HIV infection depletes the immune system of the coordinating functions of CD4+ T cells and APCs, whereas the population of CD8+ CTLs remains largely intact: functional but undirected. We have developed a humanized bispecific immunoadhesin-antibody (BIA) that redirects these remaining T cells to kill HIV-infected cells. This BIA expresses effector cell retargeting via a targeting activity that exploits the natural affinity of CD4 for gp120, and a recruiting activity that employs an anti-CD3 moiety to engage CTLs. The resultant molecule is 97% human in origin. In functional tests, this BIA mediated killing of HIV-infected cells using either pure CTL preparations, or whole PBL fractions that additionally include Fc gamma receptor-bearing large granular lymphocyte effectors. In contrast, a human anti-gp120 Ab induced target lysis via Ab-dependent cellular cytotoxicity (ADCC) only with large granular lymphocyte-containing fractions and not with CTLs. ADCC with this Ab was blocked in human serum, whereas BIA-mediated effector cell retargeting lysis of HIV-infected cells by CTLs was preserved. The affinity of the BIA for HIV-gp120 on infected cells and for CD3 epsilon on CTLs was derived in a flow cytometric Scatchard procedure. Relative to the bivalent parent molecules, CD4/gp120 affinity on cells was unchanged in the BIA (Ka 7 x 10(7) M-1), whereas the anti-CD3 affinity was diminished 50-fold (Ka 2 x 10(6) M-1 vs 1 x 10(8) M-1). Physical association of CD3+ effectors and gp120-expressing targets was confirmed by fluorescence microscopy and was dependent upon the presence of BIA and expression of target gp120. The unimpaired cytocidal activity of the BIA in the presence of serum highlights a potentially important advantage of this type of construct over native Abs for HIV-directed therapy.