AbstractIt has been hypothesized that the intercellular adhesion receptors used by normal cells could also be operative in the spreading of circulating malignant cells to target organs. In the present work, we show that genetic ablation of the ICAM-1 gene confers resistance to T cell lymphoma metastasis. Following i.v. inoculation of LFA-1-expressing malignant T lymphoma cells, we found that ICAM-1-deficient mice were almost completely resistant to the development of lymphoid malignancy compared with wild-type control mice that developed lymphoid tumors in the kidneys, spleen, and liver. Histologic examinations confirmed that ICAM-1-deficient mice, in contrast to wild-type mice, had no evidence of lymphoid infiltration in these organs. The effect of ICAM-1 on T cell lymphoma metastasis was observed in two distinct strains of ICAM-1-deficient animals. Nonetheless, lymphoma cells migrated with the same efficiency to target organs in both normal and ICAM-1-deficient mice, indicating not only that ICAM-1 expression by the host is essential in lymphoma metastasis, but also that this is so at stages subsequent to homing and extravasation into target organs. These results point to posthoming events as a focus of future investigation on the control of metastasis mediated by ICAM-1.