Histopathological analyses of the antitumor activity of anti-glypican-3 antibody (GC33) in human liver cancer xenograft models: The essential role of macrophages

Abstract

Previously, we demonstrated the antitumor efficacy of the anti-glypican-3 (GPC3)

antibody GC33 in several human liver cancer xenograft models and the important role of

antibody-dependent cellular cytotoxicity (ADCC) in the antitumor mechanism of GC33.

Involvement of other mechanisms such as modulation of the functions of GPC3 in antitumor

activity remains to be elucidated. In this study, we investigated histopathologically

time-course changes in xenografts in mice following a single administration of GC33 to

clarify the morphological changes contributing to the tumor growth inhibition of GC33,

including the changes in GPC3-related factors/components (proliferation, extracelluar

matrices (ECMs) and macrophage). Histopathological changes peaked 3–5 days after GC33

administration and included increased tumor cell death, tumor cells with round morphology,

multinucleated tumor cells and small spindle/round-like cells (mostly F4/80-positive

macrophages). No direct effects of GC33 on proliferation activity of tumor cells were

observed. Meanwhile, alteration of ECM structures and a remarkable increase in macrophages

was noted in the GC33-treated group. Increase in macrophages was observed mainly in the

outer layer of tumor nodules; the area of the increase approximately included the area where

the change in tumor cells and ECMs were observed. Interestingly, depletion of macrophages

in the xenograft models resulted in a marked reduction of the antitumor activity of GC33. In

the in vitro ADCC assay, ADCC was only slightly induced by mouse peritoneal macrophages.

These data suggest that macrophages play an essential role in the antitumor activity of GC33

and the possible involvement of macrophage-mediated non-ADCC action.