Introduction

Alloreactive T cells are important mediators of rejection or tolerance of the transplanted organ. Detection and isolation of viable alloreactive T cells at the single cell level requires a cell surface marker specifically induced upon T cell receptor activation. In this study, a member of the TNFR-family, CD137 (4-1BB) was investigated for its potential to identify circulating alloreactive T cells in combination with phenotypic and functional analysis.

Methods

Optimal conditions for sensitive and specific detection of allogeneic-induced CD137 expression on circulating T cells were established. Thereafter, CD137⁺ alloreactive T cells were phenotypically and functionally characterized by multi-parameter flow-cytometry.

Results

Alloantigen-induced CD137 expression identified both alloreactive CD8⁺ T cells (0.21±0.07%) and CD4⁺ T cells (0.21±0.05%). CD137⁺ alloreactive T cells were detected in different T cell subsets, including naÏve T cells, but were preferentially found in CD28⁺ T cells and not in the terminally differentiated T cell subsets. Upon allogeneic stimulation, the cytokine producing, but not proliferating capacity of T cells mainly resided within the CD137-expressing fraction. A minority of the CD137⁺ alloreactive cytokine producing T cells (<10%) produced any combination of IFN-Γ, IL-2 and TNF-Α. Poly-functional alloreactive T cells, defined by multiple cytokine expression, were infrequently observed. Numbers of alloreactive CD137⁺ cytokine⁺ T cells were positively associated with the number of HLA mismatches, although a substantial variation was observed.

Conclusions

Activation-induced CD137 expression allows for detection of the total cytokine producing, but not the total proliferating, alloreactive T cell compartment at the single cell level by multi-parameter flow-cytometry. CD137 expression might be a useful marker to gain more insight into the development of alloreactive T cells following kidney transplantation.

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