cells (Table E2.1). Although this small population of cells appears to be acutely activated, expression of the transient activation markers CD69 and CD25 does not appear to be significantly upregulated (our unpublished observations). Thus, T cell interaction with a weak tolerogen that leads to TCR revision appears to initiate some but not all of the events associated with full activation of T cells encountering a foreign conventional antigen.

Cells whose phenotype is consistent with their position as intermediates in a TCR revision process have also been reported in normal humans23 and, in increased numbers, in patients with defective responses to DNA damage.24 These CD4+ peripheral T lymphocytes are TCRlow, RAG-expressing cells that contain recombination intermediates at the TCR-p loci. It is unclear whether these cells are undergoing TCR revision, and if so, what triggers this response.

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