The distribution of the gp40-45 Kd antigen bound by the WR17 monoclonal antibody of IgG2 subclass in normal lymphoid tissue was characterized by immunohistochemistry and immunofluorescence staining with flow cytometric analysis. The predominant staining pattern observed was characteristic of an anti-pan-B-lymphocyte reagent. Weak reactions were observed by immunofluorescence staining of viable cell suspensions with all neutrophils and T-lymphocytes in some normal donors. In tissue sections, B-lymphocytes were stained and no cross reactions were observed with T-lymphocytes, although macrophages stained in some sections. A range of T- and B-cell malignancies were stained with WR17 and the reactivity compared to that observed with other monoclonal antibodies in the CD19, CD21 and CD22 clusters. All B-non-Hodgkin's lymphomas, B-chronic lymphocytic, prolymphocytic and hairy cell leukaemia cells examined were stained by WR17 in indirect immunofluorescence assays, whilst the T-cell tumours were negative. The same pattern was observed in cryostat sections of malignant tissue and in addition some tissue macrophages expressed the CD37 antigen cytoplasmically. Intra-tumour heterogeneity of staining was observed with all the monoclonal antibodies tested, although overall WR17 consistently stained B-cell tumours even when expression of the CD19 pan-B-lymphocyte antigen could not be detected with some monoclonals. Monoclonal antibodies, such as WR17, within the CD37 cluster and binding to the gp 40-45 Kd molecule, bind to mature B-lymphocytes and identify the majority of B-cell malignancies.
Summary We have investigated the effectiveness of three different F(ab'y)2 bispecific antibodies (BsAb) for delivering the ribosome inactivating protein (RIP) saporin via the CD7 or CD38 cell surface molecules to the human T-ALL cell lines HSB-2 and HPB-ALL. Inhibition of 3H-leucine uptake by target cells was used as the parameter of cellular cytotoxicity. Used singly against HSB-2 cells in the presence of varied concentrations of saporin, an anti-CD7 BsAb, (HB2 x DB7-18) and an anti-CD38 BsAb (OKT1O x RabSap), gave 435-and 286-fold increases in saporin toxicity, respectively. For HPB-ALL cells the anti-CD7 BsAb performed poorly giving only an eight-fold increase in toxicity whilst on the same cell line the anti-CD38 BsAb was highly potent giving an 80,000-fold increase in saporin toxicity.A combination of both BsAb used together against HSB-2 cells was ten times more effective, than the best single BsAb HB2 x DB7-18 used alone. Kinetic studies conducted with HSB-2 cells revealed that the BsAb combination also gave an increased rate of protein synthesis inactivation in comparison to either BsAb used alone. These investigations clearly demonstrate a synergistic action when both BsAb are used in combination to target saporin against CD7 and CD38 expressed on the surface of the HSB-2 cell line.
We have investigated the cytotoxic performance of two different anti-CD7/anti-saporin BsAb's (HB2 x DB7-18 and Q1.1), three anti-CD38/anti-saporin BsAb's (OKT10 x RabSap, OKT10 x DB7-18 and Q4.1) and an anti-CD7 (HB2-Sap) and anti-CD38-saporin (OKT10-Sap) immunotoxin for delivering the ribosome inactivating protein (rip) to the human T-cell acute lymphoblastic leukemia cell line HSB-2. In the case of CD7 as target molecule the immunotoxin outperformed both anti-CD7 BsAb's being six times more effective than HB2 x DB7-18 and 98 times more so than Q1.1 at effectively inhibiting protein synthesis in a dose dependent manner. The chemically constructed HB2 x DB7-18 BsAb was more effective at inhibiting protein synthesis and cell growth in target HSB-2 cells in a dose dependent manner than the quadroma produced BsAb Q1.1. Both BsAb demonstrated a prozone effect used at concentrations above 0.1 nM though this was more pronounced for Q1.1 than for HB2 x DB7-18. The prozone effect was partially though not completely reversed by increasing the concentration of saporin in the system. In the case of CD38 as target molecule the anti-CD38 IT OKT10-Sap performed poorly, never actually achieving its IC50. Two BsAb's constructed with monoclonal anti-saporin Fab arms each recognizing a different epitope on the saporin molecule also performed poorly. In contrast the BsAb OKT10 x RabSap constructed with Fab derived from a rabbit polyclonal anti-saporin antiserum performed in a dose dependent manner achieving its IC50 at a concentration of 1.3 nM. This BsAb also exhibited a prozone effect. These results exemplify the importance of cross linking adjacent target molecules on the cell surface in order to achieve effective delivery of saporin to the cell interior.
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