Serological and biochemical characterization of recombinant baculovirus carcinoembryonic antigen

Mutational analysis of Lck in CD45-negative T cells: dominant role of tyrosine 394 phosphorylation in kinase activity

The CD45 tyrosine phosphatase has been reported to activate the src family tyrosine kinases Lck and Fyn by dephosphorylating regulatory COOH-terminal tyrosine residues 505 and 528, respectively. However, recent studies with CD45- T-cell lines have found that despite the fact that Lck and Fyn were constitutively hyperphosphorylated, the tyrosine kinase activity of both enzymes was actually increased. In the present study, phosphoamino acid analysis revealed that the increased phosphorylation of Lck in CD45- YAC-1 T cells was restricted to tyrosine residues. To understand the relationship between tyrosine phosphorylation and Lck kinase activity, CD45- YAC-1 cells were transfected with forms of Lck in which tyrosines whose phosphorylation is thought to regulate enzyme activity (Tyr-192, Tyr-394, Tyr-505, or both Tyr-394 and Tyr-505) were replaced with phenylalanine. While the Y-to-F mutation at position 192 (192-Y-->F) had little effect, the 505-Y-->F mutation increased enzymatic activity. In contrast, the 394-Y-->F mutation decreased the kinase activity to very low levels, an effect that the double mutation, 394-Y-->F and 505Y-->F, could not reverse. Phosphopeptide analysis of tryptic digests of Lck from CD45- YAC-1 cells revealed that it is hyperphosphorylated on two tyrosine residues, Tyr-505 and, to a lesser extent, Tyr-394. The purified and enzymatically active intracellular portion of CD45 dephosphorylated Lck Tyr-394 in vitro. These results demonstrate that in addition to Tyr-505, CD45 can dephosphorylate Tyr-394, and that in the absence of CD45 the hyperphosphorylation of Tyr-394 can cause an increase in the kinase activity of Lck despite the inhibitory hyperphosphorylation of Tyr-505. Therefore, Lck kinase activity is determined by the balance of activating and inhibitory tyrosine phosphorylations that are, in turn, regulated by CD45.

Baculovirus expression of a functional single-chain immunoglobulin and its IL-2 fusion protein

The baculovirus expression system has been used for the production of a variety of proteins, including antibodies. Two single-gene constructs encoding single-chain immunoglobulins have recently been developed. The antibody employed was monoclonal antibody (MAb) CC49 which reacts with the pancarcinoma antigen, tumor associated glycoprotein, TAG-72. One, single-chain construct designated SCA delta CLCH1 (SCIg), consists of the CC49 sFv covalently joined to the human Fc (gamma 1) through the hinge region. The other, SCA delta CLCH1-IL-2 (SCIg-IL-2), has a human IL-2 molecule attached to the carboxyl end of the SCIg. These constructs have been used to test the feasibility of producing biologically active antibodies using the baculovirus expression system. Both constructs have been successfully expressed in insect cells and purified. The baculovirus recombinant single-chain antibodies have been designated, bV-SCA delta CLCH1 (bV-SCIg) and bV-SCA delta CLCH1-IL-2 (bV-SCIg-IL-2) they have been shown to be secreted in the culture supernatant as dimeric molecules of approximately 115 kDa and 140 kDa, respectively. The specificity and antibody dependent cellular cytolytic activity of the baculovirus recombinant single-chain antibodies were shown to be similar to that of the myeloma derived molecules. Glycosylation analysis showed that baculovirus derived proteins were N-glycosylated, but carried few if any high mannose residues. The biological activity of the IL-2 moiety was retained in bV-SCIg-IL-2, as evidenced by its stimulatory effect on the proliferation of the IL-2 dependent cell line HT-2. The observation that a significantly shorter time is required to develop baculovirus recombinant molecules as compared to myeloma derived molecules and that insect cells express single chain MAbs at acceptable levels may have implications for the production of these molecules for clinical use.