A human immunoglobulin G1 antibody originating from an in vitro-selected Fab phage antibody binds avidly to tumor-associated MUC1 and is efficiently internalized

Abstract

We describe the engineering and characterization of a whole human antibody directed toward the tumor-associated protein core of human MUC1. The antibody PH1 originated from the in vitro selection on MUC1 of a nonimmune human Fab phage library. The PH1 variable genes were reformatted for expression as a fully human IgG1. The resulting PH1-IgG1 human antibody displays a 160-fold improved apparent kd (8.7 nmol/L) compared to the kd of the parental Fab (1.4 micromol/L). In cell-binding studies with flow cytometry and immunohistochemistry, PH1-IgG1 exhibits staining patterns typical for antibodies recognizing the tumor-associated tandem repeat region on MUC1, eg, it binds the tumor-associated glycoforms of MUC1 in breast and ovarian cancer cell lines, but not the heavily glycosylated form of MUC1 on colon carcinoma cell lines. In many tumors PH1-IgG1 binds to membranous and cytoplasmic MUC1, with often intense staining of the whole-cell membrane (eg, in adenocarcinoma). In normal tissues staining is either absent or less intense, in which case it is found mostly at the apical side of the cells. Finally, fluorescein isothiocyanate-labeled PH1-IgG1 internalizes quickly after binding to human OVCAR-3 cells, and to a lesser extent to MUC1 gene-transfected 3T3 mouse fibroblasts. The tumor-associated binding characteristics of this antibody, its efficient internalization, and its human nature, make PH1-IgG1 a valuable candidate for further studies as a cancer-targeting immunotherapeutic.

Figure 1.

A: Silver staining of purified PH1-IgG1. Lane1, molecular weight marker; lanes 2 and 3, 5 μl and 2 μl of the purified antibody. B: Western blot of purified PH1-IgG1, 1 μg was loaded.

Figure 2.

BIAcore analysis of PH1-Fab (A and B) and PH1-IgG1 (C). Binding pattern of PH1 (A) and its analysis at steady state [response units equilibrium (RUeq)]) (B). C: Binding pattern of IgG1-PH1, fitted with the BIAevaluation program (version 3.01). Fitting curves at Langmuir with mass transfer limitation) are superimposed (dashed line) to the concentration curves (straight line).

Figure 3.

Flow cytometry. Comparison between HMFG1 and PH1-IgG1 for their binding characteristics to tumor cell lines. Thick line, binding with the antibody; thin line, nonspecific binding control; dotted line, inhibition with MUC1 60-mer.

Figure 4.

Immunohistochemical staining with PH1-IgG1 of cancer and normal tissues. P, Proximal tubuli; D, distal tubuli; LH, loop of Henle.

Figure 5.

Confocal microscope pictures for internalization studies of PH1-IgG1-FITC in OVCAR-3 cells. A: Zero hours after incubation with antibody. B: After 1 hour, at 37°C. C: After 3 hours, at 4°C. D: After 3 hours, at 37°C, showing the feinting of the FITC signal.