T cell clonal conditioning: a phase occurring early after antigen presentation but before clonal expansion is impacted by Toll-like receptor stimulation.

After in vivo immunization, Ag-specific T cells disappear from circulation and become sequestered in lymphoid tissue where they encounter Ag presented by dendritic cells. In the same site and just after Ag presentation, they "disappear" a second time and we investigated this process. Using a mouse model of T cell deletion (without Toll-like receptor (TLR) stimulation) vs survival (with TLR stimulation), Ag-specific T cells indeed became undetectable by flow cytometry, however were readily detected by immunohistochemistry. Thus, whether or not the activated T cells were destined to delete or survive, they were difficult to extract from lymphoid tissue and did not disappear but in fact were abundantly present. Nevertheless, profound differences were observed during this time period when tolerizing conditions were compared with immunizing conditions. TLR stimulation induced an increase in CD25 expression, acquisition of surface MHC class II, and abnormally high increases in forward and side scatter of the peptide-specific T cells. Using a modified adoptive transfer approach, we demonstrated by flow cytometry that in the presence of TLR stimulation the Ag-specific T cells were tightly coupled to dendritic cells, explaining the unusual increases in size and granularity. Ultimately, these events induced the specific T cells to differentiate into memory cells. We postulate that this is a stage where T cells are either conditioned to survive or to delete depending upon the activation status of the innate immune system.