NEW INSIGHTS INTO ICOS-B7h PATHWAY IN ALLOIMMUNE RESPONSE

Abstract

O456* Aims: Recent data indicate that the new T cell costimulatory pathway ICOS-B7h plays an important role in alloimmune responses in vivo. Although its role has been investigated in acute allograft rejection, relatively little is known of its functions in the process of chronic rejection. In this study, we investigated the role of ICOS-B7h pathway in development of chronic rejection of vascularized cardiac allografts. Methods: We utilized a MHC class II disparate cardiac transplantation model (Bm12 into C57BL/6). In this model, Bm12 cardiac allografts survive long-term but spontaneously develop moderately severe chronic vasculopathy, cellular infiltration and fibrosis by 6-8 weeks post-transplant. Results: We treated mice with a blocking anti-B7h mAb (HK5.3) either in the initiation phase (n=6, 0.25mg, every other day, 0-20 days after transplantation) or in the progression phase (n=6, 21-41 days) of chronic rejection. At 6 weeks, histopathological examination of grafts was performed by a blinded observer and scored in terms of vasculopathy (score 0-4), fibrosis (grade 0-3) and cellular infiltration (grade 0-3). Early blockade of ICOS-B7h pathway didn’t show a protective effect on chronic rejection (vasculopathy; 3.5±0.4, fibrosis; 2.2±0.3, cellular infiltration; 2.3±0.3: mean±SEM) compared to untreated controls (n=6, vasculopathy; 3.6±0.1, fibrosis; 2.7±0.2, cellular infiltration; 2.7±0.2). In contrast, delayed blockade of B7h significantly inhibited the development of chronic vasculopathy (1.8±0.4 vs. control, P=0.043), fibrosis (1.5±0.2, P=0.004) and cellular infiltration (1.7±0.3, P=0.03). To address the underlying mechanisms, we performed quantative real-time PCR analysis. There was significant decrease in local intragraft expression of cytokines, such as TNF-α, IFN-γ and IL-4, and several key chemokines, such as RANTES and IP-10, as well as adhesion molecules such as E-selectin in recipients with delayed blockade of B7h compared to controls. Moreover, there was significantly less infiltration of CD4 and CD8 T cells (p=0.007 and p=0.042 vs. control). Thus, delayed blockade of ICOS-B7h pathway downregulated local immune activation, thereby resulting in the less infiltration of alloreactive cells into cardiac grafts. Furthermore, to rule out the possibility that early blockade simply was not sufficient enough to display the protective effect on chronic rejection in this model, we treated mice with mAb in the entire period (n=6, 0-41 days, every other day). Interestingly, these mice demonstrated severe chronic rejection (vasculopathy; 3.3±0.4, fibrosis; 2.7±0.2, cellular infiltration; 2.8±0.2). Taken together, ICOS pathway may be critical in the progression phase of chronic rejection. In addition, its blockade in the initiation phase may have a detrimental effect on the prevention of chronic rejection. Conclusions: These data highlight the importance of ICOS-B7h costimulatory pathway in the pathogenesis of progression of chronic allograft rejection and provide the rationale to explore novel blockade strategy to treat this important clinical problem.