Transplant Arteriosclerosis in Humanized Mice Reflects Alloantigen Recognition of Lung Transplant Recipients and is Controlled by Autologous Regulatory T Cells

Introduction: In animal models, regulatory T cells (Treg) control rejection caused by naïve effector leukocytes but are less effective in controlling alloantigen-primed effectors. Here, we studied both naïve and alloantigen-primed T cell responses of clinical lung transplant recipients in a humanized mouse model of transplant arteriosclerosis and attempted to control these by boosting putative Treg counts.

Methods: The pericardiophrenic artery was procured from surplus tissue of donor lungs transplanted in our clinical program and was implanted into the abdominal aorta of immune deficient mice. Mice were then divided into five treatment groups. Group A mice received no human leukocyte reconstitution and served as negative controls. Group B mice received 5x10⁶ naïve allogeneic human peripheral blood mononuclear cells (naïve PBMC) from the respective lung recipient harvested at the time of transplant. Group C mice received PBMC harvested 21 days after transplantation, containing the alloantigen primed PBMC. Group D mice received naïve allogeneic PBMC enriched with additional CD4⁺CD25^high cells representing putative Treg. Group E received primed allogeneic PBMC enriched with additional CD4⁺CD25^high cells. Human leukocyte engraftment was monitored by FACS and development of transplant arteriosclerosis was histologically assessed 28 days after PBMC reconstitution.

Results: The control group A showed only mild thickening of the intima (11,66 ± 11,62%). In group C, reconstituted with alloantigen-primed PBMC, intimal thickening resulting in obliteration of the vessel lumen was significantly more severe than in group B, reconstituted with naïve PBMC (33,08 ± 24,58 versus 50,29 ± 21,03%, p = 0.005). By contrast, in group D, reconstituted with naïve allogeneic PBMC enriched with CD4⁺CD25^high cells, intimal thickening was significant less severe than in group B (3,88 ± 18,88 versus 33,08 ± 24,58%, p = 0.012). In group E, enriching alloantigen-primed Treg similarly suppressed transplant arteriosclerosis elicited by alloantigen-primed PBMC (0,16 ± 22,52 versus 50,29 ± 21,03%, p = 0,0009).

Conclusion: We conclude that alloantigen priming in clinical lung transplant recipients leads to more severe transplant arteriosclerosis in a humanized mouse model. Intriguingly however, transplant arteriosclerosis elicited by naïve allogeneic PBMC and by alloantigen-primed PBMC is similarly controlled by putative Treg, indicating a potentially important target for future interventions in lung transplantation.