Professional presentation of antigen by activated human T cells

Activated human T cells express class II molecules, but their capacity to present soluble antigens and stimulate T cells has been repeatedly questioned. Two lines of evidence indicate that T cells may indeed function as professional antigen‐presenting cells. First, T cells that have been recently activated can efficiently capture, process and present tetanus toxoid to class II‐restricted T cell clones. This capacity correlates with the rate of class II synthesis. Second, activated T cell clones express high levels of B7, are powerful stimulators in mixed lymphocyte reactions, and their stumulatory capacity is inhibited by soluble CTLA4 or anti‐B7 antibody. Furthermore, expression of B7 can be detected in vivo on T cells from biopsies of patients with liver disease. Presentation of soluble antigen by activated T cells may play a role in the amplification of the specific response, and possibly in immunopathological states.