TI  - Characterization of  <prot>adenosine deaminase</prot>  binding to human  <prot>CD26</prot>  on T cells
      and its biologic role in immune response.
PG  - 1349-55
AB  -  <prot>CD26</prot> , a T cell activation Ag, also known as  <prot>dipeptidyl peptidase IV</prot>  is
      directly associated with  <prot>adenosine deaminase</prot> (<prot>ADA</prot>)  on the surface of T
      cells and T cell lines. In the present study, we examined both the binding
      of  <prot>ADA</prot>  and  <prot>CD26</prot>  and the functional consequences of this interaction. We
      found that  <prot>ADA</prot>  was associated with  <prot>CD26</prot>  on T cell lines lacking either <prot>ADA</prot>
      or <prot>dipeptidyl peptidase IV</prot> enzymatic activity, indicating that the
      association between  <prot>dipeptidyl peptidase IV</prot>  and  <prot>ADA</prot>  did not require
      enzymatic activity. Moreover, using immunoelectron microscopy, we
      demonstrated that <prot>CD26</prot> and <prot>ADA</prot> co-localized on the cell surface, but not
      inside cells, suggesting that <prot>CD26</prot> did not transport <prot>ADA</prot> to the surface.
      In keeping with this observation, we showed that human <prot>CD26</prot>-transfected
      murine pre-B cell lines lacking human <prot>ADA</prot> acquired <prot>ADA</prot> from an
      extracellular source. More importantly, adenosine in the absence of cell
      surface <prot>ADA</prot> inhibited T cell proliferation and <prot>IL-2</prot> production induced by
      various stimuli. On the other hand, cells expressing <prot>ADA</prot> and <prot>CD26</prot> on the
      surface were much more resistant to the inhibitory effect of adenosine.
      These data suggest that <prot>ADA</prot> on the cell surface is involved in an
      important immunoregulatory mechanism by which released  <prot>ADA</prot>  binds to cell
      surface  <prot>CD26</prot> , and this complex is capable of reducing the local
      concentration of adenosine.
AD  - Division of Tumor Immunology, Dana-Farber Cancer Institute, Boston, MA
