Introduction: Numerous attempts have been made to predict transplant outcomes by making point-measurements of immunological parameters in peripheral blood. A surprising and common feature of these studies is the limited predictive value of mechanistically important biomarkers (eg. Tregs) taken in isolation. This is partly explained by small, but biologically relevant changes in such biomarkers being dwarfed by natural variation between individuals or time-points.
Materials and Methods: Serial blood samples were collected prospectively from 28 orthotopic liver transplant recipients to estimate CD4+ CD25+ CD127low FoxP3+ Treg frequencies by flow cytometry. 9/28 patients experienced one or more acute rejection episodes within 1 year of transplantation.
Results and Discussion: Rather than making ‘static’ case-control comparisons, it may be more informative to consider rate-of-change in biomarkers over time. To test this concept, serial measurements of peripheral blood CD4+ CD25+ CD127low FoxP3+ Tregs were made in orthotopic liver transplant recipients from 2 clinical trials. Patients in the RiSET-Regensburg Study received conventional triple immunosuppression. Those patients who did not experience a rejection episode (non-rejectors) during the 1st year post-transplant exhibited an early enrichment of Tregs within their total CD4+ T cell compartment, which decayed to pre-transplant levels within 8 weeks. In this group, Treg frequency at one week after transplantation was significantly elevated compared to pre-transplant, ‘baseline’ values, but not at later time points. By contrast, no relative increase in Treg frequency was observed in patients registering one or more rejection episodes (rejectors) during the 1st year post-transplant. Absolute Treg counts were not higher in non-rejectors than rejectors. Similar responses were observed in BUiLT Study participants, who received either conventional triple immunosuppression or a ‘bottom-up’ regimen: Non-rejectors showed a transient peak in Treg frequency at 2 weeks post-transplant that subsided by week 4; by comparison, no Treg enrichment was observed in rejectors. The kinetics of this response is suggestive of a primary T cell response; therefore, we tentatively propose that the enrichment and subsequent restitution of the peripheral Treg pool is evidence of an immunoregulatory response in non-rejecting patients.
Conclusion: Peaking at 2-3 weeks, the kinetics of Treg enrichment in non-rejectors is suggestive of a primary T cell response. Hence, we propose the early Treg enrichment phase represents a polyclonal activation and expansion of Treg, whereas the subsequent decay might reflect a selection of a subset of donor antigen-reactive clones. In future, “high granularity” immune monitoring studies that capture kinetic information about evolving immune responses in the early post-transplant period may be particularly useful in predicting long-term transplant outcome.