Non-invasive molecular imaging of inflammatory macrophages in allograft rejection

Macrophages represent a critical cell type in host defense, development and homeostasis. The ability to image non-invasively pro-inflammatory macrophage infiltrate into a transplanted organ may provide an additional tool for the monitoring of the immune response of the recipient against the donor graft. We therefore decided to image in vivo sialoadhesin (Sn, Siglec 1 or CD169) using anti-Sn mAb (SER-4) directly radiolabelled with (99m)Tc pertechnetate.

We used a heterotopic heart transplantation model where allogeneic or syngeneic heart grafts were transplanted into the abdomen of recipients. In vivo nanosingle-photon emission computed tomography (SPECT/CT) imaging was performed 7 days post transplantation followed by biodistribution and histology.

In wild-type mice, the majority of (99m)Tc-SER-4 monoclonal antibody cleared from the blood with a half-life of 167 min and was located predominantly on Sn(+) tissues in the spleen, liver and bone marrow. The biodistribution in the transplantation experiments confirmed data derived from the non-invasive SPECT/CT images, with significantly higher levels of (99m)Tc-SER-4 observed in allogeneic grafts (9.4 (±2.7) %ID/g) compared to syngeneic grafts (4.3 (±10.3) %ID/g) (p = 0.0022) or in mice which received allogeneic grafts injected with (99m)Tc-IgG isotype control (5.9 (±0.6) %ID/g) (p = 0.0185). The transplanted heart to blood ratio was also significantly higher in recipients with allogeneic grafts receiving (99m)Tc-SER-4 as compared to recipients with syngeneic grafts (p = 0.000004) or recipients with allogeneic grafts receiving (99m)Tc-IgG isotype (p = 0.000002).

Here, we demonstrate that imaging of Sn(+) macrophages in inflammation may provide an important additional and non-invasive tool for the monitoring of the pathophysiology of cellular immunity in a transplant model.