FREQUENCY OF HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE (HPRT(-))T-CELLS IN THE PERIPHERAL-BLOOD OF CARDIAC TRANSPLANT RECIPIENTS - A NONINVASIVE TECHNIQUE FOR THE DIAGNOSIS OF ALLOGRAFT-REJECTION

Background Histological evaluation of serial endomyocardial biopsies p erformed at fixed time intervals after cardiac transplantation is the universal method used for the detection of cardiac rejection and asses sment of the adequacy of antirejection therapy. No noninvasive methodo logy thus far investigated has achieved a high enough sensitivity and predictive accuracy to be considered as a potential replacement for en domyocardial biopsy in the detection of rejection in adults. The prese nt study exploited the finding that the rate of spontaneous mutation i n the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene is hi gher in proliferating human T cells than in resting cells. Thus, it wa s reasoned that in the posttransplantation setting, the frequency of H PRT cells in peripheral blood may provide an indirect measure of alloa ctivated T lymphocytes. Methods and Results This study consisted of de termining the clonal frequency of HPRT mutant cells (FMC/10(6) periphe ral 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 represente d 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). St atistical 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 reject ion episodes and, in appropriate patients, the recurrence of rejection episodes. Data showed that PBMCs from patients with no evidence of re jection uniformly gave low values of <6 FMC/10(6) cells, a frequency s imilar to that seen in healthy nontransplanted volunteers. In contrast , 19 of the 22 PBMC samples that were obtained from patients whose cor responding biopsy sample was diagnosed with a histological rejection g rade of greater than or equal to 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 rejecti on 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 th e onset of rejection was 81.8% and to correctly identify no rejection, 100%. In addition, a significant association between recurrent reject ion episodes and persistence of high FMC/10(6) values in the weeks aft er treated rejection episodes was noted (P=.0003). The ability of a pe rsistently elevated trend in values of FMC/10(6) cells to correctly id entify recurrent rejection was 90% and to correctly identify no reject ion, 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 treate d rejection episode correlated with recurrent rejection. This quantita tive noninvasive assay may thus serve as a useful adjunct to endomyoca rdial biopsy for monitoring post-cardiac transplantation patients, and its use as a prospective diagnostic tool merits further study.