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Interdepartmental Program in Vascular Biology and Transplantation, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, CT 06510
We have previously shown that human endothelial cells (EC) are less
efficient than professional APC, e.g., B lymphoblastoid cells (BLC), at
stimulating allogeneic CD8+ T cells to develop into CTL. In
this study we describe FACS-based limiting dilution analyses using the
dilution of the intracellular dye CFSE as an indicator of
CD8+ T cell alloactivation and expansion with significantly
increased sensitivity compared with conventional, cytotoxicity-based
assays. In addition, this assay permits the relative size of clonal CTL
populations that are generated in individual CD8+ T cell
cultures to be determined (clonal burst size). We have applied this
method to quantitatively compare the generation of CTL at the clonal
level following stimulation of allogeneic CD8+ T cells by
either BLC or HUVEC derived from the same donor. CD8+ T
cells expanded by allostimulation were identified as CD8+,
CFSElow cells and were categorized as CTL by the expression
of intracellular perforin and IFN-
. Precursor frequencies for
EC-stimulated CTL were 5- to 40-fold (mean, 7.5-fold) lower compared
with BLC-stimulated CTL (p < 0.01). Concomitantly,
the average clonal burst sizes in EC-stimulated CTL cultures were
significantly smaller than those in conventional CTL cultures,
primarily due to the occurrence of some very large clone sizes
exclusively with BLC stimulation. Although EC-stimulated CTL were
generated only from the memory subset of CD8+ T cells,
BLC-stimulated very large burst sizes of CTL were observed from both
naive and memory CD8+ T cell precursors. These data
establish that both a lower frequency of reactive precursors and more
limited clonal expansion, but not regulatory T cells, contribute to the
reduced capacity of EC to promote alloreactive CTL differentiation
compared with that of professional APC.
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