Chronic rejection, operatively defined as obliterative thickening of tubular structures such as graft vasculature, has become an intractable problem despite any current, otherwise effective, immunosuppression used in the management of transplant (Tx) recipients. A hallmark of chronic cardiac rejection is accelerated or Tx arteriosclerosis, characterized by diffuse intimal thickening due to the initial accumulation of macrophages (MO), followed by smooth muscle cells (SMC). Based on in vitro and some descriptive in vivo studies, the pathogenesis of Tx arteriosclerosis is thought to involve T cell recognition of donor MHC antigens expressed by endothelial leukocyte adhesion molecules, chemotaxis and activation of MO. Local elaboration of pro-inflammatory cytokines, and recruitment and proliferation of medial-derived SMC within the intima. Though, collectively, these events involve activation of many genes and complex intracellular pathways, most, if not all, such steps involve activation of the transcriptional factor, NFkB. Indeed, in vitro studies have led to a new paradigm in which NFkB is the critical regulator of multiple genes activated during inflammatory or immunologic responses. Thus, NFkB is considered essential to the induction of IL-2 and IL-2R in T cells; Ig synthesis by B cells; IL-1 and TNFalpha in MO; IFNg in NK cells; IL-1alpha and beta, IL-6, IL-8, MCP-1, E-selectin, ICAM-1 and VCAM- 1 in EC; and cell-matrix interactions in fibroblasts and smooth muscle cells. However, due to a lack of suitable and specific inhibitors of the NFkB pathway, little has been done towards testing this knowledge in vivo, including in the context of allograft recipients and Tx arteriosclerosis. We have developed a model of chronic cardiac allograft rejection in the mouse in which long-term allograft survival is achieved by brief CD4 monoclonal antibody (mAb) therapy post-Tx, albeit with a concomitant development of severe Tx arteriosclerosis. We propose to use this model to identify the mechanisms responsible for development of chronic rejection, focusing on the use of transgenic mice expressing a tetracycline-sensitive inhibitor of the transcription factor, NFkB. This novel system should function as a conditional NFkB/Rel """"""""knockout"""""""", in which expression of the inhibitor can be regulated by tetracycline and allow shutdown of NFkB activity at any stage of the experimental protocol. Thus, analysis of these mice should provide the first direct in vivo evidence as to the role of the NFkB pathway, and potentially lead to the development of important new therapeutic strategies of direct relevance to Tx recipients.
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