Frequency of Hypoxanthine Guanine Phosphoribosyltransferase (
HPRT
−
) T Cells in the Peripheral Blood of Cardiac Transplant Recipients
Authors: Ansari Aftab A. , Mayne Ann , Sundstrom J. Bruce , Gravanis Michael B. , Kanter Kirk , Sell Kenneth W. , Villinger Francois , Siu Cynthia O. , Herskowitz Ahvie 
Published: June 2012

Background
Histological evaluation of serial endomyocardial biopsies performed at fixed time intervals after cardiac transplantation is the universal method used for the detection of cardiac rejection and assessment of the adequacy of antirejection therapy. No noninvasive methodology thus far investigated has achieved a high enough sensitivity and predictive accuracy to be considered as a potential replacement for endomyocardial biopsy in the detection of rejection in adults. The present study exploited the finding that the rate of spontaneous mutation in the hypoxanthine guanine phosphoribosyltransferase (
HPRT
) gene is higher in proliferating human T cells than in resting cells. Thus, it was reasoned that in the posttransplantation setting, the frequency of
HPRT
−
cells in peripheral blood may provide an indirect measure of alloactivated T lymphocytes.

Methods and Results
This study consisted of determining the clonal frequency of
HPRT
−
mutant cells (FMC/10
6
peripheral blood mononuclear cells [PBMCs]) within a total of 293 peripheral blood samples representing various numbers of sequential samples from each of 27 transplant recipients. These sequential samples represented time periods when endomyocardial biopsy specimens showed either (1) no evidence of rejection (n=5 patients), (2) a single initial episode after transplantation of early (<1 year) or late (>1 year) rejection (n=12 patients), or (3) multiple rejection episodes (n=10 patients). Statistical analyses were used to quantify the time profiles of FMC/10
6
PBMCs in serial samples among transplant recipients and to determine the association of these profiles with both the onset of first rejection episodes and, in appropriate patients, the recurrence of rejection episodes. Data showed that PBMCs from patients with no evidence of rejection uniformly gave low values of <6 FMC/10
6
cells, a frequency similar to that seen in healthy nontransplanted volunteers. In contrast, 19 of the 22 PBMC samples that were obtained from patients whose corresponding biopsy sample was diagnosed with a histological rejection grade of ≥3 gave values of >6 FMC/10
6
cells, 11 of which gave values >50/10
6
cells (range, 146 to 46 982 FMC/10
6
cells). A significant association between the onset of first rejection and an increased rate of FMC/10
6
values was noted (
P
=.0001). The ability of a rising trend in FMC/10
6
values to correctly identify the onset of rejection was 81.8% and to correctly identify no rejection, 100%. In addition, a significant association between recurrent rejection episodes and persistence of high FMC/10
6
values in the weeks after treated rejection episodes was noted (
P
=.0003). The ability of a persistently elevated trend in values of FMC/10
6
cells to correctly identify recurrent rejection was 90% and to correctly identify no rejection, 100%.

Conclusions
Increasing frequencies of
HPRT
−
mutant cells in peripheral blood correlated with the onset of first rejection, and persistently elevated
HPRT
−
mutant cells in the weeks after a treated rejection episode correlated with recurrent rejection. This quantitative noninvasive assay may thus serve as a useful adjunct to endomyocardial biopsy for monitoring post–cardiac transplantation patients, and its use as a prospective diagnostic tool merits further study.