SPECIFIC BINDING OF ADENOSINE-DEAMINASE BUT NOT HIV-1 TRANSACTIVATOR PROTEIN TAT TO HUMAN CD26

Adenosine deaminase (ADA) and the HIV-1 transactivator protein Tat hav e been reported to bind to human CD26, also known as dipeptidyl peptid ase IV (DPP IV). In order to demonstrate the specificity of such bindi ng under native conditions of CD26, i.e., when expressed on the cell s urface, we established murine cell lines expressing transfected human CD26, either wild-type or mutated at its serine-630, which inactivates the DPP IV activity. This experimental system is advantageous since m urine ADA does not; bind human CD26, whereas human and bovine ADA bind . Consequently, murine cell. clones expressing either the wild-type or mutated form of human CD26 were found to bind specifically bovine I-1 25-labeled ADA with a high affinity (K-D = 12 +/- 2 nM and 11 +/- 4 nM , respectively). No specific binding of I-125-labeled ADA was observed to murine clones not expressing human CD26. The binding of I-125-labe led ADA to CD26 was further characterized by the use of monoclonal ant ibodies specific to human CD26. The results obtained were in accord wi th those reported previously using other experimental models. These ob servations indicated that the murine cells expressing human CD26 provi de a highly suitable model to investigate the potential binding of HIV -1 Tat to CD26. In contrast to previously published results, however, we could not demonstrate a specific interaction between Tat and human CD26. The I-125-labeled ADA-specific binding to human CD26 was not aff ected by Tat, even at concentrations which induced cell death. Similar ly, the binding of several monoclonal antibodies to human CD26 was not modified by the addition of Tat. More significantly, Tat binding to d ifferent murine cell clones (human CD26 negative or positive) was foun d not to be correlated with the expression of human CD26. Finally, the toxic effect of Tat on the growth of different murine cell clones was independent of human CD26 expression. Taken together, these observati ons further confirm the specific binding of ADA to human CD26 and poin t out that CD26 is not the target of HIV-1 Tat protein. (C) 1996 Acade mic Press, Inc.