Abstract
We have shown previously that rejection of preinduced rat brain tumours is possible following therapeutic immunizations with interferon-gamma (IFN gamma)-transfected glioma cells (N32-IFN gamma). In the present study we have used the same model to evaluate whether quantitative differences in tumour-infiltrating lymphocytes can be detected between animals receiving therapeutic immunizations with either IFN gamma-transfected glioma cells, wild-type glioma cells or no treatment. Since leucocyte transpedesis into the tumour can be anticipated to depend on the state of vascularization, we have mapped the development of microvessels in the tumour in parallel with the leucocyte infiltration. Our results show that microvessels start to form at day 7 and then gradually increase in number and size, indicating the establishment of an extensive vascularization by day 24. Leucocyte infiltration displays a biphasic pattern after tumour grafting. We have therefore studied the infiltration kinetics after an early immunization (1 day after intracerebral isografting) and compared the effects with those of a late immunization (10 days after intracerebral isografting) with N32-IFN gamma or wild-type N32. Our results show (1) an early infiltration of granulocytes 3 days after isografting; (2) a T-cell-receptor-positive (TCR+) T-cell infiltration starting on day 10; (3) a macrophage infiltration starting on day 13; (4) a CD8+ cell infiltration starting on day 13. The proportions of TCR+ T cells, CD8+ cells and natural killer cells differs significantly between animals immunized with N32-IFN gamma and those receiving wild-type N32, when analysed 14 days after immunization at day 10. This difference can only be detected when animals are immunized at later stages of tumour growth. We propose that this could depend on an early-immunization-independent leucocyte infiltration during tumour establishment. This has to be considered when evaluating studies of leucocyte infiltration in experimental tumours.
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