Dissemination of T cell lymphoma to target organs: a post-homing event implicating ICAM-1 and matrix metalloproteinases.

There is increasing evidence that metastasis of a tumor cell (its ability to induce the "development of a tumor" at distant sites following intravasation) is manifested only after homing to distant site(s). All tumor cells, however, do not necessarily undergo uncontrolled cellular division to form secondary tumors once they have "homed" to a target site. One of the major rate-limiting steps in metastasis is in fact related to the ability of the extravasated tumor cells to find an appropriate "nest", where favorable growth conditions will allow them to form a secondary tumor upon massive cell division (1). But to establish such a favorable nest (referred herein as the "nidification" process), tumor cells must penetrate deep into the stroma of the target tissue. This process is facilitated when tumor cells produce of specific proteases, which degrade structural proteins of the extracellular matrix (2,3). The production of proteases by stromal cells can also occur; these enzymes will degrade stroma surrounding the tumor cells, resulting in a massive remodeling of the local parenchyma that may interfere with the vital functions of a target organ as well as help nidification (4). In this review, we focus our attention on post-extravasation events involving adhesion molecules and MMP in the metastatic process of lymphoma cells. We propose that during dissemination of LFA-1-positive lymphoma cells to peripheral organs, the interaction between lymphoma cells and vascular endothelial cells upregulates the local expression of MMP and TIMPs. Since control of lymphoma metastasis appears to occur at the post-extravasation level, we hypothesize that in addition to extravasation, adhesion molecules are implicated in the control of post-extravasation events.