T cell activation: in vivo veritas

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Abstract
Phenotypic changes in CD4+ T cells undergoing antigen‐dependent activation were compared in vivo and in vitro. The most obvious difference was in expression of CD25, the alpha chain of the high affinity receptor for IL‐2. High level expression of CD25 in vivo is restricted to a small fraction of the cells at the leading edge of the cell division profile, whereas all activated cells express high levels of CD25 in cultures responding to antigen. Because IL‐2 is known to upregulate expression of CD25 in preactivated T cells, this suggests a difference in IL‐2 exposure in the two responses. A number of other markers, including CD54, show a similar difference in the pattern of expression in vivo and in vitro. Using 6‐colour flow cytometry, it was demonstrated that the small percentage of cells expressing CD25 in vivo coexpresses a very high level of a number of other activation markers, including CD38, CD44 and Ly‐6A/E, suggesting that these may also be upregulated by autocrine IL‐2.
Funding Information
  • National Health and Medical Research Council Fellowship
  • Australian Postgraduate Award
  • National Health and Medical Research Council of Australia
  • New South Wales Health Department