ACTIVATION AND PREFERENTIAL EXPANSION OF RAT CYTOTOXIC (CD8) T-CELLS IN-VITRO AND IN-VIVO WITH A BISPECIFIC (ANTI-TCR-ALPHA BETA X ANTI-CD2)F(AB')(2) ANTIBODY/

We show that a bispecific F(ab')(2) Ab (BsAb), which cross-links the T CR to CD2 ([anti-CD2 X anti-TCR]) and is highly mitogenic in vitro, al so induces marked T cell activation and proliferation when given as a small single dose (50 to 100 mu g) to rats. Interestingly, the prolife ration appeared selective for CD8 cells, increasing this compartment m ore than 20 times over 72 h. This resulted in a three- to fourfold inc rease in spleen weight, with histologic evidence of T-zone and red pul p expansion by CD8(+) blast cells. Such changes were in striking contr ast to those seen after a similar amount of the parent IgG anti-TCR mA b, R73, which, while inducing a transient increase in CD25(+) cells, f ailed to alter the lymphocyte composition. The mitogenic activity of t he BsAb was totally dependent on its ability to cross-link CD2 and the TCR, hence a mixture of anti-CD2 and anti-TCR F(ab')(2) fragments was without effect in vitro or in vivo. Unlike normal rat T cells or thos e taken from R73 IgG-treated rats, blood and splenic T cells recovered from BsAb-treated rats were highly cytotoxic against R73 hybridoma ta rgets that express surface anti-TCR mAb and consequently trigger the l ytic activity of activated CTL or a rat lymphoma line, LAMA (Thy-1(+)) , using a second BsAb, [anti-TCR X anti-Thy-1], to retarget the CTL. T his latter assay provides the basis for a future two-stage targeting s trategy for cancer immunotherapy in which a mitogenic BsAb would be gi ven to patients to activate CTL nonspecifically, and then, at an appro priate time, a second BsAb [anti-TCR X antitumor] would be given to de liver activated cells to the tumor target cells.