Analysis of lymphocyte phenotypes in cord blood from early gestation fetuses

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Abstract
SUMMARY: Using cord blood samples obtained from Fetuses between 16 and 40 weeks gestation, we have used a lysed whole blood flow cytometric technique to study the natural history of lymphocyte phenotypes known to be highly represented in cord blood at birth. The majority (51·0 ± 14·7%) of lymphocytes expressed CD45RA, a marker of ‘virgin’ cells and there was a correIation between increasing percentages of CD45R A+ lymphocytes and gestational age (r = 0·44, P<0·01). Few cells (8·5 ± 4·2%) expressed the CD45RO marker of primed lymphocytes and very few (1·0 ± 0·7%) co-expressed CD45RA and RO, indicating little traffic between the two maturation markers. The percentage of B lymphocytes co-expressing CDS was high in the fetal circuIation (55·5±10·5%) compared with healthy adults (23·2 ± 14·3%; P<0·00001) and the level of CDS+ B cells declined with gestational age in an exponential manner (r=−0·45, P<0·05). Similarly, levels of T lymphocytes expressing the γδ T cell receptor (TCR) declined exponentially (r=−0·59, P<0·005). These results demonstrate that lymphocytes remain almost entirely unprimed before birth. In addition, CD5+ B lymphocytes and TCR-γδ+ T lymphocytes decline exponentially towards birth, in a manner suggesting that they may be seeding peripheral sites such as the spleen, skin and mucosae.