title: T cell activation: in vivo veritas.
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.
authors.affiliation: Centenary Institute of Cancer Medicine and Cell Biology, Newtown, NSW 2042, Australia. B.Fazekas@centenary.usyd.edu.au
authors.forename: Barbara
authors.initials: B
authors.lastname: Fazekas de St  Groth
authors.forename: Adrian L
authors.initials: AL
authors.lastname: Smith
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Higgins
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000276:immunology; D015496:CD4-Positive T-Lymphocytes / Q000276:immunology*; D002455:Cell Division / Q000276:immunology; D008213:Lymphocyte Activation / Q000276:immunology*; D051379:Mice; D008822:Mice, Transgenic; D011948:Receptors, Antigen, T-Cell / Q000235:genetics / Q000276:immunology*; D015375:Receptors, Interleukin-2 / Q000276:immunology*
chemical_list: D015703:Antigens, CD; D011948:Receptors, Antigen, T-Cell; D015375:Receptors, Interleukin-2