EVIDENCE AGAINST T-CELL DEVELOPMENT IN THE ADULT HUMAN INTESTINAL-MUCOSA BASED UPON LACK OF TERMINAL DEOXYNUCLEOTIDYLTRANSFERASE EXPRESSION

Several lines of evidence indicate that a subset of murine intestinal intraepithelial lymphocytes (iIEL), particularly those which express t he CD8 alpha alpha homodimer, mature extrathymically. This study confi rms that a small fraction of adult human iIEL also express the CD8 alp ha alpha homodimer and demonstrates that most of these cells in the sm all intestine are T cells using the alpha beta T-cell receptor (TCR). Whether these cells or other subsets of adult human iIEL mature extrat hymically in the intestine was asses sed by measuring the expression o f terminal deoxynucleotidyltransferase (TdT), an enzyme expressed excl usively by immature lymphocytes. Very low levels of TdT message could be detected by polymerase chain reaction (PCR) amplification in some i IEL samples. The level of TdT expression was assayed by competitive PC R amplification and compared with thymocytes and peripheral blood lymp hocytes. These measurements indicated that the number of immature T ce lls expressing TdT in the intestinal epithelium was less than one cell per 10(7) lymphocytes. This demonstrates that there are few if any Td T expressing immature T cells in the adult human intestinal mucosa and indicates, therefore, that T-cell development in the intestinal mucos a does not contribute significantly to the T-cell repertoire of the ad ult human intestine.