Transplant tolerance: is it really free of concerns?

The Evolving Roles of Memory Immune Cells in Transplantation

Memory cells are the products of immune responses but also exert significant impact on subsequent immunity and immune tolerance, thus placing them in a unique position in transplant research. Memory cells are heterogeneous, including not only memory T cells but also memory B cells and innate memory cells. Memory cells are a critical component of protective immunity against invading pathogens, especially in immunosuppressed patients, but they also mediate graft loss and tolerance resistance. Recent studies suggest that some memory cells unexpectedly act as regulatory cells, promoting rather than hindering transplant survival. This functional diversity makes therapeutic targeting of memory cells a challenging task in transplantation. In this article, we highlight recent advances in our understanding of memory cells, focusing on diversity of memory cells and mechanisms involved in their induction and functions. We also provide a broad overview on the challenges and opportunities in targeting memory cells in the induction of transplant tolerance.

Memory T cells in transplantation - progress and challenges

PURPOSE OF REVIEW

Memory T cells present a different set of challenges to transplant patients; they are needed for protection against invading pathogens, especially under conditions of immunosuppression. But their presence also threatens transplant survival, as some of them are alloreactive. Efforts to resolve this paradox will be critical in the induction of transplant tolerance.

RECENT FINDINGS

There has been significant progress made in the past few years in the areas of population diversity of memory T cells, metabolic control of their induction, and mechanisms and pathways involved in memory cell exhaustion. Multiple targets on memory T cells have been identified, some of which are under vigorous testing in various transplant models.

SUMMARY

Memory T cells are both friends and foes to transplant patients, and tolerance strategies should selectively target alloreactive memory T cells and leave other memory cells unaltered. This situation remains a major challenge in the clinic.

Cross-immune tolerance: conception and its potential significance on transplantation tolerance

The diversity of alloreactive T cells in graft rejection and the presence of extensive crossreactivity among alloreactive T cells indicate that the induction of transplantation tolerance may fundamentally alter the size of host T-cell repertoire involved in protective immunity and immune surveillance, especially those that are crossreactive to conventional antigens. We herein highlight the crossreactive nature of alloreactive T cells and the potential risks of altered T-cell repertoire associated with the induction of transplantation tolerance. The possibility that T-cell tolerance to one set of antigens results in their tolerance to other unrelated antigens due to T-cell crossreactivity and/or heterogeneity is defined as 'cross-immune tolerance'. The definition and significance of this concept were discussed in details.

Isolated CD39 expression on CD4+ T cells denotes both regulatory and memory populations

Foxp3(+) regulatory T cells (Tregs) express both ectoenzymes CD39 and CD73, which in tandem hydrolyze pericellular ATP into adenosine, an immunoinhibitory molecule that contributes to Treg suppressive function. Using Foxp3GFP knockin mice, we noted that the mouse CD4(+)CD39(+) T-cell pool contains two roughly equal size Foxp3(+) and Foxp3(-) populations. While Foxp3(+)CD39(+) cells are CD73(bright) and are the bone fide Tregs, Foxp3(-)CD39(+) cells do not have suppressive activity and are CD44(+)CD62L(-)CD25(-)CD73(dim/-), exhibiting memory cell phenotype. Functionally, CD39 expression on memory and Treg cells confers protection against ATP-induced apoptosis. Compared with Foxp3(-)CD39(-) naïve T cells, Foxp3(-)CD39(+) cells freshly isolated from non-immunized mice express at rest significantly higher levels of mRNA for T-helper lineage-specific cytokines IFN-gamma (Th1), IL-4/IL-10 (Th2), IL-17A/F (Th17), as well as pro-inflammatory cytokines, and rapidly secrete these cytokines upon stimulation. Moreover, the presence of Foxp3(-)CD39(+) cells inhibits TGF-beta induction of Foxp3 in Foxp3(-)CD39(-) cells. Furthermore, when transferred in vivo, Foxp3(-)CD39(+) cells rejected MHC-mismatched skin allografts in a much faster tempo than Foxp3(-)CD39(-) cells. Thus, besides Tregs, CD39 is also expressed on pre-existing memory T cells of Th1-, Th2- and Th17-types with heightened alloreactivity.

Effects of biomaterial-induced inflammation on fibrosis and rejection

Evidence is emerging that biomaterials cause inflammation by ligating innate immune receptors on antigen presenting cells. Although inflammation is usually viewed as detrimental, it has unexpected and potentially beneficial effects on fibrosis and transplant rejection. For example, the magnitude of inflammation due to a biomaterial is not predictive of the extent of fibrosis. Similarly, biomaterials do not always show adjuvancy. Some biomaterials suppressed T cell rejection responses in vivo and in vitro, while others non-specifically stimulated T cell proliferation. Understanding these complex inter-relationships is the key to designing a biomaterial that stimulates regeneration and induces tolerance in tissue engineering applications.