BAT is an immune-modulatory monoclonal antibody that exhibits strong lymphocyte-mediated anti-tumor activity against a variety of murine and human tumors. Peptide A is a vaccine we have developed by screening a phage display peptide library on BAT monoclonal antibody. Anti-tumor activity was obtained in mice inoculated with B16 melanoma by either a single injection with BAT or immunization with peptide A. The aim of this study was to follow and compare histopathologically the process of prevention of melanoma metastases in lungs of treated and immunized mice. Mice were sacrificed on different days after tumor inoculation, their lungs were weighed and the number of metastases was counted. The lungs were then fixed in formalin, embedded in paraffin, and stained with hematoxylin and eosin. Histological examination of tumor inoculated mice on day 10 revealed the existence of microscopic melanoma lesions (0.01-0.012 mm) that increased gradually in number and size and on day 21, most of the metastases were large and spanned entire lobes, from the pleura to the hilum, measuring up to 3.5 mm and coexisting with scattered, small metastases showing the same morphology and pattern of lung involvement. On day 24, the lungs of untreated mice were massively infiltrated by coalescing metastases replacing up to 50% of the lung tissue and measuring up to 7.0 mm. The number of lung metastases and weight was dramatically decreased by a single injection of BAT monoclonal antibody ten days post tumor inoculation. The treated mice clearly had fewer and smaller metastases in different mice at the different days post tumor inoculation. On day 21, there were few small metastases measuring up to 1.6 mm and on day 24 no lung metastases were detected in this group that appeared with a completely normal lung structure. Immunization with peptide A started one day post tumor inoculation and was compared to immunization with control peptide N. Fourteen days post tumor inoculation, mice immunized with peptide A had only 1-2 metastases (0.012-0.076 mm) and on day 24 ranged up to 2 mm compared to control immunized mice where the tumor developed up to 5-7 mm. Foci of lung inflammation in both the untreated, treated or immunized mice were rare, small, and not preferentially associated with the lung metastases. They were composed mainly of small lymphocytes and a few macrophages. This study is the basis of histopathological understanding of metastases prevention in lungs of mice immunized or treated by BAT monoclonal antibody.

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