Frequency of Hypoxanthine Guanine Phosphoribosyltransferase ( HPRT − ) T Cells in the Peripheral Blood of Cardiac Transplant Recipients

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Abstract
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/106 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) rejection (n=12 patients), or (3) multiple rejection episodes (n=10 patients). Statistical analyses were used to quantify the time profiles of FMC/106 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/106 cells, 11 of which gave values >50/106 cells (range, 146 to 46 982 FMC/106 cells). A significant association between the onset of first rejection and an increased rate of FMC/106 values was noted (P=.0001). The ability of a rising trend in FMC/106 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/106 values in the weeks after treated rejection episodes was noted (P=.0003). The ability of a persistently elevated trend in values of FMC/106 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.