptive transfer of alloactivated splenocytes did not. Furthermore, while normal splenocytes caused antibody-dependent cell-mediated cytotoxicity (ADCC) of xenogeneic endothelial cells, xenoactivated splenocytes caused significantly greater endothelial cytotoxicity by antibody-independent mechanisms. Both normal and xenoactivated splenocytes were significantly less cytotoxic if adherent cells, consisting predominantly of monocytes and macrophages, were first removed. In vivo recipient macrophage depletion, using liposome-encapsulated dichloromethylene diphosphonate, did not influence DXR and this may indicate that nonphagocytic circulating monocytes may be more important in DXR. However, adoptive transfer of splenocytes from a macrophage depleted, xenoactivated donor did not accelerate xenograft rejection, while splenocytes from a nondepleted xenoactivated donor did, thereby supporting the importance of monocytes and macrophages in this phase of xenograft rejection.